excretion 2.71, 2.72B, 2.73B, 2.74B, 2.75B, 2.76B, 2.77B, 2.79B, 2.78B

Question 1

what are two excretory products in humans?

Answer 1

carbon dioxide - made in respiring cells by respiration and removed in lungs via blood plasma
urea- made in the liver cells as a result of the break down of amino acids (deamination), removed via the kidneys and skin

Question 2

how is urea formed?

Answer 2

1. amino acids are filtered into the liver along with the rest of the blood via the hepatic artery and hepatic portal vein
2. excess amino acids cannot be stored so they are broken down so they can be excreted. broken down into carbohydrates and ammonia
3. ammonia is very toxic and is converted into less toxic chemical urea
4. amino acids that are needed and the urea are released back into the bloodstream via the hepatic vein

Question 3

what’s the role of the kidney in excretion?

Answer 3

kidney filters blood and any excess materials and passes them to the bladder to be excreted

Question 4

what’s the structure of the kidney?

Answer 4

contains three main sections:
cortex, medulla, pelvis

kidney contains millions of nephrons that filter the blood

Question 5

what’s the process of ultrafiltration in the nephron?

Answer 5

1. the diameter of the efferent arteriole is smaller than the afferent arteriole at the entrance - creating a build-up of pressure in the capillaries forming the glomerulus
2. this pressure causes small molecules (urea, glucose, amino acids and salts) out of the capillaries and into the bowman’s capsule (this forms the glomerular filtrate)
3. larger molecules (protein) are too big to be filtered so stay in blood

capillaries of the glomerulus have gaps between them, this is called the basement membrane

Question 6

what’s the process of selective reabsorption?

Answer 6

1. the filtrate travels to the proximate convoluted tubule (PCT)
2. there are specialised cells within the walls of the pct and these help useful substances move back into the bloodstream
   all glucose and amino acids are reabsorbed with sone salts and 80% of water

Question 7

how are substances moved in the pct?

Answer 7

glucose and amino acids and ions - actively transported
water - osmosis
initially molecules will be moved by diffusion but mainly by active transport

Question 8

how is the pct specialised for the movement of molecules?

Answer 8

folded membrane - increase surface area
many mitochondria - atp for active transport

Question 9

what is the process of water reabsorption?

Answer 9

80% water is reabsorbed in pct
loop of henle and collecting duct reabsorb water
the loop of henle concentrates the urine by transporting salt into the blood increasing the concentration gradient for osmosis
the collecting duct responds to a hormone called ADH to vary how much water is reabsorbed
this is called osmoregulation

Question 10

what is excretion?

Answer 10

removal of metabolic waste from the body

Question 11

what are the gaps between the capillaries of the glomerulus called?

Answer 11

basement membrane, surrounding the capillaries controls which molecules can pass into bowman’s capsule

Question 12

what does urine contain?

Answer 12

water, urea, ions

Question 13

how do ADH and osmoregulation help regulate water content of blood?

Answer 13

water levels can be changed by adjusting the permeability of the collecting duct in the nephron (kidney)
the hormone ADH controls the water levels
hypothalamus monitors the level of water in blood , tells pituitary gland to release/ not release ADH
collecting duct is surrounded by capillaries so ADH diffuses out of blood and binds with receptors of the the collecting duct cells and make the wall more permeable to water
more water is reabsorbed

Question 14

what is the gland and function of ADH?

Answer 14

• pituitary gland
• reduce the amount of water lost in urine