5.5.9 controlling heart rate

Question 1

important roles of the circulatory system

Answer 1

• transport oxygen & nutrients (eg. glucose, fatty acids, amino acids) to the tissues
• removal of waste productions to prevent accumulation which may become toxic (eg. carbon dioxide)
• transport urea from liver to kidneys
• distribute heat around the body or deliver it to skin to be radiated away

Question 2

how can the heart action be modified to allow the circulatory system to adapt to meet the tissues’ needs

Answer 2

• raise or lower heart rate (number of beats per minute)
• altering force of contractions of ventricular walls
• altering stroke volume (volume of blood pumped per beat)

Question 3

what is the cardiac muscle described as

Answer 3

myogenic

Question 4

which muscle has a higher myogenic rate

Answer 4

atrial muscle has a higher myogenic rate than the ventricular muscle

Question 5

pacemaker of the heart

Answer 5

sinoatrial node (SAN)

Question 6

describe the sinoatrial node (SAN)

Answer 6

• initiates waves of excitation which usually override myogenic action of cardiac muscle
• region of tissue that can initiate an action potential
• action potential travels as a wave of excitation over the atria walls, through the AVN (atrio-ventricular node) & down the Purkyne fibres to walls of ventricles = causes them to contract

Question 7

what hormone does the heart muscle respond directly to

Answer 7

adrenaline in the blood = increases heart rate

Question 8

what is heart rate controlled by at rest

Answer 8

= SAN

• set frequency at which it initiates waves of excitation
• frequency of excitation usually 60-80 per minute
• however, frequency of excitation waves altered by output from cardiovascular centre in medulla oblongata

Question 9

cardiovascular centre & SAN

Answer 9

• nerves from cardiovascular centre in medulla oblongata of brain supply SAN
• nerves are part of autonomic nervous system
• nerves don’t initiate a contraction, but can affect frequency of contractions

Question 10

how can the nerves from the cardiovascular centre affect frequency of contractions

Answer 10

• action potentials sent down sympathetic nerve (accelerans nerve) cause release of noradrenaline at SAN = increases heart rate
• action potentials sent down vagus nerve release acetylcholine = reduces heart rate

Question 11

outline sensory input to cardiovascular centre

Answer 11

• stretch receptors in muscles detect limb movement = send impulses to cardiovascular centre, informing it extra oxygen may be needed soon (increases heart rate)
• chemoreceptors in carotid arteries, aorta & brain monitor pH of blood (when exercising, blood pH reduced as carbonic acid formed which will affect oxygen transport) = change in pH detected, sending action potentials to cardiovascular centre (increases heart rate)
• concentration of carbon dioxide in blood (when stop exercising, concentration of carbon dioxide in blood falls) = reduces activity of accelerator pathway (heart rate decreases)
• stretch receptors in walls of carotid sinus monitor blood pressure = if pressure rises too high, stretch receptors send action potentials to cardiovascular centre (heart rate decreases)

Question 12

what happens if the mechanism controlling heart rate fails

Answer 12

= artificial pacemaker fitted

• delivers electrical impulse to heart muscle
• implanted under skin/fat on chest (or sometimes within chest cavity)
• may be connected to SAN or directly to ventricle muscle