Kidney: Definition, Anatomy, Physiology, Unique Functions And Its Diseases.

Kidney Definition

The kidneys are two vital internal organs that extract waste from the blood and regulate the body’s fluid and electrolyte levels. Although only one is required, humans have two due to the organ’s importance; if one stops working, the other will take over. Nephrons are microscopic filtration systems present in the kidneys that regulate the concentrations of salt, water, glucose, and amino acids in the blood plasma filtrate that generates urine. The kidneys produce two hormones: renin and erythropoietin.

Kidney Location

The kidneys are located at the waistline and on the posterior (back) wall of the abdomen. They are somewhat concealed by the ribs. Almost 95% of the time, the left kidney is slightly greater than the right. While the right is greater than the left, additional disorders may be present.

Each kidney is protected by the rib cage, perirenal (perinephric) fat, renal capsule, and back muscles. These vital organs are retroperitoneal, placed behind the peritoneum rather than within the abdominal cavity.

Kidney Anatomy

Almost universally, kidney anatomy depicts these organs as just bean-shaped. This is how the kidney bean got its name; both form and colour are comparable.

Human kidneys are around 10 centimetres long and five centimetres wide. The urinary system is made up of the bladder, ureters and urethra.

Each organ is surrounded by a thick membrane called the renal capsule. This membrane acts as a shield for the soft inner tissue and offers protection. This capsule is surrounded by a layer of fat termed the perirenal fat capsule. This fat layer is protected by the renal fascia.

The renal hilus, alternatively referred to as the pedicle or renal hilum, is the starting point for kidney anatomy. This is the indentation that gives the shape of a bean. The inner tissue is accessed by the renal arteries, renal veins, and the hollow, muscled tube of the ureter.

If you make vertical cuts through any animal’s kidney, you’ll observe the same fundamental architecture as in a human example. A white tissue region next to the hilus This is surrounded by blobs that are practically triangular in shape. A newly dissected kidney’s outer border exhibits a rich reddish-brown colour.


The renal pelvis, alternatively referred to as the renal pelvis or pyelum, is the whitish tissue visible in the above figure. In this instance, the term “pelvis” alludes to a basin that serves as a fluid collection and drainage site. The pelvis functions as a funnel, transporting newly generated urine via the calyces to the ureter.


The renal medulla is the subsequent operational unit. It is identified by the shape of the renal pyramids. Nephron loops, segments of convoluted tubules, and gathering ducts are included within the pyramids. The ureter drains all urine from the kidney and transports it to the calyces and renal pelvis.


The renal cortex is composed of the Bowman’s capsules, the glomerulus (capillary network), and segments of the nephrons’ convoluted tubules. is the third functional structure. The hormone erythropoietin is also produced by interstitial cells in the renal cortex (EPO).

Renin-producing cells may be located near the nephrons in both the medulla and cortex. Renin, a hormone secreted by these cells, is vital for blood pressure control.

Kidney Blood Supply

The renal arteries, which are branches of the abdominal aorta, are divided into the left and right renal arteries, which provide blood to the left and right kidneys, respectively (the thick, central artery). The aorta supplies the organ with oxygenated, nutrient-rich blood. but it also transports waste materials.

At the hilus, the renal artery divides into arterioles and, subsequently, many capillaries. The kidneys are densely packed with capillaries, and at the commencement of each nephron, they create tightly wrapped networks (glomeruli).


The term “nephron” refers to individual filtration systems, and the average human kidney contains between 200,000 and 2.5 million of them. No more nephrons are formed beyond the 36th week of pregnancy.

As it travels the length of a nephron, waste materials stay in the blood plasma filtration. The final solution, urine, enters a network of collecting ducts that coalesce into single apertures (renal papillae) at the foot of each pyramid. Urine enters the calyces via these papillae.

A nephron is made up of two components: the renal corpuscle and the renal tubule. The corpuscle is used to characterise the capillary cluster (glomerulus) and Bowman’s capsule. They are located deep within the renal cortex. The Bowman’s capsule passively transports filtrate from the glomerulus. This is the stage of urine generation when it is filtered.

Tubules in the nephron absorb and release a variety of tiny chemicals and ions in precise regions. The procedure through which molecules penetrate the tubules via the Bowman’s capsule and interstitial tissue is called absorption. Reabsorption refers to the return of molecules from the filtrate to the interstitial fluid. Other compounds are secreted into the tubular fluid to assist the kidneys in maintaining pH and electrolyte levels. The components of urine—water, ions, creatinine, toxins, and urea—are all transported into the gathering ducts during excretion.

The Bowman’s capsule is connected to the proximal convoluted tubule. This site is capable of reabsorbing sodium and chlorine ions, water, amino acids, glucose, and vitamins into the bloodstream. Hydrogen and potassium ions, phosphate, citric acid, ammonia (NH3), and urea are absorbed by the tubule from the interstitial tissue.

