Excretion . Biology 39.3 Excretion. O ur bodies must get rid of waste products from the cells to maintain health. Food residues must be eliminated from the body in the form of feces.
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Biology 39.3 Excretion
Our bodies must get rid of waste products from the cells to maintain health.
Food residues must be eliminated from the body in the form of feces.
Other wastes produced as a result of metabolic reactions that occur within the body must also be eliminated. For example, water and carbon dioxide are the waste products of cellular respiration.
During the metabolism of proteins and nucleic acids, a toxic nitrogen-containing waste called ammonia is formed.
Water and metabolic wastes
To maintain health, the body must remove wastes.
It also must maintain osmotic balance and stable pH by either removing or conserving salts and water.
Excretion is the process that rids the body of toxic chemicals, excess water, salts, and carbon dioxide while maintaining osmotic and pH balance.
Water and metabolic wastes
The major organs of excretion are the lungs, the kidneys, and the skin.
Carbon dioxide (and some water vapor) is transported to your lungs by the circulatory system and excreted every time you exhale.
Excess water is secreted through the skin as sweat and through the kidneys as urine.
In the liver, ammonia is converted to a less toxic nitrogen waste called urea, which is than carried by the bloodstream to the kidneys, where it is removed from the blood.
Water and metabolic wastes
The kidneys are extremely important organs because of their role in regulating the amount of water and salts contained in blood plasma.
The kidneys are a pair of bean shaped, reddish brown organs located in the lower back. Each kidney is the size of a small fist.
The body has to maintain a certain level of salts in blood plasma and in the fluid surrounding cells, or serious harm to the body’s cells and organ systems can result.
Each kidney is a complex organ composed of roughly 1 million microscopic blood-filtering units called nephrons.
Nephrons are tiny tubes in the kidneys.
One end of a nephron is a cup-shaped capsule surrounded by a tight ball of capillaries that filters wastes from the blood, retains useful molecules, and produces urine.
Three different phases occur as the blood flows through a nephron: filtration, reabsorption and secretion.
Kidneys – Blood Filters
Filtration begins at the cup-shaped capsule called Bowman’s capsule.
Within each Bowman’s capsule an arteriole enters and splits into a fine network of capillaries called glomerulus.
The glomerulus acts as a filtration device. The blood pressure inside the capillaries forces a fluid composed of salt, water, glucose, amino acids, and urea into the hollow interior of the Bowman’s capsule.
This fluid is called filtrate. Blood cells, proteins, and other molecules too large to cross the membrane remain in the blood.
Reabsorption and secretion:
Reabsorption begins when the filtrate passes from the Bowman’s capsules into the renal tubules- long narrow tubes connected to Bowman’s capsules.
Renal tubules (arteries) bend at the center, which forms a loop. As the filtrate passes through the renal tubules, the tubules extract from the filtrate a variety of useful molecules, including glucose, ions, and some water.
These substances reenter the bloodstream through capillaries that wrap around the tubule. This arrangement prevents molecules from being eliminated from the body in the urine. Some substances can pass from the blood into the filtrate in a process called secretion.
The urine that is excreted from the body is formed from the water, urea, and various salts that are left after the absorption and secretion processes.
Collecting ducts receive fluid from several nephron segments and empty urine into areas of the kidneys that lead to the ureters.
Ureters are tubes that carry urine from the kidney to the urinary bladder. The collecting duct removes much of the water from the filtrate that passes through it. As a result, human urine can be very concentrated; up to four times more concentrated than blood plasma.
The ureters have smooth muscle in their walls. The slow, rhythmic contractions of this muscle move the urine through the ureters.
The ureters direct the urine into the urinary bladder, a hollow, muscular sac that stores urine.
The urinary bladder gradually expands as it fills, holding up to .63 of a quart of urine when filled.
The urinary bladders of males tend to be larger than those of females.
Elimination of Urine
Muscular contractions of the bladder force urine out of the body.
Urine leaves the bladder and exits the body through a tube called the urethra.
A healthy adult eliminates from about 1.6 to 2.4 quarts of urine per day from the body, depending on how much fluid has been consumed during the day.
In females, the urethra lies in front of the vagina and is only about 1 inch long.
Such a short length makes it easy for bacteria and other pathogens to invade the female urinary system, which explains why females are more prone to urinary infections than males are.
There is no connection between the urethra and the genital (reproductive) system in females.
In males the urethra passes through the penis.
In males, both sperm and urine exit the body through the urethra.
The tube that carries sperm from the testes eventually merges with the urethra.
The elimination of urine from the body through the urethra is called urination.
When the bladder fills with urine, stretch receptors in the bladder’s walls send nerve impulses to the spinal cord.
In response to these impulses from the filled bladder, these impulses cause contractions of the bladder’s muscular walls and relaxation of the rings of muscle closing off the urethra.
The bladder then empties its contents through the urethra.
In older children and adults, the brain overrides this urination reflex, which delays the release of urine until a convenient time.
Because of the vital role played by the kidneys in maintaining homeostasis, diseases affecting these organs may eventually threaten life.
If one kidney is lost in an accident or by disease, the other may enlarge and do the work of both.
Nephrotic syndrome refers to a number of signs and symptoms that result from damage to the glomeruli, which leads over time to kidney failure.
The most common causes of kidney failure are infection, diabetes, high blood pressure, and damage to the kidneys by the body’s own immune system.
Damage to the Kidneys
Because of their function in excretion, kidneys often are exposed to hazardous chemicals that have entered the body through the lungs, skin, or gastrointestinal tract.
Household substances that, in concentration, can damage kidneys include paints, varnishes, furniture oils, glues, aerosol sprays, air fresheners, and lead.
When kidneys fail, toxic wastes, such as urea, accumulate in the plasma, and blood plasma ion levels increase to potentially deadly levels.
If both kidneys fail, there are only two treatment options:
Dialysis, also called hemodialysis, is a procedure for filtering the blood by using a dialysis machine.
A dialysis machine, just as the nephrons in the kidney, sorts small molecules in the blood, keeping some and discarding others.
Dialysis machines are sometimes used for kidneys that are damaged but will eventually heal or be replaced by a kidney transplant.
A more permanent solution to kidney failure is transplanting a kidney from a healthy donor.
A major problem with kidney transplants is common to all organ transplants- rejection of the transplanted organ by the recipients immune system.
Recall that the cells of your body have “self-markers” or antigens on their surfaces that identify the cells to your immune system so it will not attack them. The combination of these antigens displayed on your body’s cells is as unique as your fingerprints.
Only identical twins have the same set of antigens.
The more closely related two individuals are, the more likely they are to have common antigens.
This is why tissue transplants are more likely to succeed if the donor and the recipient are closely related.
But even in close matches, there is some chance of organ rejection.
To reduce the chance of rejection, the recipient is treated with drugs that suppress the activity of the immune system.