5.1.2 Excretion and Homeostatic Control

Question 1

What is excretion ?

Answer 1

•The process of removing metabolic (eg CO2 and nitrogen based biproducts) to maintain metabolism.
•It enables organisms to maintain pH balance and regulate osmotic pressure.

Question 2

Describe the gross structure of the mammalian liver

Answer 2

•Liver lobules : cylinders of hepatocytes arranged in rows and connected at the centre. They connect to the hepatic vein
•Hepatic vein : takes deoxygenated blood away from the liver & is attached to hepatic portal vein
•Hepatic portal vein : contains products of digestion
•Hepatic artery : supplied liver with oxygenated blood via sinusoid capillaries.

Question 3

Outline the functions of the mammalian liver.

Answer 3

•Site of gluconeogenesis, glycosis and glycogenesis
•Stores glycogen
•Deanimates amino acids, forming ammonia organic acids
•Detoxifies chemicals eg ethanol -> ethanal -> acetic acid.

Question 4

Describe the gross structure of a mammalian kidney.

Answer 4

Fibrous capsule : protects the kidney
Cortex : outer region of Bowman’s capsule, convoluted tubules and blood vessels
Medulla : inner region of collecting ducts, loop of Henle and blood vessels.

Question 5

What are the components of a nephron

Answer 5

•Glomerulus
•Bowman’s capsule
•Proximal convoluted tubule
•Collecting duct
•Loop of henle
•Distal convoluted tubule

Question 6

Describe the process of ultrafiltration

Answer 6

•Occurs in the bowman’s capsule
•High hydrostatic pressure in glomerulus forces small molecules out of capillaries against osmotic gradient
•Basement membrane acts as a filter, large proteins (>69,000) stay in capillary.

Question 7

How are the cells of the Bowman’s capsule adapted for ultrafiltration ?

Answer 7

•There are pores between the epithelial cells of the capillaries
•Fluid can pass between and under podocytes.

Question 8

State what happens during selective reabsorption and where it occurs

Answer 8

•Useful molecules from glomerular filtrate e.g. glucose are re absorbed into the blood
•This occurs in the proximal convoluted tubule (PCT)in the kidneys.

Question 9

Outline the transport process involved in selective reabsorption

Answer 9

•Glucose from glomerular filtrate is co-transported with Na+ to the cell lining of PCT
•Active transport takes it to the intercellular spaces
•Diffusion then takes it to the capillaries.

Question 10

How does the kidney produce urine ?

Answer 10

•After selective reabsorption, filtrate passes through Loop of Henle which acts as a countercurrent
•It then passes through the distal convoluted tubule where water and mineral ions are re absorbed
•More water is reabsorbed in the collecting duct, remaining fluid is urine.

Question 11

What happens in the loop of Henle ?

Answer 11

•Active transport of Na+ and Cl- ions out of ascending limb
•Water potential of interstitial fluid decreases
•Osmosis of water out of descending limb (ascending is impermeable to water)
•Water potential of filtrate decreases going down the ascending limb

Question 12

Explain the function of the distal convoluted tubule

Answer 12

Reabsorption of :
Water via osmosis
Ions via active transport (determined by ADH)

Question 13

Define osmoregulation

Answer 13

The control of plasma water potential via negative feedback homeostatic mechanisms

Question 14

Explain the role of the hypothalamus in osmoregulation

Answer 14

•Osmosis of water out of osmoreceptors (due to water potential of plasma) causes them to shrink
•This triggers the hypothalamus to produce more ADH.

Question 15

Explain the role of the pituitary gland in osmoregulation

Answer 15

Stores and secretes ADH produced by the hypothalamus

Question 16

Explain the role of ADH in osmoregulation

Answer 16

•The hormone binds to receptors on cell surfaces in collecting duct, where it triggers activation of cAMP as a secondary messenger
•This triggers cellular processes that form aquaporins that increase reabsorption of water
•This makes urine more concentrated

Question 17

How does ADH increase reabsorption of water ?

Answer 17

•It makes the cells lining the collecting duct more permeable to water (causes vesicles with aquaporins to fuse with membrane)
•Makes cells lining collecting duct more permeable to urea, making water potential on interstitial fluid decrease.

Question 18

What can cause kidney failure ?

Answer 18

•Kidney infections cause inflammatory damage = change in glomerular filtration rate
•Kidney stones
•Diabetes
•High blood pressure, damages walls of glomerulus.

Question 19

Describe the effects of kidney failure

Answer 19

•Build up of toxic waste products e.g. urea causes symptoms like vomiting
•If kidneys cannot remove excess water swelling occurs

Question 20

Name potential treatments for kidney failure

Answer 20

•Dialysis (haemodialysis / peritoneal), short term treatment that must be repeated regularly
•Kidney transplant, long term treatment but must be followed by immunosuppressants.

Question 21

Describe haemodialysis

Answer 21

•Removes blood from the body and passes it through machine
•Blood runs countercurrent to dialysis fluid, separated by artificial membrane (diffusion gradient)
•Blood thinning agent prevents clotting outside of body.

Question 22

Describe peritoneal dialysis

Answer 22

•Dialysis fluid is put into body cavity
•The exchange of molecules happens across the body’s peritoneal membrane
•Fluid must be drained and replaced regularly.

Question 23

How can urine samples be used to test for pregnancy ?

Answer 23

•Monoclonal antibodies bind to the hormone hCG in the urine of pregnant women
•

Question 24

How can urine samples be tested for drugs such as anabolic steroids ?

Answer 24

•Gas chromatography measures the time taken for urine to pass through a column
•This is compared to how long it takes a steroid to pass through.