What are Kidneys, Structure and Anatomy?

What are Kidneys and Why Are They So Important?

The kidneys are two bean-shaped organs of the renal system that are responsible for blood filtration, removing waste items and drugs, balancing fluids, releasing hormones to regulate blood pressure, producing an active form of vitamin D for bones, and controlling the production of red blood cells.  

The kidneys are necessary organs of the urinary system. They filter the blood and remove excessive waste products in the form of urine. They lie on each side of the spine, at the back, and below the rib cage.

Each kidney filters about 120-125 millilitres of blood per minute, which is approximately 180 litres per day. It is also a major organ that makes red blood cells and is responsible for keeping bones healthy. The kidneys also maintain the human body's temperature, a process known as homeostasis.

What are Kidneys and Their Functions

Kidneys perform the following functions:

• Kidneys filter the blood and excrete waste and excess water through urine.
• Kidneys regulate blood pressure.
• Kidneys maintain acid-base balance.
• Kidneys regularly maintain electrolytes.
• Kidneys secrete certain hormones, including erythropoietin and renin.

Kidneys act as filter systems. They keep the body healthy by filtering out waste and extra fluids from the blood. The blood enters the kidneys through blood vessels called renal arteries. Inside the kidneys, blood is filtered through tiny units called nephrons, which remove waste products and excess substances.

The cleaned blood moves to the kidney through renal veins and then returns to the body. Every minute, the kidneys filter around one litre of blood, which is about one-fifth of all the blood pumped by the heart.

Renal Physiology and Function

Filtration

Filtration occurs in the glomerulus. The glomerulus is a tiny part of the kidney. It is made up of a cluster of blood vessels known as capillaries. The glomerulus is surrounded by a cup-shaped structure known as Bowman’s capsule, which is responsible for blood filtration. 

The filtration system consists of different layers, including tiny cells and a membrane.  The high pressure in these little vessels forces water, small molecules (like glucose and amino acids), and waste products out of the blood and into the Bowman's capsule. 

Due to their size, larger molecules, such as proteins and blood cells, remain in the blood. The result is a liquid called glomerular filtrate. It is similar to blood plasma but without the larger proteins and cells.

Tubular Function

After filtration, the filtrate passes through the renal tubules. The renal tubule consists of a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule, and collecting ducts. These tubules reabsorb essential substances like glucose and ions back into the blood.

They also reabsorb other substances to maintain homeostasis. Reabsorbed products include amino acids, bicarbonate, water, and phosphate. It also secretes extra waste and ions, such as hydrogen and potassium, from the blood and tubules.

Reabsorption 

When the filtrate moves through the renal tubules, reabsorption occurs. The process involves the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. During reabsorption, the blood takes back the essential nutrients. The nutrients include glucose, amino acids, and ions such as sodium, potassium, and calcium. 

This process involves both passive and active transport. Passive transport moves substances naturally from high to low concentration, while active transport requires energy to move substances against their natural flow. 

The loop of Henle plays an essential role in concentrating urine by establishing a countercurrent multiplier system. It creates a concentration gradient in the medulla of the kidney. This results in concentrated urine because the bloodstream reabsorbs required substances.

Secretion 

Secretion occurs in the distal convoluted tubule and collecting ducts. Through these ducts, additional substances are added to the filtrate from the blood. These include excess ions like potassium, hydrogen ions, creatinine, and some drugs. 

The secretion process is also responsible for maintaining the body’s acid-base balance. It removes substances that the initial filtration missed or needs to eliminate more effectively. The secretion process ensures that these extra substances are removed from the body through urine.

Excretion and Formation of Urine:

The urine formation occurs in the excretion step after filtrates pass through reabsorption and secretion. 

Excretion 

Excretion is the final step in the urine formation. The processed filtrate, known as urine, flows from the collecting ducts into the renal pelvis. It travels through the ureters to the urinary bladder, where it is temporarily stored.

During urination, the bladder contracts, pushing the urine through the urethra and out of the body. This process removes waste products and excess substances in the form of urine. The excretion also helps to maintain the body’s overall balance and health.

Hormone Secretion

The kidneys secrete hormones like erythropoietin, renin, and calcitriol. These hormones perform different functions, as highlighted below. 

Erythropoietin (EPO)

Erythropoietin is a glycoprotein hormone that stimulates red blood cells (RBC). EPO isis primarily produced by the interstitial fibroblasts in the renal cortex of the kidney. The liver also secretes this hormone, but the kidneys are the primary producers in adults.

Renin

Renin is an enzyme that converts angiotensinogen hormone into angiotensin I, leading to the production of angiotensin II. It responds to constrict blood vessels and raise blood pressure. Lymph is a fluid with white blood cells that fight infections. The body refers to the main component of extracellular fluid as interstitial fluid.

Calcitriol

Calcitriol is an active form of vitamin D that affects hormones. It helps the body to absorb more calcium from food and keeps more phosphate in the kidneys.

Blood Pressure Regulation:

The kidneys play a significant role in long-term blood pressure regulation by controlling blood volume through the renin-angiotensin-aldosterone system (RAAS), which can have both immediate and long-term effects on blood pressure. They make long-term changes to adjust the pressure in the arteries by managing the fluid outside of cells, i.e., extracellular fluid.

