The kidney contains approximately one million nephrons. These are little systems of blood vessels and tubules where filtration occurs, electrolyte balance is maintained, and body fluid levels are controlled.
Nephrons are located in the outer section of the kidney called the renal cortex. Cortical nephrons often have shorter nephron loops that may or may not extend into the middle layer of the kidney, called the renal medulla. Juxtamedullary nephrons are located near the medulla in which their nephron loops are deeply set.
Renal Corpuscle
Afferent arterioles are tiny arteries that carry blood to the nephron. They empty blood into a part of the nephron called the glomerulus. The glomerulus is a coiled network of capillaries surrounded by a capsule called the glomerular capsule. Capillaries are tiny blood vessels that connect arteries and veins. They are where nutrient and gas exchange take place between the blood and the body's tissues.
The glomerulus and the glomerular capsule, also called Bowman's capsule, make up a part of the nephron called the renal corpuscle. Filtration begins in the glomerulus. Filtered fluid moves into the renal tubule.
Renal Tubule
As the renal tubule progresses, it becomes a coiled section called the proximal convoluted tubule. This is where water, salt, and glucose leave the tubule to be absorbed by the body. The tubule extends toward the renal pelvis forming the nephron loop, also called the loop of Henle.
Nephron loops from many nephrons join together in the medulla to form a collecting duct. Urine flows through this duct and through an opening called a renal papilla and into the renal calyces. The nephron loop is not considered to be part of the nephron.
The portion of the loop extending toward the pelvis is called the descending limb and the returning section is called the ascending limb of the nephron loop. Water is lost from the descending limb by osmosis into the medullary fluid. Osmosis is the movement of water from an area of lower solute concentration to an area of higher solute concentration. Solute can be salt, for example.
Water cannot pass through the walls of the ascending limb of the nephron loop. Salt is pumped out of the ascending loop reducing the solute concentration of the fluid within the tubule. The tubule that follows the ascending nephron loop is called the distal convoluted tubule. It is responsible for the regulation of electrolytes and maintaining pH balance. The distal convoluted tubule empties into a collecting duct which drains fluid in the renal pelvis to be excreted.
Peritubular Capillary System
Efferent arterioles carry blood away from the glomerulus. They form a network of blood vessels around the nephron loop. The network is called the peritubular capillary system. These loops of capillaries are called vasa recta.
The nephron is a complex unit that is responsible for a series of exchanges. Waste products, including urea, are filtered out of the blood. Water is transferred via osmosis and is related to the movement of sodium chloride in and out of the system. Hydrogen atoms, also referred to as protons, are transferred in and out of the system to regulate pH.
The product of this process is urine. It is collected by the collecting ducts and emptied in the renal calyces and pelvis. It is transported to the bladder via the ureters for excretion from the body.
Reference
Shier, D., Butler, J., Lewis, Ricki. Hole's Human Anatomy & Physiology. New York: McGraw-Hill, 2002.
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