12.3: Kidneys

Since the kidneys are virtually identical to each other in structure and functioning, we will consider only the right kidney here.

Blood Vessels

As shown in Figure 12.1, blood vessels enter and leave the kidney where it is indented. The arteries branch into smaller vessels as they pass through the inner region (medulla) of the kidney (Figure 12.2). These branches curve over the segments of the medulla and then send smaller arterial branches to the outer region (cortex). Within the cortex, each of the smallest branches (afferent arterioles) leads into a tuft of capillaries called a glomerulus. Another tiny artery (efferent arteriole) leaves the glomerulus and leads into another group of capillaries (peritubular capillaries), which surround small kidney tubules. Blood from these capillaries is collected into veins, which carry it back through the medulla and out of the kidney.

Figure 12.2 Kidney structure. (Sources of images and videos below. Used with permission.)

The capillaries that constitute each glomerulus are much more porous than are other capillaries. Blood pressure causes much of the water and most small molecules in the blood, including both desirable and undesirable substances, to pass through the glomerular wall by the process of filtration. The filtrate that has passed through the glomerular wall is captured by a double layer of kidney cells called Bowman's capsule, which surrounds the glomerulus. The filtrate then passes into a twisted tube called the renal tubule, which has three sections, the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. (Figure 12.3). Meanwhile, blood cells, large molecules such as proteins, and some water and small molecules remain in the glomerulus and then flow through the efferent arteriole.

Figure 12.3 Nephron structure. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Tubules and Collecting Ducts

Different types of kidney cells compose each region of the renal tubule, and one region of the tubule (the loop of Henle) passes through the center of the kidney. As the filtrate passes through each region of the tubule, the tubule cells send desirable materials in the filtrate into the blood in the surrounding capillaries. These materials include essentially all the glucose and amino acids, much of the water and sodium, and smaller amounts of minerals such as calcium. This retrieval process is called reabsorption (Figure 12.4). At the same time, the tubule cells cause undesirable materials remaining in the blood to move into the fluid within the tubule by the process called secretion (excretion). Finally, more water is reabsorbed as the fluid passes through the collecting duct. The solution of wastes, toxins, and other undesirable materials remaining in the collecting duct is urine. Urine passes from the kidney into the ureter, which transports it to the urinary bladder.

Figure 12.4 Filtration, reabsorption, and secretion. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Nephrons

The kidney has approximately 1 million glomeruli, each of which is associated with a Bowman's capsule and a renal tubule. The combination of these three structures is called a nephron (Figure 12.2, Figure 12.3). All nephrons function in a similar though not an identical manner. One noteworthy difference is that nephrons with glomeruli close to the medulla (juxtamedullary nephrons) seem to be especially important for reabsorbing water.

Overall Functions

Urine formation involves the three processes of filtration, reabsorption, and secretion. The rate and amount of each of these processes are carefully adjusted so that blood leaving the kidneys can compensate for any factors that tend to disturb homeostasis with respect to waste and toxin levels, osmotic pressure, the concentrations of many individual substances, acid/base balance, and blood pressure. Adjustments are made based on the quality of blood passing through the kidney and many regulatory substances including hormones, nitric oxide (*NO), and sympathetic nerves. Tubule cells also add correct amounts of vitamin D and erythropoietin to the blood. Thus, the kidneys perform all the urinary system functions for homeostasis.

Under favorable living conditions, such as having comfortable temperatures, proper diet, and moderate exercise, as little as 30 percent of the working capacity of both kidneys is needed to maintain homeostasis. The additional reserve capacity becomes important when conditions are less favorable, such as when high temperatures cause profuse sweating or the diet contains excess water. However, even the most fully functional kidneys can be overburdened by extreme conditions such as complete water deprivation. Therefore, in healthy adults there is a range of living conditions within which the kidneys can maintain homeostasis. Conditions outside this range overwhelm the powers of compensation of the kidneys and lead to loss of homeostasis, cell and body malfunction, illness, and possibly death.