Why do we consider blood as a connective tissue?

1. Blood:

(i) Blood is a fluid connective tissue in the body.

(ii) Blood functions as a vehicle for the transport of gases, nutrients, waste products, cells and hormones throughout the body.

2. Formed elements:

(i) Blood is composed of a fluid matrix, plasma in which formed elements are suspended.

(ii) Red blood cells (RBCs, erythrocytes), white blood cells (WBCs; leukocytes) and platelets (thrombocytes) constitute the formed elements.

(iii) The process by which blood cells are formed is called haematopoiesis.

(iv) RBCs of humans are circular (round), biconcave and non-nucleated.

(v) RBCs are formed in red bone marrow.

(vi) The life-span of mammalian RBCs is about $120$ days ($4$ months).

(vii) Breakdown of RBCs occurs in the spleen, bone marrow and liver.

(viii) WBCs (leucocytes) are of five types: neutrophils, eosinophils, basophils, monocytes and lymphocytes.

(ix) Monocytes are the largest WBCs with eccentrically placed kidney-shaped nuclei.

(x) Thrombocytes or platelets are formed by the fragmentation of ‘megakaryocytes’ and contain substance that promotes blood clotting.

3. Blood group:

(i) Human blood is grouped into A, B, AB and O systems based on the presence or absence of two surface antigens, A, B on the RBCs.

(ii) Blood group AB is called universal recipient and blood group O is called universal donor.

(iii) Rh blood grouping is done based on the presence or absence of another antigen called Rhesus factor (Rh) on the surface of RBCs.

4. Coagulation of blood:

(i) When a blood vessel is ruptured, the blood loss is prevented by a mechanism called blood clotting or blood coagulation.

(ii) Blood clotting involves sequence of events resulting in the formation of thromboplastin, thrombin, fibrin and clot.

(iii) Anticoagulants are the substances that inhibit the process of clotting.

(iv) Some anticoagulants are heparin, sodium citrates, sodium oxalates and hirudin.

5. Lymph:

(i) The spaces between cells in the tissues contain a fluid derived from blood called tissue fluid.

(ii) Lymph can be defined as blood minus RBCs, and it is a clear, colourless liquid with a composition similar to blood plasma.

6. Circulatory pathways:

(i) The circulatory patterns are of two types: open and closed.

(ii) All vertebrates and a few invertebrates have a closed circulatory system.

(iii) Our circulatory system consists of the heart, arterial system, venous system and lymphatic system.

7. Human circulatory system:

(i) The human heart is situated in the thoracic cavity in the mediastinum, the area between the lungs.

(ii) Membrane surrounding the heart is pericardium or pericardial sac.

(iii) The heart wall is composed of three layers, epicardium (visceral pericardium), myocardium and endocardium.

(iv) Heart of man has four chambers with two upper thin-walled atria (sing, atrium) and two lower thick-walled ventricles.

(v) The right atrioventricular valve is called tricuspid valve and the left atrioventricular valve is therefore called bicuspid or mitral valve.

(vi) Sinoatrial node (SAN) generates the maximum number of action potentials per minute ($70-75$/min) and therefore, it sets the pace of the activities of the heart, hence called the pacemaker.

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

(viii) The systole forces the blood to move from the atria to the ventricles and to the pulmonary artery and the aorta.

8. Cardiac cycle:

(i) The cardiac cycle is formed by sequential events of systole and diastole in the heart which is cyclically repeated and is called the cardiac cycle.

(ii) A healthy person shows $72-75$ such cycles per minute.

9. Electrocardiograph:

(i) Electrocardiograph (ECG) is the recording of the electrical changes that accompany the cardiac cycle.

(ii) Electrocardiogram is the instrument used to record electrical activities of heart muscles.

10. Blood pressure:

(i) Blood pressure (BP) is a measure of the force blood exerts against blood vessel walls.

(ii) The instrument used to measure BP is the sphygmomanometer.

(iii) Normal systolic BP in a healthy adult man is $120$ mm Hg and diastolic BP is $80$ mm Hg.

(iv) Pulse is the alternate expansion and elastic recoil of an artery with each systole.

(v) The normal pulse rate ranges $70-90$ per minute.

11. Double circulation:

(i) There are two types of heart circuit in vertebrates: single heart circuit and double heart circuit.

(ii) Single heart circuit is found in fishes having two-chambered or venous heart.

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

(iv) 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.

(v) 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.

(vi) Portal veins are veins that begin in a primary capillary network, extend to some distance and end in a secondary capillary network.