2. Excretion as an example of Homeostatic control

Question 1

What is excretion?

Answer 1

The removal of metabolic waste from the body.

Question 2

What products must be excreted from the body?

Answer 2

Carbon dioxide from respiration, nitrogen-containing compounds like urea

Question 3

What are the excretory organs?

Answer 3

lungs, liver, skin and kidneys

Question 4

What is the importance of excretion?

Answer 4

A build up of metabolic products can be fatal. They interfere with cell processes by altering pH, which prevents normal metabolism

Question 5

How are the lungs involved in excretion?

Answer 5

Every cell in the body releases carbon dioxide from respiration.
It is then transported in the bloodstream to the lungs.
In the lungs, carbon dioxide diffuses into the alveoli to be excreted as you breathe out.

Question 6

How is the liver involved in excretion?

Answer 6

It has many metabolic roles and some of the substances produced will be passed into the bile for excretion with the faeces e.g. the pigment bilirubin.
It also converts excess amino acids to urea.

Question 7

How are the kidneys involved in excretion?

Answer 7

Urea is passed into the bloodstream to be transported to the kidneys.
It is transported in solution (dissolved in the plasma).
In the kidneys, urea is removed from the blood to become a part of the urine.
Urine is stored in the bladder before being excreted via the urethra.

Question 8

How is the skin involved in excretion?

Answer 8

Sweat contains a range of excretory substances including urea, uric acid and ammonia.
The loss of salt and water also maintains the water potential of the blood.

Question 9

How is carbon dioxide transported in the blood?

Answer 9

It reacts with water to form carbonic acid.
Carbonic acid dissociates to release hydrogen ions and the hydrogencarbonate ions.
This occurs in the red blood cells under the influence of carbonic anhydrase.

Question 10

What is the function of the liver?

Answer 10

It carries out many different functions...
Regulates blood sugar level
Removes toxic ammonia
Produces blood proteins (blood clotting factors)
Detoxifies drugs and alcohol
Produces bile, breaks down fats
Removes damaged red blood cells
Stores glycogen, vitamins A and D

Question 11

What is needed for the liver to carry out its function?

Answer 11

It is essential that the liver has a good supply of blood to remove excess substances from the blood and return others to maintain a concentration gradient.

Question 12

What is the role of hepatic artery?

Answer 12

Supplies the liver with oxygenated blood for aerobic respiration to carry out metabolic processes.
They have a higher oxygen concentration as it comes from the heart through the aorta.

Question 13

What is the role of hepatic portal vein?

Answer 13

Supplies the liver with deoxygenated blood rich in digestive products.
They have a lower oxygen concentration as it comes from the digestive system.

Question 14

How does blood leave the liver?

Answer 14

Via hepatic vein.

This rejoins the vena cava.

Question 15

What is the role of the bile duct?

Answer 15

Carries bile from the liver to the gall bladder, where it is stored until it is required to aid digestion in the small intestine.

Question 16

How is the hepatic artery and hepatic portal vein adapted to the function of the liver?

Answer 16

They split into smaller vessels which run through the lobules.
The blood in the two vessels is mixed and passes along sinusoid vessels in close contact with the liver cells either side of the vessel.
These cells are able to remove and supply substances to and from the blood.

Question 17

What are Kupffer cells?

Answer 17

Specialised macrophages which move about within the sinusoids.
They breakdown and recycle old red blood cells.
This produces bilirubin.

Question 18

Where does blood go after the sinusoid vessel?

Answer 18

When blood reaches the end of the sinusoid, the substances empty into the intra-lobular vessel.
These vessels leads onto the hepatic vein which drains blood from the liver.

Question 19

What are liver cells called?

Answer 19

Hepatocytes

They carry out hundreds of different metabolic reactions.

Question 20

How are liver cells adapted to their function?

Answer 20

Relatively unspecialised and have a simple cuboidal shape
Their cytoplasm is very dense and has a specialised number of organelles inside for the many different metabolic processes they carry out.
Many mitochondria – for aerobic respiration
Many ribosomes- to build up proteins (especially enzymes) involved in the many metabolic processes.

