P1 Excretion and Osmoregulation

Question 1

What are the blood vessels that go to the liver?

Answer 1

• The hepatic artery supplies oxygenated, nutrient poor blood from the heart.
• The hepatic portal vein supplies deoxygenated, nutrient rich blood from the digestive system.
• Hepatic vein carries deoxygenated blood out of the liver to the heart.

Question 2

Describe the internal structure of the liver:

Answer 2

• The liver is mainly made up of cells called hepatocytes, which are surrounded by capillaries called sinusoids.
• Sinusoids are connected to the hepatic artery, the hepatic portal vein and to the central vein (which takes blood to the hepatic vein).

Question 3

What are the main functions of the liver?

Answer 3

1. Stores glycogen, which regulates blood glucose levels.
2. Breaks down toxic substances (detoxification), eg. alcohol.
3. Breaks down excess amino acids - removes the amine group (deamination) which is converted to ammonia, and the remaining amino acid is used in respiration. Ammonia is highly toxic and highly soluble in blood, so ammonia is combined with carbon dioxide to form urea, which is less soluble and less toxic.

Question 4

What is the main role of the kidneys?

Answer 4

1. Filter blood - removing harmful waste products.
2. Produce urine - by controlling the water potential of the blood (osmoregulation). If water potential gets too low, cells will shrink, but if it gets too high, cells undergo lysis.

Question 5

Where are nephrons found and what is their role?

Answer 5

• Contained between the cortex and the medulla, they are responsible for filtering blood and producing urine.
• Each nephron contains as glomerulus, Bowmans capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and a collecting duct.

Question 6

What is the glomerulus?

Answer 6

• A mass of blood capillaries, blood is supplied by the afferent arteriole and is carried away by the efferent arteriole.
• The efferent arteriole then branches into a network of capillaries that surround the rest of the nephron, to ensure the whole nephron has a short diffusion pathway to blood.
• The glomerulus is surrounded by the Bowman’s capsule.

Question 7

How are substances transported out of the blood?

Answer 7

• Blood in the glomerulus (capillaries) is kept at a high pressure (much higher than in the Bowman’s capsule). Therefore substances in the blood pass from the glomerulus, into the Bowman’s capsule.
• The efferent arteriole (blood leaving) has a smaller diameter than the afferent arteriole (blood entering) causing hydrostatic pressure to build up in the glomerulus, creating a hydrostatic pressure gradient between the glomerulus and the Bowman’s capsule.
• Therefore substances in the blood move down this hydrostatic pressure gradient, and into the Bowman’s capsule.

Question 8

What are the three layers between the capillaries in the glomerulus and the Bowman’s capsule?

Answer 8

1. Gaps in the endothelium (lining of the capillaries).
2. The basement membrane (contains pores which act as sieves, controlling the size of substances that come through).
3. Podocytes (cells in the lining of the Bowman’s capsule).
   - These allow small substances eg. water, urea, glucose and ions to enter the Bowman’s capsule lumen, but prevent larger substances (proteins and blood cells from entering).

Question 9

What is the process of forcing substances from the capillaries into the Bowman’s capsule called?

Answer 9

Ultrafiltration

Question 10

What is the mixture of substances that passes into the lumen of the Bowman’s capsule called?

Answer 10

Glomerular filtrate

Question 11

What is selective reabsorption and where does it occur?

Answer 11

• When glomerular filtrate enters the proximal convoluted tubule, useful substances are reabsorbed back into the blood.
• Useful substances move from the lumen of the proximal convoluted tubule, through the epithelial cells and into the capillaries.
• This requires sodium ions and co-transport proteins.

Question 12

How are sodium ions and co-transport proteins involved in selective reabsorption?

Answer 12

• Sodium ions from the epithelial cells are actively transported into the capillaries, creating a concentration gradient between the proximal convoluted tubule lumen and the epithelial cells.
• Therefore, using co-transport proteins, sodium ions move from the tubule lumen to the epithelial cells (facilitated diffusion).
• Each co-transport protein can transport another useful molecule, (eg. glucose) as well as the sodium ion.
• Once in the epithelial cells, useful substances diffuse down a concentration gradient into the blood.
• Facilitated diffusion of sodium ions into the epithelial cells reduces the water potential of the epithelial cells, and increases the water potential of the tubule lumen, so water moves from the tubule lumen into the epithelial cells, and then the capillaries by osmosis.

Question 13

How is the proximal convoluted tubule adapted for selective reabsorption?

Answer 13

1. Contains mitochondria to produce ATP for the active transport of sodium ions.
2. The membrane contains a large number and variety of co-transport proteins to transport as many useful substances as possible.
3. The epithelium of the proximal convoluted tubule contain microvilli, to increase the surface area for diffusion.
4. The epithelium is one cell thick, meaning there is a short diffusion distance of substances being reabsorbed, increasing the rate of reabsorption.

Question 14

Describe what happens at the loop of Henle:

Answer 14

• The ascending limb contains enzymes that actively transport sodium ions out of the limb and into the medulla, decreasing the water potential of the medulla, creating a water potential gradient between the loop of Henle and the medulla.
• Only the descending limb is permeable to water, so water moves from the descending limb to the medulla by osmosis.
• Once sodium ions and water enter the medulla, they are reabsorbed into the blood.