This adjustment of blood pressure occurs after the release of a vasoconstrictor hormone known as Angiotensin II. This hormone is responsible for narrowing blood vessels. These hormones also play an essential role in increasing the kidneys’ absorption of sodium chloride, or salt.

This absorption effectively increases the size of the extracellular fluid compartment and raises blood pressure. Anything that alters blood pressure, including excessive alcohol consumption, smoking, and obesity, can damage the kidneys over time.

Maintaining pH

The normal blood pH range ranges from 7.35 to 7.45. If the pH goes below or above this range, it can lead to acidemia (too acidic) or alkalemia (too basic). The change in pH can cause problems and can be life-threatening. Also, fluctuation in pH can cause the breakdown of proteins and enzymes to stop working properly. 

Both organs—the lungs and kidneys—are responsible for maintaining the pH level to normal. The lungs control blood pH by adjusting the amount of carbon dioxide in the blood. The kidneys help by balancing pH by reabsorbing bicarbonate and producing urine based on the pH level. If the body needs to reduce acid, the kidneys can make more bicarbonate by excreting excess acid.

Osmolality Regulation

Osmolality refers to the concentration of solutes (mainly sodium, chloride, and other ions) per kilogramme of solvent (water) in the body fluids. It maintains the water and electrolyte balance. Dehydration is a primary cause of electrolyte imbalance.

If the blood osmolality level increases, the brain’s hypothalamus notices and sends a signal to the pituitary gland. This gland then releases an antidiuretic hormone (ADH).

When ADH is released, the kidneys make several changes to help balance.

• The kidneys make urine more concentrated by reducing the amount of water in it.
• They absorb more water back into the body.
• They open channels that usually block water from passing through, allowing more water to be absorbed.
• Kidneys keep urea (a waste product), which helps pull more water into the bloodstream.

Detailed Renal Anatomy

How much do my kidneys weigh? 

The chart below highlights individuals' weights by age group and gender. These values are approximate averages. Kidney weight can vary depending on factors such as body size, overall health, and individual physiology.

The weight of the kidney generally decreases slightly with age due to changes in renal structure and function.

Age Group	Gender	Kidney Weight (g)
Newborn	Male/Female	4 - 5 grams
Adult (18-65 years)	Male	125 - 170
	Female	115 - 155
Elderly (65+ years)	Male	110 - 160
	Female	100 - 150

 

 

In-Depth Anatomy:

 

Kidneys are reddish-brown in colour, with a smooth outer surface and a curved inner surface. Each kidney is about 11-14 cm long, 6 cm wide, and 4 cm thick. They are positioned at the T12 to L3 levels of the vertebral spine. The kidneys are surrounded by three layers: the renal cortex, renal medulla, and renal pelvis.

Kidney Capsule (Renal Capsule):

The kidney capsule (renal capsule) is a thin, tough, and fibrous connective tissue layer that surrounds the outer surface of the kidney. It provides structural support and protection and helps maintain the kidney's shape. It also acts as a barrier against infections and physical trauma. The capsule also contains blood vessels and nerves that supply the kidney.

Renal Artery:

The renal artery is a major blood vessel that supplies blood to the kidneys. It arises from the abdominal aorta and enters each kidney at the hilum. The renal artery carries oxygenated blood from the heart to the kidneys. Inside the kidneys, the blood is filtered, with waste products and excess fluids removed. 

Renal Cortex: 

The renal cortex is the outermost granular layer of the kidney. It is situated between the renal capsule and the renal medulla. The renal cortex has grains due to glomeruli and convoluted tubules and extends inward towards the medulla and renal column. The renal cortex is primarily responsible for the filtration of blood and the formation of urine.

Renal Medulla

The renal medulla is the inner layer of the kidneys. It consists of 8–18 cone-shaped structures, known as renal pyramids. These pyramids are composed of renal tubules and collecting ducts. The renal medulla concentrates urine and regulates electrolyte balance. 

The pyramids collect urine from the nephrons and funnel it into the renal pelvis. The base of the renal pyramid is the wider, more expansive section orientated towards the renal cortex. The renal columns are extensions of the renal cortex that project inward between the renal pyramids (also known as renal medullary pyramids). 

Renal Papilla

The base of each pyramid faces outwards, forming a tip known as the renal papilla. The papilla points inward towards the renal pelvis. The medulla looks striped because the tubules and blood vessels inside it are lined up in parallel rows. 

Urine produced in the collecting ducts drains through the renal papilla into the minor renal calyx. From the minor calyx, the urine flows into the major renal calyx, a larger, funnel-shaped cavity that collects urine from several minor calyces. The major calyces channel the urine into the renal pelvis, where it is directed through the ureter to be excreted from the body. Each kidney has two or three major calyces.

Renal Pelvis

The renal pelvis is a funnel-shaped area in the kidney that collects urine. It gathers urine from the renal papillae and sends it into the ureter, which carries it to the bladder. The renal pelvis splits into two or three large branches called calyces. Each calyx further divides into smaller branches called minor calyces, which surround each renal papilla.

Renal Vein:

The renal vein is a major blood vessel that drains deoxygenated blood from the kidneys and returns it to the heart. Each kidney has its renal vein, which exits the kidney at the hilum. The renal vein collects the filtered blood from the kidneys and carries it back to the inferior vena cava, which then returns it to the heart.