Question 21

How is glycogen stored in the liver?

Answer 21

It is stored in glycogen granules in the cytoplasm of the hepatocytes.
It can be broken down to release glucose into the blood as required.

Question 22

How does detoxification take place in the liver?

Answer 22

Toxins are rendered harmless by oxidation, reduction methylation or by combination with another molecule.
Liver cells contain many enzymes that render toxic molecules less toxic.

Question 23

What molecules in the liver render substances less toxic?

Answer 23

catalase converts hydrogen peroxide to oxygen and water.

Cytochrome P450 is a group of enzymes used to break down drugs.

Question 24

How is alcohol broken down?

Answer 24

It is broken down in the hepatocytes by the enzyme ethanol dehydrogenase.
The resulting compound in ethanal.
This is further broken down by ethanal dehydrogenase.
The final product is ethanoate (acetate).

Question 25

What happens to ethanoate (acetate) after it has been produced from the breakdown of ethanol?

Answer 25

It combines with coenzyme A to form acetyl coenzyme A, which enters the process of aerobic respiration.
The hydrogen atoms released combine with NAD to produce reduced NAD.

Question 26

What impact does the consumption of too much alcohol have on the liver?

Answer 26

It uses up all the stores of NAD.
There is not enough left to break down fatty acids.
So the fatty acids are converted back to lipids and stored as fat which enlarges the liver.
This can lead to cirrhosis.

Question 27

What are the two steps of breaking down amino acids in the liver?

Answer 27

deamination and the ornithine cycle

Question 28

What is deamination?

Answer 28

It removes the amino group and produces ammonia.
Ammonia is highly toxic so must not accumulate.
It also produces a keto acid which enters respiration.

Question 29

What is the ornithine cycle?

Answer 29

Ammonia and carbon dioxide combine with Ornithine to produce Citrulline.
This is converted to arginine by addition of further ammonia.
This is re-converted to ornithine by the removal of urea.

Question 30

How is blood supplied to and leave the kidney?

Answer 30

They are supplied with blood from the renal artery.

The blood is drained by the renal vein.

Question 31

How does blood travel around the kidney?

Answer 31

The renal artery splits into many different arterioles which each lead to a glomerulus.
Blood from the glomerulus continues into an efferent arteriole which carries the blood to more capillaries.
These eventually meet and flow into the renal vein.

Question 32

What is the general structure of the kidney?

Answer 32

There are two kidneys.
The outer region is called the cortex.
The inner region is the medulla
The centre is the pelvis, which leads to the ureter.
The bulk consists of nephrons (there are around 1 million)

Question 33

What is the function of the kidneys?

Answer 33

Excretion.
They remove waste products from the blood and produce urine.
The urine passes out of the kidney down the ureter and to the bladder.

Question 34

What is the role of the Bowman’s capsule?

Answer 34

Ultrafiltration

Question 35

What is ultrafiltration?

Answer 35

When fluid from the blood in the glomerulus is pushed into the bowman’s capsule to be drained into the proximal convoluted tubule.

Question 36

How is the Bowman’s capsule adapted to ultrafiltration?

Answer 36

The barrier between the blood in the capillary and the lumen of the Bowman’s capsule consists of three layers all adapted to ultrafiltration.
These layers are: the endothelium of the capillary, the basement membrane, and the epithelial cells of the bowman’s capsule

Question 37

How is the endothelium of the capillary adapted to its function?

Answer 37

There are narrow gaps between the cells of the endothelium.
The cells also contain pores called fenestrations.
This allows blood plasma and substances to pass out of the capillary.

Question 38

How is the basement membrane adapted to its function?

Answer 38

Consists of a fine mesh of collagen fibres and glycoproteins.
This prevents the passage of molecules through with a molecular mass larger than 69,000.
This means most proteins and all blood cells stay in the glomerulus.

Question 39

How is the epithelial cells of the bowman’s capsule adapted to its function?

Answer 39

The cells (called podocytes) have finger-like projections called major processes that hold the cells away from the endothelium of the capillary.
Fluid from the blood in the glomerulus can pass between these cells into the lumen of the bowman's capsule.

