Consider statements about the cytoskeleton: 1. Microtubules are made of tubulin. 2. Microfilaments are made of actin. 3. Intermediate filaments are made of cellulose. Which are correct? MCQ with Answer and Explanation

Explanation:
The heart is a muscular pump that circulates blood through the cardiovascular system, delivering oxygen and nutrients to tissues and removing waste. The lungs are the primary organs of the respiratory system, where gas exchange occurs: oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be exhaled. The kidneys are the primary excretory organs, filtering blood to remove metabolic waste, excess water, and ions, producing urine. All statements are correct.

Explanation:
The counter-current mechanism involves the Loop of Henle and the vasa recta (blood capillaries surrounding the loop). The flow of filtrate in the two limbs of the Loop of Henle is in opposite directions, and the flow of blood in the two limbs of vasa recta is also counter-current. This mechanism helps maintain a high osmolarity gradient in the medullary interstitium, essential for concentrating urine.

Explanation:
Vitamins are classified by their solubility. Vitamins A, D, E, and K are fat-soluble, meaning they are absorbed with dietary fats and can be stored in the body's liver and fatty tissues. Vitamins B-complex and C are water-soluble, meaning they dissolve in water and are generally not stored in significant amounts. Vitamin D (cholecalciferol) is unique because it can be synthesized endogenously in the skin when exposed to ultraviolet B (UVB) radiation from sunlight. All statements are correct.