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Biology
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38–3 The Excretory System
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3. Functions of the Excretory System
Every cell produces metabolic wastes.
The process by which these wastes are eliminated is called excretion
- Skin excretes excess water and salts in the form of sweat.
- Lungs excrete carbon dioxide
- Kidneys also play a major role in excretion.
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4. Functions of the Excretory System
The Kidneys What are the functions of the kidneys?
remove waste products from the blood.
maintain blood pH.
regulate the water content of blood - therefore, blood volume
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The Kidneys
The Kidneys
- located on either side of the spinal column
- Ureter carries urine to the urinary bladder
- Urinary bladder is where urine is stored
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The Kidneys
Structure of the Kidneys
Kidney
Nephron
Kidneys are made up of nephrons. Blood enters the nephron, where impurities are filtered out and emptied into the collecting duct. The purified blood leaves the nephron through the renal vein.
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The Kidneys
Kidney Structure 
A kidney has two distinct regions:
The inner part is called the renal medulla.
The outer part is called the renal cortex.
Blood enters the kidney through the renal artery
The kidney removes urea, excess water, and other waste products and passes them to the ureter
The clean, filtered blood leaves the kidney through the renal vein and returns to circulation
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The Kidneys
Cortex
Medulla
Renal artery
Renal vein
Ureter
To the bladder
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The Kidneys
Nephrons: - functional units of the kidney - located in the renal cortex
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The Kidneys
Capillaries
Bowman’s capsule
Glomerulus
Kidneys are made up of nephrons. Blood enters the nephron, where impurities are filtered out and emptied into the collecting duct. The purified blood leaves the nephron through the renal vein.
Collecting duct
Vein
Artery
To the ureter
Loop of Henle
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The Kidneys
Capillaries
Each nephron has its own blood supply:
an arteriole (artery)
a venule (vein)
a network of capillaries connecting them
Collecting duct
Vein
Artery
To the ureter
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The Kidneys
How is Blood filtered? As blood enters a nephron, impurities are filtered out and emptied into the collecting duct The purified blood exits the nephron through the venule (vein) The mechanism of blood purification involves two distinct processes: filtration and reabsorption
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The Kidneys
Filtration 
- Passing a liquid or gas through a filter to remove wastes
- mainly takes place in the glomerulus
The glomerulus is a small network of capillaries encased in the top of the nephron by a hollow, cup-shaped structure called Bowman's capsule.
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The Kidneys
Reabsorption 
Most of the material removed from the blood at Bowman's capsule makes its way back into the blood.
The process in which liquid is taken back into a vessel is called reabsorption
Almost 99% of the water that enters Bowman’s capsule is reabsorbed into the blood.
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The Kidneys
As the kidney works, purified blood is returned to circulation while urine is collected in the urinary bladder.
Urine is stored here until it is released from the body through a tube called the urethra.
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16. Control of Kidney Function
The activity of the kidneys is mostly controlled by the composition of the blood.
In addition, regulatory hormones are released in response to the composition of blood.
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17. Control of Kidney Function
For Example:
When you drink a liquid, it is absorbed into the blood - As a result, the concentration of water in the blood increases - As the amount of water in the blood increases the rate of water reabsorption in the kidneys decreases
Therefore: Less water is returned to the blood, and excess water is sent to the urinary bladder to be excreted as urine
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18. Homeostasis by Machine
Humans have two kidneys, but can survive with only one.
If both kidneys are damaged by disease or injury, there are two options:
a kidney transplant
kidney dialysis
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19. Homeostasis by Machine
Kidney dialysis works as follows:
Blood is removed by a tube and pumped through special tubing that acts like nephrons
Tiny pores in the tubing allow salts and small molecules to pass through
Wastes diffuse out of the blood into the fluid-filled chamber, allowing purified blood to be returned to the body
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20. Homeostasis by Machine
Kidney Dialysis
Blood in tubing flows through dialysis fluid
Blood pump
Vein
Artery
Shunt
Used dialysis fluid
For people with damaged kidneys, dialysis machines can perform many of the functions of the kidneys.
Dialysis machine
Fresh dialysis fluid
Compressed air
Air detector
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38–3
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38–3
A dialysis machine performs the function of which structure in the excretory system?
nephron
ureter
urethra
glomerulus
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38–3
In the human body, the kidneys play an important role in
producing digestive enzymes.
circulating the blood.
destroying old red blood cells.
maintaining homeostasis.
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38–3
In the nephron, most filtration occurs in the
renal tubule.
capillaries.
glomerulus.
loop of Henle.
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38–3
Urine leaves the body through the
loop of Henle.
glomerulus.
urethra.
bladder.
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38–3
Materials filtered out of the blood include all of the following EXCEPT:
water.
urea.
amino acids.
plasma proteins.
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27. END OF SECTION