Nephron:

The nephron is the functional unit of the kidney that filters blood and produces urine. Each kidney contains about a million nephrons. A nephron consists of a renal corpuscle, which includes the glomerulus, Bowman's capsule, and a renal tubule, which processes filtered fluid. It is the leading site of filtration, reabsorption, secretion, and excretion, which are all essential to urine formation. Blood enters the nephron through the glomerulus, where waste and excess substances are filtered out into Bowman's capsule. The filtered fluid then passes through the tubule, where essential nutrients and water are reabsorbed, and waste is concentrated into urine. The urine flows to the collecting ducts, the renal pelvis, and into the ureter.

 

Glomerulus :

The glomerulus is a key component of the kidney's filtration system. It consists of a network of tiny blood vessels (capillaries) located within the Bowman capsule. The glomerulus filters blood by allowing water, salts, and small molecules to pass through its porous walls while retaining larger molecules like proteins and blood cells. This filtration process removes waste and excess substances from the blood to form urine. Each kidney contains around one million glomeruli, making them essential for maintaining fluid and electrolyte balance and overall kidney function.

Blood Supply:

Blood flows to the kidneys through the renal arteries, which come from the abdominal aorta. Each renal artery splits into smaller arteries: segmental, interlobar, arcuate, and interlobular. The interlobular arteries then lead to small blood vessels called afferent arterioles, which are responsible for bringing blood to the glomeruli.

After the blood is filtered in the glomeruli, it passes through efferent arterioles, which branch into tiny capillaries. The peritubular capillaries surround the renal tubules and the vasa recta around the loop of Henle. The capillaries merge into interlobular veins, then arcuate veins, interlobar veins, and finally, the renal veins, which carry the blood to the inferior vena cava.

Nerve Supply

Nerves innervate the kidneys through the renal plexus, which includes two types of nerve fibres: sympathetic and parasympathetic.

• Sympathetic Nerves: These nerves originate from the thoracic and lumbar spinal cord (from the middle and lower parts of the spine). The sympathetic nerve is responsible for regulating blood flow and GFR rate. It also filters blood and releases the renin hormone to manage blood pressure.
• Parasympathetic Nerves: These have minimal functions and mainly affect the renal pelvis (the part where urine collects) and the ureter (the tube that carries urine to the bladder). They help move urine through these areas.

Development and Growth

 

In the developing embryo, the kidneys go through three stages of formation: the pronephros, mesonephros, and metanephros. All of these stages originate from the urogenital ridge. 

Pronephros 

The pronephros is an early, non-functional stage that appears around the 4th week but is quickly replaced by the mesonephros. Its development begins in the cervical region of the embryo. They form tiny tubes called nephrotomes of a total of 6-10 pairs. 

These tubules join into the pronephric duct that extends from the cervical region to the cloaca. This early kidney system is not functional and disappears by the end of week 4.

Mesonephros 

The mesonephros develop below the pronephros. The presence of the pronephric duct stimulates nearby cells from the thoracolumbar region to form mesonephric tubules. These tubes receive blood vessels for filtering and drain into a mesonephric duct. 

This stage serves as the embryo's primitive excretory system. However, most of these tubes diminish by the end of the second month. Additionally, the mesonephric duct grows a bud known as the ureteric bud, which induces the development of the definitive kidney.

Metanephros 

The metanephros is the final and functional kidney. It starts forming in the 5th week of development and becomes functional around the 12th week. A bud from the mesonephric duct connects with a part of the embryo called the metanephric blastema. It leads to the development of mature, functional kidneys.

Factors that Influence Kidney Development and Growth

Various factors influence the development and growth of kidneys.

 

Factor	Description
Genetic Factors	Genes regulate kidney tissue formation and differentiation. Mutations can cause kidney disorders.
Embryonic Signals	Signalling molecules (e.g., FGFs, BMPs) guide kidney growth and differentiation.
Interactions with Other Tissues	Close interaction between developing kidney tissues (e.g., metanephric blastema and ureteric bud) is essential for proper formation.
Blood Supply	Adequate blood flow provides nutrients and oxygen necessary for kidney development.
Hormones	Hormones (e.g., aldosterone and angiotensin) regulate kidney maturation and function.
Environmental Factors	Maternal health, nutrition, and exposure to toxins can affect kidney development.
Mechanical Forces	Physical forces and the mechanical environment impact kidney growth and positioning.

 

Kidneys in the Urinary System

How Kidneys Fit into the Broader Urinary System

The kidneys are a central part of the urinary system. They filter blood, remove waste products, and maintain fluid and electrolyte balance. The urinary system consists of two kidneys, two ureters,  the bladder, and the urethra.

Each organ of the urinary system performs different functions.

• Kidneys: Filters blood excrete fluid
• Ureters: These tubes connect kidneys to the bladder.
• Bladder: An organ for storing urine.
• Urethra: A tube connects the bladder to the body's surface. Its primary role is to allow urine to be excreted from the bladder out of the body.

Interaction of Kidney with Other Urinary Organs

The urinary system consists of the kidneys, ureters, bladder, and urethra. These organs work together to filter, store, and remove liquid waste from the body. The urinary system is located in the abdominal and pelvic regions.

The urinary system consists of a pair of kidneys, each about the size of a fist. They filter around 120 to 150 quarts (113.6 to 141.95 litres) of blood every day, making about 1 to 2 quarts (0.95 to 1.95 litres) of urine.

