Page 7 - class-XI-ch.8-Lesson notes

2. Structure of the Heart: The heart is divided by a septum into two sides, left and right; each side of the heart is
further sub-divided into two chambers. The upper chambers are called atrium or auricles. The lower chambers
are called ventricles so; there are four chambers in the heart—left auricle, right auricle, left ventricle and right
ventricle. The walls of auricles are thinner in comparison to those of ventricles. Right auricle communicates
with right ventricle, whereas left auricle communicates with left ventricle. Blood is controlled by various valves.

3. Function of the Heart: The circulation of blood throughout the body mainly depends upon the heart, the
course of the blood from the left ventricle through arteries, arterioles and capillaries, and then its return to the
right auricle by veins is called systemic circulation. The course of blood from the right ventricle through the
lungs to the left auricle is called pulmonary circulation. Systemic circulation has greater force and pulmonary
circulation has lesser force.

(a) Systemic Circulation: In systemic circulation, blood comes out from the left ventricle of the heart through aorta.
Then, it passes through small arteries which carry it to various parts of the body. The arteries are further
divided and sub-divided into arterioles to supply blood. These have narrow Passages and thus, resist the flow of
blood. These perform two functions, viz.) Maintenance of arterial blood pressure and regulation of the flow of
blood into capillaries, these capillaries have very thin walls so that exchange can take place between the plasma
and the interstitial fluid. Then, capillaries unite and form venues which in turn, become veins and eventually
become superior and inferior vena cava (see figure below).
(b) Pulmonary Circulation: From the right auricle, blood passes into the right ventricle. When right ventricle
contracts, the blood goes into pulmonary artery. This is divided to carry the blood to the right and left lungs.
The blood is resisted very little in lungs. In lungs, each artery breaks up into smaller arteries, then into arterioles
and lastly into pulmonary capillaries. Blood gets enriched with 02 there and leaves behind CO2. Then pulmonary
capillaries unite until veins are formed. Then blood comes back to the heart by four pulmonary veins which
release the purified blood into left auricle. Again, the blood passes into left ventricle from where it is sent into
aorta for systemic circulation again. In this way, blood circulation takes place throughout the body.

Arteries:

Contain blood under high pressure and are, therefore, elastic, muscular and thick-walled. The structure and
relative thickness vary with the size of artery. The great extensibility of the arteries enables them to receive additional
amount of blood forced into them during each contraction of the heart. If these arteries were rigid, the systolic blood
pressure would be markedly increased. These arteries do not collapse when they are empty only the muscular coat
contracts to some extent in the region of opening.

Classification of Arteries:

Arteries may be divided into three classes.
(a) Elastic Arteries: These arteries include those large arteries leading directly from the heart (the aorta and
pulmonary artery). These are also called conducting arteries because they conduct blood from the heart to the
muscular arteries. These are very large and may be up to 3 cm in diameter. These have more elasticity to bear
the shock of contraction of heart.
(b) Muscular Arteries: These arteries are of medium size. These are also called distributing arteries because these
distribute the blood to the various organs.

(c) Arterioles: These are also muscular arteries but these are much smaller in size. They maintain the blood pressure
of the body in general.