Homeostasis (waste products)

Question 1

What is excretion?

Answer 1

the removal of waste substances produced in metabolic reactions.

Question 2

What substances does urea aim to excrete from the body?

Answer 2

• Water
• Nitrogenous substances
• Salts

Question 3

Explain what urea is and how it is excreted: (5)

Answer 3

Urea is a nitrogenous waste substance produced in the liver and is excreted by the kidneys as urine to remove excess amino acids from our body
Amino acids as deaminated in the liver
The amine group from the amino acid is removed from the molecule
This forms the nitrogenous substance ammonia which is highly toxic so is converted into urea, a less toxic substance
The rest of the molecule can be converted into other stores or substances or used in respiration.
The urea travels into the blood to be filtered out by the kidneys, creating urine which we excrete.

Question 4

State and describe the process involved in the production of urea:

Answer 4

Deamination is when the amine group is removed from the amino acid molecule, forming highly toxic ammonia which is quickly converted into less toxic urea.

Question 5

How does our body increase our water content?

Answer 5

drinking
eating
respiration

Question 6

How does our body decrease our water content?

Answer 6

urine
egestion
sweating
breathing

Question 7

Explain why it is important for our body to remove CO₂ from our body:

Answer 7

High levels of CO₂ causes the pH of our blood to drop (become more acidic) which can denature enzymes effecting our metabolic reactions.

Question 8

Explain why it is important for our body to remove excess amino acids from our body:

Answer 8

When our liver deaminates excess amino acids, it forms ammonia which converts into urea, both are toxic substances that poison our body.

Question 9

How does the kidneys create normal urine?

Answer 9

1. Blood from the aorta enters the kidneys via the renal artery.
2. The kidney contains many nephrons, in the first part (Bowman’s capsule) under high pressure, small molecules such as water, salts, sugars and urea undergo ultrafiltration, where they are filtered out of the blood into the nephron’s tubule.
3. Later on in the loop of Henlé, selective reabsorption of vital substances such as glucose are reabsorbed into the blood, and the water content in the blood is restored to normal.

Question 10

What waste products does urine expel with it?

Answer 10

water, salts and urea

Question 11

Explain how our body maintains water balance after drinking lots of water.

Answer 11

1. the body absorbs the water into the blood from the small intestine, making the blood’s plasma more dilute.
2. osmoreceptors in the hypothalamus detect the change in the concentration of the blood.
3. this causes the pituitary gland to release less ADH into the blood.
4. the kidneys detect the low levels of ADH, so the walls of the collecting duct in the nephrons become less permeable, therefore less water is reabsorbed back into the blood, causing the filtrate to be dilute and have a larger volume.
5. this leaves the blood that comes out of the kidneys to be more concentrated with glucose and salts and less dilute.
6. this is important because having too dilute blood may causes cells to become swollen and at risk of bursting, as excess water from the blood may travel via osmosis into the cells from a dilute to concentrates solution, this damages cells and stops metabolic reactions from occurring.

Question 12

Explain how our body maintains water balance after not drinking for the last 8 hours:

Answer 12

1. the blood’s plasma becomes too concentrated with glucose and salts, risking cells becoming shrunken as water moves out of the cells into the blood via osmosis, this damages cells, making them unable to metabolise.
2. osmoreceptors in the hypothalamus detect the too concentrated levels in the blood and cause the pituitary gland to release more ADH into the blood.
3. the kidneys detect higher levels of ADH in the blood, causing the walls in the collecting ducts to become more permeable therefore are able to reabsorb water from the filtrate, causing the filtrate to become more concentrated in urea and the blood to become more dilute.

Question 13

Explain how a kidney machine works:

Answer 13

1. a catheter is inserted into a blood vessel (the artery).
2. blood is pumped out of the body with a pump and blood thinners are added to prevent clotting.
3. the blood (concentrated with urea) enters the dialyser, which contains dialysis fluid (consisting of water, glucose and salts) and partially permeable tubes.
4. using counter current flow, the blood and dialysis fluid are pumped in opposite directions to ensure that a concentration gradient is kept to enable urea to diffuse out of the blood into the fluid to be carried away. since the fluid already contains glucose and salts of a typical sample of blood, glucose and salts from the blood don’t diffuse out.
5. this removes only the urea from the blood.
6. the blood exits the dialyser and is pumped through a filter and bubble trap, to remove air in the blood before in reenters the body.