The renal corpuscle, located in the renal cortex, is made up of a network of capillaries known as the glomerulus and the capsule, a cup-shaped chamber that surrounds it, called the glomerular or Bowman’s capsule.

The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function. The first part is called the proximal convoluted tubule (PCT) due to its proximity to the glomerulus; it stays in the renal cortex. The second part is called the loop of Henle, or nephritic loop, because it forms a loop (with descending and ascending limbs) that goes through the renal medulla. The third part of the renal tubule is called the distal convoluted tubule (DCT) and this part is also restricted to the renal cortex. The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons and fuse together as they enter the papillae of the renal medulla.

The capillary network that originates from the renal arteries supplies the nephron with blood that needs to be filtered. The branch that enters the glomerulus is called the afferent arteriole. The branch that exits the glomerulus is called the efferent arteriole. Within the glomerulus, the network of capillaries is called the glomerular capillary bed. Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network, which surrounds and interacts with parts of the renal tubule. In cortical nephrons, the peritubular capillary network surrounds the PCT and DCT. In juxtamedullary nephrons, the peritubular capillary network forms a network around the loop of Henle and is called the vasa recta.

Concept in Action

Go to this website to see another coronal section of the kidney and to explore an animation of the workings of nephrons.

Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus. Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called glomerular filtration. Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network. Water is also reabsorbed during this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is, in essence, the opposite process to tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis. This entire process is illustrated in Figure 22.7.

Glomerular Filtration

Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.

Concept in Action

To learn more about the vascular system of kidneys, click through this review and the steps of blood flow.

Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones. Sodium (Na⁺) is the most abundant ion and most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na⁺ is actively transported out of the tubule, water follows it to even out the osmotic pressure. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries. However, every solute has a transport maximum and the excess is not reabsorbed.

In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, as illustrated in Figure 22.8, it acts as a countercurrent multiplier. The vasa recta around it acts as the countercurrent exchanger.

Loop diuretics are drugs sometimes used to treat hypertension. These drugs inhibit the reabsorption of Na⁺ and Cl^– ions by the ascending limb of the loop of Henle. A side effect is that they increase urination. Why do you think this is the case?

By the time the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. If the body requires additional water, all of it can be reabsorbed at this point. Further reabsorption is controlled by hormones, which will be discussed in a later section. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct. Kidneys also maintain an acid-base balance by secreting excess H⁺ ions.

Although parts of the renal tubules are named proximal and distal, in a cross-section of the kidney, the tubules are placed close together and in contact with each other and the glomerulus. This allows for exchange of chemical messengers between the different cell types. For example, the DCT ascending limb of the loop of Henle has masses of cells called macula densa, which are in contact with cells of the afferent arterioles called juxtaglomerular cells. Together, the macula densa and juxtaglomerular cells form the juxtaglomerular complex (JGC). The JGC is an endocrine structure that secretes the enzyme renin and the hormone erythropoietin. When hormones trigger the macula densa cells in the DCT due to variations in blood volume, blood pressure, or electrolyte balance, these cells can immediately communicate the problem to the capillaries in the afferent and efferent arterioles, which can constrict or relax to change the glomerular filtration rate of the kidneys.

Nephrologist

A nephrologist studies and deals with diseases of the kidneys—both those that cause kidney failure (such as diabetes) and the conditions that are produced by kidney disease (such as hypertension). Blood pressure, blood volume, and changes in electrolyte balance come under the purview of a nephrologist.

Nephrologists usually work with other physicians who refer patients to them or consult with them about specific diagnoses and treatment plans. Patients are usually referred to a nephrologist for symptoms such as blood or protein in the urine, very high blood pressure, kidney stones, or renal failure.

Nephrology is a subspecialty of internal medicine. To become a nephrologist, medical school is followed by additional training to become certified in internal medicine. An additional two or more years is spent specifically studying kidney disorders and their accompanying effects on the body.

Summary

The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the osmolarity of body fluids at 300 mOsm. They are surrounded by three layers and are made up internally of three distinct regions—the cortex, medulla, and pelvis.

The blood vessels that transport blood into and out of the kidneys arise from and merge with the aorta and inferior vena cava, respectively. The renal arteries branch out from the aorta and enter the kidney where they further divide into segmental, interlobar, arcuate, and cortical radiate arteries.

The nephron is the functional unit of the kidney, which actively filters blood and generates urine. The nephron is made up of the renal corpuscle and renal tubule. Cortical nephrons are found in the renal cortex, while juxtamedullary nephrons are found in the renal cortex close to the renal medulla. The nephron filters and exchanges water and solutes with two sets of blood vessels and the tissue fluid in the kidneys.

There are three steps in the formation of urine: glomerular filtration, which occurs in the glomerulus; tubular reabsorption, which occurs in the renal tubules; and tubular secretion, which also occurs in the renal tubules.

