|
|
Excretory systems are special structures in organisms through which waste products of METABOLISM are rid and the proper balance of water and salts in the blood and other body fluids is maintained at the same time. Defecation is the elimination of undigested material from the body and is a function of the DIGESTIVE SYSTEM.
Urinary System
The major excretory organs in vertebrates are the KIDNEYS, which are two paired organs, one on each side of the body. The kidneys and associated organs that store and eliminate urine are collectively called the urinary system. The kidneys dispose of toxic wastes, such as ammonia, urea, and uric acid, and excess salt and water in the form of urine. The heart, in a sense, pumps blood blindly, but the kidneys monitor the quality of blood so that the organism is not poisoned by the end-products of its own metabolism and the proper volume and composition of its body fluids is maintained.
The kidneys continually form urine by filtering many materials from the blood or body fluids and then returning some of these as necessary. Just what the kidneys do and how they do it varies from one vertebrate to another. In general, however, a two-way traffic route prevails between the blood, for example, and urine-forming units in the kidney. The bulk of water extracted from the blood is put back into the bloodstream, as are necessary nutrients and salts. On the other hand, wastes are removed and added to the now much concentrated urine.
In humans, about one-fourth of the blood pumped by the heart per minute passes through, and is filtered by, the kidneys. Approximately 1,600 l (1,700 qt) of blood flow through the kidneys daily, but only about a thousandth is converted into urine. Human urine is approximately 95 percent water; the rest is largely urea and small amounts of salt. The urine of birds and many reptiles, by contrast, is often a dry paste composed mostly of URIC ACID.
The kidneys of the lower vertebrates, such as fish and amphibians, lie in the forepart of the body, open into the body cavity, and filter materials from both the bloodstream and fluids in the cavity, or coelom. In mammals, birds, and reptiles, the kidneys lie in the pelvic region and filter only the bloodstream.
The urine-forming unit of the kidney is called the nephron, which is a coiled tubule full of complicated twists and turns and too small to be seen by the naked eye. Each human kidney contains approximately 1 million nephrons, and the total filtering surface of a single kidney is as large as the surface of the entire body.
The tubules of the various nephrons empty urine into collecting tubules, which in turn empty into the central cavity of the kidney, or kidney pelvis. In frogs and other amphibians, birds, and reptiles, a large duct finally empties the urine into the cloaca, which is a common chamber for eliminating urine and feces and for copulation. Mammals have no cloaca; instead, ducts, called ureters, drain into the urinary bladder. The bladder is a storage organ that is periodically emptied to the outside through another duct, the urethra.
Other Excretory Organs
Vertebrates also use the RESPIRATORY SYSTEM, special glands, and the SKIN to eliminate water, salt, and carbon dioxide, which is a relatively harmless end-product of metabolism. Carbon dioxide and some excess water are discharged through the lungs; the sweat glands of mammals get rid of salts and water; many marine fish eliminate excess salt through their gills; and a rectal gland in the shark eliminates excess salt. Seabirds, such as gulls, albatross, and penguins, take in large quantities of salt with their food. They can also drink seawater, which no mammal can do. For these birds, the excess salt is not eliminated through the kidneys, but through specialized glands in the head. These glands have a structure entirely different from that of the kidney and can excrete a salt solution up to twice the concentration of seawater. The solution flows into the nasal cavity, runs out through the nostrils, and drips from the tip of the beak. Large sea turtles and marine iguanas of the Galapagos Islands have similar salt glands close to the eyes, and the ducts open into corners of the eye.
The human body has no alternative excretory mechanisms, such as special salt glands. If a human adrift at sea drinks salt water, thirst will be aggravated because more water will be excreted than is drunk in the attempt to remove salt from the body. A human would not be better off eating raw fish; despite the relatively low salt content of fish, much water will be excreted in eliminating urea from the body, which would be formed from the breakdown, or metabolism, of protein in the fish.
