Kidneys
From Encyclopædia
The two kidneys are the major
organs of excretion in vertebrates. Excess water, toxic waste products of
metabolism such as UREA and URIC ACID, and inorganic salts are disposed by the kidneys in the form of URINE. Because of their excretory function, the kidneys are also largely responsible for maintaining the water balance of the body and the acidity (pH) of the blood (see
EXCRETORY SYSTEM). The kidneys and associated
organs that produce and eliminate urine are collectively called the urinary system. The kidneys also play important roles in other body activities, such as in releasing the protein erythropoietin--which stimulates the bone marrow to increase the formation of red blood
cells--and in helping to
Control blood
pressure. Some
drugs or their breakdown products in the body are eliminated through the kidney.ANATOMYHuman kidneys are paired, reddish brown, bean-shaped structures about 11 cm (4.4 in) long. They are located in back of the body cavity, one on each side of the spine just above the waist. The kidneys are loosely held in place by a
Mass of fat (perirenal fat) and by two layers of fibrous tissue (renal fascia) between which the kidneys are placed. The outer margin is convex and the inner border concave. Located on the inner surface is a slit called the hilus, through which passes the arteries, veins, nerves, and the renal pelvis, a funnel-like structure. Urine from each kidney is collected in the renal pelvis and passes into a hollow tube, the ureter, which is 40-45 cm (16-18 in) long and extends downward, emptying into the urinary bladder. A shorter, single tube, the urethra, eliminates urine from the bladder through the penis in the male and above the vaginal orifice in the female.The cut surface of the kidney reveals two distinct areas: the cortex--a dark
band along the outer border, about 1 cm (0.4 in) in thickness--and the inner medulla. The medulla is divided into 8 to 18 conical masses of tissue termed renal pyramids. The apex of each pyramid, the papilla, extends into the renal pelvis, through which urine is discharged from the kidney tissue. The cortex arches over the bases of the pyramids (cortical arches) and extends
Down between each pyramid as the renal
columns.NephronsEach kidney contains at least 1 million microscopic structures called nephrons, or renal tubules. These are the basic functional, or urine-forming, units of the kidney. Each nephron, about 4 cm (1.6 in) long, has thin walls of epithelial
cells and is divided into a Bowman's capsule and a tubule. Each nephron begins with a renal corpuscle, which is a granular body found in the cortex. Each corpuscle is composed of a double-walled, cuplike structure called Bowman's capsule, and this surrounds a tuft or knot of blood vessels called a glomerulus (pl. glomeruli). A winding, convoluted section of the nephron tubule extends from Bowman's capsule toward the medulla region of the kidney and is called the proximal convoluted tubule. This region of the tubule is followed by Henle's loop. This loop, a relatively straight portion of the tubule, extends into the medulla and loops back in a sharp turn to the cortex. The final section, the distal convoluted tubule, is also convoluted and winding and is located close to the proximal convoluted portion. It continues on to connect other tubules in a branching structure called the collecting tubules. The collecting tubules descend into the medulla and terminate at the papillae, thus
conducting the formed urine to the renal pelvis.Blood CirculationArterial blood enters the kidneys, at the hilus, by renal arteries, which subdivide into smaller and yet smaller arteries, and finally to afferent arterioles. They
lead into the 30 to 40
capillary loops of the glomerulus. Blood is recollected from each glomerulus by the efferent arterioles, which are much smaller in diameter than the afferent arterioles and therefore create a relatively higher backup blood
pressure in the capillaries of the glomerulus than in other capillaries of the body. This is significant in the efficient filtering function of the nephron. The efferent vessels divide into capillaries surrounding the tubules, thus supplying blood to the medulla. Eventually they rejoin into veins and exit through the renal hilus.URINE PRODUCTIONThe initial site of urine production is the glomerulus. Arterial blood
pressure, originally generated from the heart, drives a filtrate of plasma across the porous
capillary walls of the glomeruli into the open space around the
capillary tuft, called Bowman's space, and is collected in Bowman's capsule. The filtered plasma, now called glomerular filtrate, is mainly water but also contains salts,