The Henlé descending and ascending loops are found in the medulla of the kidney. Water reabsorption is the primary function of the descending loop. From close-by collecting ducts, the ascending loop exteacts salt ions and urea as well as chlorine. Water molecules cannot pass via Henlé’s ascending loop.

The salt (NaCl), calcium ions, and water are reabsorbed by the distal convoluted tubule, which emerges into a collecting duct. Bicarbonate, hydrogen, and potassium ions, as well as ammonia, are all absorbed by the tubule. To keep the pH of the body stable, hydrogen and bicarbonate should be in balance. The arterial blood pH ranges from 7.35 to 7.45, which is a very small range.

Kidney Function

Kidney function entails more than simply the elimination of waste materials, although it is critical. Without medical assistance, we would die if we didn’t have at least one working kidney.

Fluid Balance

As previously stated, the Henlé loop is critical for fluid management (water homeostasis). On average, our blood is purified fifteen times every day. When we are dehydrated, the descending loop of Henlé absorbs little water, allowing water molecules to be reabsorbed into the interstitial tissue. Urine will appear darker in hue.

The pituitary gland secretes antidiuretic hormone (ADH), which regulates water balance in the kidneys. ADH enhances the reabsorption of water in the Henlé descending loop when water levels are low.

Blood Pressure Regulation

The kidneys are part of the renin-angiotensin-aldosterone system (RAAS). It manages blood pressure and fluid balance. While fluid balance has a strong influence on blood pressure regulation, unlike fluid balance, which is predominantly regulated by ADH, blood pressure regulation is influenced by some other hormones.

Renin is the initial stage in the RAAS system. The renal cortex secretes renin in response to lower sodium levels or lower blood volume. Renin is essential for the conversion of angiotensinogen to angiotensin I in the liver. Angiotensin-converting-enzyme is another enzyme generated in the lungs that converts angiotensin I to angiotensin II. Angiotensin II raises blood pressure by causing vasoconstriction in the peripheral blood arteries.

At the same time, angiotensin II stimulates the adrenal glands to secrete aldosterone. The adrenal glands are distinct organs that are located above the kidneys. Aldosterone instructs the nephrons to re-absorb salt and water into interstitial tissue while excreting potassium into the urine.

Electrolyte Balance

Athletes are often observed drinking electrolyte-fortified beverages. Sweating removes vital elements dissolved in water from the body (electrolytes). This loss might also happen as a result of vomiting or diarrhoea.

Sodium, chloride, potassium, magnesium, phosphate, and bicarbonate are the most prevalent electrolytes in the body. Each of these minerals serves a variety of functions.

Water has a great affinity for sodium and chloride, and functioning kidneys are efficient at flushing the body of excess salt.If you consume an extremely salty meal, you are likely to become thirsty and require a restroom visit within a short amount of time. This is because the kidneys excrete salt components, and salt excretes a significant amount of water. Excess water fills the bladder, and the lack of reabsorbed water results in the release of an antidiuretic hormone, which makes you thirsty.

Cell signalling and muscle contraction both need sodium and chloride. Sodium and potassium have opposing effects and are linked to cardiovascular disease when they are out of balance. Phosphate is a mineral that is necessary for the health of the bones, teeth, nerves, and muscles. Magnesium is involved in more than 300 metabolic events in the body.

Bicarbonate is a natural alkali that aids the body’s pH balance. While carbon dioxide and hydrogen ions are acidic, additional acids must be neutralised or eliminated before they may be breathed out. pH equilibrium in the body is maintained by alkaline bicarbonate ions and acidic hydrogen ions, which may be taken in or reabsorbed from urine.

Toxin Removal

The kidneys, together with the liver, work hard to keep us safe from pollutants. Blood clots as a result of poisonous snake bites, and components of the clotting process aggregate in kidney collecting ducts. Poisoning may cause severe renal damage or chronic kidney failure even if it is treated immediately.

Toxins are molecules that might be tiny, medium, or huge in size. The majority of molecules and cells are too big to pass through a normal Bowman’s capsule and into the bloodstream. These molecules are broken down by the liver into smaller ones.

Toxins can be any type of waste product, including degraded dead cells or remnants of cellular respiration. Urine removes a wide variety of poisons from the body.

Due to the high permeability of damaged nephrons, the presence of larger protein molecules such as albumin and/or red blood cells in the urine typically suggests that one or both kidneys have been damaged.

Erythropoietin Production

EPO (erythropoietin) is a hormone that promotes the synthesis of red blood cells.

When the body recognises reduced oxygen levels, it produces extra red blood cells to carry the oxygen to the tissues. Above sea level, the air contains around 21% oxygen; at 6,000 feet, it contains just 9.5 percent. High-altitude residents have more red blood cells.

