The Kidneys – Anatomy, Structure, Functions

Location and External Anatomy of the Kidneys

The kidneys are located at the rear wall of the abdominal cavity and are protected by the ribcage.

The Kidneys

The kidneys are the primary functional organ of the renal system. They are essential in homeostatic functions such as the regulation of electrolytes, maintenance of acid-base balance, and the regulation of blood pressure (by maintaining salt and water balance). They serve the body as a natural filter of the blood and remove wastes that are excreted through the urine.

They are also responsible for the reabsorption of water, glucose, and amino acids, and will maintain the balance of these molecules in the body. In addition, the kidneys produce hormones including calcitriol, erythropoietin, and the enzyme renin, which are involved in renal and hematological physiological processes.

Anatomical Location

The kidneys are a pair of bean-shaped, brown organs about the size of your fist. They are covered by the renal capsule, which is a tough capsule of fibrous connective tissue. Adhering to the surface of each kidney are two layers of fat to help cushion them.

The asymmetry within the abdominal cavity caused by the liver typically results in the right kidney being slightly lower than the left, and the left kidney is located slightly more medial than the right. The right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen.

 

The kidneys: Human kidneys are viewed from behind with the spine removed.

Resting on top of each kidney is an adrenal gland (adrenal meaning on top of renal), which is involved in some renal system processes despite being a primary endocrine organ. The upper parts of the kidneys are partially protected by lower ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia.

The kidneys are located at the rear wall of the abdominal cavity just above the waistline and are protected by the ribcage. They are considered retroperitoneal, which means that they lie behind the peritoneum, the membrane lining of the abdominal cavity.

There are a number of important external structures connecting the kidneys to the rest of the body. The renal artery branches off from the lower part of the aorta and provides the blood supply to the kidneys. Renal veins take blood away from the kidneys into the inferior vena cava. The ureters are structures that come out of the kidneys, bringing urine down into the bladder.

Internal Anatomy of the Kidneys

The cortex and medulla makeup two of the internal layers of a kidney and are composed of individual filtering units known as nephrons.

There are three major regions of the kidney

• Renal cortex
• Renal medulla
• Renal pelvis

The renal cortex is a space between the medulla and the outer capsule. The renal medulla contains the majority of the length of nephrons, the main functional component of the kidney that filters fluid from the blood. The renal pelvis connects the kidney with the circulatory and nervous systems from the rest of the body.

Renal Cortex

The kidneys are surrounded by a renal cortex, a layer of tissue that is also covered by renal fascia (connective tissue) and the renal capsule. The renal cortex is granular tissue due to the presence of nephrons—the functional unit of the kidney—that are located deeper within the kidney, within the renal pyramids of the medulla.

The cortex provides a space for arterioles and venules from the renal artery and vein, as well as the glomerular capillaries, to perfuse the nephrons of the kidney. Erythropoietin, a hormone necessary for the synthesis of new red blood cells, is also produced in the renal cortex.

 

Kidney structure: The kidney is made up of three main areas: the outer cortex, a medulla in the middle, and the renal pelvis.

Renal Medulla

The medulla is the inner region of the parenchyma of the kidney.
The medulla consists of multiple pyramidal tissue masses, called the renal pyramids, which are triangle structures that contain a dense network of nephrons.

At one end of each nephron, in the cortex of the kidney, is a cup-shaped structure called the Bowman’s capsule. It surrounds a tuft of capillaries called the glomerulus that carries blood from the renal arteries into the nephron, where plasma is filtered through the capsule.

After entering the capsule, the filtered fluid flows along the proximal convoluted tubule to the loop of Henle and then to the distal convoluted tubule and the collecting ducts, which flow into the ureter. Each of the different components of the nephrons is selectively permeable to different molecules and enables the complex regulation of water and ion concentrations in the body.

Renal Pelvis

The renal pelvis contains helium. The hilum is the concave part of the bean shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters—the urine-bearing tubes that exit the kidney and empty into the urinary bladder. The renal pelvis connects the kidney to the rest of the body.

Supply of Blood and Nerves to the Kidneys

The renal veins drain the kidney and the renal arteries supply blood to the kidney.

Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum.

