Excretion

Question 1

What two sources supply the liver with blood?

Answer 1

The hepatic artery

The hepatic portal vein

Question 2

What is the role of the hepatic artery?

Answer 2

Oxygenated blood from the aorta -> Supplies the liver with the oxygen needed for respiration

Question 3

What is the role of the hepatic portal vein?

Answer 3

Deoxygenated blood from the digestive system enters the liver containing toxic compounds absorbed from the intestines.
before it can circulate the rest of the body the concentrations of such compounds must be adjusted.

Question 4

How do substances leave the liver?

Answer 4

• Blood leaves via the hepatic vein which connects to the vena cava
• Via the bile duct, bile is excreted from the liver to the gall bladder where it is stored until required to aid the digestion of fats in the small intestine

Question 5

Where is blood mixed from the hepatic artery and the hepatic portal vein?

Answer 5

It is passed through a chamber called the sinusoid which is lined with hepatocytes to remove/return substances to and from the blood

Question 6

What is the structure of the liver/where does blood enter to be cleansed?

Answer 6

The liver is divided into lobes which are further divided into cylindrical shaped lobules
The hepatic artery and hepatic portal vein divide into smaller vessels running between lobules -> inter-lobular vessels
At intervals these vessels branch into sinusoid where blood from both vessels are mixed
The sinusoid is lined with hepatocytes which absorb substances

Question 7

What are kupffer cells?

Answer 7

They are specialised macrophages that move about within the sinusoids and breakdown old red blood cells.
A product of haemoglobin breakdown is bilirubin and is a component of bile

Question 8

Where does bile and clean blood leave the liver?

Answer 8

Bile is made in the liver cells and released into the bile canaliculi which join together to form the bile duct, this transports blood to the gall bladder

At the centre of each lobule is an intra-lobular vessel; a branch of the hepatic vein the sinusoids empty into these vessels which join to form the hepatic vein where blood is drained from the liver

Question 9

How does the liver undergo detoxification of alcohol?

Answer 9

Ethanol dehydrogenase breaks down ethanol in the hepatocytes resulting in ethanal, this is further dehydrogenated by the enzyme ethanal dehydrogenase producing ethanoate(acetate).
This is combined with coenzyme A to form acetyl coA which enters the process of aerobic respiration
The hydrogen atoms released from alcohol are combined with NAD (another coenzyme) to form reduced NAD

Question 10

Which enzymes catalyse the breakdown of recreational and medicinal drugs?

Answer 10

Cytochrome P450

Question 11

What is the role of catalase in detoxification within the liver?

Answer 11

Converts hydrogen peroxide to oxygen and water

Question 12

How is the liver used for storage?

Answer 12

It is used for the storage of glycogen which forms granules in the cytoplasm of hepatocytes, this glycogen can be broken down to release glucose into the blood as required

Question 13

What is deamination?

Answer 13

This is a process whereby oxygen and amino acids react to remove the amino group, this produces ammonia (a very soluble and highly toxic compound) and a keto acid (which can enter respiration directly to release its energy)

Question 14

Why do amino acids undergo treatment instead of excreting whole amino acids?

Answer 14

Amino acids contain a lot of useful energy however excess cannot be stored because the amino groups make them toxic

Question 15

What is the ornithine cycle?

Answer 15

Converts toxic and soluble ammonia to urea which can be passed back into the blood
Ammonia and carbon dioxide combine to produce citrulline, this is converted to arginine by addition of further ammonia, the arginine is re-converted to ornithine by the removal of urea

Question 16

What supplies each kidney with blood?

Answer 16

The renal artery

Question 17

What drains each kidney of blood?

Answer 17

The renal vein

Question 18

What sections is the kidney made up of?

Answer 18

The outer region is called the cortex
The inner region is called the medulla
The centre is the pelvis which leads to the ureter

Question 19

What is ultrafiltration?

Answer 19

When fluid from the blood in the glomerulus is pushed into the bow and capsule

Question 20

What is the afferent arteriole?

Answer 20

The renal artery splits off to form the afferent arteriole which is where the blood travels before it reaches the glomerulus (knot of capillaries)

Question 21

What is the efferent arteriole?

Answer 21

Blood from the glomerulus continues into the efferent arteriole

Question 22

What is a nephron?

Answer 22

Tiny tubules which begin in the cortex as a cup-shaped structure called the Bowman’s capsule. The coiled tubule passes through the cortex loops down into the medulla, back into the correx before joining a collecting duct that passes down into the medulla

Question 23

What are the 3 layers adapted to enable ultrafiltration across the blood capillary and lumen of the Bowman’s capsule?

Answer 23

• The endothelium of the capillary
• The basement membrane
• The epithelial cells of the Bowman’s capsule

Question 24

How is the endothelium of the capillary adapted to enable ultrafiltration?

Answer 24

It consists of endothelial cells which have pores called fenestrations which allow blood plasma and substances dissolved in it to pass out of the capillary.

