The Cardiac Cycle

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  • Created by: lauraxxx
  • Created on: 02-01-15 16:21

Relaxation of the heart (diastole)

Blood returns to the atria, as they fill the pressure rises causing the atrioventricular valves to open allowing blood to move into the ventricles. Both the atria and ventricular walls are relaxed.

The relaxation of the ventricles and the atria reduces pressure in the ventricle, this causes the pressure to lower than that in the aorta and the pulmonary artery and so the semi-lunar valves in the aorta and pulmonary artery close

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Contraction of the Heart (Atrial/ventricular systo

Atria

Muscles of the atria wall contract forcing the remaining blood in the ventricles

The ventricle walls remain relaxed

Ventricles

After a short delay the walls contract simultaniously, this increases the blood pressur forcing shyt the atrioventricular valves to prevent back flow.

With the valve shut the pressure rises further forcing the semi-lunar valves open and pushing blood into the artery and the aorta.

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Valves and the control of blood flow

Atrioventricular valvesBetween the artias and ventricles, they prevent the back flow of blood when the ventrivles are contracting so the ventricular pressure exceddes artial pressure.

The closing of these valves stops the blood flowing back into the atria instead of the aorta and pulmonary artery when the ventricles contract.

Semi-lunar valves

In the aorta and the pulmonarary artery.These prevent backflow of blood into the ventricles when the recoil action of the elastic walls of these vessels creates a greater pressue in the vessles than the ventricles.

Pocket valves

Occur throughout the veins, they make sure that when the veins are squeezed e.g. when the sketal muscles contract, to make sure blood flows back to the heart instead of away from it.

All valves are constructed the same. They have a cusp-shaped and are made of flexiable, fiberous material.When pressure occurs on the convex side they open and on the concave side they close, blood collects in the concave and forces it shut.

To prevent the valves from becoming inverted they have string like tendons that are attached to pillars of the muscles in the ventricle wall.

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How is the cardiac cycle controlled?

The cardiac muscle is myogenic ( the contraction is stimulated from within the muscle rather then from nervous impulses

The wall of the right atrium is the sinoatrial node (SAN) this is were the originala stimulus comes from, this is why it is known as the pacemaker.

- The impulse from the SAN spreads across the whole of both atria causing them to contract

- The layer of non-conductive tissue atrioventricular spetum prevents the impulse from travelling to the ventricles

- The wave of secondary impluses travels through the atrioventricular node (AVN) which is between the atria

- After a short delay the AVN convays the electrical ipulse thrugh the ventrivles along the bundle of His

- The bundle of His conducts the wave through the atriovenricular septum to the base of the ventricles and into the pukijy fibres

- The pukmjy fibres relase this impulse causing the ventricles to contract quickly simultaniously from the apex to the heart upwards

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Pressure and Volume Changes of the Heart

Aortic Pressure

RISES when the ventricles contract as blood is forced into the aorta it then GRADUALLY FALLS but never below 12kPa because of the elasticity of its walls, which reacts a RECOIL action - it produces a temporary rise in pressure during the relaxation pior to provide a constant delievery of blood.

Atrial Pressure

ALWAYS RELIVILY LOW because of its thin walls cannot create much force, HIGHEST when theyre contracting and FALLS when the left artioventricular valve closes and the walls relax. The atria then FILL WITH BLOOD which GRADUALLY BUILDS UP PRESSURE until a slight DROP when the valve opens and the blood moves to the ventricles.

Ventricular Pressure

LOW AT FIRST but then GRADUALLY INCREASES as the ventricles fill with blood as the atria contract. The left atrioventricular valve closes and PRESSURE RISES DRAMATICALLY as the thick muscular walls of the ventricles contract. AS PRESSURE RISES ABOVE THE AORTA blood is forced into the aorta as the semilunar valves are forced open. PRESSURE THEN FALLS as the walls relax and are emptied

Ventricular Volume

RISES AS THE ATRIA CONTRACT and the ventricles fill with blood and then DROPS SUDDENLY as blood is forced OUT OF INTO THE AORTA when the semilunar valve opens. VOLUME INCCREASES again as ventricles fill with blood.

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