Renal Arteries

The renal arteries branch off of the abdominal aorta and supply the kidneys with blood. The arterial supply of the kidneys is variable from person to person, and there may be one or more renal arteries supplying each kidney.

Due to the position of the aorta, the inferior vena cava, and the kidneys in the body, the right renal artery is normally longer than the left renal artery. The renal arteries carry a large portion of the total blood flow to the kidneys—up to a third of the total cardiac output can pass through the renal arteries to be filtered by the kidneys.

Renal blood supply starts with the branching of the aorta into the renal arteries (which are each named based on the region of the kidney they pass through) and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys, which then split into several arterioles.

These afferent arterioles branch into the glomerular capillaries, which facilitate fluid transfer to the nephrons inside the Bowman’s capsule, while efferent arterioles take blood away from the glomerulus, and into the interlobular capillaries, which provide tissue oxygenation to the parenchyma of the kidney.

Renal Veins

The renal veins are the veins that drain the kidneys and connect them to the inferior vena cava. The renal vein drains blood from venules that arise from the interlobular capillaries inside the parenchyma of the kidney.

Renal Plexus

The renal plexus are the source of nervous tissue innervation within the kidney, which surrounds and primarily alters the size of the arterioles within the renal cortex. Input from the sympathetic nervous system triggers vasoconstriction of the arterioles in the kidney, thereby reducing renal blood flow into the glomerulus.

The kidney also receives input from the parasympathetic nervous system, by way of the renal branches of the vagus nerve (cranial nerve X), which causes vasodilation and increased blood flow of the afferent arterioles. Due to this mechanism, sympathetic nervous stimulation will decrease urine production, while parasympathetic nervous stimulation will increase urine production.

 

Blood supply to the kidneys: The renal arteries branch off of the abdominal aorta and supply the kidneys with blood.

Nephron, Parts, and Histology

The nephron of the kidney is involved in the regulation of water and soluble substances in the blood.

A Nephron

A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine. Its function is vital for the homeostasis of blood volume, blood pressure, and plasma osmolarity. It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.

 

The basic physiology of a nephron within a kidney:

The labels are:

• The glomerulus,
• Efferent arteriole,
• Bowman’s capsule,
• Proximal tube,
• Cortical collecting tube,
• Distal tube,
• Loop of Henle,
• Collecting duct,
• Peritubular capillaries,
• Arcuate vein,
• Arcuate artery,
• Afferent arteriole, and
• Juxtaglomerular apparatus.

The Glomerulus

The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman’s capsule.

A group of specialized cells known as juxtaglomerular apparatus (JGA) is located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.

Bowman’s capsule (also called the glomerular capsule) surrounds the glomerulus. It is composed of visceral (simple squamous epithelial cells; inner) and parietal (simple squamous epithelial cells; outer) layers. The visceral layer lies just beneath the thickened glomerular basement membrane and only allows fluid and small molecules like glucose and ions like sodium to pass through into the nephron.

Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. However, in some injuries, they may be able to pass through and can cause blood and protein content to enter the urine, which is a sign of problems in the kidney.

Proximal Convoluted Tubule

The proximal tubule is the first site of water reabsorption into the bloodstream and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na⁺/K⁺/ATPase pumps that actively transports sodium across the basolateral membrane.

Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport. Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.

Fluid leaving this tubule generally is unchanged due to the equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma.

The Loop of Henle

The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle’s loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.

Distal Convoluted Tubule and Collecting Duct

The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.

Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.

The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolarity of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.

Mechanism

Glomerular Filtration

Glomerular filtration is the initial process in urine production. It is a passive process in which hydrostatic pressure pushes fluid and solute through a membrane with no energy requirement. The filtration membrane has three layers: fenestrated endothelium of the glomerular capillaries which allow blood components except the cells to pass through; basement membrane, which is a negatively charged physical barrier that prevents proteins from permeating; and foot processes of podocytes of the glomerular capsule that creates more selective filtration. The outward and inward force from the capillaries determines how much water and solutes cross the filtration membrane. Hydrostatic pressure from the glomerular capillaries is the major filtration force with a pressure of 55mmHg. The other potential filtration force is the capsular space colloid osmotic pressure, but it is zero because proteins are not usually present within the capsular space. Then the capsular space hydrostatic pressure and the colloid osmotic pressure in glomerular capillaries negate the filtration force from the hydrostatic pressure in the glomerular capillaries, creating a net filtration pressure which plays a big role in the glomerular filtration rate (GFR).[rx]

