Cardiovascular System: Anatomy And Physiology

Cardiovascular System Anatomy & Physiology
The heart is the pump responsible for maintaining adequate circulation of oxygenated blood around the vascular network of the body. It is a four-chamber pump, with the right side receiving deoxygenated blood from the body at low presure and pumping it to the lungs (the pulmonary circulation) and the left side receiving oxygenated blood from the lungs and pumping it at high pressure around the body (the systemic circulation).
The myocardium (cardiac muscle) is a specialised form of muscle, consisting of individual cells joined by electrical connections. The contraction of each cell is produced by a rise in intracellular calcium concentration leading to spontaneous depolarisation, and as each cell
is electrically connected to its neighbour, contraction of one cell leads to a wave of depolarisation and contraction across the myocardium.
This depolarisation and contraction of the heart is controlled by a specialised group of cells localised in the sino-atrial node in the right atrium- the pacemaker cells.
1. These cells generate a rhythmical depolarisation, which then spreads out over the atria to the atrio-ventricular node. 2. The atria then contract, pushing blood into the ventricles. 3. The electrical conduction passes via the Atrio-ventricular node to the bundle of His, which divides into right and left branches and then spreads out from the base of the ventricles across the myocardium. 4. This leads to a 'bottom-up' contraction of the ventricles, forcing blood up and out into the pulmonary artery (right) and aorta (left). 5. The atria then re-fill as the myocardium relaxes. |
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The 'squeeze' is called systole and normally lasts for about 250ms. The relaxation period, when the atria and ventricles re-fill, is called diastole; the time given for diastole depends on the heart rate.
The ECG
The Electrocardiograph (ECG) is clinically very useful, as it shows the electrical activity within the heart, simply by placing electrodes at various points on the body surface. This enables clinicians to determine the state of the conducting system and of the myocardium itself, as damage to the myocardium alters the way the impulses travel through it.
When looking at an ECG, it is often helpful to remember that an upward deflection on the ECG represents depolarisation moving towards the viewing electrode, and a downward deflection represents depolarisation moving away from the viewing electrode. Below is a normal lead II ECG. * The P wave represents atrial depolarisation- there is little muscle in the atrium so the deflection is small.
* The Q wave represents depolarisation at the bundle of His; again, this is small as there is little muscle there. * The R wave represents the main spread of depolarisation, from the inside out, through the base of the ventricles. This involves large ammounts of muscle so the deflection is large. * The S wave shows the subsequent depolarisation of the rest of the ventricles upwards from the base of the ventricles. * The T wave represents repolarisation of the myocardium after systole is complete. This is a relatively slow process- hence the smooth curved deflection.
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The Coronary Circulation

The heart needs its own reliable blood supply in order to keep beating- the coronary circulation. There are two main coronary arteries, the left and right coronary arteries, and these branch further to form several major branches (see image). The coronary arteries lie in grooves (sulci) running over the surface of the myocardium, covered over by the epicardium, and have many branches which terminate in arterioles supplying the vast capillary network of the myocardium. Even though these vessels have multiple anastomoses, significant obstruction to one or other of the main branches will lead to ischaemia in the area supplied by that branch.