Parathyroid hormone and calcitonin results in the fluctuation of $\mathrm{Ca}^{2+}$ level. Parathyroid Hormone (PTH) increases the $\mathrm{Ca}^{2+}$ levels in the blood. PTH acts on the bones and stimulates the process of bone resorption (dissolution/demineralisation). PTH also stimulates reabsorption of $\mathrm{Ca}^{2+}$ by the renal tubules and increases $\mathrm{Ca}^{2+}$ absorption from the digested food. Calcitonin is a 32-amino acid linear polypeptide hormone, that is produced in humans primarily by the parafollicular cells of the thyroid. It acts by reducing blood calcium $\left(\mathrm{Ca}^{2+}\right)$, levels opposing the effect of Parathyroid Hormone (PTH).

Rahul has gained weight because his muscle shape has changed. Regular exercise increase the body muscle as there is an enlargement of muscles due to increase in the amount of sarcoplasm and mitochondria and the strength he to developed led him gain the mass and size of body muscle and reduction in fat content.

(a) Due, to continuous exercise her muscles got fatigues because of the accumulation of lactic acid within skeletal muscles. Pain is also oftenly experienced in the fatigued muscles.

(b) Her breathing rate changes from normal to high as during exercise, her body muscle require more oxygen for the ATP production, than the normal value, hence her breathing enhances, to lake most oxygen from the atmosphere.

Gout is a disease, caused due to defect in purine metabolism. It causes accumulation of excess of uric acid and its crystals in the joints. The level of uric acid and crystals of its salts get raised in blood causing their accumulation in the joints causes gouty arthritis. The excess of urates in blood can also lead to the formation stones in the kidneys.

ATP (Adenosin Triphosphate) is the source of energy for muscle contraction. The head of each myosin molecule contains an enzyme called myosin ATPase.

In the presence of this enzyme along with $\mathrm{Ca}^{2+}$ then, and $\mathrm{Mg}^{2+}$ ions the ATP molecule breaks down into ADP and inorganic phosphate, thus releasing energy in the head of myosin.

$$\text { ATP } \longrightarrow \text { ADP }+P_i+\text { Energy }$$

Energy from ATP causes energised myosin to cross bridges and to bind with actin and in this way initiates muscle contraction.