biology- control of heart rate 🟣

Describe how heart controls and coordinates the regular contraction of the atria and ventricles



1. Both atria fill

2. SAN generates an impulse. Both atria contract (atrial systolé). Ventricles fill

3. The AVN picks up the impulse, delays it (allowing ventricles to fill) and channels it down the Bundle of His to the Purkinje fibres which causes ventricle systolé. Pressure in ventricle increases → AV valve shuts

4. All 4 valves are now shut. Pressure rises quickly

5. Pressure in ventricle exceeds pressure in artery so SL valves open

6. Arteries fill with blood

7. As soon as ventricular pressure drops below that in arteries, SL valve shuts

8. Pressure in ventricles drops rapidly to below pressure in atria so AV valve opens again

What is SA node

• Cardiac muscle cells located in the wall of the right atrium near where the vena cavae enter it

• Determines basic heart rate (pacemaker) 70bpm

Pressure changes in heart

• Atrioventricular valves close when the pressure in the ventricles is higher than pressure in the atria

• They open when the pressure is higher in the atria than in the ventricles

Formula for cardiac output

Cardiac output = stroke volume x heart rate

How are changes to cardiac output effected

Thorough the autonomic nervous system (ANS)

Within the medulla of the brain is the cardioregulatory centre which has 2 parts:

Cardio-accelatory centre- linked by the sympathetic nervous system to the SA node. When stimulated these nerves cause an increase in cardiac output

Sympathetic → Neurotransmitter- noradrenaline

Cardio-inhibitory centre- linked by parasympathetic fibres within the vagus nerve to the SA node, AV node and bundle of His. Stimulation from these nerves decrease the cardiac output.

Parasympathetic → Neurotransmitter- acetylcholine

What does which of the centres stimulate the heart depend on

Blood pH

What happens when CO2 level in blood falls

• blood pH rises

• Chemoreceptors stimulate the cardio-inhibitory centre

• Sends impulses along parasympathetic nerves that slow down the activity of the SNA and hence the heart rate

• Will reduce the heart rate to its normal or resting rate

• Parasympathetic nerve secretes acetylcholine from its nerve endings

What happens when CO2 level in blood rises (eg as a result of strenuous exercise)

• Increases H+ conc

• pH lowered

• Sensory receptors- chemoreceptors in the carotid artery and aortic sinuses detect this change

• And send nervous impulses to the cardio-acceleratory centre

• Which increases heart beat

• Therefore increases the rate at which CO2 is delivered to lungs for removal

What happens when stretch receptors in the aorta, carotid artery and vena cava are simulated

• Aorta and carotid artery: Indicates there is distention of these vessels as a result of increased blood flow to them

• This causes the cardio-inhibitory centre to stimulate the heart to reduce cardiac output

• Simulation of receptors in the vena cava and the wall of the right atrium indicates increased venous return, probably as a result of muscular activity increasing the rate at which blood is returned from the tissues

• Under these conditions the cardiac centres in the brain increase the cardiac output by increasing the heart rate. A high venous return also stretches the wall of the left ventricle causing the ventricles to contract more strongly, giving a greater stroke volume

• Enables the heart to adjust the strength and rate of its contractions to the volume according to the volume of blood passing through it at any time

What happens when blood pressure rises/falls

• Pressure receptors (baroreceptors) in the carotid artery detect blood pressure changes and relay impulses to the medulla

• If blood pressure falls the medulla sends impulses along sympathetic nerves to the arterioles

• The muscles in the arterioles contract causing vasoconstriction → blood pressure increases

• If blood pressure rises the medulla sends impulses along parasympathetic nerves to the arterioles

• The muscles in the arterioles relax causing dilation → blood pressure reduces

What does a rise in blood CO2 also cause

• A rise in blood pressure

• This increases the speed with which blood is delivered to the lungs and so helps remove the CO2 more quickly

• Hormones like adrenaline similarly raise blood pressure

How can resting HR be measured

Is this method accurate for subject that is:

resting

recovering after period of exercise

Count pulse rate over 30s and multiply this by 2 to give beats per minute

Or electronic heart rate monitor

• Resting- fair reflection of pulse

• Recovering after period of exercise- no, as HR will be decreasing so the pulse rate measured in the first 30s will be higher than in the following 30s, which is not measured

• therefore doubling the first measurement to five a rate per minute will give a higher value for HR- over estimate

What is homeostasis

• Maintenance of constant internal conditions (blood glucose, blood WP, blood pH, blood/tissue fluid) , despite changes externally or internally

• In the case of heart rate and breathing rate these are changed to ensure that blood pH/H+/carbon dioxide levels are maintained at the optimum levels