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Heart

The heart is a hollow, muscular organ that pumps blood through the vessels
and cavities of the body by its repeated, rhythmic contractions. In the
human body the heart is situated in the middle of the thorax and weighs
about 300-350g in an adult. It consists of four chambers, the two upper
atria and the two lower ventricles. Mammals have a double circulatory
system, meaning that two separate circulatory cycles occur simultaneously.
The first cycle, termed pulmonary circulation, consists of deoxygenated
blood being pumped out of the right ventricle into the lungs, where it
exchanges unwanted carbon dioxide and water for oxygen. The blood then
travels back into the left side of the heart, from where it is pumped all
round the body in systemic circulation. The body removes oxygen and
nutrients from the blood to be used in respiration and the cycle continues.

Structure

The atria chambers receive blood from the veins, and the ventricles pump it
from the heart into the arteries. The left ventricle is much thicker than
the right because it must pump blood around the entire body, this involves
exerting a considerable force to overcome the pressure caused by the body.
As the right ventricle must just pump blood to the lungs it requires less
muscle.

The wall of the heart is very muscular and does not tire. It consists of
three distinct layers. The first is the outer epicardium which is composed
of a layer of flattened epithelial cells and connective tissue. Beneath this
is a much thicker myocardium made up of cardiac muscle. The endocardium is a
further layer of flattened epithelial cells and connective tissue.

Cardiac muscle is myogenic. This means that unlike skeletal muscle which
requires either conscious or reflex nervous stimuli, cardiac muscle is
self-exciting. The rhythmic contractions occur spontaneously, although the
frequency can be changed by nervous or hormonal influences such as exercise
or the perception of danger. A large blood supply is necessary to power the
heart and this is supplied by the left and right coronary arteries, which
branch off from the aorta.

The cardiac cycle

Every beat of the heart involves a sequence of events called the cardiac
cycle. This consists of three major stages: the atrial systole, the
ventricular systole and the complete cardiac diastole. The atrial systole
consists of the contraction of the atria and the corresponding influx of
blood in to the ventricles. Once the blood has fully left the atria
atrioventricular valves, which are situated between the atria and
ventricular chambers, close. This prevents any backflow into the atria. It
is the sound of the valves closing which produces the familiar beating
sounds of the heart.

The ventricular systole consists of the contraction of the ventricles and
flow of blood in to the atria. Again, once all the blood has left, the
pulmonary and aortic semilunar valves close. Finally complete cardiac
diastole involves the relaxation of the atria and ventricles in preparation
for new blood to enter the heart.

Regulation of the cardiac cycle

The heart is self-regulating, the rhythmic sequence of contractions is
coordinated by the sinoatrial node and atrioventricular nodes. The
sinoatrial node, often known as the pacemaker, is located in the upper wall
of the right atrium and is responsible for the wave of electrical
stimulation (See action potential) that initiates the atria to contract.
Once the wave reaches the atrioventricular node, situated in the wall
between ventricular chambers, it is conducted through the bundles of His and
causes contraction of the ventricles. The time taken for the wave to reach
this node from the sinoatrial nerve creates a delay between contraction of
the two chambers and ensures that each contraction is coordinated
simultaneously throughout all of the heart.

Heartbeat

Smaller animals have faster heartbeat. This is evident within a species as
well, as the young beat their hearts faster than the adults. Gray Whale
beats 9 times per minute, Harbour Seal 10 when diving, 140 when on land,
elephant 25, human 70, sparrow 500, shrew 600, and hummingbird 1,200 when hovering.

Heart, is also the name of a Pop Group

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