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The kidney is one of the most vital organs in the human body, playing a central role in maintaining life and health. It belongs to the urinary system and is responsible for filtering blood, removing waste products, and regulating the balance of water, electrolytes, and acids and bases. By performing these essential functions, the kidney ensures that the internal environment of the body remains stable, a condition known as homeostasis.

Each human normally has two kidneys, located on either side of the vertebral column in the posterior part of the abdominal cavity. Despite their relatively small size, kidneys perform highly complex tasks that are indispensable for survival. They not only excrete unwanted substances but also produce important hormones such as erythropoietin, which stimulates red blood cell formation, renin, which helps regulate blood pressure, and calcitriol, which is the active form of vitamin D essential for calcium metabolism.

Nephron, which is specialized in filtering plasma and forming urine. Through processes of filtration, reabsorption, and secretion, the nephron carefully controls the composition of blood and prevents the accumulation of toxic substances. Because of this, the kidney is often described as both an excretory and an endocrine organ.

Anatomy of the Kidney

The kidney is a paired, bean-shaped organ situated in the posterior abdominal wall on either side of the vertebral column. Each kidney is about 11 centimeters long, 6 centimeters wide, and 3 centimeters thick, with an average weight of 120 to 150 grams in adults. The right kidney is slightly lower than the left because of the presence of the liver. The upper pole of each kidney is in contact with the diaphragm and adrenal gland, while the lower pole extends toward the iliac crest. The kidneys lie retroperitoneally, which means they are positioned behind the peritoneum, and extend approximately from the level of the twelfth thoracic vertebra to the third lumbar vertebra.

The external surface of the kidney shows two poles, two borders, and two surfaces. The upper pole is rounded and broad, capped by the suprarenal gland. The lower pole is narrower and pointed. The lateral border of the kidney is convex, while the medial border is concave and shows a vertical slit called the hilum. Through the hilum, important structures enter and leave the kidney. These include the renal vein, renal artery, lymphatics, nerves, and the ureter, arranged in that order from anterior to posterior. The anterior surface of the kidney is irregular and related to various abdominal organs such as the liver, stomach, spleen, pancreas, and intestines, depending on the side. The posterior surface is more uniform and rests on the diaphragm, psoas major, quadratus lumborum, and transversus abdominis muscles, separated from them by the renal fascia and perirenal fat.

The kidney is covered by several layers of protective tissues. The innermost covering is a thin, fibrous renal capsule that closely adheres to the surface and can be peeled off in healthy kidneys. Outside the capsule lies the perirenal fat, which cushions the organ against trauma. This is enclosed by the renal fascia, a fibrous sheath that anchors the kidney to surrounding structures. Beyond this lies the pararenal fat, providing further protection. Together, these coverings maintain the kidney’s position and safeguard it from injury.

On sectioning, the kidney reveals two distinct regions: the outer cortex and the inner medulla. The renal cortex is reddish-brown, granular, and extends as columns of Bertin between the medullary pyramids. The medulla is darker and arranged into 8 to 18 conical masses called renal pyramids. The broad base of each pyramid faces the cortex, while the apex, known as the renal papilla, projects into a minor calyx. These pyramids together with the cortex form the renal lobes. The cortex and medulla together contain the nephrons, the structural and functional units of the kidney.

The collecting system of the kidney begins at the renal papilla. Urine formed in the nephrons drains into the collecting ducts, which converge at the papilla to empty into the minor calyces. Several minor calyces unite to form a major calyx, and two or three major calyces join to form the funnel-shaped renal pelvis. The renal pelvis narrows downwards to continue as the ureter, which transports urine to the urinary bladder.

Internally, the kidney also contains the renal sinus, a central cavity occupied by the renal pelvis, calyces, renal vessels, lymphatics, and fat. The renal sinus communicates with the outside through the hilum.

The microscopic anatomy is defined by the nephron, which is the basic functional unit. Each nephron consists of a renal corpuscle, made up of Bowman’s capsule and the glomerulus, followed by a tubular system including the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts. Together, these structures filter blood, reabsorb essential substances, and excrete waste products in the form of urine.

