Structure and Function of the Kidneys: Overview

The kidneys have three basic mechanisms for separating the various components of the blood: filtration, reabsorption, and secretion. These three processes occur in the nephron (Figure 2), which is the most basic functional unit of the kidney. Each kidney contains approximately one million of these functional units. The nephron contains a cluster of blood vessels known as the glomerulus, surrounded by the hollow Bowman's capsule. The glomerulus and Bowman's capsule together are known as the renal corpuscle. Bowman's capsule leads into a membrane-enclosed, U-shaped tubule that empties into a collecting duct. The collecting ducts from the various nephrons merge together, and ultimately empty into the bladder.

	Nephron Segment	Function
	Renal Corpuscle: Glomerulus Bowman's Capsule	Filtration: Glomerulus filters proteins and cells from the blood. All other blood components pass into Bowman's capsule, then into the tubule.
	U-Shaped Tubule	Reabsorption and Secretion: Semipermeable membranes surrounding the tubule allow selective passage of particles back into the blood (reabsorption), or from the blood into the tubule (secretion).
	Collecting Duct	Collection: Collects all material that has not returned to the blood through the tubular membranes. This material exits the kidney as urine.
Figure 2 This is a schematic diagram of the nephron.		Table 1 This table summarizes the essential functions of the major segments of the nephron.

Renal Corpuscle

Blood first enters the kidney through the renal artery (see Figure 1), which branches into a network of tiny blood vessels called arterioles. These arterioles then carry the blood into the tiny blood vessels of the glomerulus. It is here, in the renal corpuscle, where filtration occurs. The glomerulus filters proteins and cells, which are too large to pass through the membrane channels of this specialized component, from the blood. These large particles remain in the blood vessels of the glomerulus, which join with other blood vessels so that the proteins remain circulating in the blood throughout the body. The small particles (e.g., ions, sugars, and ammonia) pass through the membranes of the glomerulus into Bowman's capsule. These smaller components then enter the membrane-enclosed tubule in essentially the same concentrations as they have in the blood. Hence, the fluid entering the tubule is identical to the blood, except that it contains no proteins or cells.

Tubule

The tubule functions as a dialysis unit, in which the fluid inside the tubule is the internal solution and the blood (in capillaries surrounding the tubule) acts as the external solution. Particles may pass through the membrane and return to the blood stream in the process known as reabsorption, which is analogous to the movement of particles from the internal to the external solution in the dialysis experiment you performed in lab. The reabsorption of many blood components is regulated physiologically, as discussed below. Alternatively, particles may pass through the membrane from the blood into this tubule in the process known as secretion, which is analogous to the movement of particles from the external solution into the dialysis bag in the experiment you performed in lab. The most important particles that are secreted from the blood back into the tubules are H⁺ and K⁺ ions, as well as organic ions from foreign chemicals or the natural by-products of the body's metabolism.

Collecting Duct

The blood components that remain in the nephron when the fluid reaches the collecting duct are excreted from the body.The collecting duct from one nephron meets up with many others to feed into the ureter. The ureters (one from each kidney) enter the bladder, which leads to the urethra, where the liquid waste is excreted from the body. Hence, the material that is filtered and secreted from the blood into the tubule, less the amount that is reabsorbed into the blood, is ultimately excreted from the body.

The localization of each of these processes within specific components of the nephron is summarized in Table 1, above.