The Excretory System
The word excretion means the removal of waste substances from the body.
Several organs are involved with the excretory system, including the kidneys, liver, sweat
glands, and lungs, however, the primary organs of excretions are the kidneys.
Excretion is vital to the health of the body because the wastes are poisonous.
As you know, carbon dioxide is removed by the lungs.
Urea, salts and nitrogen wastes are removed by the kidneys and sweat glands.
Nitrogen wastes are a by-product of protein metabolism. Amino groups are removed from amino
acids. The NH2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH 3).
Ammonia is a very toxic nitrogen waste. Ammonia is converted to urea, a compound the body
can tolerate at higher concentrations than ammonia. Urea is dumped into the blood and
concentrated by the kidneys.
Excretory System Functions
1. Collect water and filter body fluids.
2. Remove and concentrate waste products from body fluids.
3. Return substances to body fluids as necessary for homeostasis.
4. Eliminate excretory products from the body.
Many organs play a part in the excretion of waste from the body and each has its own important
1. The Lungs – where gas exchange occurs and CO2 is removed from the body
Every time we exhale, CO2 is given off as a waste product
CO2 diffuses into the bloodstream, where it circulates to the lungs
In the lungs, CO2 diffuses from the blood into the alveoli
When we exhale the CO2 and water vapor leave the body
2. The Liver – proteins and other compounds are broken down by bile excreted by the liver
an end product of this is urea
3. The Skin – secretes perspiration through pores and cools down the body
Sweat is a mixture of 3 metabolic wastes: water, salt and urea
Wastes diffuse from the blood into a sweat gland
When body temp rises, sweat is released from the gland
It travels through a tube (duct) and exits through openings in the skin (pores)
4. The Urinary System – made up of kidneys, ureters, urinary bladder and the urethra
A. Kidneys - bean shaped organs located on either side of your body
Made up of microscopic filtering units called nephrons
Function is to produce urine and filter the blood big filters
Four major metabolic wastes are filtered from the blood of the kidneys: water,
salt, urea, and CO2.
sweat and urine are made of basically the same ingredients
The kidneys have 3 layers of skin: the cortex (outer region), the medulla (inner region),
and the pelvis (cavity that collect urine, leads to ureter).
B. After the kidneys filter waste from the blood, ureters carry liquid waste from the
kidneys to the bladder.
C. The bladder stores the liquid waste.
D. Then the urethra carries liquid waste from the bladder out of the body.
5. The Nephron – functional unit of the kidneys
It is a long thin tube, closed at one end, has two twisted regions and a long loop, ends
in a long straight portion and is surrounded by capillaries
A. The parts of the nephron are as follows:
Glomerulus/Bowman's capsule - This closed end at the beginning of the nephron is
located in the cortex.
Proximal convoluted tubule or proximal tubule - The first twisted region after the
Bowman's capsule; it is in the cortex.
Loop of Henle - A long loop after the proximal tubule, it extends from the cortex down
into the medulla and back.
Distal convoluted tubule or distal tubule - This second twisted portion of the nephron
after the loop of Henle is located in the cortex.
Collecting duct - This long straight portion after the distal tubule that is the open end of
the nephron extends from the cortex down through the medulla.
B. Urine Production – 3 step process
1. Filtration: Glomerulus/Bowman’s capsule filters water and solutes from the blood.
2. Absorption: Water is reabsorbed back into the blood through proximal tubule.
3. Processing: Ions and other waste products are secreted into distal tubule.
Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16
times a 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.
Infection, environmental toxins such as mercury, and genetic disease can disrupt kidney
Many kidney problems can be treated by dialysis, where a machine acts as a kidney.
Kidney transplants are an alternative to dialysis.
1. Acute Renal Failure – the kidneys suddenly stop working
Caused by severe blood loss, infection, injury, poison and dehydration
Treated with antibiotics and dialysis to help kidneys recover
50% of patients will regain normal kidney function
If permanent kidney damage occurs, it may lead to chronic kidney disease.
2. Kidney stones – small masses found in the kidneys or urinary tract, block the flow of urine
Excess wastes crystallize forming hard stones
They can grow and become very painful
Treated by drinking water and passing the stone through the urethra
In some cases, they become a painful irritant that may require surgery or ultrasound
3. Polycystic Kidney Disease – inherited disorder characterized by many grape-like clusters of
cysts that make the kidneys larger over time.
Treated by medicine and surgery to reduce pain, antibiotics to cure infections, dialysis
and transplant to replace failed kidneys.
Inside Your Kidneys
If you were to cut a kidney in half, you would see the following parts:
renal capsule - a thin, outer membrane that helps protect the kidney
cortex - a lightly colored outer region
medulla - a darker, reddish-brown, inner region
renal pelvis - a flat, funnel shaped cavity that collects the urine into the ureters
Diagram showing the parts of the kidney and the nephron
If you look closely at the cortex and medulla, you can see many tiny, tubular structures that stretch
across both regions perpendicular to the surface of the kidney. In each kidney, there are one million
of these structures, called nephrons. The nephron is the basic unit of the kidney. It is a long thin tube
that is closed at one end, has two twisted regions interspaced with a long hair-pin loop, ends in a long
straight portion and is surrounded by capillaries.