Download The heart is a muscular organ responsible for pumping blood

Heart
The heart is a muscular organ responsible for pumping blood through the blood
vessels by repeated, rhythmic contractions, or a similar structure in the annelids, mollusks,
and arthropods. The term cardiac (as in cardiology) means "related to the heart" and comes
from the Greek καρδία, kardia, for "heart." The heart is composed of cardiac muscle, an
involuntary muscle tissue which is found only within this organ. The average human heart
beating at 72 BPM, will beat approximately 2.5 billion times during a lifetime of 66 years.
In the human body, the heart is usually situated in the middle of the thorax with the largest
part of the heart slightly offset to the left (although sometimes it is on the right, see
dextrocardia), underneath the breastbone (see diagrams). The heart is usually felt to be on the
left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The left
lung is smaller than the right lung because the heart occupies more of the left hemithorax. The
heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. The
pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective
tissue; and a double membrane structure containing a serous fluid to reduce friction during
heart contractions (the serous pericardium). The mediastinum, a subdivision of the thoracic
cavity, is the name of the heart cavity.
The function of the right side of the heart (see right heart) is to collect de-oxygenated blood,
in the right atrium, from the body and pump it, via the right ventricle, into the lungs
(pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas
exchange). This happens through the passive process of diffusion. The left side (see left heart)
collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood
moves to the left ventricle which pumps it out to the body. On both sides, the lower ventricles
are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is
thicker than the wall surrounding the right ventricle due to the higher force needed to pump
the blood through the systemic circulation.
The pulmonary arteries carry blood from the heart to the lungs. They are the only arteries
(other than umbilical arteries in the fetus) that carry deoxygenated blood.
In the human heart, the pulmonary trunk (pulmonary artery or main pulmonary artery)
begins at the base of the right ventricle. It is short and wide - approximately 5 cm (2 inches) in
length and 3 cm (1.2 inches) in diameter. It then branches into two pulmonary arteries (left
and right), which deliver deoxygenated blood to the corresponding lung.
Systemic circulation
Systemic circulation is the portion of the cardiovascular system which carries
oxygenated blood away from the heart, to the body, and returns deoxygenated
blood back to the heart.
Arteries always take blood away from the heart, regardless of their oxygenation, and veins
always bring blood back. In general, arteries bring oxygenated blood to the tissues; veins
bring deoxygenated blood back to the heart. In the case of the pulmonary vessels, however,
the oxygenation is reversed: the pulmonary artery takes deoxygenated blood from the heart to
the lungs, and oxygenated blood is pumped back through the pulmonary vein to the heart. As
blood circulates through the body, oxygen and nutrients diffuse from the blood into cells
surrounding the capillaries, and carbon dioxide diffuses into the blood from the capillary cells.
The release of oxygen from red blood cells or erythrocytes is regulated in mammals. It
increases with an increase of carbon dioxide in tissues, an increase in temperature, or a
decrease in pH. Such characteristics are exhibited by tissues undergoing high metabolism, as
they require increased levels of oxygen.