Because blood has to pass into the heart against the gravity so it leakes into interstitial spaces.

In human being haemoglobin is present in the RBC , but in earthworm it is present in plasma. So it attains less surface area to bind to oxygen.

Main blood vessel in which subsidiary venules join to transfer the secretion at the target site. Hepatic portal vein plays vital role in the digestive system the impurities and to detoxify the harmful materials.

Pulmonary vein carries oxygenated blood from lungs to the heart. Other veins in the body carry deoxygenated blood, hence pulmonary vein is different from them.

Platelets are also called thrombocytes, are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains 1,500,00-3,500,00 platelets mm^-3. Platelets can release a variety of substances most of which are involved in the coagulation or clotting of blood. A reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body.

The person suffering from haemophilia lacks clotting factors in blood and as a result, the clotting mechanism is defective. So in case of injury, the person is at a risk of blood loss.

WBCs play an important role in immune system. They engulf and destroy foreign materials entering our body and thus help in warding off disease.

If a patient's ECG reveals an abnormally long delay between P-wave and the QRS deflection, it suggests that there is a delay of condution from the atria to the ventrickes. Hence, the stimulation from SA nodes are conductiong stimuli to the ventricles very slowly.

A fish's heart pumps only deoxygenated blood as it is a two-chambered heart with an atrium and a ventricle. The heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to other body parts, from where deoxygenated blood is returned to the heart.

In electrocardiography, P-Q interval is the time taken by the impulse to travel through atria, AV node and the rest of the conducting tissues. The normal PR interval lasts for about 0.16 s. The ST interval is the representation of time between the end of the spread of impluse through ventricles and its repolarisation.

We can hear with a stethoscope, the two main heart sounds(i.e., lubb and dupp), which repeat rythmically. These sounds result from the closure of the heart valves. The first sound (lubb), which is of longer duration (0.16-0.90 s) and a louder one is created by the closure of the atrioventricular valves immediately after the start of the ventricular systole. The second sound is of shorter duration (0.10 s) and is created by the closure of the semilunar valve at the end of the ventricular systole.

We can hear with a stethoscope, the two main heart sounds(i.e., lubb and dupp), which repeat rythmically. These sounds result from the closure of the heart valves. The first sound (lubb), which is of longer duration (0.16-0.90 s) and a louder one is created by the closure of the atrioventricular valves immediately after the start of the ventricular systole. The second sound is of shorter duration (0.10 s) and is created by the closure of the semilunar valve at the end of the ventricular systole.

When the blood supply to myocardium of the heart is greatly reduced, then the cardiac muscle fibres die. This reduced blood supply can be due to the blockage of the coronary muscles can be thrombus(blood clot formed in the coronary artery).

Atherosclerosis affects the vessels that supply blood to the heart muscle. It is caused by the deposits of calcium, fat, cholesterol and fibrous tissues, which make the lumen of arteries narrower.

Causes of high blood pressure are as follows
(i) A diet full of oily and greasy products, incresse cholesterol level, which can cause thickening of the arteries due to which BP many rise.
(ii) Tobacco smoking also speeds up the heart rate and contracys blood vessels.
(iii) Mental tension is also one of the main causes of hypertension.