The Heart

• The Heart is made from cardiac muscle
• The muscle never gets fatigued (tired)
• The bicuspid and tricuspid valves are called atrioventricular as they control the passages of the blood

• The heart is divided into left and right by the septum
• Cardiac muscles has its own suppy of blood - coronary circulation
• When the muscles contract, its called systole
• When the muscle relax, it called diastole

1.  Blood enters atria but bicuspid and tricuspid valves are closed

2. Walls of atria contract which raises the blood pressure which forces the valves open and the blood goes through the ventricles

3. When the ventricles are full, they contract to increase the blood pressure and this closes the valves so the blood can't go back into the atria

4. The ventricles continue to contract until the semi-lunar valves are open and the blood is ejected into the arteries

5. As the ventricles empty, high pressure closes the valves and the cycle begins again

• Normally the heart beats around 70 times a min

• Changes in the heart rate are brought by nerve impulses from part of the brain called the medulla
• When we start to exercise, our muscles produce more carbon dioxide in aerobic respiration
• Sensors in the aorta and the carotid artery (artery leading to the head) detect this increase
• The medulla responds by sending nerve impulses along the accelerator nerve
• When carbon dioxide production returns to normal, the medulla recieves fewer impulses
• It responds by sending nerve impulses along a deaccelerator nerve

• When we exercise, muscles release more energy and they need an increased supply of oxygen for aerobic respiration.

• To deliver the extra oxygen, both the heart rate and the volume of blood pumped (stroke volume) increases

• When stressed, the rate increases. The increased output supplies extra blood to muscles, enabling them to release extra energy through aerobic respiration

• This allows us to fight or run away ("fight or flight response")

• It is triggered by secretion of adrenaline from adrenal glands

• The coronary arteries are amoung the narrowest in the body so they are easiy blocked up with fatty substances.
• This can cut off the blood supply to the cardiac muscle

Factors that make coronary heart disease more likely:

• Heredity - some people inherit a tendancy to heart disease
• High blood pressure - puts more strain on the heart
• Diet - eating more saturated fat is likely to raise cholesterol levels
• Smoking - raises blood pressure and makes blood clots more likely to form
• Stress - raise blood pressure
• Lack of exercise - regular exercise helps to reduce blood pressure and strengthens the heart