Platelets are also called thrombocytes that are produced from megakaryocytes (special cells in bone marrow) and lack nuclei. Blood normally contains 1,50,000 - 3,50,000 platelets mm^-3 of blood. They secrete substances involved in coagulation or clotting of blood. The reduction in platelet number can lead to clotting disorders that result in excessive loss of blood from the body.

Stroke volume SV = EDV - ESV
EDV - Amount of blood getting collected in ventricle during diastole.
ESV - Volume of blood remaining in ventricle after contraction.
1. Heart's pumping action maintains balance between cardiac output and venous return, Heart is a double pump.
2. Each side of the heart can fail independently of the other.
3. Increase in EDV can increase SV, On the other hand decrease in venous return and EDV can decrease Sv.
4. Similarly changes ·of EDV and ESV on SV are not independent.

1. Elastic fibres allow the artery to stretch and recoil as blood passes through it.
2. Arteries closer to the heart experience more pressure than arteries further away from the heart.
3. Therefore arteries close to the heart have more elastic fibres in their walls to cope with high pressure of blood coming from the heart.

1. Cornea lacks blood vessels which keeps the eye clear for vision.
2. It receives nutrients by diffusion from the tear fluid and aqueous humour in the eye.
3. There are no blood vessels in cartilage Nutrients diffuse through a dense connective tissue surrounding the cartilage.

There are usually two heart sounds they are
(i) 'lub' sound: It is the 1^st sound produced by the closure of atrio ventricular valves (both tricuspid and bicuspid valves).
(ii) 'dub' sound: It is the 2^nd sound produced by the closure of semilunar valves (both pulmonary aortic valves).
(iii) A 3^rd abnormal sound may be produced with vibrations as "murmuring" under the dis-order of heart.

(i) The colourless fluid tissue circulated within the body is lymph.
(ii) A large number of lymphocytic cell concentrations in various lymphoid organs help to maintain immunity and disposal of cell debris.
(iii) Lacteals of lymphatic system absorb and transport some substances like fatty acids, glycerol, and vitamins A, D, E & K.

Open circulation	Closed circulation
1. No blood vessels.	Blood vessels are present.
2. Blood flows in sinuses.	Blood flows in vessels.
3. No respiratory pigment.	Respiratory pigment present.
4. Eg: insects.	Eg: Earthworms, Molluscans, Echinoderms and Chordates.

Reason : None of these functions can be carried out by RBC. Red Blood Cells (RBC) contain haemoglobin pigment and play a major role in transport of respiratory gases.

If a person diet does not contain iron, the synthesis of haemoglobin is lowered in bone marrow because each haemoglobin consists of 4 iron atoms to accept 4 O2

S.No. Arteries veins 1. Transport oxygenated blood except
pulmonary artery Transport deoxygenated blood except
pulmonary vein 2. Deeply seated Superficially seated 3. Branches of aorta Branches of venacavae 4. Have no valves Abdominal veins have only valves 5. Transports blood away from the heart Transport blood towards the heart 6. Walls are thicker Wall are thinner 7. Blood samples may rarely be taken Blood samples may be usually taken 8. Lumen is narrow Lumen is wider 9. Exhibit systolic pressure Exhibit diastolic pressure