The body must rid itself of the waste products of cellular activity. This process of removing metabollic waste, called excretion, is just as vital as digestion to maintaining the body’s internal enviroment. Thus the excretory system not only eliminates wastes but also plays a critical role in maintaining homeostasis by regulating balance of water and other substances in the blood.
The main waste products that the body must eliminate are, first, carbon dioxide and water from cellular respiration and, second, nitrogenous compoundsfrom the breakdown of proteins. The lungs excrete most of the carbon dioxide and some of the water through respiration. Most water and nitrogenous wastes are eliminated by the kidneys, the main excretory organs of the body.
The most common nitrogenous waste is ammonia, a substance so toxic that it could not remain long in the body without harming cells. The body is protected from ammonia poisoning by the liver, which removes ammonia from blood and converts it into a less harmfull substance called urea. The urea enters the bloodstream and then is removed by the kidneys.
Humans have two kidneys, bean-shaped excretory organs each about the size of a clenched fist. The kidneys are located in the small of the back, one behind the stomach and the other behind the liver. There are three parts of the kidney. The cortex is the outermost portion of the kidney, which makes up about a third of its tissue mass. The medulla is the inner two-thirds of the kidney. The renal pelvis is a funnel-shaped structure in the center of the kidney. Blood enters a kidney through renal artery and leaves through a renal vein.
A complex series of processes in the kindey removes waste from the blood and adjusts its chemical makeup. The substances - toxins, urea, water, and mineral salts - removed from the blood by these processes form an amber-colored liquid called urine. The urine flows from the kidneys into the urinary system for storage and eventual elimination from the body.
The functional unit of the kidney is the nephron. Each kidney consists of more than a million nephrons. Each nephron has a cup-shaped structure called Bowman’s capsule that encloses a bed of capillaries. This capillary bed, called a glonerulus, receives blood from the renal artery. High pressure forces fluids from the blood through the capillary walls into the Bowman’s capsule. The material filtered from the blood then flows through the renal tubule, a long tube with permeable walls. The remaining blood flows through a network of cappilaries that wrap around the renal tubule.
As the blood and fluid flow through a nephron, the composition of both is modified by the exchange of materials among the renal tubule, the capillaries, and the extracellular fluid. Various types of exchange take place in the four different types of the renal tubule: the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct. To understand how the structure of each part of the nephron is related to its function, the three major processes that take place in the nephron are: filtration, reabsorbtion, and secretion.
The process through which materials from the blood are forced out of the glomerulus and into Bowman’s capsule is called filtration. Pressure forces water, nitrogenous wastes, glucose, and mineral salts through the thin capillary walls. About one-fifth of the fluid portion of the blood filters into the capsule. The rest remains in the cappilaries along with proteins and cells, which are too large to pass through the capillary walls. Thus the filtrate - the fluid that enters the nephron - resembles blood plasma without the large protein molecules found in plasma.
The body needs to retain many of the substances that have been removed from the blood by filtration. Thus as the filtrate flows through the renal tubule, these materials return to the blood by passing through the walls of the tubule and entering the surrounding capillaries. This process is called reabsorbtion. Most reabsrorbtion occurs in the proximal convulated tubule. In this region about 75 percent of the water in the filtrate returns to the cappilaries by osmosis. Glucose and minerals such as sodium, potassium, and calcium are returned to the blood by active transport.
Some additional reabsobtion occurs in the distal convoluted tubule. Also in this region of the tubule some substances pass from the blood into the filtrate through a process called secretion. The pH of the blood is also adjusted by secreting hydrogen ions into the filtrate. The fluid and wastes that remain in the renal tubule form urine. The urine from several renal tubules flows into a collecting duct.
In the collectig duct urine is further concentrated by the osmosis of water through the wall of the duct into the extracellular fluid. This process allows the body to conserve water. In fact, osmosis in the collecting duct, together with reabsorbtion in other parts of the tubule, returns to the blood about 99 of every 100 ml of waterin the filtrate.