Blood Supply

The kidneys receive about 20 to 25 percent of the cardiac output, reflecting their importance in blood purification. The renal artery, a direct branch of the abdominal aorta, enters through the hilum and divides into segmental arteries. These further branch into interlobar, arcuate, and interlobular arteries, eventually forming afferent arterioles that supply the glomeruli. The efferent arterioles leaving the glomerulus form a network of peritubular capillaries and vasa recta, which play a crucial role in reabsorption and urine concentration. Venous blood drains through corresponding veins into the renal vein, which opens into the inferior vena cava.

Functions of Kidney

The kidney is not only an organ of excretion but also a regulator of the body’s internal environment and a producer of important hormones. Its functions are complex and interrelated, and together they ensure homeostasis, which means maintaining a stable internal condition despite changes in the external environment.

The most important function of the kidney is the excretion of waste products. Metabolic processes in the body, such as the breakdown of proteins, generate waste substances like urea, uric acid, creatinine, and ammonia. If these products were allowed to accumulate in the blood, they would become toxic and disrupt cellular function. The kidney filters these substances from the blood through the glomeruli and eliminates them in the urine, thereby detoxifying the body and maintaining a healthy chemical balance.

Along with excretion, the kidney plays a crucial role in the regulation of water balance. Every day, the body takes in water through food and drink, while also losing water through sweating, respiration, and excretion. The kidney adjusts urine output depending on the body’s needs. When water intake is high, the kidneys produce dilute urine to remove the excess. When water is scarce, they conserve water by producing concentrated urine under the influence of antidiuretic hormone. This fine adjustment prevents dehydration or overhydration and keeps the body’s fluid balance stable.

Electrolyte regulation is another key task. The kidney controls the concentration of essential ions such as sodium, potassium, calcium, chloride, bicarbonate, magnesium, and phosphate. Sodium balance is particularly important because it influences blood pressure and volume. Potassium regulation is essential for normal muscle and nerve function. Calcium and phosphate balance, under the influence of parathyroid hormone and calcitriol produced by the kidney, is critical for bone health and many enzymatic reactions.

The kidney is also central to maintaining the acid–base balance of the body. Normal cell function depends on keeping the blood pH within a narrow range. The kidney contributes by excreting hydrogen ions and reabsorbing bicarbonate ions. This ensures that the body is protected against dangerous conditions like acidosis or alkalosis, which can occur when blood pH falls too low or rises too high.

A vital role of the kidney is its contribution to blood pressure regulation. This is achieved by the renin–angiotensin–aldosterone system. Specialized juxtaglomerular cells in the kidney secrete renin when blood pressure or sodium levels are low. Renin triggers a cascade of reactions that result in the production of angiotensin II, a powerful vasoconstrictor, and stimulation of aldosterone secretion, which increases sodium and water reabsorption. Together, these actions raise blood pressure and restore circulation to normal.

Beyond these regulatory tasks, the kidney also functions as an endocrine organ. It secretes erythropoietin, a hormone that stimulates the bone marrow to produce red blood cells. This ensures adequate oxygen-carrying capacity in the blood. It also activates vitamin D by converting it into calcitriol, the hormonally active form. Calcitriol is essential for the absorption of calcium and phosphate from the intestine and for maintaining healthy bones.

The kidney has an additional role in metabolic homeostasis. It helps in gluconeogenesis, the process of generating glucose from non-carbohydrate sources, particularly during fasting. This ensures a continuous supply of glucose to vital organs such as the brain. The kidney also produces prostaglandins and other local mediators that influence renal blood flow and systemic vascular resistance.

Histology of Kidney

The kidney shows a highly complex microscopic organization that allows it to carry out the processes of filtration, reabsorption, secretion, and urine concentration. Its fundamental unit is the nephron, and about one million nephrons are present in each kidney. Histologically, the kidney is divided into the cortex and the medulla, each region containing characteristic structures that together contribute to renal function.

The renal cortex appears granular when observed under the microscope. This is due to the presence of numerous renal corpuscles and convoluted tubules. The renal corpuscle is made up of two parts: the glomerulus and Bowman’s capsule. The glomerulus is a tuft of capillaries lined by fenestrated endothelial cells, allowing blood plasma to filter through. Surrounding the glomerulus is Bowman’s capsule, a double-layered cup-like structure. The inner layer of Bowman’s capsule is formed by specialized epithelial cells known as podocytes, which possess interdigitating foot processes that create filtration slits. The outer parietal layer is formed by simple squamous epithelium. The space between these two layers, called Bowman’s space, is where the initial filtrate from the blood is collected.

