Maintaining a stable internal environment, or homeostasis, is essential for the survival of complex animals. Living organisms must maintain a salt and water balance, and they must continually remove the toxic byproducts generated by metabolism.
Organisms have evolved a variety of strategies to maintain a more or less stable internal environment. In animals that have circulatory systems, the blood typically passes through excretory organs, commonly termed kidneys. In terrestrial animals, the kidneys not only play a major role in the removal of wastes but are also the primary organs of osmoregulation.
In this animation, we look at the function of the mammalian kidney.
The major excretory organ of mammals is the kidney. Humans have two kidneys located in the upper posterior region of the abdominal cavity. The urine they produce is conducted to the urinary bladder through the ureters. The urethra drains the bladder. The internal structure of the kidney includes an outer cortex and an inner medulla. The ureter divides into branches, the ends of which envelop medullary tissues called renal pyramids.
The actual work of the kidney is carried out by functional units called nephrons. Each human kidney contains about a million nephrons. Each nephron consists of vascular and tubule components. An afferent arteriole carries blood to a knot of capillaries called the glomerulus. Draining each glomerulus is an efferent arteriole that gives rise to the peritubular capillaries, most of which surround the cortical portions of the nephron tubules.
Blood pressure forces water and small molecules to be filtered from the glomerulus and collected in Bowman's capsule. The initial segment of a renal tubule is called the proximal convoluted tubule. The glomerulus, Bowman's capsule, and proximal convoluted tubule of each nephron are located in the cortex.
From the proximal convoluted tubule, the nephron tubule turns down into the medulla. The portion of the tubule in the medulla is called the loop of Henle. Where the ascending limb of the loop of Henle reaches the cortex, it becomes the distal convoluted tubule.
The distal convoluted tubules of many nephrons join a common collecting duct. The collecting ducts run parallel with the loops of Henle down through the medulla and empty into the ureter at the tips of the renal pyramids.
A few peritubular capillaries run into the medulla, in parallel with the loop of Henle and the collecting duct, and form the vasa recta. These capillaries carry away the molecules that are reabsorbed from the tubules. All the peritubular capillaries join back together into a venule that eventually leads to the renal vein.
Nephrons regulate the composition of blood and urine by a combination of filtration, secretion, and reabsorption. Viewed schematically, we will see how these processes are facilitated by the regular arrangement of segments of the nephron.
The proximal convoluted tubule is responsible for most of the reabsorption of water and solutes from the glomerular filtrate. The cells of this section of the nephron actively transport sodium and other solutes, such as glucose and amino acids, out of the tubule fluid.
The active transport of solutes out of the proximal convoluted tubule into the tissue fluid causes water to follow by diffusion. The water and solutes moved into the tissue fluid are taken up by the peritubular capillaries and returned to the venous blood leaving the kidney.
Despite the large volume of water and solutes reabsorbed out of the proximal convoluted tubule, the overall concentration, or osmolarity, of the fluid that enters the loop of Henle is similar to that of blood plasma, although its composition is quite different. The ability of the kidney to produce urine that is hypertonic to the blood plasma is due to the loop of Henle. The loop of Henle does not concentrate the urine directly; rather, it functions as a countercurrent multiplier creating a concentration gradient in the surrounding medulla.
To understand the countercurrent multiplier mechanism, it is easiest to move backward through the tubule, starting with the thick ascending limb. The thick ascending limb actively transports sodium from the tubule fluid and moves it into the surrounding tissue fluid. Chloride ions follow passively.
The thick ascending limb is not permeable to water, so the reabsorption of sodium and chloride out of this part of the tubule is not followed by the outward diffusion of water. This reabsorption of sodium and chloride raises the concentration of solutes in the surrounding tissue fluid.
The descending limb, in contrast, is permeable to water, but not very permeable to sodium and chloride. Since the surrounding fluid has been made more concentrated, water leaves the tubule by osmosis. As a result, the fluid in the descending limb becomes more concentrated as it flows toward the bottom of the medulla.
The thin ascending limb is not permeable to water. It is, however, permeable to sodium and chloride. Since the tubule fluid is more concentrated than the surrounding tissue, sodium and chloride diffuse out. The thick ascending limb continues to move sodium and chloride to the medulla by active transport.
As a result of this process, the tubule fluid reaching the distal convoluted tubule is less concentrated than the blood plasma, and the solutes that have been left behind in the renal medulla have created a concentration gradient in the surrounding tissue fluid.
Since the fluid entering the distal convoluted tubule is less concentrated than the surrounding cortex, the tubule loses water osmotically as it flows toward the collecting duct.
The tubule fluid entering the collecting duct is at the same concentration as the blood plasma. However, since sodium and chloride have been moved out of the tubule fluid, urea and other waste products make up a greater proportion of its total solute content. As the collecting duct descends from the cortex to the tip of the renal pyramid, the concentration gradient established by the loop of Henle increases. This increasing solute concentration causes more and more water to be absorbed from the fluid, thus concentrating the urine in the collecting duct.
The function of the mammalian kidney may be summarized as follows:
• The glomeruli filter large volumes of blood plasma. Most of this volume, along with valuable molecules such as glucose and amino acids, is reabsorbed from the proximal convoluted tubules.
• The loops of Henle create a concentration gradient in the medulla of the kidney by a mechanism called the countercurrent multiplier. As the filtrate flows up through the thick segment of the ascending limb, NaCl is transported out of the filtrate and into the extracellular spaces of the medulla. The resulting increase in the osmotic concentration of the extracellular fluids draws water out of the filtrate in the thin descending limb making it more concentrated. As this more concentrated filtrate flows up through the thick ascending limb, more NaCl is transported out and the extracellular fluid becomes even more concentrated. Thus, the opposing—or countercurrent—directions of flow through the two limbs of the loop of Henle, along with differential permeability and transport capacities of these limbs results in "multiplying" the concentration difference between the filtrate and the extracellular fluid of the medulla.
• The concentration difference is not uniform throughout the medulla, but forms a gradient that increases from the region next to the cortex to the tip of the renal pyramid. In response to this gradient, as the urine flows down through the collecting duct from the cortex to the tip of the renal pyramid, the urine can lose water osmotically and become more concentrated. This countercurrent multiplier mechanism makes it possible for mammals to produce a urine that is hypertonic to their blood plasma.
Mammals inhabit an enormous range of environments on Earth, including some of the most arid. The major adaptation that allows mammals to maintain homeostasis in the face of a wide range of osmotic stress is the variable ability of their kidneys to concentrate urine. Thus, when they take in lots of water in their food, they can excrete the excess water by producing dilute urine, and when they are exposed to very arid conditions, they can conserve water by producing a highly concentrated urine.