The Heart Beat
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The heart muscle contracts rhythmically with a period of rest between each contraction. The contraction period is called the systole & the relaxation period is called the diastolE.

The heart is really two muscles, one on top of the other; a strong membrane called the valve plain separates them. All the hearts valves are positioned in line here. Starting from the right these valves are called the tricuspid, lunar valve, aorta valve and the bicuspid valve. There is a dole in the middle of the membrane for the HISS a bundle of nerve fibres to pass through. This membrane also stops nerve impulses admitted by the senatorial nod from returning to the top of the heart.

1, Atria systole

The atria contract and force blood into the ventricles. The contraction occurs around the openings of the veins into the atria first so that these are closed up to prevent back flow into them.

2, Ventricular systole

The ventricles contract and force the blood under pressure, past the semi lunar valves into the arteries. The closing of the bicuspid and tricuspid valves prevents back flow.

At the same time the atria and ventricles relax and blood begins to flow back into them from the veins.

3, Diastole

The muscles of both atria and ventricles relax. The heart fills with blood, which is prevented from flowing straight out again by closing the seminar valves in the arteries.

Rate of heartbeat 

The autonomic nerves control the pace of the contractions of the heart muscle. These supply a group of special cells close to the opening of the superior venal Cava. This is called the senatorial node.

Once stimulated waves of contraction pass from fibre to fibre in the arterial muscle but are stopped from going straight on to the ventricles by the membrane of connective tissue.

They are transmitted towards the atria ventricle node and the bundle of HISS. This splits into two and runs down the septum and branches into the ventricular muscle, each branch ending in purkinje fibres. These directly contract the muscle cells of the myocardium.

The average rate of heartbeat is 72 beats per minute. Thus each complete cardiac cycle takes 0:8 seconds. This is divided into systole and diastole as follows: -

Atria systole: 0:1 seconds 

Atria diastole: 0:7 seconds 

Ventricle systole: 0:3 seconds 

Ventricle diastole: 0:5 seconds 

Complete systole: 0:4 seconds

Complete diastole: 0:4 seconds

When heart rate increases, the period of complete diastole is shortened. This occurs during activity, inspiration and response to some drugs e.g. caffeine. Heart rate decreases during sleep, expiration and some drugs e.g. alcohol. 

Cardiac out put

This is the amount of blood flowing from the heart at a given time. It depends on: -

1, Stoke volume i.e. output/ventricles/beat.

2, Heart rates i.e. number of beats per minute.

Cardiac out put = stroke volume X heart rate.

At rest: 6:6L/minute = 110cm3/beat X 60 beats/min.

After sever exercise: 40L/minute = 2000cm3/beat X 200 beats/min, so an increase in stroke volume or heart rate can increase cardiac output.

Regulation of cardiac output by cardiac centre

This can occur in two ways: -

1, Altering heart beat, this alters by means of the autonomic nervous system.

The ANS has two branches involved: -

A, the sympathetic nervous system. This stimulates the sinu-alvial node with the accelerator nerve. This secretes small amounts of nor-adrenaline, which increases heart rate.

B, the parasympathetic nervous system. This stimulates the sinu-atrial node with the vague nerve. This secretes small amounts of acetylcholine, which decreases the heart rate.

2, Alluring stroke volume

If the amount of blood returning through the veins to the heart increases, the cardiac muscle is stretched during diastole and a greater amount of blood is put out into the arteries by the left ventricle.

This results in an increase in cardiac output, this occurs during exercise.

Other Notes in this Category

1. Abortion Methods
2. Atmosphere and Pollution
3. Bacteria
4. Composition of Blood - Plasma, White Blood Cells, Red Blood Cells and Platelets
5. Down Syndrome
6. Enzyme Quiz
7. Fungi
8. Hereditary Diseases
9. Respiration and Osmosis
10. The Defences of the Human Body
11. The Heart Beat
12. Virii

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