(a) Excretion is a necessary process of our body as it is necessary to eliminate certain waste and toxic products, such as carbon dioxide, urea etc. When these toxic materials are not removed from the body, they get mixed with blood and can cause poisoning that slows down critical chemical reactions and damage the cells of the body. Hence it is necessary to remove such poisonous waste materials from our body.
(b) The Bowman's capsule and the proximal convoluted part of all the nephrons lie in the cortex of kidneys giving it a 'dotted' appearance.
(c) During summer when the surrounding temperature is high, we lose a considerable part of water through perspiration (i.e., in the form of sweat, direct evaporation, etc.). This means that the kidneys have to reabsorb more water from the urine. Due to this reason, we urinate fewer times in summer than in winter and the urine passed is generally thicker.
(d) Henle's loops and collecting ducts lie in the medulla of kidneys giving it a 'striped' appearance.