→ Vertebrates circulate blood, a fluid connective tissue in their body, to transport essential substances to the cells and to carry waste substances from there. Another fluid, lymph (tissue fluid) is also used for the transport of certain substances.

→ Blood comprises a fluid matrix, plasma and the formed elements.

→ Red blood cells (RBCs, erythrocytes), White blood cells (WBCs, leucocytes) and platelets (thrombocytes) constitute the formed elements.

→ Blood of humans is grouped into A, B, AB and O systems based on the presence or absence of 2 surface antigens, A, B on the RBCs. Another blood grouping is also done based on the presence or absence of another antigen called Rhesus factor (Rh) on the surface of RBCs.

→ The spaces between cells in the tissues contain a fluid derived from the blood called tissue fluid. This fluid called lymph is almost similar to blood except for the protein content and the formed elements.

→ All vertebrates and a few invertebrates have a closed circulatory system. Our circulatory system consists of the muscular pumping organ, heart, a network of vessels and the fluid, blood.

→ The heart has two atria and two ventricles.

→ Cardiac musculature is auto-excitable.

→ Sino-atrial node (SAN) generates a maximum number of action potentials per minute (70-75/min) and therefore it sets the pace of the activities of the heart. Hence it is called the pacemaker.

→ The action potential causes the atria and then the ventricles to undergo contraction (systole) followed by their relaxation (diastole).

→ The systole forces the blood to move from the atria to the ventricles and to the pulmonary artery and aorta.

→ The sequential event in the heart which is cyclically respected is called the cardiac cycle.

→ A healthy person shows 72 such cycles per minute.

→ About 70ml of blood is pumped out by each ventricle during a cardiac cycle and it is called the stroke or beat volume.

→ The volume of blood pumped out by each ventricle of the heart per minute is called the cardiac output and it is equal to the product of stroke volume and heart rate (approx 5 litres).

→ The electrical activity of the heart can be recorded from the body surface by using an electrocardiograph and the recording is called an electrocardiogram (ECG) which is of clinical importance.

→ We have a complete double circulation i.e., two circulatory pathways namely pulmonary and systemic are present.

→ The pulmonary circulation starts by the pumping of deoxygenated blood by the right ventricle which is carried to the lungs where it is oxygenated and returned to the left atrium. The systematic 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.

→ Though the heart is auto-excitable, its functions can be moderated by neural and hormonal mechanisms.

→ Serum: Plasma without the clotting factors is called serum.

→ Haemoglobin: They have a red coloured, iron-containing complex protein called haemoglobin.

→ Thrombocytes: Platelets also called thrombocytes, are cell fragments produced from megakaryocytes.

→ Universal donors: Group ‘O’ blood can be donated to a person with any other blood group and hence ‘O’ group individuals are called ‘Universal donors’.

→ Universal recipients: A person with an ‘AB’ group can accept blood from persons with AB as well as the other group of blood. Therefore such persons are called ‘Universal recipients’.

→ Rh-positive and Rh-negative: Such individuals are called Rh-positive (Rh+ve) and those in whom this antigen is absent are called Rh negative (Rh-ve).

→ Erythroblastosis Foetalis: In the case of her subsequent pregnancies, the Rh antibodies from the mother (Rh-ve) can leak into the foetus or could cause severe anaemia and jaundice to the baby. This condition is called erythroblastosis foetal.

→ Prothrombin: Thrombins, in turn, are formed from another inactive substance present in the plasma called prothrombin.

→ Lymphatic system: An elaborate network of vessels called the lymphatic system collect this fluid and drains it back to the major veins.

→ Sinuses: Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.

→ Atria and ventricles: Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles.

→ Nodal tissue: A specialised cardiac musculature called the nodal tissue is also distributed in the heart.

→ Sino-atrial node: A patch of nodal tissue is present in the right upper comer of the right atrium called the sino-atrial node.

→ Atrio ventricular node: Another mass of this tissue is seen in the lower left comer of the right atrium close to the atrioventricular septum called the atrioventricular node.

→ Purkinje fibres: These branches give rise to minute fibres throughout the ventricular musculature of the respective sides and are called Purkinje fibres.

→ Pacemaker: The SAN can generate a maximum number of action potentials.e., 70-75 per minute and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the Pacemaker.

→ Stroke volume: During a cardiac cycle, each ventricle pumps out approximately 70ml of blood which is called the stroke volume.

→ Hepatic portal system: A unique vascular connection exists between the digestive tract and liver called the hepatic portal system.

→ Myogenic: Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic.

→ Congestive heart failure: Congestion of the lungs is one of the main symptoms of this disease.