21.3: Microscopic Structures of the Kidneys - Nephrons

Microscopic Structures of the Kidneys - Nephrons

Nephrons are functional units located in the kidneys responsible for forming urine. Nephrons are complexes of a few components: a filtration unit, the renal corpuscle (composed of a glomerulus and Bowman's capsule/glomerular capsule) that filters the blood, kidney tubules that allow adjustment of urine contents and opportunities for reabsorption of materials into the blood, and blood vessels that bring blood to and from the nephron and reabsorb materials from the filtrate (fluid in the process of becoming urine) that had been filtered out of the blood.

Above: Structure of a nephron.

The kidneys maintain the chemical balance of body fluids by removing metabolic wastes, excess water and electrolytes. Three physiological process occur in nephrons to produce urine:

1. Filtration - blood is initially filtered in the glomerulus

2. Reabsorption

• as the filtrate moves through the proximal convoluted tubule, 60-70% of water and 100% of the organic nutrients (glucose, amino acids) are reabsorbed into the blood
• the loop of Henle conserves water and salt while concentrating the filtrate for modification by the distal convoluted tubule
• reabsorption in the distal convoluted tubule is controlled by two hormones, aldosterone and antidiuretic hormone (ADH)

3. Secretion - most secretion takes place in the distal convoluted tubule

Microscopic Kidney Structure	Location	Function
Bowman's capsule / glomerular capsule	microscopic structure in the cortex of the kidney; part of a nephron; surrounds the glomerulus	collects the waste fluid filtered out of the glomerulus. The waste fluid then travels through a series of tubules where water and electrolytes will be reabsorbed into the body.
collecting ducts	ducts traveling through the renal cortex, to the renal medulla, and ending at the renal papilla	carries filtrate from the nephrons to the renal papilla and minor calyx; reabsorbs a small amount of water and solutes; mainly, secretes solutes from the blood into the filtrate
cortex (renal)	a region of the kidney where nephrons are mostly located	region of the kidney where blood is filtered and urine is formed; contain nephrons
glomerulus	microscopic bundle of capillaries in the cortex of the kidney; part of a nephron	high-pressure leaky capillaries create raw filtrate (fluid that will become urine); fluid and solutes are pushed out of these capillaries from the blood and into the Bowman's capsule / glomerular capsule
juxtaglomerular cells	modified in the afferent arteriole	regulates blood pressure and blood volume
macula densa	modified cells in the wall of the distal convoluted tubule located adjacent to the afferent arteriole	regulates blood pressure and blood volume
medulla (renal)	a region of the kidney between the cortex and the minor calyces	contain nephron loops of juxtamedullary nephrons; balancing water and salt concentrations in the blood
nephron	microscopic structures mostly found in the renal cortex, but some components found in the medulla; composed of kidney tubules	filters the blood to remove wastes and fluid; produces urine
peritubular capillaries	surrounding the kidney tubules of cortical nephrons and the proximal and distal convoluted tubules of juxtamedullary nephrons	the peritubular capillaries surround the loop of Henle to reabsorb water, concentrating the waste fluid.
podocyte	cells on the surface of the glomerulus	create filtration slits that aid in filtration of the blood as fluid pushes out of the glomerular capillaries into the Bowman's capsule
renal corpuscle	microscopic structures in the renal cortex; composed of the glomerulus and Bowman's capsule	filters blood to produce raw filtrate; filtrate composed of water, nitrogenous waste, and nutrients; blood cells and platelets are NOT filtered and remain in the blood
renal tubules (proximal and distal convoluted tubules; loop of Henle)	proximal convoluted tubule occurs between renal corpuscle and descending limb of the nephron loop loop of Henle consists of descending limb and ascending limb; occurs between the proximal convoluted tubule and the distal convoluted tubule distal convoluted tubule occurs between the ascending limb of the loop of Henle and the collecting duct	modifies the filtrate through reabsorption and secretion to form urine proximal convoluted tubule: the major site for reabsorption of water and solutes from the filtrate into the interstitial fluid and then into blood capillaries; reabsorb 100% of most organic solutes (amino acid, glucose, etc.); 65% of water, sodium ions, and potassium ions; and 50% of chloride ions descending limb of nephron loop: the nephron loop is also called the loop of Henle; 15% of the water in the filtrate is reabsorbed by osmosis into the interstitial fluid; the descending limb is impermeable to solutes ascending limb of nephron loop: reabsorbs solutes; impermeable to water distal convoluted tubule: reabsorbs a small amount of water and solutes; mainly, secretes solutes from the blood into the filtrate; drains into a collecting duct
vasa recta	capillaries surrounding the loop of Henle in juxtamedullary nephrons	water reabsorption from filtrate in juxtamedullary nephrons

