Biology Today - May 2018

• The mammalian heart is derived from embryonic mesoderm that differentiates after gastrulation into mesothelium,
  endothelium, and myocardium. Myocardium develops into heart muscle.

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External Structure
Human heart is 4 chambered, consisting of two
atria and two ventricles. The left and right atria are
separated externally by a shallow vertical interatrial
groove. The atria are demarcated externally
from the ventricles by an oblique groove called
atrioventricular sulcus. There are also present
coronary sulcus, anterior interventricular
sulcus and posterior interventricular sulcus.
These have coronary arteries, through which the heart
receives blood.

The left atrium is smaller than the right atrium. Each
atrium has an appendage called an auricle which
increases its surface area. The superior vena cava,
inferior vena cava and coronary sinus open into right
atrium. The left atrium receives four openings of
pulmonary veins.
Ventricles are thick walled and the left ventricle is longer and narrower than the right ventricle. Its walls are about three times
thicker than the right ventricle. The pulmonary trunk arises from the right ventricle. It divides into left and right pulmonary arteries
that carry deoxygenated blood to the lungs. The aorta arises from the left ventricle.

Internal Structure
Atrium
The two thin walled atria are separated by interatrial septum. The right atrium receives blood from superior vena cava, inferior
vena cava and coronary sinus. The superior vena cava carries blood from upper body and the inferior vena cava carries blood
from the lower body region. Coronary sinus carries blood from the heart itself.

The right atrium receives deoxygenated blood while the left atrium receives oxygenated blood from the lungs through two
pairs of pulmonary veins. An oval depression known as fossa ovalis is present in the right atrium near interatrial septum. It marks
the position of an opening between two atria in the fetus, i.e., foramen ovale but in the adult it persists only as a depression.

Ventricle
The two ventricles are separated from each other by a thick, curved partition, the interventricular septum. The inner surface of the
ventricles is raised into a network of low, muscular ridge called the columnae carneae or trabeculae carneae, and a few large,
conical, muscular elevations termed the papillary muscles (musculi papillares). A prominent muscular trabeculum, called the
moderator band, extends from the interventricular septum to the anterior papillary muscle in the right ventricle.
The left and right pulmonary arteries carry deoxygenated blood to the lungs. The aorta arising from the left ventricle is divisible into the
ascending aorta, arch of aorta and descending aorta. The right and left coronary arteries arise from the ascending aorta.
The arch of the aorta (also called aortic arch) gives rise to the brachiocephalic artery (innominate artery), left common
carotid artery and left subclavian artery. The descending aorta runs through the thorax and abdomen and hence it is divisible
into thoracic and abdominal parts.

The pulmonary trunk is connected with the aorta by the ligamentum arteriosum that represents the remnant of an embryonic
connection between the pulmonary trunk and aorta. In embryo the ligamentum arteriosum is called ductus arteriosus.

Coronary arteries arise from the ascending aorta and supply blood to the heart. These arteries exit from behind the aortic valve
cusps in the very first part of the aorta and lead to a branching network of small arteries, arterioles, capillaries, venules and veins
similar to those in other organs. Most of the cardiac veins drain into a single large vein, the coronary sinus, which empties into
the right atrium.

Superior vena

cava

Right pulmonary

artery

Right pulmonary

veins

Right coronary

artery

Right ventricle

Anterior cardiac

vein

Small cardiac vein

Inferior vena cava

Right auricle

Aorta

Ligament arteriosum

Left pulmonary artery

Pulmonary trunk

Left pulmonary veins

Anterior interventricular

artery (in anterior

interventricular sulcus)

Great cardiac vein

Left coronary artery

Left ventricle

Apex of the heart

Left auricle

Fig.: Human heart in front view