Exercises

1. Which of the following statements about the kidney is false?
   1. The renal pelvis drains into the ureter.
   2. The renal pyramids are in the medulla.
   3. The cortex covers the capsule.
   4. Nephrons are in the renal cortex.
2. Which of the following statements about the nephron is false?
   1. The collecting duct empties into the distal convoluted tubule.
   2. The Bowman’s capsule surrounds the glomerulus.
   3. The loop of Henle is between the proximal and distal convoluted tubules.
   4. The loop of Henle empties into the distal convoluted tubule.
3. The macula densa is/are:
   1. present in the renal medulla.
   2. dense tissue present in the outer layer of the kidney.
   3. cells present in the DCT and collecting tubules.
   4. present in blood capillaries.
4. The osmolarity of body fluids is maintained at ________.
   1. 100 mOsm
   2. 300 mOsm
   3. 1000 mOsm
   4. it is not constantly maintained
5. The gland located at the top of the kidney is the ________ gland.
   1. adrenal
   2. pituitary
   3. thyroid
   4. thymus
6. Loop diuretics are drugs sometimes used to treat hypertension. These drugs inhibit the reabsorption of Na⁺ and Cl^– ions by the ascending limb of the loop of Henle. A side effect is that they increase urination. Why do you think this is the case?
7. Why are the loop of Henle and vasa recta important for the formation of concentrated urine?
8. Describe the structure of the kidney.

Answers

1. C
2. A
3. C
4. B
5. A
6. Loop diuretics decrease the excretion of salt into the renal medulla, thereby reducing its osmolality. As a result, less water is excreted into the medulla by the descending limb, and more water is excreted as urine.
7. The loop of Henle is part of the renal tubule that loops into the renal medulla. In the loop of Henle, the filtrate exchanges solutes and water with the renal medulla and the vasa recta (the peritubular capillary network). The vasa recta acts as the countercurrent exchanger. The kidneys maintain the osmolality of the rest of the body at a constant 300 mOsm by concentrating the filtrate as it passes through the loop of Henle.
8. Externally, the kidneys are surrounded by three layers. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule, which helps anchor the kidneys in place. The third and innermost layer is the renal capsule. Internally, the kidney has three regions—an outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney, which is the concave part of the “bean” shape.

Glossary

afferent arteriole

arteriole that branches from the cortical radiate artery and enters the glomerulus

arcuate artery

artery that branches from the interlobar artery and arches over the base of the renal pyramids

ascending limb

part of the loop of Henle that ascends from the renal medulla to the renal cortex

Bowman’s capsule

structure that encloses the glomerulus

calyx

structure that connects the renal pelvis to the renal medulla

cortex (animal)

outer layer of an organ like the kidney or adrenal gland

cortical radiate artery

artery that radiates from the arcuate arteries into the renal cortex

cortical nephron

nephron that lies in the renal cortex

countercurrent exchanger

peritubular capillary network that allows exchange of solutes and water from the renal tubules

countercurrent multiplier

osmotic gradient in the renal medulla that is responsible for concentration of urine

descending limb

part of the loop of Henle that descends from the renal cortex into the renal medulla

distal convoluted tubule (DCT)

part of the renal tubule that is the most distant from the glomerulus

efferent arteriole

arteriole that exits from the glomerulus

glomerular filtration

filtration of blood in the glomerular capillary network into the glomerulus

glomerular filtration rate (GFR)

amount of filtrate formed by the glomerulus per minute

glomerulus (renal)

part of the renal corpuscle that contains the capillary network

hilum

region in the renal pelvis where blood vessels, nerves, and ureters bunch before entering or exiting the kidney

inferior vena cava

one of the main veins in the human body

interlobar artery

artery that branches from the segmental artery and travels in between the renal lobes

juxtaglomerular cell

cell in the afferent and efferent arterioles that responds to stimuli from the macula densa

juxtamedullary nephron

nephron that lies in the cortex but close to the renal medulla

kidney

organ that performs excretory and osmoregulatory functions

lobes of the kidney

renal pyramid along with the adjoining cortical region

loop of Henle

part of the renal tubule that loops into the renal medulla

macula densa

group of cells that senses changes in sodium ion concentration; present in parts of the renal tubule and collecting ducts

medulla

middle layer of an organ like the kidney or adrenal gland

nephron

functional unit of the kidney

perirenal fat capsule

fat layer that suspends the kidneys

peritubular capillary network

capillary network that surrounds the renal tubule after the efferent artery exits the glomerulus

proximal convoluted tubule (PCT)

part of the renal tubule that lies close to the glomerulus

renal artery

branch of the artery that enters the kidney

renal capsule

layer that encapsulates the kidneys

renal column

area of the kidney through which the interlobar arteries travel in the process of supplying blood to the renal lobes

renal corpuscle

glomerulus and the Bowman’s capsule together

renal fascia

connective tissue that supports the kidneys

renal pelvis

region in the kidney where the calyces join the ureters

renal pyramid

conical structure in the renal medulla

renal tubule

tubule of the nephron that arises from the glomerulus

renal vein

branch of a vein that exits the kidney and joins the inferior vena cava

segmental artery

artery that branches from the renal artery

transport maximum

maximum amount of solute that can be transported out of the renal tubules during reabsorption

tubular reabsorption

reclamation of water and solutes that got filtered out in the glomerulus

tubular secretion

process of secretion of wastes that do not get reabsorbed

ureter

urine-bearing tube coming out of the kidney; carries urine to the bladder

urinary bladder

structure that the ureters empty the urine into; stores urine

urine

filtrate produced by kidneys that gets excreted out of the body

vasa recta

peritubular network that surrounds the loop of Henle of the juxtamedullary nephrons