Above: Renal corpuscles (formed by the glomerulus and Bowman's capsule) are located in the convoluted regions of the cortex and are composed of a capillary tuft, the glomerulus, surrounded by a double-walled capsule, Bowman’s capsule. The space between the capsular walls, Bowman’s space, receives the fluid passing through the filtration barrier. The distal convoluted tubule associated with this renal corpuscle loops back to its own vascular pole, where it lies adjacent to the afferent arteriole. Modified cells in the wall of the tubule, the macula densa, and cells in the wall of the afferent arteriole, juxtaglomerular cells, comprise the juxtaglomerular apparatus, which regulates blood pressure and blood volume. The tissue in the bottom image is magnified by 400x.

The following are the structures filtrate passes through from its creation to its passage from the kidney as urine:

1. Filtrate is created in the renal corpuscle
2. Proximal convoluted tubule
3. Descending limb of nephron loop
4. Ascending limb of nephron loop
5. Distal convoluted tubule
6. Collecting duct
7. Renal papilla
8. Minor calyx
9. Major calyx
10. Renal pelvis
11. Ureter

The following list gives the order of blood vessels that blood passes through in the cortex to reach a nephron and then to exit the renal cortex:

1. Arcuate artery
2. Cortical radiate artery
3. Afferent arteriole
4. Efferent arteriole
5. Peritubular capillaries
6. Cortical radiate vein
7. Arcuate vein

Above: Structure and position of cortical nephrons versus juxtamedullary nephrons. Note that blood vessels are not shown in this figure, only the renal corpuscles and kidney tubules.

There are two different types of nephrons: cortical nephrons and juxtamedullary nephrons. Cortical nephrons are primarily located in the cortex of the kidneys, although the nephron loops may cross into the medulla. Cortical nephrons make up about 85% of all nephrons in a kidney. Juxtamedullary nephrons are located closer to the medulla (juxta = near to) and they have longer nephron loops that are largely in the medulla and travel deep within it. Juxtamedullary nephrons make up about 15% of the nephrons in a kidney. Juxtamedullary nephrons enable a greater amount of water reabsorption by the blood from the filtrate, thereby producing more concentrated urine. Additionally, the capillaries networking around the loop of Henle in juxtamedullary nephrons are not connected to the peritubular capillaries as in the cortical nephrons and are instead associated with blood vessels of the renal medulla. As such, the capillaries surrounding the juxtamedullary nephron loop are not called peritubular capillaries and instead are called vasa recta.

Attributions

• "Anatomy 204L: Laboratory Manual (Second Edition)" by Ethan Snow, University of North Dakota is licensed under CC BY-NC 4.0
• "Digital Histology" by Department of Anatomy and Neurobiology and the Office of Faculty Affairs, Virginia Commonwealth University School of Medicine and the ALT Lab at Virginia Commonwealth University is licensed under CC BY 4.0
• "Kidney Nephron.png" by Holly Fischer is licensed under CC BY 3.0
• "Renal Anatomy" by Dongho Kim is licensed under CC BY-NC-SA 4.0