heart-rate regulation

myogenic

contraction of the heart is initiated within the muscle itself rather than by nerve impulses

state the name and location of the 2 nodes involved in heart contraction

1. Sinoatrial node (SAN)

   • within wall of right atrium

2. Atrioventricular node (AVN)

   • near lower end of right atrium, in the wall that separates the 2 atria

role of SAN

1. sends out electrical impulses

2. initiates the heartbeat/acts as a pacemaker/stimulates atria to contract

describe how heartbeats are initiated and coordinated

1. SAN initiates heartbeat and sends wave of impulses across atria

   • causing atrial contraction

2. non-conducting tissue prevents immediate contraction of ventricles

3. AVN delays impulse while blood leaves atria and ventricles fill

   • allowing atria to empty before ventricles contract

4. AVN sends wave of impulses down bundle of His

   • causing ventricles to contract from the base up

   • ventricular systole

when a wave of electrical activity reaches the AVN, there is a short delay before a new wave leaves the AVN. Explain the importance of this short delay.

Allows atria to empty and ventricles to fill before ventricles contract

When the heart beats, both ventricles contract at the same time. Explain how this is coordinated in the heart after the initiation of the heartbeat by the SAN.

• impulses only pass through bundle of His

• wave of impulses passes through both ventricles at the same time

explain how the heart muscles and the heart valves maintain one-way flow of blood from the left atrium to the aorta

1. atrium has higher pressure than the ventricle, causing atrioventricular valves to open

2. ventricles has higher pressure than atrium, causing atrioventricular valves to close

3. ventricle has higher pressure than aorta, causing semi-lunar valve to open

4. higher pressure in aorta than ventricles as the heart relaxed

5. muscle contraction causes increase in pressure

cardiac output formula

cardiac output (CO) = stroke volume (V) x heart rate (R)

sympathetic and parasympathetic nervous system

1. sympathetic: involved in ‘fight or flight’ response

   • stimulates effectors to speed up activity

2. parasympathetic: involved in normal resting conditions

   • inhibits effectors to slow down activity

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

1. baroreceptors detect a rise/increase in blood pressure

2. so baroreceptors sends impulses to medulla/cardio-inhibitory centre, which send more frequent impulses to the SAN along parasympathetic neurones

3. causes the release of ACh (acetylcholine), which decreases the impulses to the AVN

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

1. baroreceptors detect a fall in blood pressue

2. baroreceptors send more impulses to medulla/cardio-acceleratory centre (cardiac control centre), which send more frequent impulses to SAN via sympathetic nervous system

3. stimulates release of noradrenaline, which increases heart rate and strength of contraction

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

1. chemoreceptors detect pH decrease and send more impulses to cardioacceleratory centre of medulla oblongata

2. more impulses to SAN via sympathetic nervous system

3. heart rate increases

   • so rate of blood flow to the lungs increases

   • so rate of gas exchange and ventilation rate increase

some drugs inhibit the transmission of nerve impulses to the heart.

explain how these drugs reduce high blood pressure

1. the drugs inhibit impulses in sympathetic nerves/ impulses from cardio-acceleratory centre

2. therefore SAN not stimulated

explain how nervous control in a human can cause increased cardiac output during exercise

1. coordination via medulla

2. increased impulses along sympathetic nerve to SAN

3. more impulses sent from SAN

A woman does moderate exercise. Explain what causes her heart rate to increase while she exercises.

1. rate of respiration increases in muscle cells

2. CO2 concentration increases

   • pH falls

   • acidity increases

3. chemoreceptors detect rise in CO2

4. chemoreceptors in medulla send impulses to cardioacceleratory centre

5. increased frequency/sends more of impulses by sympathetic pathway to SAN

explain why increased cardio output is an advantage during exercise

1. more energy released for aerobic respiration

2. higher cardiac output increases oxygen/glucose supply to muscles