The Heart
Overview
The heart can be described as two fluid pumps side by side.
The heart pumps oxygen-poor blood into the pulmonary circulation (to the lungs) and oxygen-rich blood into the systemic circulation (to the rest of the body).
In the capillary network of the systemic system the oxygen it carries is depleted
Fibres of the sino-atrial (SA) node automatically generate rhythmic impulses, which then spread throughout the remainder of the heart's conduction system.
Cardiac function is influenced by the amount of blood returning to it via veins, the
autonomic nervous system, hormones, and the the body's fluid and electrolyte balance.
Although it is not much larger than a clenched fist and weighs less than a pound, (approximately 300 grams) the human heart is a truly incredible organ.
Throughout history the heart has represented the rhythm of life itself. Its continuous and rhythmic beating ensures circulation of the blood throughout the entire body. This in turn leads to the delivery of oxygen, nutrients, hormones and other regulatory molecules to, as well as the removal of waste products from, each of our organs. During an average lifetime the heart beats approximately 2.5 billion times and pumps about 300 million litres of blood.
This
video shows a human heart. Note the fatty tissue deposited around the
ventricles. It does take a little time to download so please be patient. To start the
animation just place your mouse cursor over the picture.
It is so responsive to the changing needs of our body that cardiac output can vary from as little as 5 to a maximum of 35 litres of blood per minute, a sevenfold change, over a very short interval.
The heart is a four-chambered, hollow, muscular organ that has a conical shape. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. Approximately two-thirds of its mass lies to the left of the body's midline.
Diagram 1 Exterior view of the heart showing gross anatomy as viewed from the front
Anatomically it lies in the mediastinum, the region between the two lungs that holds several organs.
The heart rests on the muscular diaphragm and is attached via its covering to this muscular partition that separates the chest and abdominal cavities. It measures about 120mm (4.75") in length, 90mm (3.5") in width at its broadest part, and 60mm (2.4") in thickness.
Perversely,the base of the heart is the uppermost part of the heart, it tilts slightly backwards (towards the spinal column) and from it emerge two of the great vessels, the aorta and the pulmonary trunk; the superior vena cava and inferior vena cava enter its right side (see diagram 1 above). Opposite the base is the apex of the heart which is made up largely of the tip of the left ventricle. It is directed to the left and is located about 80mm ( from the body's midline in the fifth intercostal space (between the fifth rib above and the sixth below). Although its exact position varies somewhat from individual to individual and with respiratory movements, the apex beat may be palpated or heard best at this point.
The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.
Within the adult heart are two parallel independent systems, each consisting of an auricle, or atrium, and a ventricle; from their anatomical positions these systems are often designated the right heart and the left heart.
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This page last updated on Sunday, 11 July 1999 10:34 +0100
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