Blood is a special connective tissue consisting of fluid matrix, plasma and formed elements.
- Plasma: Plasma is a straw coloured, viscous fluid constituting nearly 55% of blood.
- Formed elements: RBCs, WBCs and platelets constitute the formed elements.
The circulatory system of humans consists of a muscular pumping organ called heart, a network of vessels and a fluid i.e. blood.
Heart has two atria and two ventricles. Cardiac musculature is auto-excitable. Sino-atrial Node (SAN) generates the maximum number of action potentials per minute and therefore sets the pace of activities of the heart. Hence, it is known as the pacemaker. The action potential causes the atria and the ventricles to undergo contraction (systole) followed by their relaxation (diastole). The systole forces the blood out of atria to the ventricles and to the pulmonary artery and the aorta. About 70 mL of blood is pumped out by each ventricle during a cardiac cycle and it is known as the stroke or beat volume.
We have a complete double circulation i.e. two circulatory pathways, namely, pulmonary and systemic are present. The pulmonary circulation starts with the pumping of deoxygenated blood by the right ventricle which is carried out to the lungs where it is oxygenated and returned to the left atrium. The systemic circulation starts with the pumping of oxygenated blood by the left ventricle to the aorta which is carried to all the body tissues and the deoxygenated blood from there is collected by the veins and returned to the right atrium