Biology Today - May 2018

the cuff is then slowly released, causing the pressure in the cuff and on the artery to decrease. When cuff pressure has decreased to a
value just below the systolic pressure, the artery opens slightly and allows blood flow for a brief time at the peak of systole.

During this interval, the blood flow through the partially compressed artery occurs at a very high velocity because of the small
opening and the large pressure difference across the opening. The high-velocity blood flow is turbulent and, therefore, produces
vibrations called Korotkoff’s sounds that can be heard through the stethoscope. Thus, the pressure at which sounds are first heard
as the cuff pressure decreases is identified as the systolic blood pressure. As the pressure in the cuff decreases further, the duration
of blood flow through the artery in each cycle becomes longer. When the cuff pressure reaches the diastolic blood pressure, all sound
stops because flow is now continuous and non-turbulent through the open artery. Therefore, diastolic pressure is identified as the
cuff pressure at which sounds disappear.

The sounds heard during measurement of blood pressure are not the same as the heart sounds described earlier, which are due to
closing of cardiac valves.

BLOOD CIRCULATION

In human, there are two circuits of blood circulation for greater efficiency and to completely prevent the mixing of oxygenated and
deoxygenated blood. Usually, it is called double circulation that is defined as the passage of same blood twice in the heart through
separate pathways for completing one cycle. It consists of pulmonary circulation and systemic circulation.

Right atrium

Right ventricle

Systemic veins

Aorta and branches

Systemic arteries

Left atrium

Left ventricle

Oxygen rich,

CO 2 poor blood

Capillary bed of all body tissues

where gas exchange occurs

Oxygen poor,

CO 2 rich blood

Pulmonary artery carries deoxygenated

blood from heart to lungs.

Vena cavae receive deoxygenated

blood from all veins of systemic

circulation and open into right atrium.

Pulmonary vein supplies

oxygenated blood to the heart.

Pulmonary circuit: The flow of deoxygenated blood

from the right ventricle to the lungs and the return of

oxygenated blood from the lungs to the left atrium.

Fig.: Double circulation

Capillary bed of lungs where gas exchange occurs,

and deoxygenated blood becomes oxygenated.

Systemic circuit: It involves flow of oxygenated blood from

the left ventricles to all parts of the body and deoxygenated

blood from various body parts to the right atrium.

• A portal system is named after the organ to which it carries blood. The vertebrates possess two or three portal systems: Hepatic
  portal, renal portal and hypophysial portal systems.

Coronary circulation

The flow of oxygenated blood from the

ascending aorta to the heart muscle and the

return of deoxygenated blood from the heart

muscle to the right atrium is called coronary

(cardiac) circulation. The coronary veins bring

deoxygenated blood to the coronary sinus.

Hepatic portal circulation

It is flow of deoxygenated blood from

the digestive organs to the liver before

returning to the heart. Excess glucose

present in absorbed food is converted into

glycogen in liver, which is stored for later

use, during deficiency of food.

Hypophysial portal circulation

This portal system consists of two hypophysial

portal veins which carry blood from the

hypothalamus of the brain to the anterior lobe

of the pituitary gland (hypophysis). This portal

system enables the hormones of hypothalamus

to reach the anterior lobe of the pituitary gland.

Circulation through special regions

The advantage of double circulation is that the blood can be sent to the lungs to pick up oxygen and then returned to the
heart to be pumped again before travelling around the body. The blood therefore is pumped through the capillary bed (which slows
it down and reduces its pressure) then receives another pump before it enters another capillary bed. Double circulatory systems are
therefore, high pressure system. In this type of circulation there is no mixing of the oxygen rich blood and oxygen poor blood in the
heart. Oxygenated blood carries more oxygen to different body parts as well as more CO 2 is carried with deoxygenated blood for the
removal through lungs.