7.5 The Cardiac Cycle

• The cardiac cycle refers to the sequence of events in the heart that is repeated in humans around 70 times each minute when at rest.
• There are two main phases to the beating of the heart : contraction (systole) and relaxation (diastole).
• Contraction occurs separately in the ventricles and the atria and is therefore described in two stages.
• Relaxation often takes place in all chambers of the heart simultaneously so it is referred to as a single phase.
• Relaxation of the heart (diastole) (Stage 1) : Blood returns to the atria of the heart via the pulmonary veins and vena cava. The pressure rises as volume of blood in the atria increases.
• Relaxation of the heart (diastole) (Stage 2) : Once the pressure in the atria exceeds the pressure in the ventricles, the atrioventricular valves open and allow blood into the ventricles.
• Relaxation of the heart (diastole) (Stage 3) : The muscles of the atria and ventricles are now relaxed, and the relaxation of the ventricles causes them to reduce the pressure inside, meaning the pressure drops to below that of the atrium.
• Relaxation of the heart (diastole) (Stage 4) : Due to the pressure difference, the semi-lunar valves in the aorta and pulmonary artery close, accompanied by the 'dub' sound of the heartbeat.
• Contraction of the atria (atrial systole), combined with the recoil of the ventricle walls, forces any remaining blood into the ventricles from the atria. The ventricle walls remain relaxed throughout atrial systole.
• Contraction of the ventricles (ventricular systole) (Stage One) : after a short delay, once the ventricles have filled with blood, the ventricular walls contract to increase blood pressure, and force the atrioventicular valves shut, preventing backflow.
• Contraction of the ventricles (ventricular systole) (Stage Two) : once the atrioventricular valves are closed, the pressure in the ventricles further increases. Once it exceeds the pressure in the aorta and pulmonary artery, blood is forced from the ventricles into these vessels due to the thick muscular walls.
• Contraction of the ventricles (ventricular systole) (Stage Three) : the ventricular contraction creates high pressure needed to pump blood to around the body via the aorta and pulmonary artery. The thick left ventricle wall pumps blood under higher pressure, whilst the right ventricle has a thinner wall to pump blood to the lungs.
• Blood is kept flowing in one direction through the heart and around the body by the pressure created by the heart muscle.
• Blood will always move from a region of higher pressure to a region of lower pressure.
• There can be situations in the circulatory system when pressure differences would result in blood flowing in the opposite direction from that which is desirable. Valves are used to prevent this back flow of blood.
• Valves in cardiovascular system are designed so that they open whenever the difference in blood pressure either side of them favours the movement of blood in the required direction.
• When pressure differences are reversed and blood flows in the opposite direction, the valves are designed to close.
• Atrioventricular valves exist between the left atrium and left ventricle, and the right atrium and right ventricle. These prevent back flow of blood when contraction of ventricles causes ventricular pressure to exceed atrial pressure.
• Closing the atrioventricular valves ensures that when the ventricles contract, blood within them moves to the aorta and pulmonary artery rather than back into the atria.
• Semi-lunar valves are in the aorta and pulmonary artery. These prevent the back flow of blood into the ventricles when the pressure in these vessels exceeds the ventricles.
• The pressure in the semi-lunar valves arises when the elastic walls of the vessels recoil, increasing the pressure in them. Simultaneously, the ventricle walls relax to reduce pressure in the ventricles.
• Pocket valves are in veins that are located in the venous system. These valves ensure that when the veins are squeezed (i.e. the skeletal muscles contract), blood flows back towards the heart rather than away from it.
• Valves are universally designed to have a number of flaps made of tough but flexible fibrous tissue. These flaps are cusp shaped. When pressure is greater on the convex side of these cusps, they move apart to allow blood to pass between them.
• When pressure is greater on the concave side of valve cusps, blood collects in the ‘bowl’ of the cusps, which pushes them together to form a tight fit which blocks the passage of blood.
• Mammals have a closed circulatory system, meaning that blood is confined to a network of vessels. This allows the pressure to be maintained and regulated.
• Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute. It is usually measured in dm3 min-1.
• Cardiac output depends on two factors : the heart rate and the stroke volume.
• Heart rate refers to the rate at which the heart beats
• Stroke volume refers to the volume of blood pumped out at each heart beat.