EPO is illegally used by certain elite sportsmen to improve oxygen flow to their muscles. Mariem Alaoui Selsouli, a Moroccan runner, was banned from the sport for two years in 2009 after testing positive for EPO. Roberto Barbi, an Italian marathon runner, has been banned for life after testing positive for EPO in 2001 and 2008.

Vitamin D Activation

The kidney is an important part of the vitamin D activation process. Vitamin D is delivered to the liver and transformed into calcidiol after being obtained by food or sun exposure. Many receptors for calcidiol exist in healthy kidneys. This converts it to calcitriol, a type of vitamin D that is active and useful.

Calcitriol plays an important role in bone health, calcium absorption, cell development, muscular function, and immunity. People with chronic renal illness may need calcitriol supplementation; it is pointless to give them the inactive form of vitamin D since the kidneys convert it to the active form.

Kidney Disease

Kidney illnesses and problems are widespread; the numerous small components may easily be destroyed, and being a crucial organ, any disruption in the blood flow can be fatal.

Kidney Infection

The majority of kidney infections are caused by untreated or resistant lower urinary tract infections. Kidney infection impairs function and produces excruciating discomfort. Specific (narrow-spectrum) antibiotics are frequently used to treat kidney infections.

Kidney Stones

When some meals are consumed with minimal water or in conjunction with diuretics, mineral accumulation called kidney stones or renal calculi occurs. Small kidney stones cause no symptoms and are excreted in the urine. Removing a kidney stone is harmless in this scenario.

Additional mineral layers may enlarge a kidney stone that has not been emptied. On one side of the body, painful back, flank, and lower abdomen pain arises (the afflicted side). Such a sensation is induced by an obstruction and increased pressure inside the organ; stagnant urine may result in kidney infection. A kidney stone that obstructs the excretion of urine is a medical crisis.

Calcium, oxalate, and uric acid are the main causes of kidney stones, or at least the most prevalent ones. Crystals stick together in huge amounts without enough water to dissolve them, forming kidney stones. Larger crystals are treated with soundwaves (lithotripsy), which shatter them without requiring more complex or intrusive operations. If this does not work, surgical extraction will be required. Following surgery, a kidney stent may be placed to keep the damaged ureter dilated, reducing the likelihood of additional stones causing a blockage.

Cola, almonds, beans, beer, chocolate, organ meat, chicken, and dark green vegetables are all high in oxalate and phosphate, which may induce kidney stones.

Polycystic Kidney Disease

Polycystic kidney disease is another condition that affects these critical organs (PKD). PKD is a rare autosomal dominant genetic disorder that affects around one person in every 1,000. Cysts filled with fluid develop on and within the kidneys in polycystic kidney disease. Two symptoms include chronically elevated blood pressure and an excess of waste materials in the blood. Kidney infection, damage, failure, or malignancy are all possible outcomes of PKD. A new kidney is the sole “treatment,” and many people join kidney transplant waiting lists.

Horseshoe Kidney

A horseshoe kidney is generated when two kidneys are connected in a horseshoe configuration. It’s rather frequent—about one child in every 500 is born with this congenital disorder. Abdominal discomfort, nausea, and an increased risk of kidney stones and infections are all signs of horseshoe kidneys. A person with a horseshoe kidney is also regarded to have a greater chance of acquiring kidney cancer.

Kidney Cancer

Kidney cancer is a rather prevalent condition. According to the American Cancer Society, the risk of cancer increases between the ages of 65 and 74. Men are more prone than women to developing kidney cancer, particularly if they smoke, are overweight, or have high blood pressure.

Acute Kidney Injury

Acute kidney injuries occur suddenly, often as a result of trauma or an untreated infection, and may last anywhere from a few hours to a few days. Even so, hemodialysis is often used to supplement the kidney’s filtration function during this time.

Kidney Failure

Acute or chronic renal disease may lead to kidney failure in both kidneys. If only one kidney fails, the remaining healthy kidney can perform all its functions on its own. Hemodialysis or peritoneal dialysis will be required if both kidneys are damaged. It takes approximately five years to locate a healthy, tissue-matched kidney. Throughout this time span, regular dialysis (three times per week) is essential.


  • Ogobuiro I, Tuma F. Physiology, Renal. [Updated 2019 Feb 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538339/
  • Bikle D. Vitamin D: Production, Metabolism, and Mechanisms of Action. [Updated 2017 Aug 11]. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK278935/
  • Yu ASL, Chertow GM, Luyckx VA, et al. (2019) Brenner & Rector’s The Kidney E-Book: Eleventh Edition. New York, Elsevier.
  • Chambers D, Huang C, Matthews G. (2019). Basic Physiology for Anaesthetists: Second Edition. Cambridge, Cambridge University Press.
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