6.5. The excretory system

Respiration rids the body of water and carbon dioxide, a waste product of metabolism. Other metabolic wastes, especially ni­trogen compounds, are eliminated from the body through the process of excretion.

Nitrogen compounds in the form of ammonia are released as the body breaks down excess amino acids. In concentrated form, ammonia is a poison. The body has two processes for making the ammonia less toxic. In one, the ammonia is mixed with great quantities of water. In the other, the ammonia is changed to a less harmful form.

The liver, operating as an excretory organ, combines the ammonia with carbon dioxide to form a less toxic compound called urea. Urea enters the blood and circulates throughout the body. Some urea is excreted through the skin in the form of perspiration, which is a mixture of water, minerals, and urea. Most of the urea the body produces, however, is eliminated by the excretory system.

The Kidneys

The kidneys are the major organs of the excretory system. They are two bean-shaped organs, each about 11 cm long, 6 cm wide, and 2.5 cm thick. They are located on either side of the spine in back of the abdominal cavity. Their combined weight is less than 0.5kg. The kidneys are held in position by tough connective tissue and protected by a layer of fatty tissue.

The main functions of the kidneys are to remove urea and other wastes, regulate the amount of water in the blood, and adjust the concentrations of various substances in the blood. Thus, the kidneys play a vital role in maintaining homeostasis, or balance among elements in the body.

Three main sections of a kidney are: an outer layer called the cortex, a middle layer called the medulla, and a central cavity called the renal pelvis. Blood enters the kidneys through renal ar­teries and leaves through renal veins.

The basic functional unit in each kidney, called a nephron, straddles both the cortex and the medulla. Each kidney contains an estimated one million nephrons.

The nephron consists partly of a glomerulus which is a mass of capillaries that form a tight ball. Each glomerulus is surrounded by a hol­low, cup-shaped sac called a Bowman's capsule. The glomeru­lus and Bowman's capsule, located in the cortex, are responsi­ble for filtering wastes from the blood. The Bowman's capsule is the first part of the renal tubule, much of which is a coiled tube extending into the medulla and back again. The last part of the renal tubule is called the collecting duct. The collecting duct is a straight tube leading to the renal pelvis. Water and mineral composition of the blood are regulated primarily by the coiled part of the renal tubule.

Filtration

The process of removing urea and other wastes from the blood is called glomerular filtration. The process starts as blood from the renal arteries flows into the glomerular capillaries. Blood in the glomerulus is under high pressure as it is pumped with great force from the heart into the tiny capillaries. This pressure forces water, urea, glucose, and minerals—a mixture called filtrate—into the Bowman's capsule. Red blood cells, white blood cells, and protein molecules do not pass out of the capillaries.

Blood passes through the kidneys at a rate of 0.25 L per minute. In other words, all the blood in the human body passes through the kidneys once every 30 minutes. A total of 170 L of filtrate is produced daily.

Reabsorption

If the kidneys only filtered the blood, a person would soon die, because along with metabolic wastes, filtration removes glucose, water, and other substances needed for life. However, these materials are returned to the blood in the renal tubule, the second part of the nephron. Tubular reabsorption is the process by which these vital materials are returned to the blood. Because reabsorption is important to the normal func­tioning of the body, the kidneys may be more accurately de­scribed as organs of regulation rather than excretion.

Reabsorption occurs as materials cross the walls of a renal tubule into a web of surrounding capillaries. Glucose and such chemicals as sodium, potassium, hydrogen, magnesium, and calcium are reabsorbed through active transport. As much as 99 percent of the water in filtrate may return to the blood through osmosis. When blood volume is low, a large amount of water is reabsorbed. When blood volume becomes normal, the rate of osmosis decreases.

In addition to reabsorption, a process called tubular secre­tion occurs in the renal tubule. In the process of tubular secretion, tubule cells actively remove certain substances from the blood and secrete these substances into the filtrate. Penicillin is an example of a substance that is secreted in this way.

Urea, other metabolic wastes, and water that remain in the renal tubule form an amber-colored liquid called urine. The urine from several tubules flows into a single collecting duct located in the medulla. In turn, all of the collecting ducts chan­nel urine into the funnel-shaped renal pelvis. From the renal pelvis the urine flows into the urinary system to be removed from the body.

The Urinary System

From the renal pelvis, urine enters a long, narrow tube calledthe ureter. The ureter from each kidney connects to the urinary bladder, a sac of smooth muscle that can hold approximately 400 to 500mL of urine. When the bladder is full, special nerves in the bladder wall send messages to the brain. The brain’s response causes sphincter muscles to relax. This relaxation in turn causes the bladder to contract. Urine is forced from the bladder through the urethra, a tube to the outside of the body. The process of expelling urine from the body is called urination.