Question 40

Where does the fluid from the bowman’s capsule go?

Answer 40

Into the proximal convoluted tubule, the loop of henle then the distal convoluted tubule.
It is then collected in the collecting duct which passes down the medulla to the centre of the kidney.

Question 41

How is the afferent arteriole adapted to ultrafiltration?

Answer 41

It has a wider diameter than the efferent arteriole which carries blood away from the glomerulus.
This ensures that blood pressure in the glomerulus is higher than the Bowman’s capsule.
This pushes fluid into the Bowman’s capsule to be excreted.

Question 42

What is filtered out of the blood into the bowman’s capsule?

Answer 42

water, amino acids, glucose, urea, inorganic mineral ions

Question 43

What substances are left in the capillary?

Answer 43

Blood cells and proteins.

This means the blood has a very low water potential to help with reabsorption later.

Question 44

What is the role of the proximal convoluted tubule?

Answer 44

About 85% of fluid is reabsorbed (all sugars, most mineral ions and some water).

Question 45

How is the proximal convoluted tubule adapted to its function?

Answer 45

The cells of these tubules are highly folded forming microvilli and a brush border which increases surface area.
Sodium potassium pumps in the membrane pump sodium ions out of the cell and potassium ions in.
The cytoplasm contains many mitochondria.

Question 46

How are glucose and amino acids selectively reabsorbed?

Answer 46

It is driven by the concentration gradient created by pumping sodium ions out of the cells into the blood.
Sodium ions diffuse into the cell and cotransport glucose or amino acids from the proximal tubule.
This reduces water potential of the cells and increases water potential in the tubule.
Therefore, water is drawn from the tubule by osmosis.

Question 47

What is the difference between the ascending and descending limb?

Answer 47

The ascending limb is impermeable to water while the descending limb allows water to pass through.

Question 48

What is the role of the loop of Henle in the production of urine?

Answer 48

Its role is to decrease water potential in the medulla to encourage water to diffuse out of the loop of Henle so it is not excreted with the urine.

Question 49

How does the Loop of Henle decrease water potential in the medulla?

Answer 49

The ascending limb actively pumps sodium and chloride ions into the medulla to decrease its water potential.
This causes water in the descending limb to move, by osmosis, out of the tubule, down the concentration gradient back into the blood.

Question 50

What happens in the distal convoluted tubule?

Answer 50

Additional ion reabsorption.

The urea concentration rises as water is removed further.

Question 51

How is the distal convoluted tubule adapted to its function?

Answer 51

Plasma membranes of DCT cells have many pumps and transporter proteins for active transport and facilitated diffusion.

Question 52

What is the role of the collecting duct?

Answer 52

Carries fluid back through the medulla to the pelvis.
The fluid still has a high water potential so continues to move by osmosis out of the collecting duct into the medulla.
The amount of water reabsorbed is determined by the permeability of the collecting duct walls.

Question 53

What is osmoregulation?

Answer 53

The control of water potential in the body.

Question 54

How do the kidneys help in osmoregulation?

Answer 54

They alter the permeability of the collecting duct walls.
Conserve less water- the walls become less permeable so less water is reabsorbed and more urine is produced.
Conserve more water- the walls become more permeable so more water is reabsorbed and less urine is produced.

Question 55

What triggers the changing permeability of the walls of the collecting ducts?

Answer 55

ADH in the blood.

It binds to receptors on the cells of the walls of the collecting duct.

Question 56

What effect does ADH have on the water potential of the blood?

Answer 56

They bind to receptors on the cell surface.
Vesicles containing aquaporins fuse with the cell surface membrane and make it more permeable to water.
The more ADH in the blood the more channels inserted and the more water reabsorbed into the blood.

Question 57

How is water potential of the blood detected?

Answer 57

By osmoreceptors in the hypothalamus in the brain.
When the water potential is too low, they lose water by osmosis and shrink.
This causes ADH to be produced by the hypothalamus, and be released by the posterior pituitary gland where is is excreted into the blood by exocytosis.