Download The Heart - DocShare.tips

The Heart
Diastole: heart at rest.
Systole: contraction.
Cardiac Cycle:
- Atrial diastole: Blood flows from veins into atria. Atrioventricular valves closed.
- Atrial systole: Atria contract. Pressure in atria is higher than pressure in
ventricles. Atrioventricular valves open. Blood flows into ventricles. Semilunar
valves are closed.
- Atrial & Ventricular systole: Atria complete contraction. Ventricles begin to
contract. Semilunar valves closed. Ventricular pressure becomes higher than
atrial pressure. Atrioventricular valves close (held by valve tendons). Volume in
ventricles remains constant as pressure increases. Atria become diastolic.
- Ventricular systole: Ventricles still contracting. Ventricular pressure becomes
higher than arterial pressure. Semilunar valves open. Blood flows into pulmonary
artery and aorta. Aorta accommodates extra blood (elasticity of wall). Ventricles
empty and stop contracting. Ventricular pressure rapidly falls below arterial
pressure. Semilunar valves close. Pressure in aorta falls slowly due to elastic
recoil of arterial wall. Ventricles become diastolic.
- As ventricular pressure falls below atrial pressure, atrioventricular valves open.
Both sides of the heart contract at the same time. The right side of the heart
pumps blood to the lungs. The left side of the heart pumps blood to the aorta
(and the rest of the body). The pressure in the left ventricle is higher than that in
the right as the blood has to be pushed further. The thicker wall of the left side of
the heart enables this.
If pressure is beneath 0, there is suction. This helps blood to enter the atria from
the veins.
Regulation of heart rate:
- The heart is myogenic.
- Electrical activity begins at the sinoatrial node (SAN) which is located in the all
of the right atrium.
- Electrical impulses spread from the SAN across atria walls, causing both atria to
contract simultaneously.
- Electrical activity cannot pass directly into ventricles due to layer of fibrous
tissue which acts as electrical insulation. This ensures the atria and ventricles
contract alternately.
- Impulses from the SAN stimulate the atrioventricular node (AVN). After a brief
delay which allows the ventricles time to fill, this generates more electrical
activity.
- Impulses from the AVN pass down the bundle of His (made from Purkinje fibres)
which splits into two sections.
- These reach the apex of the heart and spread back up the walls of the
ventricles.
- The ventricles contract upwards due to the impulses from the AVN which force
the blood upward towards the arteries.
The heart rate can be modified (eg increased during exercise) due to nervous
stimulation and hormones (adrenaline). The top speed of the heart can be
calculated as 220 - age in years.
Cardiac output = heart rate (bpm) x stroke volume (cm³)