glucose, amino acids, nitrogenous wastes such as urea, and a small amount of ammonia. Proteins, fats, and cellular elements (red blood
cells, white blood
cells, and platelets) are filtered out so that they remain in the general blood circulation. In normal kidneys 100 to 140 milliliters (0.21 to 0.29 pt) of filtrate is formed each minute, for a total of about 170 liters (180 qt) per day.As the glomerular filtrate passes along the proximal convoluted tubule, the majority of its water content and some of its dissolved materials are reabsorbed through the walls of the tubule into the blood of the surrounding capillaries.? This reabsorption process is highly selective. Water,
sodium and
chloride ions, most of the bicarbonate, and all of the
glucose are reabsorbed into the bloodstream, while other products, such as urea and ammonia, remain in the tubule.During the later
stage, through Henle's loop and the distal convoluted tubule, most of the remaining filtrate is further selectively reabsorbed, so that only about 1 percent of the volume of the original filtrate is finally excreted as urine. The urine is considerably different in composition from the original filtrate.The kidneys excrete 400 to 2,000 milliliters (0.84 to 4.2 pt) of urine or more per day; excretions varies in volume and composition depending on the needs of the host. Kidneys maintain the internal fluid environment within narrow limits and are capable of adapting to a wide range of environmental situations. The
cells lining the tubules are under the influence of regulating factors, such as the hormones aldosterone (from the adrenal gland), antidiuretic hormone, parathyroid hormone, and atrial natriuretic factor (from the heart).The distal tubule regulates the overall acidity of the urine, and ultimately of the blood, by excretion of hydrogen ions. Ammonia combines with hydrogen to form ammonia ions that are secreted into the urine. The removal of hydrogen ions decreases the acidity.All the blood
glucose will be removed unless the blood
glucose exceeds normal concentrations by a considerable amount. Other mechanisms remove most, if not all, of the other solutes, such as
sodium. Much of the
sodium ion in kidney filtrate is transported back to the blood, but 3 to 5 g (0.1 to 0.17 oz) pass into the urine each day. As a result, most animals have strict salt requirements and must consume several grams of
sodium chloride daily in order to live. The retention of
sodium is enhanced by the presence of aldosterone. This hormone is secreted into the bloodstream when the body's supply of
sodium falls below normal. When there is an excess of
sodium, aldosterone secretion is reduced and more
sodium is excreted.When excessive amounts of fluid are lost from the body, or the blood
pressure falls below normal, the kidneys release the enzyme renin into the blood, where it promotes the formation of angiotensin. Within minutes, angiotensin causes vasoconstriction, which increases blood
pressure and stimulates the secretion of aldosterone.EVOLUTION OF THE KIDNEYThe structure of the nephron is basically the same in the kidneys of all vertebrates and has even been identified in the fossils of the oldest known vertebrates. The kidney has gone through a series of changes, however, in the course of evolution from invertebrates. In invertebrates, the excretory
organ is a simple absorbing tubule called a nephridium. Among lower vertebrates, such as the fishes, there exists a primitive kind of kidney, called the holonephros, that consists of three to five kidney tubules near the heart. Most adult fishes and amphibians have a posterior kidney, or opisthonephros, containing a more complicated type of tubule. Adult
reptiles,
birds, and
mammals have the most advanced form of kidney, called a metanephros.During human embryonic development, a holonephros appears in the third week of gestation and begins degenerating by the fourth week without ever having functioned as a kidney. A true kidney, an opisthonephros, forms during the fourth week, and a metanephros takes its place during the fifth or sixth week and remains as the functional kidney for life.It is generally believed that the kidney first evolved in the original vertebrates, which were freshwater life-forms requiring some means for pumping excess water from the body. In keeping with the laws of OSMOSIS, there is a continual flow of water into the bodies of freshwater fish. This excess water must be disposed of once it has entered. At the same time that the nephron first evolved and developed, it became capable of reabsorbing
glucose, salts, and other materials that otherwise would have been lost in pumping out surplus water.
Cate?ory:Encyclopædia
