26.

(a) Write two functions each of:

(i) Amniotic fluid (ii) Human placenta

(b) Briefly explain the procedure followed for haemodialysis.

(c) Explain the origin and conduction of heart beat in man.

(a) i) Amniotic fluid : provides cushioning and protects the foetus from physical injuries and prevents temperature fluctuations and allows the movement of the foetus.

(ii) Human placenta allows the exchange of gases and diffusion of nutrients and immune cells from the mother's blood to the baby’s. It also allows the disposal of waste products from the baby’s blood to that of mother's. It also serves the purpose of respiration.

(b) Removal of waste materials and toxic substance and restoration of normal volume and composition of body fluid by means of an artificial kidney is called haemodialysis. In this process the blood is taken out from an artery of the patient cooled at 0°C mixed with anticoagulant (heparin) and then pumped into the artificial kidney. In this machine blood flows through channels or bubs bounded by cellophane membrane. The membrane is impermeable to macromolecules like plasma proteins but permeable to small molecules like urea, uric acid, creatinine and mineral ions. The membrane separates the blood flowing inside the channels from dialyzing fluid flowing outside the membrane. Microscopic waste particles like urea uric acid and creatinine pass through the cellophane into the solution and the purified blood is returned to the patient’s body. After clearing the considerable waste particles, the blood coming out of the machine is warmed to body temperature mixed with an antiheparin to restore its normal coagulability and returned to a vein of the patient.

c. 

The sinoatrial (SA) node (also referred to as the pacemaker of the heart) contracts generating nerve impulses that travel throughout the heart wall. This causes both atria to contract.
When the impulses from the SA node reach the AV node they are delayed for about a tenth of a second. This delay allows the atria to contract and empty their contents into the ventricles.
The impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.
At the base of the heart the atrioventricular bundles start to divide further into Purkinje fibers. When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta.

Cardiac conduction is the sequence of events that occur when the heart beats. 

Deoxygenated blood enters the right atrium and oxygenated in the left atrium. Atrium contracts and sends blood to the ventricle. The blood in ventricle passes to aorta . The blood is expelled from ventricle and the AV valve gets closed. This closing develop sound termed as ‘lub’. At the end of ventricular systole ventricular diastole starts. With onset of diastole the pressure falls in the ventricles. The high pressure tends to force some blood back towards the ventricles and this closes the semilunar valves. Hence the sound of ‘dub’ is heard.