In tracing the flow of filtrate through the nephron, I have skipped over the loop of Henle. The function of the loop of Henle is closely related to that of the collecting duct because the concentration of sodium chloride. is higher in the extracellular fluid. Positively charged sodium ions follow the chloride ions into the fluid through electrical attraction. This ensures that the NaCl concentration of the extracellular fluid remains high and thus promotes the reabsorbtion of water from the collecting duct.
The permeability of the collecting duct is determined by the concentration of antidiuretic hormone (ADH) in the blood. When the concentration of ADH is increased, the collecting ducts become more permeable to water, and more water is reabsorbed. A decrease in ADH, conversely, results in less reabsorbtion of water and thus in the excretion of a larger volume of more dilute urine.
ADH is produced by neurons in the hypothalamus and is secreted from the posterior pituitary gland. The secretion of ADH is stimulated when osmoreceptors in the hypothalamus respond to an increase in blood osmotic pressure. During dehydration, therefore, when the plasma becomes more concentrated, increased secretion of ADH promotes increased permeability of the collecting ducts to water. In severe dehydration only the minimal amount of water needed to eliminate the body’s wastes is excreted.
Urine from the collecting ducts flows through the renal pelvis and into a narrow tube called a ureter. A ureter leads from each kidney to the urinary bladder, a muscular sac that stores urine. Muscular contractions of the bladder force urine out of the body through a tube called the urethra.
Disease can affect any of the parts of the closely related urinary and genital systems. Both can be infected or malfunction because of a shortcoming in development.
Kidney Inflammations
Glomerulonephritis is a serious inflammatory disease of the kidneys. It usually is triggered by a prior infection, often by streptococcal bacteria, which inflames the glomeruli, the tiny tufts through which blood is filtered. The inflammation may go away after a few weeks or may slowly destroy all the glomeruli. In the early stages, the inflammation may reduce filtration enough to cause blood to retain some excess fluid, salts, and wastes. Blood pressure might also rise. If the inflammation persists, the glomeruli are destroyed, blood pressure soars, and urine formation may stop. Mechanical means must be taken to cleanse the blood.
Pyelonephritis is a bacterial infection of the inner portions of the kidneys and the urine. If quickly treated, the infection can be cured. If untreated, however, the infection may scar and eventually destroy kidney tubules, resulting in a need for mechanical cleansing of the blood. Once damaged by a bout of pyelonephritis, the kidneys are easily reinfected.
Toxemia of pregnancy is a disorder stemming from other kidney problems experienced by some women in the last half of pregnancy. During a pregnancy, the kidneys must work more than usual. However, a woman entering pregnancy with a kidney disease such as glomerulonephritis may not be able to step up kidney function enough to meet the new demands. In severe cases of toxemia, the fetus may die or have to be aborted to save the mother's life. In lesser cases, however, medical treatment poses little risk to either life. Once a woman develops toxemia, she is likely to develop it again in later pregnancies.
Calculi and Other Urinary Disorders
Calculus disease occurs when certain substances in urine crystallize into compact stones called calculi. A stone may be formed within a kidney and become swept by urine into the ureters and the bladder. It may cause pain, obstruct urine flow, or grow large enough to damage the kidney or bladder. Small calculi may be passed in urine, and large ones can be pulverized without surgery by means of energetic sound waves. Calculi can consist of calcium, urates, cystine, or other crystals. The tendency to form kidney stones sometimes runs in families.
Polycystic disease, an inherited failure of normal kidney development, strikes infants as well as adults. Many fluid-filled cysts spring up throughout the kidneys and cause them to malfunction. Polycystic disease sufferers eventually become uremic.
Uremia means "urine in blood." It describes the condition in which the kidneys almost totally fail to operate. The blood then retains the nitrogenous products of protein metabolism instead of having them removed by the kidneys. Also, the concentration of many of the electrolytes, or salts, in the blood rises too high. The breath or perspiration of affected persons smells of urine. Each of the previously mentioned kidney ailments could cause uremia. Artificial kidneys have been developed to cleanse the blood of uremic patients. In some cases, patients with destroyed kidneys can receive a human kidney transplant.
