THE HEART

The structure of the mammalian heart

The heart is a muscular organ about the size of a closed fist that functions as the body’scirculatory pump. It takes in deoxygenated blood through the veins and delivers it to the lungsfor oxygenation before pumping it into the various arteries. The heart is located in the thoraciccavity between the two lungs.

The external structure of the mammalian heart is as shown in the labelled diagram below:

The mammalian heart is broader at the top and narrower at the bottom. It is enclosed by a double layer of tough and elastic membranes called pericardium. These membranes prevent the heart from ever-expanding when beating very fast. Also the pericardium secrets a fluid which enables the membranes to move smoothly against each other.

The walls of the ventricles are thicker than those of the auricles because the ventricles pumpblood a greater distance than the auricles. Auricles pump blood to the ventricles while theventricles pump blood to the other parts of the body.

The left ventricle is thicker than the right ventricle because the right ventricle pumps blood to the lungs while the left ventricle pumps blood to the rest of the body parts.The heart consists of four chambers, right and left atria and right and left ventricles.

 The functions of each part and the associated structures are as follows:

1.                  The right atrium links to the right ventricle by the tricuspid valve. This valve prevents backflow of the blood into the atrium above, when the ventricle contracts.

2.                  The left atrium links to the left ventricle by the bicuspid valve. This valve also prevents backflow of the blood into the atrium above, when the ventricle contracts.

3.                  Semi-lunar (pocket) valves are found in the blood vessels leaving the heart (pulmonary artery and aorta). They only allow exit of blood from the heart through these vessels following ventricular contractions.

4.                  Ventricles have thicker muscular walls than atria. When each atrium contracts, it only needs to propel the blood a short distance into each ventricle while ventricles pump blood to distant body parts.

5.                  The left ventricle has even thicker muscular walls than the right ventricle. The left ventricle needs a more powerful contraction to propel blood to the systemic circulation (all of the body apart from the lungs). The right ventricle propels blood to the nearby lungs. So, the contraction does not need to be so powerful.

INTERNAL STRUCTURE OF THE MAMMALIAN HEART

The heart has several valves. And these valves have flaps that ensure that blood flows in one direction only.

These valves include the following:

1.                  The tricuspid valve; found between the right auricle and right ventricle

2.                  The bicuspid valve: found between left auricle and left ventricle

3.                  Semi-lunar valves which are located at the bases of the pulmonary artery and aorta to preventblood from flowing back into the ventricles.

These valves will close if the blood flows back. The valves are held in place by tendons which prevent the flaps from turning inside out.The right and left sides of the heart are separated by septum which is a thick muscular wall which prevents mixing of oxygenated and deoxygenated blood.

Functions of parts of the mammalian heart

Part of the heart	Function
Aorta	The largest artery in the body; it conducts freshly oxygenated blood from the heart to the tissues.
Superior venacava	Large vein that brings deoxygenated blood from the upper parts of the body to the right atrium
Inferior venacava	Large vein that brings deoxygenated blood from lower regions of the body to right atrium
Pulmonary artery	Carries deoxygenated blood from the right ventricle to the lungs
Pulmonary vein	Blood vessel that carries oxygenated blood from the lungs to the left atrium
Right atrium	This chamber of the heart receives deoxygenated blood from the body (from the superior and inferior venacava).
Left atrium	This chamber of the heart receives oxygenated blood from the lungs
Tricuspid valve	Located on the right side of the heart between the right atrium (RA)and right ventricle (RV)
Bicuspid valve	Located on the left side of the heart between the left atrium (LA) and the left ventricle (LV)
Right ventricle	The chamber of the heart that pumps deoxygenated blood to the lungs
Left ventricle	Receives blood from the left atrium and pumps it into the aorta for transport to the body cells
Septum	Divides the right and left chambers of the heart

The Adaptations of the Parts of the Mammalian Heart to their Functions

1.                  The cardiac muscle is adapted to be highly resistant to fatigue.

2.                  The heart has a large number of mitochondria enabling continuous supply of energy to theheart and numerous myoglobins (oxygen storing pigment).

3.                  The presence of the cardiac muscles enables the heart to beat rhythmically.

4.                  The pericardium which surrounds and protects the heart from physical damage.

5.                  Pericardial fluid which prevents friction when the heart beats.

6.                  The outer layer of the pericardium attaches to the breastbone and other structures in the chestcavity and thus helps to hold the heart in place.

7.                  Bicuspid and tricuspid valves between atria and ventricles which prevent the backflow ofblood.

8.                  Septum which prevents the mixing of deoxygenated blood in the right and oxygenated bloodin the left chambers of the heart.

9.                  Its own blood supply for supplying nutrients and removing waste.

10.             The left ventricle has thick muscular wall to pump blood at a higher pressure to the distantbody tissues,

11.             The heart is supplied with the nerves which control the rate of heartbeat depending on thebody requirements.