Ureters: These are two thin tubes that carry urine from the kidneys to the bladder. Each ureter is about 9 inches long. The left ureter carries urine from the left kidney to the urinary bladder. It is part of the urinary system and is paired with the right ureter, which performs the same function for the right kidney.

Urinary Bladder: The bladder stores urine until it is excreted from the body. It’s a narrow, hollow muscle shaped like a triangle. It can hold about 2 cups (500 millilitres) of urine. The brain controls the bladder, signalling it when to hold urine and when to empty. The bladder becomes empty when the urine is excreted from the body.

Urethra: The urine is excreted from the bladder through the tube called the urethra.  Two sphincter muscles keep the urethra closed to hold urine until it is excreted. These muscles open and close to let urine out. If these muscles weaken, it becomes harder to control urine.

Urinary incontinence occurs when urine is accidentally or involuntarily lost from the bladder. To urinate normally, all parts of the urinary tract must work together in proper order.

Maintaining Healthy Kidneys

It is essential to maintain an active lifestyle and eat a nutritious diet. Healthy dietary options improve kidney health and reduce further complications. Here are some tips to help keep the kidneys healthy.

Healthy Kidney Tips

Stay Active and Eat Nutritious Food

Exercise is not just for losing weight. It lowers the risk of kidney disease, reduces blood pressure, and improves heart health. Eating a balanced diet with less salt and processed foods can protect the kidneys. Choose fresh foods like cauliflower, blueberries, fish, and whole grains.

Control Blood Sugar Level

High blood sugar levels are harmful to the kidneys. Kidneys work harder to filter blood for people who have diabetes or high blood sugar. It can cause damage to the kidneys over time. Keeping blood sugar in check can reduce this risk and help prevent further damage.

Monitor Blood Pressure

High blood pressure can damage the kidneys. It’s more risky for people who have diabetes, heart disease, or a high lipid profile. The normal blood pressure is about 120/80 mm/Hg. If the blood pressure increases, changes in lifestyle and dietary improvements can reverse it. 

If the readings remain consistently above 140/90 mm/Hg, it is advised to consult a doctor to manage it. Taking medication is also considerable, with consultation advice if needed.

Stay Hydrated 

Staying hydrated improves kidney health. For proper kidney functioning, it is recommended to drink 1.5 to 2 litres of water a day. Water helps the kidneys remove toxins and lowers the risk of kidney disease. Drinking more water also helps to remove kidney stones. 

Avoid Smoking 

Smoking is harmful to the kidneys. It damages the blood vessels and reduces the blood flow to the kidneys. The reduction of blood flow increases the risk of kidney cancer. It is recommended to avoid smoking to avoid further kidney complications.

Be Aware of Over-the-Counter Medications 

Taking too many over-the-counter painkillers, like ibuprofen or naproxen, can damage kidneys. According to the National Kidney Foundation, it is recommended not to use painkillers for more than 10 days or 3 days for fever. If someone needs pain relief often, talk to a doctor about safer options.

People with a Higher Risk of Kidney Damage

A regular kidney test is necessary if a person is at a higher risk of kidney damage. Regular screening may benefit people over 60 or who were born with a low birth weight. It is also beneficial for those who have heart disease, high blood pressure, or are obese. Regular tests can help detect kidney problems at an earlier stage and prevent further damage.

Measuring Kidney Function:

It is important to understand kidney functions in order to diagnose and manage renal conditions. The primary tests to evaluate renal function include:

• Estimated glomerular filtration rate (eGFR)
• Electrolytes
• Blood urea nitrogen (BUN) or urea
• Creatinine levels and creatinine clearance
• Cystatin C levels
• Imagining Tests 
• Kidney Biopsy
• Lab Tests

Estimated Glomerular Filtration Rate (eGFR)

The eGFR measures how well the kidneys filter waste from the blood. The eGFR value can be calculated by taking serum creatinine level, age, sex, and race. This test helps assess overall kidney function and stage of chronic kidney disease.

Electrolytes

Electrolyte tests measure the levels of essential minerals in the blood, such as sodium, potassium, and chloride. The levels of electrolytes indicate how well the kidneys maintain fluid balance and acid-base status.

Blood Urea Nitrogen (BUN) or Urea

BUN measures the amount of nitrogen in the blood from waste product urea. The increased value of BUN levels indicates impaired kidney function. It is because the kidneys are less able to remove urea from the blood.

Creatinine Levels and Creatinine Clearance

Creatinine is a waste product from muscle metabolism. Creatinine levels or clearance tests measure kidney function by determining how much the kidneys filter creatinine from the blood. A lower clearance rate refers to reduced kidney function and vice versa. 

Calculate creatinine clearance to measure kidney functioning

Cystatin C Levels

Cystatin C is a protein that is filtered by the kidneys. The increased cystatin C levels in the blood indicate impaired kidney function. Cystatin C starts accumulating due to improper functioning of the kidneys. 

Lab tests

Lab tests include urinalysis, which involves testing a urine sample. A urine sample is taken for analysis during urinalysis. The test shows whether the urine contains blood and minerals that form kidney stones or not. The presence of white blood cells and bacteria in the urine shows urinary tract infection (UTI).

Imaging Tests 

Imaging tests include ultrasound and CT scans. They provide visual information about the kidney's structure and can identify kidney abnormalities. Imaging tests can detect kidney stones, tumours, or structural defects that affect kidney function.

