The Kidneys

Location and External Anatomy of the Kidneys

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

Learning Objectives

Describe the external anatomy and location of the kidneys

Key Takeaways

Key Points

  •  The kidneys lie in the lower abdominal cavity, on its rear wall.
  • Adhering to the surface of each kidney are two layers of fat to help cushion them.
  • 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 renal artery connects the kidneys to the aorta, while the renal vein connects the kidneys to the inferior vena cava.
  • The adrenal glands lie superior to the kidneys.

Key Terms

  • adrenal gland: Two glands that are responsible for releasing hormones in response to stress, such as catecholamines, and that also produce aldosterone, a renal system hormone.

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 hemotological 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 left kidney being 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.

This is a drawing of human kidneys, viewed from behind with the spine removed. The left kidney is slightly higher than the right one due to the asymmetry within the abdominal cavity caused by the liver.

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

Resting on top of each kidney is an adrenal gland (adrenal meaning on top of renal), which are involved in some renal system processes despite being a primarily 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 downward into the bladder.

Internal Anatomy of the Kidneys

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

Learning Objectives

Distinguish between the cortex and medulla in the internal anatomy of the kidney

Key Takeaways

Key Points

  • The renal cortex, renal medulla, and renal pelvis are the three main internal regions found in a kidney.
  • Nephrons, masses of tiny tubules, are largely located in the medulla and receive fluid from the blood vessels in the renal cortex.
  • The renal cortex produces erythropotein.
  • The kidneys are made up by three external layers, which include the renal fascia (the outermost layer), the perirenal fat capsule, and lastly, the innermost layer, the renal capsule, which then surround the space of the renal cortex.
  • The Bowman’s or renal capsule, located at one end of each nephron in the cortex of the kidney, is the blood-filtering region of the nephron.
  • The renal pelvis contains a hilium —the concave site in which the renal artery and vein and nerves enter the kidney and the ureter leaves the kidney.

Key Terms

  • renal medulla: The inner-most region of the kidney, arranged into pyramid-like structures, that consists of the bulk of nephron structure.
  • renal cortex: The outer region of the kidney, between the renal capsule and the renal medulla, that consists of a space that contains blood vessels that connect to the nephrons.
  • nephron: The basic structural and functional unit of the kidney that filters the blood in order to regulate chemical concentrations and produce urine.

There are three major regions of the kidney: 

  1. Renal cortex
  2. Renal medulla
  3. 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 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. Erythropotein, a hormone necessary for the synthesis of new red blood cells, is also produced in the renal cortex.

This is a drawing of the kidney. It highlights the kidney's three main areas, which are the outer cortex, a medulla in the middle, and the renal pelvis.

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 are selectively permeable to different molecules, and enable the complex regulation of water and ion concentrations in the body.

Renal Pelvis

The renal pelvis contains the hilium. 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.

Learning Objectives

Summarize the blood and nerve supply of the kidney

Key Takeaways

Key Points

  • The renal arteries branch off of the abdominal aorta and supply the kidneys with blood. The arterial supply of the kidneys varies from person to person, and there may be one or more renal arteries to supply each kidney.
  • The renal veins are the veins that drain the kidneys and connect them to the inferior vena cava.
  • The kidney and the nervous system communicate via the renal plexus. The sympathetic nervous system will trigger vasoconstriction and reduce renal blood flow, while parasympathetic nervous stimulation will trigger vasodilation and increased blood flow.
  • Afferent arterioles branch into the glomerular capillaries, while efferent arterioles take blood away from the glomerular capillaries and into the interlobular capillaries that provide oxygen to the kidney.

Key Terms

  • renal artery: These arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood.
  • renal vein: The veins that drain the kidney and connect the kidney to the inferior vena cava.

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 surround and primarily alter 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.

This is a photograph of the kidneys and their surroundings. The renal arteries are seen to branch off of the abdominal aorta and supply the kidneys with blood.

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 blood.

Learning Objectives

Identify the parts and histology of kidney nephrons

Key Takeaways

Key Points

  • The glomerulus is the site in the nephron where fluid and solutes are filtered out of the blood to form a glomerular filtrate.
  • The proximal and distal tubules, the loop of Henle, and the collecting ducts are sites for the reabsorption of water and ions.
  • All of the glucose in the blood is reabsorbed by the proximal convoluted tubule through ion cotransport.
  • The loop of Henle (sometimes known as the nephron loop) is a U-shaped tube that consists of a descending limb and ascending limb, which differ in permeability.
  • The collecting duct and distal convoluted tubule are normally impermeable to water, but this is altered due to hormone stimulus during homeostasis.

Key Terms

  • Bowman’s capsule: A cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney.
  • proximal tubule: The first and primary sort of water and ion reabsorption in the kidney, where all glucose in the blood is reabsorbed.
  • glomerulus: A small, intertwined group of capillaries within the nephrons of the kidney that filter the blood to make urine.

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 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.

An illustration of the basic physiology of a nephron within a kidney The labels on the figure are: 1. Glomerulus, 2. Efferent arteriole, 3. Bowman's capsule, 4. Proximal tube, 5. Cortical collecting tube, 6. Distal tube, 7. Loop of Henle, 8. Collecting duct, 9. Peritubular capillaries, 10. Arcuate vein, 11. Arcuate artery, 12. Afferent arteriole, and 13. Juxtaglomerular apparatus.

The basic physiology of a nephron within a kidney: The labels are: 1. Glomerulus, 2. Efferent arteriole, 3. Bowman’s capsule, 4. Proximal tube, 5. Cortical collecting tube, 6. Distal tube, 7. Loop of Henle, 8. Collecting duct, 9. Peritubular capillaries, 10. Arcuate vein, 11. Arcuate artery, 12. Afferent arteriole, and 13. 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) are 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.

The 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.