topic 6

Question 1

control of cardiac cycle

Answer 1

• SAN releases wave of depolarisation across walls of atria, causing them to contract
• non conductive layer of collagen tissue prevents wave from being passed directly from atria to ventricles
• wave is passed from SAN to AVN
• AVN passes wave to bundle of His but not before a slight delay to ensure the atria have emptied
• bundle of His passes wave down septum to Purkyne fibres in ventricle walls, causing apex and ventricle walls to contract simultaneously from the bottom up
• cells depolarise and cardiac muscle relaxes

Question 2

control of heart rate

Answer 2

pressure
too high: may cause damage to artery walls
too low: insufficient supply of oxygenated blood to respiring cells
- detected by baroreceptors (found in aorta + carotid artery)

pH
too high/low: causes enzymes in blood to denature
- detected by chemoreceptors (found in aorta, carotid artery + medulla)

Question 3

when blood pressure is too high

Answer 3

• increased pressure stretch baroreceptors
• respond by increasing frequency of impulses sent along sensory neurones to medulla
• medulla increases frequency of impulses sent along parasympathetic neurones
• these secrete acetylcholine which binds to receptors on SAN, decreasing the frequency of impulses + slowing down heart rate, reducing BP back to normal

Question 4

when blood pressure is too low

Answer 4

• baroreceptors are less stretched
• respond by increasing frequency of impulses sent along sensory neurones to medulla
• medulla increases frequency of impulses sent along sympathetic neurones
• these secrete noradrenaline which bind to receptors on SAN, increasing the frequency of impulses + speeding up heart rate, increasing BP back to normal

Question 5

when blood pH is too low

Question 6

when blood pH is too high

Question 7

resting potential

Answer 7

• Na-K pump moves 3 Na+ out for every 2 K+ in
• creates an electrochemical gradient
• so Na+ begin to move back in whilst K+ begins to leave via FD
• however, the axon membrane is more permeable to K+ as there are more K+ channels open
• so more K+ leaves than Na+ enters
• meaning more +tive ions are moving out than entering
• so outside of membrane is more +tively charged compared to the inside
• creating a potential difference of -70mV

Question 8

action potential

Question 9

refractory period