Kidney Biopsy

A kidney biopsy involves taking a small sample of kidney tissue to examine under a microscope. This test can diagnose kidney complications and also determine the extent of damage or disease.

Clinical Significance of Kidney Health

Acquired Diseases

Chronic kidney disease (CKD) and kidney stones are referred to as acquired diseases. 

Chronic Kidney Disease (CKD)

Chronic kidney diseases refer to damaged kidneys that can’t filter blood. Diabetes, high blood pressure, heart disease, and a family history of kidney failure are the main risk factors for developing kidney diseases.

What are the Causes of CKD?

Chronic kidney disease is usually caused by other conditions that strain the kidneys. It often results from a combination of different problems. CKD can be caused by

• Increased blood pressure 
• Uncontrolled diabetes 
• High lipid profile and cholesterol 
• Kidney infections
• Glomerulonephritis (kidney inflammation).
• Autosomal dominant polycystic kidney disease.
• Consistent urine blockage.
• Long-term usage of non-steroidal anti-inflammatory drugs (NSAIDs)

What are the Symptoms of Chronic Kidney Diseases (CKD)?

The symptoms of CKD that appear in the advanced stage include

• Fatigue and tiredness
• Swelling in ankles, feet, or hands
• Shortness of breath
• Mild fever or feeling sick
• blood in the urine

How to Diagnose CKD?

CKD can be diagnosed through blood tests to measure kidney function, urine tests for protein levels, and imaging to assess kidney structure. A kidney biopsy can also be used for a detailed diagnosis of chronic kidney disease.

How to Manage CKD?

It is necessary to take preventive steps to protect the kidneys' health. The most important step in treating kidney disease is controlling blood pressure. Healthy habits can also help to manage kidney disease.

How to Treat CKD?

The main treatment for CKD depends on the functioning and severity of kidney health. Treatment options include

• Medications to control other complications, including high blood pressure and high cholesterol
• Dialysis (for manual filtration of blood)
• Kidney transplant 

 

Kidney Stones

Kidney stones are hard, pebble-like deposits that form in the kidneys due to high levels of certain minerals. These stones vary in size and shape, from as small as a grain of sand to as large as a golf ball. They may be smooth or jagged and are typically yellow or brown.

Small kidney stones may pass through the urinary tract with minimal pain. Larger stones may become lodged and make it difficult to pass through urine. Blocking urine flow can cause severe pain or sometimes bleeding.

How to Diagnose Kidney Stones?

Kidney stones can be diagnosed with severe pain in the kidneys and urine obstruction. The confirmation of kidney tests includes:

• estimated glomerular filtration rate (eGFR)
• electrolytes
• blood urea nitrogen (BUN) or urea
• creatinine levels and creatinine clearance
• cystatin C levels

How to Manage Kidney Stones

To manage kidney stones, follow these steps.

• To manage kidney stones, drink plenty of water to help flush them out. 
• Eat a balanced diet and avoid foods high in oxalates. 
• Use pain relievers for discomfort and follow the doctor’s advice. 
• Regular check-ups can help track the stones, and medical procedures may be needed for large or painful stones.

Kidney Injury and Failure

Acute Kidney Injury

The term acute kidney injury (AKI) refers to the kidneys suddenly stopping to work properly. AKI can range from a minor loss of kidney functioning to complete kidney failure. It usually results from complications of other serious illnesses.

AKI is common in older people whose kidneys are damaged due to other complications. So, it must be detected early and treated on time to avoid further complications. AKI may also affect and reduce the ability of other organs to work properly. If it is not treated properly, abnormal levels of salts and chemicals start to build up in the body. 

It can also affect children and young people. The common symptoms of acute kidney injury (AKI) include:

 

• Fever or feeling sick 
• Diarrhoea
• Dehydration
• Excreting less urine than usual
• Confusion and drowsiness

If the acute kidney injury is not treated well, it may lead to chronic kidney disease. The failure of kidneys may lead to temporary support from dialysis, kidney transplant, or ultimately death.

What are the Causes of Acute Kidney Injury (AKI)?

 

AKI is usually caused by reduced blood flow to the kidneys due to other health complications. The following 8 reasons could cause the reduced blood flow:

 

• Low blood volume after bleeding 
• Excessive vomiting or diarrhoea
• Severe dehydration
• Heart pumping out less blood than normal due to heart failure, 

• Liver failure or sepsis
• Intake of certain medicines to reduce blood pressure, such as ACE inhibitors, diuretics, or NSAIDs
• Inflammation of glomerulonephritis, the blood vessels (vasculitis), or other structures in the kidney.
• Blockage of an enlarged prostate, tumour in the pelvis such as an ovarian or bladder tumour, kidney stones

How to Diagnose Acute Kidney Injury (AKI)

AKI can easily be diagnosed with urine and blood tests. 

The blood test measures creatinine levels, which reflect a chemical waste produced by the muscles. Increased creatinine levels in the blood are diagnosed as CKI due to improper kidney function. Urine analysis is done to analyse protein, blood cells, sugar, and waste products. The presence of these substances indicates kidney malfunction.

How to Treat Acute Kidney Injury (AKI)

The treatment of AKI depends on the causes and severity of the illness. Proper diagnosis is necessary before treatment. These steps need to be followed for the treatment of acute kidney injury.

