9.3 Homeostasis

Question 1

State the name and location of the 2 nodes involved in heart contraction.

Answer 1

Sinoatrial node (SAN) = within the wall of the right atrium
Atrioventricular node (AVN) = near lower end of right atrium in the wall that separates the 2 atria

Question 2

Describe how heartbeats are coordinates

Answer 2

1. SAN initiates wave of depolarisation (WOD)
2. WOD spreads across both atria = atrial systole
3. Layer of fibrous, non-conducting tissue delays impulse while ventricles fill and valves close
4. AVN conveys WOD down septum via Bundle of His, which branches into Purkinje fibres along ventricles
5. Causes ventricles to contract from apex upwards

Question 3

Name the receptors involved in changing heart rate and state their location.

Answer 3

Baroreceptors: detect changes in blood pressure; carotid artery
Chemoreceptors: detect changes in pH e.g. due to increase in CO2 concentration; carotid & aortic body

Question 4

How does the body respond to an increase in blood pressure?

Answer 4

1. Baroreceptors send more impulse to cardio inhibitory centre in the medulla oblongata
2. More impulses to SAN down vagus nerve via parasympathetic nervous system
3. Stimulates release of acetylcholine, which decreases heart rate

Question 5

How does the body respond to a decrease in blood pressure?

Answer 5

1. Baroreceptors send more impulses to cardioacceleratory centre in the medulla oblongata
2. More impulse to SAN via sympathetic nervous system
3. Stimulates release of noradrenaline, which increases heart rate and strength of contraction

Question 6

How does the body respond to an increase in CO2 concentration?

Answer 6

1. Chemoreceptors detect pH decrease and send more impulses to cardioacceleratory centre of medulla oblongata
2. More impulses to SAN via sympathetic nervous system
   Heart rate increases, so rate of blood flow to lungs increases = rate of gas exchange and ventilation rate increase

Question 7

Describe the gross structure of the mammalian urinary system.

Answer 7

Contains kidney, renal vein, renal artery, ureter, urethra and bladder

Question 8

Describe the cross structure of a mammalian kidney.

Answer 8

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

Question 9

Describe the structure of the nephron.

Answer 9

Contains:
Golmerulus
Bowman’s capsule
Proximal convoluted tubule
Collecting duct
Afferent arteriole
Efferent arteriole
Loop of Henle
Distal convoluted tubule

Question 10

Describe the blood vessels associated with a nephron.

Answer 10

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

Question 11

Describe the sections of a nephron.

Answer 11

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

Question 12

How is urea produced?

Answer 12

1. Hepatocytes deaminate excess amino acids to form ammonia
2. Ornithine cycle in liver cells converts ammonia to urea, which is less toxic

Question 13

How is urea removed from the bloodstream?

Answer 13

Ultrafiltration in Bowman’s capsule:

High hydrostatic pressure in glomerulus forces small molecules out of capillary fenestrations against osmotic gradient
Basement membrane acts as a filter - blood cells & large molecules

Question 14

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

Answer 14

Fenestrations between epithelial cells of capillaries
Fluid can pass between & under folded membrane of podocytes

Question 15

State what happens during selective reabsorption and where it occurs.

Answer 15

Useful molecules from glomerular filtrate
Occurs in proximal convoluted tubule

Question 16

How are cells in the proximal convoluted tubule adapted for selective reabsorption?

Answer 16

Microvilli = large SA for co-transporter proteins
Many mitochondria = ATP for active transport of glucose into intercellular spaces
Folded basal membrane = large SA

Question 17

What happens in the loop of Henle?

Answer 17

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
   Water potential of filtrate decreases going down descending limb

Question 18

Explain the role of the DCT.

Answer 18

Reabsorption:
1. Of water via osmosis
2. Of ions via active transport

Permeability of walls is determined by action of hormones

Question 19

Explain the role of the collecting duct.

Answer 19

Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins

Question 20

Explain how the loop of Henle acts as a countercurrent multiplier.

Answer 20

Ensures filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential
Maintains water potential gradient for maximum reabsorption of water

Question 21

Define osmoregulation.

Answer 21

Control of plasma water potential via negative feedback homeostatic mechanisms

Question 22

What might cause blood water potential to change?

Answer 22

Level of water intake
Level of ion intake in diet
Level of ions used in metabolic processes or excreted
Sweating

Question 23

Explain the role of the hypothalamus in osmoregulation.

Answer 23

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

Question 24

Explain the role of the posterior pituitary gland in osmoregulation.

Answer 24

Stores and secretes the ADH produced by the hypothalamus

Question 25

Explain the role of ADH in osmoregulation.

Answer 25

Forms hormone-receptor complexes 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 26

How does ADH increase reabsorption of water?

Answer 26

1. Makes cells lining collecting duct more permeable to water:

Binds to receptors —> activates phosphorylase —> vesicles with aquaporins on membrane fuse with cell-surface membrane

2. Make cells lining collecting duct more permeable to urea: water potential in interstitial fluid decreases

Question 27

What is an ectotherm?

Answer 27

Organisms that cannot increase its respiration rate to increase the internal production of heat
Relies on external sources to regulate its body temperature

Question 28

What is an endotherm?

Answer 28

Organisms that can regulate its body temperature independently of external sources
Thermoreceptors send signals to the hypothalamus, which triggers a physiological or behavioural response

Question 29

Outline behavioural methods endotherms use to regulate their body temperature.

Answer 29

Basking in the sun
Pressure against warm surfaces
Digging burrows
Hibernation/aestivation
Panting

Question 30

How does the autonomic nervous system enable endotherms to thermoregulate?

Answer 30

Via negative feedback - thermoreceptors in the hypothalamus detect changes in blood temperatures
Hypothalamus sends impulses to effectors - may result in: vasodilation/constriction, sweating, piloerection, shivering or an increase in metabolic rate

Question 31

Explain the role of the skin in thermoregulation.

Answer 31

Vasodilation/constriction of arterioles supplying skin capillaries controls heat loss to skin surface
Hair erector muscles contract & follicles protrude to trap air for insulation
Evaporation of sweat cools skin surface