Question 25

How is the basement membrane adapted to enable ultrafiltration?

Answer 25

Membrane consists of a fine mesh of collagen fibres and glycoproteins which prevent blood cells and proteins from leaving the capillary

Question 26

How are the epithelial cells of the Bowman’s capsuleadapted to enable ultrafiltration?

Answer 26

These cells are called podocytes which have projections that allow fluid to pass between and into the lumen of the Bowman’s capsule

Question 27

What makes up the rest of the nephron tubule after the Bowman’s capsule?

Answer 27

It leads on to the proximal convoluted tubule which descends down to the loop of Henle. It then travels back up to the distal convoluted tubule and finally into the collecting duct

Question 28

What is filtered out of the blood by the kidney?

Answer 28

The blood plasma pushed out of the arterioles contains:

• Water
• Amino acids
• Glucose
• Urea
• Inorganic mineral ions

Question 29

How is the lining of the proximal convoluted tubule adapted for selective reabsorption?

Answer 29

• cell surface membrane in contact with tubule fluid has microvilli - increased SA
• cell surface membrane contains cotransporter proteins which transport glucose or amino acids back into the cell
• cell cytoplasm has many mitochondria(providing ATP for active transport)
• opposite membrane of the cell contains sodium/potassium pumps

Question 30

Explain the mechanism of reabsorption

Answer 30

1. Sodium ions pumped out of the cells lining the tubule into the blood decreasing conc of sodium ions in the cell cytoplasm
2. Sodium ions diffuse into the cell from the tubule through a cotransport protein carrying glucose or an amino acid at the same time
3. This reduces the water potential of the cells so water is drawn in from the tubule by osmosis
4. Glucose and amino acids diffuse into the blood from the cells

Question 31

How is the reabsorption of water controlled within the loop of Henle? What is the effect on the water potential?

Answer 31

• Mineral ions (sodium and chloride ions) are transferred from the ascending limb to the descending limb this decreases the water potential in the descending limb as conc increases and water moves out
• increased effect as it descends into the medulla
• at the base of the ascending limb mineral ions diffuse out
• higher up the ascending limb active transport is used to move mineral ions out. Upper portion of ascending limb is impermeable to water

Question 32

What is the water potential of the tissue fluid in the medulla like?
What does this allow for when fluid flows through the collecting duct?

Answer 32

Very low
This low water potential will create a concentration gradient allowing water to move through osmosis from the tubule into the surrounding tissue

Question 33

What will a graph show on the concentration changes in the tubule fluid?

Answer 33

1. Glucose will decrease as it is selectively reabsorbed from the proximal tubule
2. sodium ions diffuse into descending limb of Henle and are pumped out of the ascending limb so increase then decrease
3. urea conc rises as it is actively moved into the tubule

Question 34

What is osmoregulation? Explain the mechanism

Answer 34

Control of the water potential in the body.

• Permeability of collecting duct walls decrease on a cool day or when you’ve drunk a lot of fluid.
• permeability of collecting duct walls increase on a hot day or when little fluid has been drunk to allow reabsorption of water into the blood

Question 35

How does the antidiuretic hormone (ADH) effect membrane permeability?

Answer 35

The cells in the walls of the collecting duct have membrane bound receptors for ADH. The ADH binds causing a chain of enzyme controlled reactions which eventually results in vesicles containing water-permeable channels (aquaporins) to fuse with the cell surface membrane.

Question 36

What happens if the level of ADH in the blood falls?

Answer 36

The cell surface membrane folds inwards to create new vesicles which remove water-permeable channels from the membrane.
Thus giving a larger volume of urine with a higher water potential.

Question 37

How does the hypothalamus control the concentration of ADH in the blood?

Answer 37

Osmoreceptors are sensory receptors in the hypothalamus that monitor the water potential by responding to the effects; they shrink and stimulate neurosecretory cells in the hypothalamus
These cells are specialised neurones that produce and release ADH which moves down the axon to the terminal bulb in the posterior pituitary glad where it’s stored in vesicles. ADH is released by exocytosis and enters the blood capillaries it’s transported around the body and acts on the cells of the collecting ducts.

Question 38

How can kidney function be assessed?

Answer 38

By estimating the glomerular filtration rate (GFR) a measure of how much fluid passes into the nephrons each minute.

Question 39

What causes kidney failure?

Answer 39

Diabetes type 1 and 2, heart disease, hypertension and infection.

Question 40

What is haemodialysis?

Answer 40

An example of renal dialysis:
Blood from an artery or vein is passed into a machine that contains an artificial dialysis membrane shaped to form capillaries which are surrounded by dialysis fluid that flows in the opposite direction to the blood (countercurrent). Any bubbles are removed before the blood returns to the body.

Question 41

What is peritoneal dialysis?

Answer 41

Surgeon Implants a permanent tube in the abdomen. Dialysis solution is poured through the tube and fills the space between the abdominal wall and organs. The used solution is drained after several hours.