• Increase water intake and other liquid fluids in case of dehydration.
• Use antibiotics in cases of infection prescribed by a doctor.
• Do not take over-the-counter (OTC) medicines if the illness has been resolved.
• In cases of bladder blockage, doctors might use a urinary catheter (a thin tube that drains the bladder).

The majority of people recover from acute kidney injury, but some are unable to. Depending on the severity of their symptoms, they might develop chronic kidney disease (CKD). In CKD, a dialysis machine filters blood to excrete waste products from the body. 

Dialysis

Due to the severity of chronic kidney disease (CKD), the kidneys may stop working over time. This process is usually slow, giving time to prepare for the next steps in treatment. When CKD reaches this point, one option is dialysis. Dialysis helps remove waste and extra fluid from the blood.

There are 2 types of dialysis.

 

• Haemodialysis 
• Peritoneal dialysis

Haemodialysis

Haemodialysis is a treatment that filters blood using a machine called a haemodialyzer. It is usually done 3 times a week at the hospital or home. Blood is entered from the body, cleaned by the machine, and then returned to the body. During a haemodialysis session, needles are placed in a blood vessel, often in the arm. 

Blood flows through a dialyser, filters, and removes waste and extra fluid. The cleaned blood is then returned to the body. Each session lasts about 3-4 hours and is typically done three times a week. Haemodialysis is usually performed at a dialysis centre, but it can also be done at home with the right equipment and training.

Haemodialysis is effective for removing waste. It allows for more regular monitoring of health. With proper setup, it can also be done at home.

Peritoneal Dialysis

Peritoneal dialysis uses the abdomen’s lining to filter the blood. A special fluid is used to absorb waste from the blood. In peritoneal dialysis, a catheter is placed into the abdomen. Through this catheter, a fluid called dialysate is introduced. Waste and extra fluid from the blood move into the dialysate. 

After a few hours, the used fluid is drained and replaced with fresh fluid. This process can be done manually several times a day or automatically using a machine while sleeping. Peritoneal dialysis can be done at home, which offers flexibility and convenience. It also provides a more continuous form of treatment.

When Dialysis is Used?

Dialysis is used when the kidneys fail to perform their basic functions. It usually occurs in advanced stages of kidney failure, termed end-stage renal disease (ESRD) or stage 5 chronic kidney disease (CKD).

Indicators for Dialysis Include:

• Severe Decline in Kidney Function: When the kidneys’ ability to filter blood drops below 10-15% of normal function.
• Build-up of Waste and Fluids: When waste products and excess fluid accumulate in the blood, causing symptoms such as swelling, fatigue, or confusion.
• Electrolyte Imbalance: When levels of potassium, sodium, or other electrolytes are increased.
• Persistent Symptoms: When symptoms like severe nausea, vomiting, or shortness of breath cannot be managed with medication alone.

Why Dialysis is Used

Dialysis is a lifesaving treatment that performs some of the important functions of healthy kidneys:

• Removes Waste Products: Dialysis filters out harmful waste products from the blood that healthy kidneys would normally remove.
• Controls Fluid Balance: It helps regulate the amount of fluid in the body, preventing fluid overload that can lead to heart failure or lung problems.
• Maintains Electrolyte Balance: Dialysis helps control levels of electrolytes like potassium and sodium, which are vital for heart and muscle function.
• Regulates Blood Pressure: By managing fluid levels and removing waste, dialysis can help control high blood pressure.

Without dialysis or a kidney transplant, kidney failure can lead to serious complications and can be life-threatening. Therefore, dialysis is necessary for maintaining health and quality of life when kidney function is significantly impaired.

 

Congenital Diseases

Polycystic Kidney Disease (PKD)

Polycystic Kidney Disease (PKD) is a genetic condition in which many fluid-filled cysts form in the kidneys. PKD is an inherited disorder that causes clusters of cyst formations in the kidneys. These cysts are noncancerous and fluid-filled. The size of fluid-filled cysts may vary from small to very large, causing harm to the kidneys.

The severity of PKD can also lead to cyst formation in the liver and other parts of the body. It can cause serious complications, including high blood pressure and kidney failure. The severity of PKD can vary, but some complications can be prevented with lifestyle changes. Treatments for certain complications help protect kidney function.

Signs and Symptoms of Polycystic Kidney Disease (PKD)

The majority of people suffering from PKD don’t notice any symptoms until the age of 30 to 40 years. About 25% of people with PKD have a heart valve problem that makes their chest flutter and can cause chest pain. These symptoms usually disappear with time but can be an early sign of PKD.

The most common sign of PKD is high blood pressure. Sometimes, patients might get headaches from high blood pressure, or their doctor might find high blood pressure during a routine check-up. It’s important to treat high blood pressure because it can damage the kidneys and lead to kidney failure if not managed properly. Other signs of PKD include:

• Back or side pain
• A bigger abdomen
• Blood in the urine
• Frequent kidney infections
• Persistent high blood pressure
• Chest pain/fluttering in the chest

Diagnosis of Polycystic Kidney Disease (PKD)

Ultrasound is the reliable and most affordable way to diagnose PKD. If someone at risk for PKD is over 40 and has a normal ultrasound of their kidneys, they likely don’t suffer from it. A computed tomography (CT scan) or magnetic resonance imaging (MRI) test helps to diagnose the smaller cysts. MRI can also measure and track the growth of cysts in the kidney.