Also within the cortex are the proximal and distal convoluted tubules. The proximal convoluted tubule is lined by cuboidal epithelium with a dense brush border of microvilli, which provides a large surface area for reabsorption. It is here that most of the filtered water, electrolytes, and nutrients are reabsorbed into the blood. The distal convoluted tubule is also lined by cuboidal epithelium but lacks a prominent brush border. It is mainly involved in selective reabsorption and secretion and functions under the influence of hormones such as aldosterone. Another important histological feature in the cortex is the juxtaglomerular apparatus. This structure includes juxtaglomerular cells, which are modified smooth muscle cells of the afferent arteriole that secrete renin, the macula densa, which is a cluster of specialized cells in the distal convoluted tubule that sense sodium concentration in the filtrate, and the extraglomerular mesangial cells, which provide structural support and help in signaling. The juxtaglomerular apparatus plays a vital role in regulating blood pressure and the glomerular filtration rate.

The renal medulla appears striated due to the parallel arrangement of tubules and blood vessels. It contains the straight segments of nephrons, loops of Henle, collecting ducts, and the vasa recta. The loop of Henle has descending and ascending limbs, each with specialized epithelial linings. The thin segment of the loop is lined by simple squamous epithelium, which facilitates passive diffusion of water and solutes, while the thick ascending limb is lined by cuboidal epithelium capable of active ion transport. Collecting ducts are lined by cuboidal to columnar epithelial cells and gradually converge into larger ducts, known as ducts of Bellini, which open at the tips of the renal papillae into the minor calyces. These ducts play an essential role in the final adjustment of water reabsorption under the influence of antidiuretic hormone.

Between the tubules and ducts lies the interstitial tissue, which contains fibroblast-like cells and interstitial macrophages. These cells secrete signaling molecules that support renal physiology and tissue repair. In the medulla, the blood vessels form specialized structures called the vasa recta, which run parallel to the loops of Henle and maintain the osmotic gradient necessary for concentration of urine.

Development of Kidney

The development of the kidney is a gradual and highly organized process that begins early in embryonic life. It originates from the intermediate mesoderm, a part of the mesodermal layer that lies between the paraxial and lateral plate mesoderm. From this source, the urinary system and the kidneys arise in three successive stages: the pronephros, the mesonephros, and the metanephros. These three stages do not function equally; the first two appear temporarily and regress, while the third stage, the metanephros, develops into the permanent kidneys of the adult.

The pronephros is the earliest form of kidney development. It appears during the fourth week of embryonic life in the cervical region. It consists of a few cell clusters and tubular structures that soon disappear without forming a functional organ. Although it is non-functional in humans, it is important because its ducts persist and serve as the basis for the development of the later stages.

The mesonephros develops after the regression of the pronephros, appearing in the thoracic and lumbar regions of the embryo. It consists of mesonephric tubules connected to the mesonephric duct, also known as the Wolffian duct. The mesonephros is partly functional for a short period in early embryonic life, producing a small amount of urine that enters the amniotic cavity. In males, portions of the mesonephric duct persist and later contribute to parts of the reproductive system, such as the epididymis, vas deferens, and seminal vesicles. In females, most of the mesonephric system degenerates, leaving only small vestigial structures.

The metanephros is the final stage of kidney development and appears in the fifth week of gestation. It develops from two main sources: the ureteric bud and the metanephric mesenchyme, also called the metanephric blastema. The ureteric bud is an outgrowth of the mesonephric duct near its entry into the cloaca. This bud elongates and branches repeatedly to form the ureter, renal pelvis, major and minor calyces, and collecting ducts. The metanephric mesenchyme condenses around the tips of the branching ureteric bud and differentiates into the nephron, which includes Bowman’s capsule, the proximal and distal convoluted tubules, and the loop of Henle.

The interaction between the ureteric bud and the metanephric mesenchyme is essential for normal kidney formation. Signaling molecules from the mesenchyme stimulate branching of the ureteric bud, while signals from the ureteric bud induce the mesenchyme to differentiate into nephrons. This process of reciprocal induction continues until the complete architecture of the kidney is established.

By the tenth to twelfth week of development, the metanephros begins to produce urine, which is excreted into the amniotic cavity and contributes to the volume of amniotic fluid. Although the kidneys are functional at this stage, their role in waste excretion is limited, since the placenta is the main organ responsible for removing fetal waste products.

