45% of blood contains formed elements while 55% of blood is plasma. The formed elements are erythrocytes or red blood cells, leukocytes or white blood cells and thrombocytes or platelets.

In fishes, heart pumps out deoxygenated blood which is oxygenated by gills and is supplied to body parts where deoxygenated blood is returned to heart. This type of circulation is known as single circulation.

Cardiac output is product of stroke volume and heart rate. It is total amount of blood pumped by each ventricle per minute. The average cardiac output of a healthy adult is 5000ml.

Thrombocytes or blood platelets are formed from megakaryocytes, which are special cells present in bone marrow. Thrombocytes are cell fragments which are involved in clotting of blood.

The P-wave represents electrical excitation of depolarisation of atria which leads to contraction of both atria. The T-wave represents repolarisation of ventricles.

During atrial systole, both atria contract simultaneously while ventricles remain relaxed. It occurs due to action potential generated by SAN. The tricuspid and bicuspid valves open.

The human heart is divided into four chambers- two atria and two ventricles. They pump blood to different parts of body. The atrio-ventricular septum separates atria and ventricles. It is made of fibrous tissue.

RBCs are formed in red bone marrow in adults. RBCs are devoid of nucleus in most of mammals and are biconcave in shape. They have a red coloured iron-containing pigment called haemoglobin.

The placenta prevents mixing of blood of fetus with blood of mor. The placenta provides oxygen and nutrients to fetus and removes carbon dioxide and waste products.

Blood can be classified on basis of several factors. Apart from ABO blood grouping system, Rh blood grouping is widely used. Around 80% of people are Rh positive (Rh+ve).