Genetic testing is also used to diagnose PKD. It involves a blood test to investigate faulty genes that cause PKD. This test is costly, and it also fails to detect PKD in about 15% of people who have it. Genetic testing can be helpful if someone:

• Has an unclear diagnosis from imaging tests
• Has a family history of PKD and wants to donate a kidney
• Is under 30, has a family history of PKD, a negative ultrasound, and is planning to start a family

Treatment of Polycystic Kidney Disease (PKD)

There is no cure for PKD, but research is ongoing. Recent studies suggest that drinking plain water throughout the day and avoiding caffeine can decrease cyst growth. Research is also improving the concepts and understanding of the genetic causes of PKD.

Some treatments decrease the progression of kidney diseases, but more research is needed. Meanwhile, the symptoms of PKD can be managed by following ways to protect the kidney from further complications.

• Keep blood pressure under control.
• Treat bladder or kidney infections with antibiotics promptly.
• Drink plenty of fluids if the blood is observed in the urine.
• Use painkillers carefully; consult the doctor about safe over-the-counter options.
• Adopt a healthy lifestyle by quitting smoking, exercising, controlling weight, and reducing salt intake.
• Keep hydrated by drinking a lot of plain water throughout the day.
• Avoid caffeine in all beverages. 

Medication of Polycystic Kidney Disease (PKD)

In April 2018, the FDA approved a new drug called tolvaptan to treat autosomal dominant polycystic kidney disease (ADPKD). This drug helps slow down the loss of kidney function in adults with this type of PKD. Consult a healthcare professional to learn more about this treatment and determine if it's suitable for the disease.

Diagnosis

There are two methods for diagnosing kidney disease: imaging and biopsy. Although imaging technology has advanced dramatically over the past 100 years, old imaging tests are still considerable.

Imagining

Imagining includes the following diagnostic methods:

Computed radiography (digital X-ray)

Computed radiography is used for the initial assessment of kidney stones. It also determines their shape and size. 

Ultrasonography (Ultrasound) 

Ultrasound uses high-frequency sound waves to create images of the kidneys. It helps diagnose conditions by evaluating kidney size, shape, and structure and detecting cysts and tumours. It also helps assess kidney stones and monitor kidney health without radiation, congenital anomalies, swelling, and blockage of urine flow.

Computed tomography (CT scan)

A CT scan of the kidneys provides detailed cross-sectional images to diagnose conditions such as kidney stones, tumours, and trauma. It helps detect abnormalities, assess kidney structure, and guide procedures.

Magnetic resonance imaging (MRI)

MRI provides detailed, high-resolution images of the kidneys without ionising radiation. It evaluates kidney tumours, cysts, and blood vessels. MRI is a safe, non-invasive method for comprehensive kidney assessment.

Kidney Biopsy

A kidney biopsy is used to diagnose kidney conditions, such as glomerulonephritis, infections, or cancer. It also assesses the extent of damage in chronic kidney disease (CKD) and determines the cause of abnormal renal function test results (RFT).

Doctors perform a kidney biopsy to collect small pieces of kidney tissue for testing. Usually, they use ultrasound to take several samples. In some cases, doctors may use CT guidance or perform the biopsy surgically. The doctor might suggest a kidney biopsy if the patient has:

• A sudden decrease in kidney function without a clear reason.
• Persistent blood in the urine.
• The presence of protein during a urine test.
• A transplanted kidney that needs regular monitoring.
• Is very overweight
• Has unusual kidney conditions
• Has a history of bleeding issues
• Has a kidney with many cysts
• Has a kidney close to another organ like the bowel

Why is Kidney Biopsy Done?

A kidney biopsy might be done to:

• Find out what’s causing a kidney problem when other tests don’t show it.
• Help create a treatment plan based on how the kidney is doing.
• Observe the progress of kidney disease getting worse.
• Determine the extent of damage kidney disease or another illness has caused.
• Evaluate if the treatment for kidney disease is working.
• Monitor the health of a transplanted kidney or figure out why it might not be working properly.

Not all problems lead to a kidney biopsy. The decision is based on the signs and symptoms, test results, and overall health of the individual.

The Procedure of Kidney Biopsy

The procedure of kidney biopsy includes

Preparation:

• Pre-procedure Testing: Pre-procedure testing requires blood tests, imaging studies (ultrasound), or other diagnostic tests. It evaluates the kidney function and determines the best approach for the biopsy.
• Medication Review: Patients need to inform their doctor about all medications, including over-the-counter drugs and supplements. They are advised to stop certain medications, especially blood thinners, to reduce the risk of bleeding.
• Fasting: Depending on the specific instructions given by the healthcare provider, patients may be asked to fast for a few hours before the procedure.

Procedure Types:

• Percutaneous Biopsy (Most Common): This type of biopsy is done via skin using a needle.
• Open Biopsy: An open biopsy is performed during surgery, where a larger incision is made. It is less common and is usually performed if a percutaneous biopsy isn't possible.

Steps for Percutaneous Biopsy:

• Anaesthesia: Local anaesthesia is administered to numb the area where the biopsy needle will be inserted. Sedation may also be provided for relaxation.
• Positioning: The patient will be positioned, usually lying on the stomach or side, to provide the best access to the kidney.
• Imaging Guidance: An ultrasound or CT scan is used to locate the kidney and guide the needle to the correct position.
• Needle Insertion: A thin, hollow needle is inserted through the skin and into the kidney. The doctor may use a spring-loaded device to obtain the tissue sample.
• Sample Collection: Several samples may be taken to ensure adequate tissue for analysis.
• Post-Procedure Care: The needle is withdrawn, and pressure is applied to the site to stop bleeding. A bandage or dressing is applied.

