CBSE Class 10 Answered
1. The amount of filtration that occurs is known as the glomerular filtration rate or GFR. It is about 120 ml/min (180 l/day). This is an enormous volume but it is important to note that it is only a small fraction (20%) of the total plasma which is delivered to the kidneys in the blood. The volume of plasma in the circulating blood is only about 3 litres, yet we filter 180 litres per day of it. This paradoxical situation is possible because, after filtration, almost all (99%) of the plasma is reabsorbed along the nephron, so can be filtered again and again, almost 60 times a day. And that is why, although the average weight of an adult human being is 70-80 kg, then also it is possible that 180 litres of glomerular filtrate is produced in the body every day.
2. Although the average surface area of an adult in 2.5 m2, still the lungs with surface area 80 m2 can fit into the body. This is because of the spherical and folded compact shape of the lungs and adaptations of the alveoli. The alveoli are flattened with a dense capillary network which aids in efficient gas exchange.