During development, the kidneys undergo a change in position. Initially, they are located in the pelvic region, but as the fetus grows, they gradually ascend to the lumbar region of the posterior abdominal wall. This ascent is due to differential growth of the body rather than active movement. At the same time, the kidneys rotate medially so that the hilum, which initially faces anteriorly, eventually comes to face medially. During this migration, the kidneys receive blood supply from successive arteries branching from the aorta, but as they ascend, these vessels regress, and new renal arteries develop at higher levels.

Regulation of Kidney Function

The kidney performs a wide range of vital functions, and to maintain accuracy and efficiency, its activity is finely regulated by multiple mechanisms. Regulation ensures that glomerular filtration, tubular reabsorption, and secretion occur at rates appropriate to the body’s needs. This control involves intrinsic autoregulatory mechanisms within the kidney, as well as extrinsic influences from the nervous and endocrine systems. Together, these mechanisms allow the kidneys to adapt to changes in blood pressure, fluid intake, electrolyte balance, and metabolic demands, while keeping homeostasis stable.

The first and most important regulatory process is autoregulation of renal blood flow and glomerular filtration rate. Despite fluctuations in systemic blood pressure, the kidneys are able to maintain relatively constant blood flow and filtration within a mean arterial pressure range of about 80 to 180 mmHg. Autoregulation occurs mainly through two mechanisms. The myogenic mechanism depends on the ability of the afferent arteriole to constrict when stretched by increased blood pressure and to relax when blood pressure falls. This automatic adjustment prevents large swings in glomerular filtration rate. The second mechanism is tubuloglomerular feedback, which is mediated by the juxtaglomerular apparatus. When sodium chloride concentration in the filtrate rises, the macula densa cells of the distal convoluted tubule detect this and signal the afferent arteriole to constrict, reducing glomerular filtration. Conversely, when sodium chloride levels fall, the macula densa stimulates dilation of the afferent arteriole and the release of renin from juxtaglomerular cells, both of which increase filtration pressure.

The renin–angiotensin–aldosterone system is a major hormonal regulator of kidney function. When renal perfusion pressure drops or when sodium concentration is low, the juxtaglomerular cells release renin. Renin acts on angiotensinogen, a plasma protein, to produce angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme, mainly in the lungs. Angiotensin II is a potent vasoconstrictor that raises systemic blood pressure, but it also specifically constricts the efferent arteriole of the nephron, thereby increasing glomerular pressure and maintaining filtration. In addition, angiotensin II stimulates the adrenal cortex to release aldosterone, which acts on the distal convoluted tubule and collecting ducts to increase sodium and water reabsorption. This restores blood volume and blood pressure.

Antidiuretic hormone, also called vasopressin, is another key regulator. It is secreted by the posterior pituitary gland in response to increased plasma osmolality or decreased blood volume. ADH acts on the collecting ducts of the nephron, increasing their permeability to water by inserting aquaporin channels in the tubular membrane. As a result, more water is reabsorbed, urine becomes concentrated, and plasma osmolality is corrected. In the absence of ADH, the collecting ducts remain impermeable to water, leading to the production of dilute urine.

Atrial natriuretic peptide provides a counter-regulatory influence on kidney function. It is secreted by the atria of the heart when blood volume and atrial pressure rise. This hormone dilates the afferent arteriole and constricts the efferent arteriole, thereby increasing glomerular filtration rate. It also inhibits sodium and water reabsorption in the distal tubules and collecting ducts, and suppresses the secretion of renin and aldosterone. The overall effect is enhanced excretion of sodium and water, reducing blood volume and pressure.

The sympathetic nervous system contributes significantly to the regulation of kidney function, especially during stress or blood loss. Sympathetic activation causes constriction of renal blood vessels, particularly the afferent arterioles, thereby reducing renal blood flow and glomerular filtration rate. This response conserves fluid and diverts blood to essential organs such as the brain and heart. Sympathetic stimulation also promotes renin release, linking the neural response to the hormonal control of blood pressure.

In addition to these major mechanisms, local factors within the kidney, such as prostaglandins, nitric oxide, and kinins, exert fine control over renal circulation. Prostaglandins, for example, cause vasodilation of renal vessels and protect the kidney from ischemia during excessive sympathetic activity or angiotensin II action. Nitric oxide, produced by endothelial cells, also acts as a vasodilator to maintain adequate renal blood flow.