After the Procedure

• Monitoring: The patient will be monitored for a few hours to ensure there are no immediate complications.
• Activity Restrictions: As advised by the doctor, the patient may need to avoid strenuous activities for a period of time.
• Pain Management: Mild pain or discomfort at the biopsy site is common and can usually be managed with over-the-counter pain medications.
• Follow-Up: The doctor will discuss the biopsy results once they are available and plan further treatment or management based on the findings.

Risks and Complications:

• Bleeding: There’s a risk of bleeding in the kidney or surrounding area.
• Infection: There is a risk of infection at the biopsy site.
• Pain: Some discomfort or pain at the biopsy site is possible.
• Damage to the Kidney: Although rare, there’s a risk of damage to the kidney or surrounding organs.

 

Gene and Protein Expression in Kidney

Genes and proteins play key roles in kidney development and function. Different genes help form kidney structures, while proteins ensure the proper functioning of the kidney. Errors with these genes or proteins can cause serious kidney diseases, affecting kidney structure and function. So, it is necessary to know how these processes work to help diagnose and treat kidney disorders.

 

Role of Gene and Protein Expression in Kidney Development and Function

Genes and proteins are crucial for the development and function of the kidneys. During early development, specific genes like PAX2, WT1, and SIX2 respond to the formation of kidney cells. These genes build the kidney's structure and are essential for developing functional nephrons.

In the later stages, proteins such as aquaporins and sodium-potassium ATPase are vital for kidney functions like water reabsorption and maintaining ion balance. These proteins are involved in filtering blood, reabsorbing necessary substances, and balancing acids and bases.

For example, nephrin and podocin are important for keeping the filtration barrier in the kidneys, preventing protein loss in urine. Transport proteins like NCC and SGLT2 in the renal tubules help reabsorb nutrients and ions. The Renin-Angiotensin-Aldosterone System (RAAS) manages blood pressure and fluid balance with proteins like Renin and Angiotensin II.

Abnormalities in Genes and Proteins lead to Kidney Diseases

When genes and proteins don't work properly, it can lead to kidney diseases. The polycystic kidney disease (PKD) results from mutations in genes involved in kidney development and causes cysts and kidney problems. 

In diabetic nephropathy, disturbances with proteins that handle glucose reabsorption and filtration lead to functional problems. Disruptions in the Renin-Angiotensin-Aldosterone System (RAAS) pathway can cause high blood pressure and kidney damage. Understanding these processes helps in diagnosing and treating kidney diseases.

 

FAQs

How much do my kidneys weigh?

The weight of each kidney in male adults is 125-170g and for females is 115-155g.

How big is a kidney?

On average, each kidney is about 11-14 cm long, 6 cm wide, and 4 cm thick.

What colour are the kidneys?

Healthy kidneys are usually reddish brown and have a smooth surface. The reddish colour is due  to the rich blood supply, and the texture is due to the outer cortex and inner medulla.

What causes kidney damage?

There are many causes of kidney damage, including hypertension, uncontrolled diabetes, certain medications or drugs, kidney infections, and genetic conditions.

What are the first signs of kidney problems?

The early warning signs of kidney failure include swelling in the ankles, frequent urination, blood in the urine, fatigue and weakness, and changes in urine colour.

What are common tests to assess kidney health?

The routine test to assess kidney health involves 

• Urinalysis
• Blood tests, including serum creatinine and blood urea nitrogen (BUN), to assess kidney function.
• Imaging: ultrasound or CT scans to visualise kidney structure and detect any abnormalities.

How do I maintain kidney health?

Maintaining kidney health is necessary for its proper functioning. Eat a healthy diet with low salt and balanced nutrients. Stay hydrated, but avoid excessive water intake. Exercise regularly. Monitor and manage blood pressure and blood sugar levels. Avoid overusing over-the-counter painkillers and toxins.

Is water intake necessary for kidney health?

Sufficient water intake helps kidneys filter waste from the blood and prevent urinary tract infections (UTI) and kidney stones. Generally, about 2 to 3 litres (8 to 12 cups) of water per day is recommended for proper kidney functioning and maintenance. 

How can I differentiate kidney pain from back pain?

Kidney pain is felt in the lower back or sides and may be associated with urinary symptoms or fever. However, back pain is often more diffuse and can be related to muscle strain, poor posture, or spinal issues.

When to see a healthcare provider?

• Severe pain in the lower back or sides.
• Changes in urine colour, frequency, or volume.
• Swelling in ankle, hand or feet.
• High blood pressure or uncontrolled diabetes.

 How can I improve my kidney function naturally?

Maintain a healthy diet, stay hydrated, exercise regularly, and avoid excessive use of toxins and medications.

What foods are good for kidney health?

Berries, fish rich in omega-3 fatty acids, leafy greens, and whole grains are beneficial foods for kidney health.

How often should I get my kidneys checked?

It depends on individual risk factors, but generally, annual check-ups or more frequent testing are recommended for people with risk factors such as diabetes or hypertension.