GFR is the volume of fluid filtered in a minute, and it depends on the net filtration pressure, the total available surface area for filtration, and filtration membrane permeability. The normal GFR is between 120 to 125ml/min. It is regulated intrinsically and extrinsically to maintain the GFR. The intrinsic control function by adjusting its own resistance to blood flow via a myogenic mechanism and a tubuloglomerular feedback mechanism. The myogenic mechanism maintains the GFR by constricting the afferent arteriole when the vascular smooth muscle stretches due to high blood pressure. It dilates the vascular smooth muscle when pressure is low within the afferent arteriole allowing more blood to flow through. Then the tubuloglomerular feedback mechanism function to maintain the GFR by sensing the amount of NaCl within the tubule. Macula densa cells sense NaCl around the ascending limb of the nephron loop.[rx] When blood pressure is high, the GFR will also be high; this decreases the time needed for sodium reabsorption, and therefore sodium concentration is high in the tubule. The macula densa cell senses it and releases the vasoconstrictor chemicals which constricts the afferent arteriole and reduces blood flow. Then when the pressure is low, Na gets reabsorbed more causing its concentration in the tubule to below, and macula densa do not release vasoconstricting molecules.[rx][rx]

The extrinsic control maintains the GFR and also maintains the systemic blood pressure via the sympathetic nervous system and the renin-angiotensin-aldosterone mechanism. When the volume of fluid in the extracellular decreases excessively, norepinephrine and epinephrine get released and cause vasoconstriction leading to a decrease in blood flow to the kidney and the level of GFR. Also, the renin-angiotensin-aldosterone axis gets activated by three means when the blood pressure drops. The first is the activation of the beta-1 adrenergic receptor, which causes the release of renin from the granular cells of the kidney. The second mechanism is the macula densa cells which senses low NaCl concentration during decreased blood flow to the kidney and trigger the granular cells to release renin. The third mechanism is the stretch receptor around the granular cells senses decreased tension during decreased blood flow to the kidney and also triggers the release of renin, therefore, regulating the glomerular filtration.[rx]

Tubular Reabsorption

The four different tubular segments have unique absorptive properties. The first is the proximal convoluted tubule (PCT). The PCT cells have the most absorptive capability. In the normal circumstance, the PCT reabsorbs all the glucose and amino acids as well as 65% of Na and water. The PCT reabsorb sodium ions by primary active transport via a basolateral Na-K pump. It reabsorbs glucose, amino acids, and vitamins through secondary active transport with Na and an electrochemical gradient drives passive paracellular diffusion. The PCT reabsorbs water by osmosis that is driven by solute reabsorption. It also reabsorbs lipid-soluble solutes via passive diffusion driven by the concentration gradient created by the reabsorption of water. Reabsorption of urea occurs in the PCT as well by passive paracellular diffusion driven by a chemical gradient.[rx]

From the PCT, the non-reabsorbed filtrates move on to the nephron loop. The nephron loop functionally divides into a descending and an ascending limb. The descending limb functions to reabsorb water via osmosis. This process is possible due to the abundance of aquaporins. Solutes do not get reabsorbed in this region. However, in the ascending limb, Na moves passively down its concentration gradient in the thin segment of the ascending limb, and also sodium, potassium, and chlorides get reabsorbed together through a symporter in the thick segment of the ascending limb. The presence of Na-K ATPase in the basolateral membrane keeps this symporter functional by creating an ionic gradient. There is also the reabsorption of the calcium and magnesium ions in the ascending limb via passive paracellular diffusion driven by the electrochemical gradient. No water reabsorption in the ascending limb.[rx]

The next tubular segment for reabsorption in the distal convoluted tubule (DCT). There is a primary active sodium transport at the basolateral membrane and secondary active transport at the apical membrane through Na-Cl symporter and channels. This process is aldosterone regulated at the distal portion. There is also calcium reabsorption via passive uptake controlled by the parathyroid hormone. Aldosterone targets the cells of the distal portion of the DCT causing synthesis and retention of apical Na and K channel as well as the synthesis of Na-K ATPase.[rx]

