- Cardiac muscle in the heart wall needs a good supply of blood supply to provide nutrients and oxygen for contraction. This is achieved by the presence of a dense capillary network that received blood from the right and left coronary arteries.
The Cardiac cycle - the complete contraction and relaxation of the heart is a single heartbeat.
- Systole = period of contraction.
- Diastole = period of relaxation. (This is longer than systole)
- Blood flows from an area of high pressure to an area of low pressure unless the blow is blocked by a valve.
- Pressures are lower on the right as there is more muscle on the left side as the blood has to travel further.
- The atria and ventricles are in diastole.
- Blood in the veins flows into the atria.
- This increases the pressure inside the empty atria as they fill.
- some blood goes into the open atrioventricular vales into the relaxed ventricles below
- Both the atria contract and blood passes down the ventricles.
- The atrioventricular calves open due to blood pressure.
- 70% of the blood flows passively down to the ventricles so the atria do not have to contract a great amount.
- The aria relax
- The ventricle walls contract, forcing blood out
- the pressure of the blood forces the atrioventricular valves to shut
- the pressure of the blood opens the semi-lunar valves
- blood passes into the aorta and pulmonary arteries
- The ventricles relax
- pressure in the ventricle falls below that in the arteries
- blood under high pressure in the arteries causes semi-lunar vales to shut.
- during diastole all the muscle in the heart relaxes
- Blood from the vena cava and pulmonary veins enter the atria
- Cycle starts again
Control of heart rate
The mechanical work in pumping blood is carries out by the cardiac muscles in the walls of the four heart chambers aka cardiac cycle. Cardiac muscle has certain feature that are distinct in the other types of muscles. It contracts rhythmically without any nervous stimulation - it is myogenic.
The initiation of this rhythm comes from a patch of muscle fibres in a small part of the right atrium. This is called the sino-atrial node S.A.N and is also known as the pacemaker. - from the pacemaker waves of electrical activity spread out rapidly over both atria. Each wave contracts the atria muscle forcing blood in the atria through the ventricular vales into the ventricles.
The atrioventricular septum between the atria and the ventricles does not conduct the cardiac impulse from the pacemaker. however, there is another specialised group (node) of cardiac muscle cells in the wall of the right atrium. this node is called atrioventricular node A.V.N and it picks up the atrial impulse and transmit it along a bundle to modified cardiac muscle fibres in the interventricular septum. When the impulses reach the apex of the heart, it spreads rapidly up the ventricular walls in a network of conductive fibred called purkinje fibres. Impulses causes heart to contract.
Blood must be put under pressure as:
- it enables the blood to reach all the cells in all parts of the body.
- it takes deoxygenated blood to the lungs to enable gas exchange it delivers certain molecules e.g. oxygen and glucose.
- it removes waste material such as CO2 and urea.
Electrocardiogram ECG
- Electrical impulses in the heart originate in the sinoatrial node and travel through the intrinsic conduction system to the heart muscle.
- The impulses stimulate the myocardial muscle fibres to contract and induce systole.
- The electrical waves can be measured at selectively places electrodes on the kin.
- electrodes on different sides of the heart measure the activity of different parts of the heart muscle. An ECG displays the voltage between pairs of these electrodes.
- Displays indicate the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
- it is the best way to measure and diagnose abnormal rhythm of the heart.
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