Biology Today - May 2018

Aorta

Left pulmonary artery

Left subclavian artery

Brachiocephalic artery Left common carotid artery

Superior vena cava

Right pulmonary artery

Right pulmonary

veins

Pectinate muscles

(horizontal muscle ridges)

Fossa ovalis

Tricuspid valve

Right ventricle

Chordae tendineae

(held by papillary muscles)

Trabeculae carneae

(irregular muscle ridges)

Inferior vena cava

Right atrium

Pulmonary trunk

Left atrium

Left pulmonary veins

Mitral (bicuspid) valve

Aortic semilunar valve

Pulmonary semilunar valve

Interventricular septum

Endocardium

Myocardium

Epicardium

Left ventricle

Papillary muscles

Fig.: Internal structure of human heart

Heart valves

Location and functions of heart valves have been summarised in the given table.

Table: Valves of the heart

Name Location Function

1. Bicuspid valve or Mitral valve
   (Left AV)

Between left atrium and ventricle

(has two flaps or cusps)

Allows the blood to move from left atrium to left

ventricle

2. Tricuspid valve
   (Right AV)

Between right atrium and

ventricle (has three flaps)

Allows the blood to move from right atrium to right

ventricle

3. Aortic semilunar valve Between aorta and left ventricle Allows unidirectional flow of oxygenated blood from
   left ventricle to aorta and prevents back flow.


4. Pulmonary semilunar valve Between pulmonary artery and
   right ventricle

Allows unidirectional flow of deoxygenated blood from

right ventricle to pulmonary artery and prevents back flow

5. Eustachian valve Right atrium Guards the opening of inferior vena cava


6. Thebasian valve (Coronary valve) Right atrium Guards the opening of coronary sinus

There are no valves at the entrances of the superior and inferior venae cavae (plural of vena cava) into the right atrium and of
the pulmonary veins into the left atrium. However, atrial contraction pumps very little blood back into the veins because atrial
contraction constricts their sites of entry into the atria, greatly increasing the resistance to backflow. Actually, a little blood is
ejected back into the veins and this accounts for the venous pulse that can often be seen in the neck veins when the atria are
contracting.

Functioning of valves

The opening and closing of the AV valves are passive processes resulting from pressure differences across the valves. When the
blood pressure in an atrium is greater than in the corresponding ventricle, the valve is pushed open and blood flows from atrium to
ventricle. In contrast, when a contracting ventricle achieves an internal pressure greater than that in its connected atrium, the AV
valve between them is forced closed. Therefore, blood does not normally move back into the atria and is forced into the pulmonary
trunk from the right ventricle and into the aorta from the left ventricle.