5.1.2: Excretion and homeostatic control

Question 1

What is excretion?

Answer 1

Process of removing metabolic wastes e.g. carbon dioxide & nitrogen-based byproducts to maintain metabolism.
Enables organisms to maintain pH balance & regulate osmotic pressure.

Question 2

Describe the gross structure of the mammalian liver.

Answer 2

Liver lobules (cylinders of hepatocytes arranged in rows & connected at the centre) are connected to: hepatic vein takes deoxygenated blood away from the liver &
is attached to → hepatic portal vein: contains products of
digestion & hepatic artery: supplies oxygenated blood via
Sinusoid capillaries.
And bile duct: transports bile to gall bladder for storage.

Question 3

Outline the functions of the mammalian liver.

Answer 3

● Site of gluconeogenesis, glycolysis, glycogenesis.
● Stores glycogen.
● Deaminates excess amino acids, forming ammonia &
organic acids. Acids can be respired or converted into glycogen. Ammonia is detoxified by addition of CO2 in
ornithine cycle.
● Detoxifies chemicals e.g. converts alcohol to ethanal
then acetic acid

Question 4

Describe the gross structure of the mammalian

urinary system

Answer 4

A: kidney
B: renal vein
C: renal artery
D: ureter
E: urethra
F: bladder

Question 5

Describe the gross structure of a mammalian kidney.

Answer 5

Fibrous capsule: protects kidney.
Cortex: outer region consists of Bowman’s capsules, convoluted tubules, blood vessels.
Medulla: inner region consists of collecting ducts, loops of Henle, blood vessels

Question 6

Describe the structure of a nephron.

Answer 6

A: glomerulus
B: Bowman’s capsule
C: proximal convoluted tubule
D: collecting duct
E: afferent arteriole
F: efferent arteriole
G: loop of Henle
H: distal convoluted tubule

Question 7

Describe the blood vessels associated with a

nephron.

Answer 7

Wide afferent arteriole from renal artery enters renal
capsule & forms glomerulus: branched knot of capillaries which combine to form narrow efferent arteriole.
Efferent arteriole branches to form capillary network
that surrounds tubules.

Question 8

Describe the histology of the kidney.

Answer 8

1: glomerulum
2: proximal tubule
3: distal tubule

Question 9

Describe the sections of a nephron.

Answer 9

Bowman’s capsule at start of nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes.
Proximal convoluted tubule (PCT): series of loops surrounded by capillaries, walls made of epithelial cells with microvilli.
Loop of Henle: hairpin loop extends from cortex into medulla.
Distal convoluted tubule : similar to PCT but fewer capillaries.
Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney

Question 10

Describe the process of ultrafiltration.

Answer 10

Occurs in Bowman’s capsule.
High hydrostatic pressure in glomerulus forces small
molecules (urea, water, glucose, mineral ions) out of
capillary fenestrations AGAINST osmotic gradient.
Basement membrane acts as filter. Blood cells & large
molecules e.g. proteins remain in capillary.

Question 11

How are cells of the Bowman’s capsule adapted for

ultrafiltration?

Answer 11

● Fenestrations between epithelial cells of capillaries.

● Fluid can pass between & under folded membrane of podocytes.

Question 12

State what happens during selective reabsorption

and where it occurs.

Answer 12

Useful molecules from glomerular filtrate e.g. glucose are reabsorbed into the blood.
Occurs in proximal convoluted tubule.

Question 13

Outline the transport processes involved in selective reabsorption.

Answer 13

glucose from glomerular filtrate
↓co transport with Na+ ions
cells lining proximal convoluted tubule
↓active transport
intercellular space
↓diffusion
blood capillary lining tubule

Question 14

How does the kidney produce urine?

Answer 14

After selective reabsorption, filtrate passes through
Loop of Henle, which acts as countercurrent multiplier &
then through distal convoluted tubule, where water &
mineral ions are reabsorbed.
More water is reabsorbed in collecting duct. Remaining
fluid (urine) contains only waste materials & water.

Question 15

What happens in the loop of Henle?

Answer 15

1. Active transport of Na+ & Cl- out of ascending limb.
2. Water potential of interstitial fluid decreases.
3. Osmosis of water out of descending limb (ascending limb is impermeable to water).
4. Water potential of filtrate decreases going down descending limb: lowest in medullary region, highest at top of ascending limb

Question 16

Explain the role of the distal convoluted tubule.

Answer 16

Reabsorption:
a. of water via osmosis
b. of ions via active transport
permeability of walls is determined by
action of hormones

Question 17

Define osmoregulation.

Answer 17

Control of plasma water potential via negative feedback homeostatic mechanisms.

Question 18

Explain the role of the hypothalamus in osmoregulation.

Answer 18

1. Osmosis of water out of osmoreceptors in hypothalamus causes them to shrink.
2. This triggers hypothalamus to produce more antidiuretic hormone (ADH).

Question 19

Explain the role of the posterior pituitary gland in

osmoregulation

Answer 19

Stores and secretes the ADH produced by the hypothalamus.

Question 20

Explain the role of ADH in osmoregulation.

Answer 20

Forms hormone-receptor complex on surface membrane of cells in collecting duct. Triggers activation of cAMP as secondary messenger.
Triggers cellular processes that increase reabsorption of water. Urine becomes more concentrated

Question 21

How does ADH increase reabsorption of water?

Answer 21

1. Makes cells lining collecting duct more permeable to
   water: Binds to receptor → activates phosphorylase →
   vesicles with aquaporins on membrane fuse with
   cell-surface membrane.
2. Makes cells lining collecting duct more permeable to
   urea: water potential in interstitial fluid decreases.

Question 22

What can cause kidney failure?

Answer 22

● Kidney infections cause inflammatory damage =
change in glomerular filtration rate (rate at which filtrate flows through kidney).
● Kidney stones.
● Uncontrolled diabetes.
● High blood pressure damages capillaries of glomeruli = larger molecules pass into urine.

Question 23

Describe the effects of kidney failure.

Answer 23

● Build-up of toxic waste products e.g. urea causes symptoms such as vomiting.
● If kidneys cannot remove excess water from blood, fluid accumulation leads to swelling.
● Disruption to electrolyte balance can make bones more brittle or increase water retention.

Question 24

Name potential treatments for kidney failure.

Answer 24

● Renal dialysis: haemodialysis/ peritoneal dialysis
(short-term solution repeated several times a week so toxic waste products do not accumulate).
● Kidney transplant (long-term solution, but difficult to find suitable donor with same blood/ tissue type & patient requires immunosuppressants)

Question 25

Describe haemodialysis.

Answer 25

Removes blood from body & pumps it through a
machine.
Blood runs countercurrent to dialysis fluid. Artificial
membrane separates fluids = diffusion gradient enables
molecules to move.
Add blood thinning agent to avoid clotting outside body

Question 26

Describe peritoneal dialysis.

Answer 26

Dialysis fluid is put into body cavity.
Exchange of molecules happens across the body’s own peritoneal membrane.
Fluid must be drained and replaced.

Question 27

How can urine samples be used to test for pregnancy?

Answer 27

Monoclonal antibodies bind to the hormone
human chorionic gonadotropin (hCG) in
the urine of pregnant women

Question 28

How can urine samples be used to test for drugs

such as anabolic steroids?

Answer 28

Gas chromatography measures the time it takes for the urine sample to pass through the column compared to the time taken for a steroid to pass through

Question 29

What are anabolic steroids?

Answer 29

Drugs used to build muscle mass.

Banned from sporting events due to their dangerous side effects & to ensure competition is fair