Introduction to Nephrons
Within the renal lobes of each kidney are approximately 0.8 to 1.5 million microscopic structures called nephrons, which are the kidney’s functional units. The nephrons perform several vital functions.
- They help remove wastes and toxins.
- They help maintain fluid, electrolyte, and pH balances,
- They help regulates blood pressure.
A nephron consists of a cup-like Bowman’s capsule in the renal cortex and a coiled renal tubule that extends from the capsule. The renal tubule consists of three sections. The first section, the proximal convoluted tubule (PCT), and the last section, the distal convoluted tubule (DCT), are located in the renal cortex. Connecting the PCT and DCT is the loop of Henle, which curves in and out of the renal medulla.
The Bowman’s capsule surrounds a tuft of capillaries called a glomerulus. Together, these structures form the renal corpuscle. The capsule filters the blood in the glomerular capillaries, and the filtrate flows into the renal tubule.
As the filtrate courses through the renal tubules, most substances return to the bloodstream by reabsorption. However, many substances remain in the tubules, and additional substances enter from nearby capillaries through secretion.
The filtrate eventually flows from the distal convoluted tubule into one or more collecting ducts. These elongated passageways carry the filtrate from the renal cortex to the apical tips of the renal pyramids, where the filtrate drains into the renal calyces and out of the kidney (excretion).
There are two types of nephrons in the kidneys, which are categorized based on the location of their renal corpuscles.
Most kidney nephrons (about 85%) are categorized as cortical (superficial) nephrons because their renal corpuscles arise in the superficial (outer) renal cortex. Cortical nephrons also have relatively short loops of Henle that do not extend beyond the outer renal medulla.
The other 15% of nephrons are classified as juxtamedullary nephrons because their renal corpuscles occur near (juxta-) the renal medulla. Additionally, the loops of Henle in these nephrons are longer, extending deep into the medulla.
The cortical nephrons carry out most of the regulatory and excretory functions of the kidneys, and the juxtamedullary nephrons provide additional support in regulating urine concentration.
Nephron Blood Supply
About 20% – 25% of the cardiac output passes through the kidneys each time the blood circulates through the body. Most blood entering the kidney (about 94%) goes to the renal cortices, where nephrons exchange substances (filtration, reabsorption, and secretion) with nearby capillaries to form urine.
Blood travels into the kidney from the descending aorta by the renal artery. Branches of the renal artery distribute the blood inside the kidney, eventually delivering it to the renal lobes.
Arcuate arteries transport blood into the renal lobes and distribute it along the border between the renal cortex and renal medulla.
In the renal lobes, cortical radiate arteries (interlobular arteries) branch at right angles from the arcuate arteries and transport blood into the renal cortex.
Arterioles and Glomerulus
In the renal cortex, the cortical radiate arteries give off branches called afferent arterioles that deliver blood to the nephrons.
Each afferent arteriole delivers blood to capillaries within a nephron’s Bowman’s capsule called the glomerulus. As blood passes through the glomerular capillaries, it is filtered. The interior of the Bowman’s capsule collects the filtrate and then passes it to the renal tubule. Reabsorption and secretion adjust the filtrate composition to form urine as the filtrate flows along the tubule.
The glomerular capillaries merge to form a second arteriole called the efferent arteriole, which transports blood from the Bowman’s capsule.
The efferent arterioles of the cortical and juxtamedullary nephrons deliver filtered blood to the peritubular capillaries, which wind around the renal tubules in the renal cortex. These vessels exchange substances with the tubular filtrate through reabsorption and secretion.
The efferent arterioles of the juxtamedullary nephrons also provide blood to a second network of capillaries called the vasa recta. These capillaries extend deep into the renal medulla and coil around the loops of Henle, where they help adjust the concentration of the tubular filtrate.
The blood from the renal capillaries moves into the cortical radiate (interlobular) veins, which join with the arcuate veins at right angles. The arcuate veins carry the blood to the outer edges of the renal lobes, where they merge with larger veins that transport the blood from the kidneys to the inferior vena cava.
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References and Attributions
American Journal of Physiology (Renal Physiology) – “Medullary and Cortical Thick Ascending Limb: Similarities and Differences“
NIH (National Library of Medicine) – “Thick Ascending Limb of the Loop of Henle“
NIH (National Library of Medicine) – “Physiology, Renal Blood Flow and Filtration“
OpenStax (Anatomy and Physiology) – “Gross Anatomy of the Kidney” Access OpenStax for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
ScienceDirect – “Juxtamedullary Nephron“
ScienceDirect – “Nephron“
Wikipedia – “Kidney“
Wikipedia – “Nephron“