Question 15

Describe the counter current mechanism:

Answer 15

• As the filtrate moves up the ascending limb, is looses more sodium ions, meaning that up the limb, the water potential of the filtrate increases.
• Whereas in the descending limb, the filtrate gradually looses water as it moves down, so the water potential of the filtrate moving down the limb decreases.
• This means the water potential in the medulla is always lower than the water potential in the loop of Henle.
• Since the filtrate is moving in opposite directions, it is a countercurrent mechanism, increasing the efficiency that substances are exchanged between the loop of Henle and the medulla, maintaining the water potential gradient.

Question 16

What differences would animals with little access to water (eg. camels) have to their loop of Henle?

Answer 16

They would have a longer loop of Henle:
- More sodium ions are actively transported out of the ascending limb, so there is a greater water potential gradient in the medulla.
- Therefore more water leaves the descending limb by osmosis, so more water is reabsorbed into the blood.

Question 17

What happens at the distal convoluted tubule, and the collecting duct?

Answer 17

• More water is reabsorbed from the filtrate, but the amount of water depends on the body’s needs.
• When the body needs more water, permeability of the distal convoluted tubule and the collecting duct increases, so more water can be reabsorbed into the blood. Therefore urine produced is more concentrated.
• When the body needs less water permeability decreases, and urine is less concentrated.

Question 18

What is formed at the collecting duct?

Answer 18

Urine

Question 19

What is reabsorbed during selective reabsorption?

Answer 19

• Water
• Small useful molecules and ions

Question 20

What is reabsorbed at the loop of Henle?

Answer 20

Water and sodium ions

Question 21

What is reabsorbed at the distal convoluted tubule/collecting duct?

Answer 21

Water

Question 22

What do osmoreceptors do and where are they found?

Answer 22

• Detect changes in the bloods water potential.
• Found in the hypothalamus and they produce ADH, which is then stored in the pituitary gland.
• ADH is constantly released by the pituitary gland, but the amount released depends on the bloods water potential.

Question 23

Describe the negative feedback system/hormonal control involved in regulating bloods water potential:

Answer 23

• When blood water potential decreases, osmoreceptors detect this change and stimulate the pituitary gland to release more ADH into the blood.
• In the kidneys, the presence of ADH increases the reabsorption of water by increasing the permeability of the distal convoluted tubule and the collecting duct.
• This increases the water potential of the blood, and produces a smaller volume of more concentrated urine.
• Once the water potential of the blood returns to normal, the pituitary gland decreases the secretion of ADH.
• When osmoreceptors detect an increase in the bloods water potential, the opposite happens.

Question 24

What are the three main uses of urine samples?

Answer 24

1. Pregnancy tests
2. Drug tests
3. Medical diagnosis

Question 25

How are urine samples used in pregnancy tests?

Answer 25

Pregnant women produce the hormone hCG. This hormone enters the urine, and pregnancy tests give a positive result if they detect hCG:
- The pregnancy test contains monoclonal antibodies with coloured beads attached. The monoclonal antibodies are complementary to hCG and as the sample of urine moves through the strip, they bind to hCG.
- hCG bound to the immobilised enzymes move up the strip until they reach immobilised antibodies that are complementary to antibodies already bound to hCG so bind, creating a blue line.
- Antibodies that aren’t bound to hCG move further up the strip to reach immobilised antibodies that are complementary to unbound antibodies which bind, producing a second line (which acts as a control, to make sure the test is working correctly).

Question 26

How are urine samples used to test for drugs?

Answer 26

• In the body, many drugs are broken down to substances with are excreted in urine.
• Some of these substances can be identified using complementary antibodies, while others can be identified using gas chromatography (turning urine into gas to separate and identify the different substances in it) eg. anabolic steroids can be identified this way

Question 27

How are urine samples used in medical diagnosis?

Answer 27

• Eg. a high level of glucose in urine can diagnose diabetes, a high level of nitrites in urine can diagnose a bacterial infection, and a low level of urea can diagnose kidney damage.
• When kidneys start to fail, the rate at which they filter blood decreases (the glomerular rate decreases). This is estimated by measuring the level of creatine (a waste product) in the blood.
• A high concentration of creatine indicates the blood is not being filtered properly, which could indicate kidney damage.
• The most common treatment for kidney failure is dialysis.

Question 28

What are the two types of dialysis?

Answer 28

1. Haemodialysis: involves passing a patients blood through a dialysis machine.
   - This machine contains an artificial, partially permeable membrane, that separates the blood from the dialysis fluid.
   - The dialysis fluid has a similar composition to blood, but does not contain urea - to ensure glucose does not diffuse out but urea does.
   - To ensure all urea is removed, blood and dialysis fluid move in opposite directions to maintain a concentration gradient along the length of the machine.
2. Peritoneal dialysis: takes place inside the body - dialysis fluid is inserted inside the stomach, and substances are exchanged through the lining of the abdomen (peritoneal membrane).

Question 29

Advantages and disadvantages of a kidney transplant:

Answer 29

Advantages:
- no dialysis
- therefore less expensive

Disadvantages:
- not enough donors
- body may reject transplants
- immunosupressents (drugs given to reduce changes of rejection) increase disease susceptibility