THE EXCRETORY SYSTEM

One of the most important ways your body maintains homeostasis is by keeping the right amount of water and salts inside your body. If you have too much water in your body, your cells can swell and burst. If you have too little water in your body, your cells can shrivel up like an old apple. Either extreme can cause illness and death of cells, tissues, and organs. The organs of your excretory system help to keep the correct balance of water and salts within your body.

Your body also needs to remove the wastes that build up from cell activity and from digestion. These wastes include carbon dioxide, urea, and certain plant materials. If these wastes are not removed, your cells can stop working and you can get very sick. The excretory system can also help to release wastes from the body. Excretion is the process of removing wastes from the body.

 

THE KIDNEYS

The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters, and collects in the bladder. The bladder can distend to store urine that eventually leaves through the urethra.

 

The nephron has three functions:

1. Glomerular filtration of water and solutes from the blood.
2. Tubular reabsorption of water and conserved molecules back into the blood.
3. Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.

Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16 times each day. In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters forms urine.

Urine Production

1. Filtration in the glomerulus and nephron capsule.
2. Reabsorption in the proximal tubule.
3. Tubular secretion in the Loop of Henle.

Components of The Nephron

• Glomerulus: mechanically filters blood
• Bowman's Capsule: mechanically filters blood
• Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino acids
• Loop of Henle: Countercurrent exchange, which maintains the concentration gradient
• Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs.

Kidney Stones

In some cases, excess wastes crystallize as kidney stones. They grow and can become a painful irritant that may require surgery or ultrasound treatments. Some stones are small enough to be forced into the urethra, others are the size of huge, massive boulders (or so I am told).

Kidney Function

Kidneys perform a number of homeostatic functions:

1. Maintain volume of extracellular fluid
2. Maintain ionic balance in extracellular fluid
3. Maintain pH and osmotic concentration of the extracellular fluid.
4. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

Hormone Control of Water and Salt

Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative feedback. ADH is released from the pituitary gland in the brain. Dropping levels of fluid in the blood signal the hypothalamus to cause the pituitary to release ADH into the blood. ADH acts to increase water absorption in the kidneys. This puts more water back in the blood, increasing the concentration of the urine. When too much fluid is present in the blood, sensors in the heart signal the hypothalamus to cause a reduction of the amounts of ADH in the blood. This increases the amount of water absorbed by the kidneys, producing large quantities of a more dilute urine.

Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron to the blood. When sodium levels in the blood fall, aldosterone is released into the blood, causing more sodium to pass from the nephron to the blood. This causes water to flow into the blood by osmosis. Renin is released into the blood to control aldosterone.

Disruption of Kidney Function |

Infection, environmental toxins such as mercury, and genetic disease can have devastating results by causing disruption of kidney function. Many kidney problems can be treated by dialysis, where a machine acts as a kidney. Kidney transplants are an alternative to dialysis.

THE LIVER

The liver is a large, important organ.  In fact it is the largest internal organ in our bodies.  Its numerous functions make it "part" of the circulatory, digestive, and excretory systems.

Excretory Function	Some proteins & other nitrogenous compounds are broken down in the liver by a process called deamination.  As a result of these reactions, a nitrogenous waste called urea is formed.
Digestive Function	The liver produces bile, which is temporarily stored in the gall bladder before being released into the small intestine where it helps "emulsify" (break down) lipid molecules.
Circulatory System	The liver removes & breaks down old red blood cells.  It is also responsible for maintaining "normal" levels of glucose in the blood. When stimulated by insulin, the liver removes glucose form the blood & converts it to glycogen for storage.  When stimulated by the hormone glucagon, the liver does the opposite: it breaks down glycogen, producing glucose, which is released into the bloodstream.  The liver is also responsible for removing potentially hazardous chemicals from the blood.  It "detoxifies" the blood.  For this reason, alcoholics and other types of addicts have a higher incidence of liver disease.

 

THE LUNGS

Cellular respiration occurs in every living cell in your body. It is the reaction that provides energy (in the form of ATP molecules) for cellular activities.  If respiration stops, the cell no longer has energy for cellular activities & the cell dies. The two lungs excrete carbon dioxide and water vapour. As respiration occurs carbon dioxide is produced as a waste product.  As the carbon dioxide accumulates in body cells, it eventually diffuses out of the cells & into the bloodstream, which eventually circulates to the lungs.  And here, in the alveoli of the lungs, carbon dioxide diffuses from the blood, into the lung tissue, and  then leaves the body every time we exhale.  We should note that some water vapor also exits the body during exhalation.

 

THE SKIN

Sweat comes out of pores in your skin.  Sweat is a mixture of three metabolic wastes: water, salts, & urea.  So as sweat, body accomplishes two things:

1) sweating has a cooling effect on the body, and

2) metabolic wastes are excreted.

the skin is that there are two layers.  The thin epidermis at the top, and the thicker dermis below.  The inner layer of skin (dermis) is where we find oil glands, hair follicles, fatty layers, nerves, and sweat glands.

Notice that the sweat gland is a tubular structure tangled with capillaries (the smallest of blood vessels).  This close association of tubes allows wastes (namely water, salts & urea) to diffuse from the blood  & into the sweat gland.  And then, when body temperature rises, the fluid (sweat) is released from the gland, travels through the tube  (duct), & reaches the skin surface through openings called pores. The skin excretes sweat, which although more dilute, contains the same excretory substances as urine. Foreign substances can be excreted in the hair and the nails.