(a) There is a difference in the rate of breathing between the aquatic organisms and terrestrial organisms because the amount of dissolved oxygen in the water in much less than the amount on land, so they have to breathe more in order to get more oxygen. The breathing rate of aquatic animals is faster than that of terrestrial animals because the amount of oxygen available in dissolved form is extremely low in concentration as compared to oxygen available in the air and the aquatic animals acquire oxygen that is dissolved in water. Oxygen is the necessary element for the survival of all organisms on earth except anaerobes.
OR
(a) The human excretory system comprises of the following structures:
(i) 2 Kidneys
(ii) 2 Ureters
(iii) 1 Urinary bladder
(iv) 1 Urethra
(b) Urine formation is carried out by the excretory system. The nephrons play a major role in this. They carry out urine formation in various steps like glomerular filtration, selective reabsorption and tubular secretion.
(i) Each nephron has a glomerulus, the site of blood filtration. The glomerulus is a network of capillaries surrounded by a cup like structure, the glomerular capsule (or Bowman's capsule). As blood flows through the glomerulus, blood pressure pushes water and solutes from the capillaries into the capsule through a filtration membrane. This glomerular filtration begins the urine formation process.
(ii) Inside the glomerulus, blood pressure pushes fluid from capillaries into the glomerular capsule through a specialized layer of cells. In this layer, the filtration membrane, allows water and small solutes to pass but blocks blood cells and large proteins. Those components remain in the bloodstream. The filtrate flows from the glomerular capsule further into the nephron.
(iii) The resulting filtrate contains waste, but also other substances the body needs such as essential ions, glucose, amino acids and smaller proteins. When the filtrate exits the glomerulus, it flows into a duct in the nephron called the renal tubule.
(iv) Waste ions and hydrogen ions pass from the capillaries into the renal tubule. This process is called secretion. The secreted ions combine with the remaining filtrate and become urine. The urine flows out of the nephron tubule into a collecting duct. It passes out of the kidney and down to the bladder. Urine is a liquid or water waste product that passes through your urinary tract and is expelled from the body through the urethra. It contains 90−95% water.