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The skin, liver, lungs, and intestines eliminate some waste products; however, if the kidneys fail to function, these other excretory organs cannot adequately compensate. The kidneys perform the following functions:
List the functions performed by the kidneys, and briefly describe each.
Excretion. The kidneys filter blood and produce a large volume of filtrate. Large molecules, such as proteins and blood cells, remain in the blood, whereas smaller molecules and ions enter the filtrate. As the filtrate flows through the kidneys, it is slowly modified until it is converted into urine. This conversion requires the reabsorption of most of the filtrate volume back into the blood, along with useful molecules and ions. Metabolic wastes, toxic molecules, and excess ions remain in a small volume of filtrate. Additional waste products are secreted into the filtrate, eventually forming urine.
Regulation of blood volume and pressure. The kidneys play a major role in controlling the extracellular fluid volume in the body by producing either a large volume of dilute urine or a small volume of concentrated urine. Consequently, the kidneys regulate blood volume and hence blood pressure.
Regulation of the concentration of solutes in the blood. The kidneys help regulate the concentration of the major ions—Na+, Cl−, K+, Ca2+, HCO3−, and HPO42−.
Regulation of extracellular fluid pH. The kidneys secrete variable amounts of H+ to help regulate the extracellular fluid pH.
Regulation of red blood cell synthesis. The kidneys secrete the hormone erythropoietin, which regulates the synthesis of red blood cells in bone marrow (see chapter 19).
Regulation of vitamin D synthesis. The kidneys play an important role in controlling blood levels of Ca2+ by regulating the synthesis of vitamin D (see chapter 6).
Describe the location, size, and shape of the kidneys.
The kidneys are bean-shaped, and each is about the size of a tightly clenched fist. They lie behind the peritoneum on the posterior abdominal wall on each side of the vertebral column near the lateral borders of the psoas major muscles (figure 26.2). The kidneys extend from the level of the last thoracic (T12) to the third lumbar (L3) vertebrae, and the rib cage partially protects them. The liver is superior to the right kidney, causing the right kidney to be slightly lower than the left.
Describe the renal capsule and the structures that surround the kidney.
A renal capsule, a layer of fibrous connective tissue, surrounds each kidney. A dense layer of adipose tissue engulfs the renal capsule. This adipose tissue cushions the kidneys against mechanical shock.
surrounds the adipose tissue and helps anchor the kidneys and surrounding adipose tissue to the abdominal wall. Adipose tissue surrounds the renal fascia.
List the structures found at the hilum and in the renal sinus of a kidney.
The hilum (hī′lŭm) is a small area where the renal artery and nerves enter, and the renal vein and ureter exit, the kidney. It is located on the concave, medial side of the kidney. The hilum opens into the renal sinus, a cavity filled with adipose tissue and connective tissue. Structures that enter and leave the kidney pass through the renal sinus
Distinguish between cortical and juxtamedullary nephrons.
There are approximately 1.3 million nephrons in each kidney. Most nephrons measure 50-55 mm in length. Nephrons whose renal corpuscles lie near the medulla are called juxtamedullary (juks′ta-med′ŭ-lār-ē; next to medulla) nephrons. They have long loops of Henle, which extend deep into the medulla. Only about 15% of the nephrons are juxtamedullary nephrons. The remainder of the nephrons are called cortical nephrons, and their loops of Henle do not extend deep into the medulla
The glomerulus, which looks like a wad of yarn, fills the indentation. Fluid is filtered from the glomerulus into the Bowman capsule. The filtered fluid then flows into the proximal convoluted tubule, which carries it away from the Bowman capsuleA Bowman capsule has an outer layer, called the parietal layer, and an inner layer, called the visceral layer (figure 26.5b). The parietal layer is constructed of simple squamous epithelial cells. The epithelial cells become cube-shaped at the beginning of the proximal convoluted tubule. The visceral layer is constructed of specialized cells called podocytes, which wrap around the glomerular capillaries.