Right after the DCT, there is a collecting tubule where the final stage of reabsorption occurs. The reabsorptions that occur here include primary active sodium transport at basolateral membrane; secondary active transport at apical membrane via Na-Cl symporter and channels with aldosterone regulation; passive calcium uptake via PTH-modulated channels in the apical membrane; and primary and secondary active transport in the basolateral membrane.[rx]

Tubular Secretion

Tubular secretion function is to dispose of substances such as drugs and metabolites that bind to plasma protein. Tubular secretion also functions to eliminate undesirable substances that were reabsorbed passively such as urea and uric acids. Elimination of excess potassium via aldosterone hormone regulation at collecting duct and distal DCT are part of tubular secretion function. There is an elimination of hydrogen ions when the blood pH drops below the normal range. Then when the blood pH increases above the normal range, reabsorption of chloride ions occurs as carbonic acid gets excreted. The secretion of creatinine, ammonia and many other organic acids and basics occur.[rx]

Storage of Urine

Once the production of urine is complete, it travels through a structure called the ureter for urine storage in the bladder. There are two ureters in a human body; one on each side; left and right. They are slender tubes with three-layered walls: the mucosa that contains a transitional epithelial tissue; muscular that is composed of the internal longitudinal layer and the external circular layer; and adventitia that is a fibrous connective tissue that covers the ureter’s external surface. As urine make its way to the ureters, the stretching of the ureter’s smooth muscle results in peristaltic contractile waves that help move the urine into the bladder.[rx] The oblique insertion of the ureter at the posterior bladder wall prevents backflow of urine. Once the urine is in the bladder, the bladder’s unique anatomy allows for efficient storage of urine.

The bladder is essentially a muscular sac with three layers. Its three layers are similar to the ureter except that the muscular layer has muscle fibers organized in inner and outer longitudinal layers and a middle circular layer. The muscular layer is also known as the detrusor muscle. The distensibility of the bladder allows it to hold a maximum capacity of up to 1000ml, though normal functional capacity is 300 to  400mL.[rx] The bladder has three openings at the smooth triangular region of the bladder; this is called the trigone. Two of the openings are where the distal portions of the ureters insert, and the other opening is the orifice for the urethra.

The urethra is a thin-walled muscular tube that functions to drain urine out of the bladder. Its mucosa lining consists of the mostly pseudostratified columnar epithelium through the proximal portion has transitional epithelial tissue. The thickening of the detrusor muscle at the bladder-urethra junction forms the internal urethral sphincter which has an autonomic nervous system control. The urethra has an additional function for males as it transports semen. In males, the urethra is approximately 22.3 cm long with three regions which include the prostatic urethra, membranous urethra, and the spongy urethra.[rx] Females, on the other hand, has a urethra that is approximately 3.8 to 5.1 cm long with an external urethral orifice that lies anterior to the vaginal opening and posterior to the clitoris.[rx]

Micturition Process

The micturition process entails contraction of the detrusor muscle and relaxation of the internal and external urethral sphincter. The process is slightly different based on age. Children younger than three years old have the micturition process coordinated by the spinal reflex. It starts with urine accumulation in the bladder that stretches the detrusor muscle causing activation of stretch receptors. The stretch sensation is carried by the visceral afferent to the sacral region of the spinal cord where it synapses with the interneuron that excites the parasympathetic neurons and inhibits the sympathetic neurons. The visceral afferent impulse concurrently decreases the firing of the somatic efferent that normally keeps the external urethral sphincter closed allowing reflexive urine output. However, after the age of 3, there is an override of reflexive urination where there is the conscious control of the external urethral sphincter.[rx] High bladder volume activates the pontine micturition center which activates the parasympathetic nervous system and inhibits the sympathetic nervous system as well as triggers awareness of a full bladder; consequently leading to relaxation of the internal sphincter and a choice to relax the external urethral sphincter once ready to void. Low bladder volume activates the pontine storage center which activates the sympathetic nervous system and inhibits the parasympathetic nervous system cumulatively allowing the accumulation of urine in the bladder.[rx]

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