The pressure of flow of blood in the aorta and its main branches is defined as blood pressure. The heart has to develop a high pressure so that blood can be pumped through the fine capillaries.
This blood pressure varies from one part of body to another. In the contraction or systolic phase, it is equal to that exerted by a column of 120 mm of mercury (Hg). In the relaxation or diastolic phase it is about 80 mm of mercury.
It is typically 120/80 in the artery of a resting adult person. It varies from person to person and is affected by age, sex, heredity, physical and emotional states and other factors. Blood pressure is measured by an instrument called sphygmomanometer.
There are three routs for the circulation of blood in man. They are the systematic route, the pulmonary route and the portal route. Systematic circulation begins with a very large aorta which arises from the left ventricle and passes through arteries, capillaries, small veins, and subsequently to large veins. All these veins of systemic circulation pour their venous blood into superior and inferior vena cava which empty into the right atrium.
Pulmonary circulation arises from the right ventricle and takes the venous blood to the lungs for aeration. The veins from various organs of digestive tract like stomach, pancreas, small intestine and the large intestine unite to from hepatic portal vein which enters the liver. In the liver, the blood passes through a network of capillaries and is finally carried to the inferior vena cava by the hepatic vein.