Because the kidneys' main function is to filter the blood, the glomerulus has several unique characteristics that make these capillaries especially permeable. Numerous, windowlike openings, called fenestrae (fe-nes′trē), are in the endothelial cells of the glomerular capillaries. Gaps, called filtration slits, are between the cell processes of the podocytes that make up the visceral layer of the Bowman capsule (figure 26.5c). A basement membrane lies sandwiched between the endothelial cells of the glomerular capillaries and the podocytes of the Bowman capsule. Together, the capillary endothelium, the basement membrane, and the podocytes of the Bowman capsule form the kidney's filtration membrane (figure 26.5d), which performs the first major step in urine formation. Urine formation begins when fluid from the glomerular capillaries is filtered across the filtration membrane into the lumen, or space, inside the Bowman capsule.
Describe the structure of the afferent and efferent arterioles, and the juxtaglomerular apparatus. What is the function of the juxtaglomerular apparatus?
An afferent (af′er-ent) arteriole supplies blood to the glomerulus, and an efferent (ef′er-ent) arteriole drains it (figure 26.5a). A layer of smooth muscle lines both the afferent and efferent arterioles. At the point where the afferent arteriole enters the renal corpuscle, the smooth muscle cells form a cufflike arrangement around the arteriole. These cells are called juxtaglomerular cells. Lying between the afferent and efferent arterioles adjacent to the renal corpuscle is part of the distal convoluted tubule of the nephron. Specialized tubule cells in this section are collectively called the macula (mak′ū-lă) densa. The juxtaglomerular cells of the afferent arteriole and the macula densa cells are in close contact with each other. Coupled together, they are called the juxtaglomerular apparatus (figure 26.5b). The juxtaglomerular apparatus secretes the enzyme renin and plays an important role in the regulation of filtrate formation and blood pressure.
loop of Henle,
The loops of Henle (nephron loops) are continuations of the proximal convoluted tubules. Each loop has two limbs: the descending limb and the ascending limb. The first part of the descending limb is similar in structure to the proximal convoluted tubules. The loops of Henle that extend into the medulla become very thin near the end of the loop (figure 26.6a, c). The lumen in the thin part narrows, and an abrupt transition occurs from simple cuboidal epithelium to simple squamous epithelium. Like the descending limb, the first part of the ascending limb is thin and made of simple squamous epithelium. Soon, however, it becomes thicker, and simple cuboidal epithelium replaces the simple squamous epithelium. The thick part of the ascending limb returns toward the renal corpuscle and ends by giving rise to the distal convoluted tubule near the macula densa.
distal convoluted tubule,
The distal convoluted tubules are not as long as the proximal convoluted tubules. The epithelium is simple cuboidal, but the cells are smaller than the epithelial cells in the proximal convoluted tubules and do not possess a large number of microvilli (figure 26.6d).
The distal convoluted tubules of many nephrons connect to the collecting ducts, which are composed of simple cuboidal epithelium (figure 26.6c). The collecting ducts, which are larger in diameter than other segments of the nephron, form much of the medullary rays and extend through the medulla toward the tips of the renal pyramids.
Explain the blood supply for the kidney.
A system of blood vessels allows the exchange of materials that occurs in the kidneys. A renal artery branches off the abdominal aorta and enters the renal sinus of each kidney (figure 26.7a). Segmental arteries diverge from the renal artery to form interlobar arteries, which ascend within the renal columns toward the renal cortex. Branches from the interlobar arteries diverge near the base of each pyramid and arch over the bases of the pyramids to form the arcuate (ar′kū-āt) arteries. Interlobular arteries project from the arcuate arteries into the cortex, and afferent arterioles are derived from the interlobular arteries or their branches. The afferent arterioles supply blood to the glomerular capillaries of the renal corpuscles. Efferent arterioles arise from the glomerular capillaries and carry blood away from the glomeruli. After each efferent arteriole exits the glomerulus, it gives rise to a plexus of capillaries, called the peritubular capillaries, around the proximal and distal convoluted tubules. Associated with the juxtamedullary nephrons are specialized sets of peritubular capillaries called the vasa recta (vā′să rek′tă) (figure 26.7b). The peritubular capillaries drain into interlobular veins, which in turn drain into the arcuate veins. The arcuate veins empty into the interlobar veins, which drain into the renal vein. The renal vein exits the kidney and connects to the inferior vena cava.
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