control of the heart rate

how is the heart myogenic?

it contracts and relaxes on its own

HOWEVER- rate of contraction is controlled by wave of electrical activity

what is the SAN?

the sinoatrial node is a tissue that is located in the RA- pacemaker

release wave of electricity (depolarisation)- when this hits cardiac muscle, the heart contracts

what is the AVN?

the atrioventricular node- located near border of left and right ventricle, within atria still

what are the bundle of His?

conductive tissues running through septum (middle) of heart and up walls of ventricles

what are the purkyne fibres

conductive tissues all through walls of ventricles

What are the steps of the heart rate?

1. SAN releases wave of depolarisation across atria= contraction

2. AVN releases another wave of depolarisation when the first one hits it, does not directly travel down to ventricles due to non-conductive layer between atria & ventricles preventing depolarisation from travelling (bus insulating)

3. INSTEAD, bundle of His (running through septum) conducts & passes wave of depolarisation down septum & Purkyne fibres in all walls of ventricles

4. THEREFORE, apex (top of heart) and then walls of the ventricles contract, short delay before this happens, whilst AVN transmits second waves of depolarisation

5. allows enough time for atria to pump all blood into ventricles

6. Finally, cells repolarise, and cardiac muscle relaxes

what is the medulla oblongata?

Area in brain that controls heart rate via autonomic nervous system

what are the 2 parts of the medulla oblongata?

• one centre increases heart rate via sympathetic nervous system

• another decreases heart rate via parasympathetic nervous system

what factors alter heart rate? And how?

pH and blood pressure

these stimuli are detected by chemoreceptors and pressure receptors (baroreceptors) in the aorta and carotid artery

what are the dangers of blood pressure being too high?

If the blood pressure is too high this can cause damage to the walls of the arteries and it is important to put mechaniss in place to reduce the blood pressure

what are the dangers of blood pressure being too low?

If the blood pressure is too low, there may be insufficient supply of oxygenated blood to respiring cells and removal of waste

how does pH decrease and how does this impact heart rate?

The pH of the blood will decrease during times of high respiratory rate, due to the production of carbon dioxide or lactic acid. Excess acid must be removed from the blood rapidly to prevent enzymes denaturing.

This is achieved by increasing the heart rate, so carbon dioxide can diffuse out into the alveloli more rapidly.

how does heart rate respond to high blood pressure?

• baroreceptors (pressure receptors) detect high BP

• send impulses along sensory neuron to medulla

• sends impulses via parasympathetic NS to SAN to decrease frequency of electrical impulses

• SAN contracts less, decreasing contraction, decreasing BP

how does heart rate respond to low blood pressure?

• baroreceptors in wall of aorta & carotid artery not stretched when low BP

• send impulses along sensory neuron to medulla

• sends impulses via sympathetic NS to SAN to increase frequency of electrical impulses

• SAN contracts more, increasing contraction, increasing BP

how does heart rate respond to high blood pH/ low CO2 levels?

• chemoreceptors detect chemical changes in blood

• send impulses along sensory neuron to medulla, sending impulses along parasympathetic NS to SAN to decrease frequency of electrical impulses

• increases contraction of heart

• Decrease heart rate to delivery blood to lungs rapidly to remove carbon dioxide

how does heart rate respond to low blood pH/ high CO2 levels?

• chemoreceptors detect chemical changes in blood

• send impulses along sensory neuron to medulla, sending impulses along sympathetic NS to SAN to increase frequency of electrical impulses

• increases contraction of heart

• Increase heart rate to delivery blood to lungs rapidly to remove carbon dioxide

Exercise causes an increase in heart rate.

Describe the role of receptors and of the nervous system in this process. (4)

1. Chemoreceptors detect rise in COz / H+ / acidity / carbonic acid / fall in pH

OR

Baro / pressure receptors detect rise in blood pressure;

2. Send impulses to cardiac centre / medulla;

3. More impulses to SAN;

4. By sympathetic (nervous system for chemoreceptors / COz)

OR

By parasympathetic (nervous system for baro / pressure receptors / blood pressure);

Describe how a heartbeat is initiated and coordinated. (5)

1. SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
   Accept excitation

2. Non-conducting tissue prevents immediate contraction of ventricles / prevents impulses reaching the ventricles;

3. AVN delays (impulse) whilst blood leaves atria / ventricles fill;

4. (AVN) sends wave of electrical activity / impulses down Bundle of His

Allow Purkyne fibres / tissue

5. Causing ventricles to contract from base up;