Kidneys help to regulate the volume of fluid and various metabolites waste products in the body.

Normally, kidney filter about 180 L of fluid daily but the volume actually excreted out is only a litre or two a day.

Patients with kidney failures can be saved by dialysis and kidney transplant.

(a) Write the structure and function of Bowman's capsule. [1]

(b) Although kidneys filter a large amount (about 180 L daily) of fluids, still the excretion from the body is only about a litre or two. Why? [1]

(c) What is excretion? Why is it necessary for any living organism? [2]

OR

(c) State two similarities between lungs and kidneys. [2]

From the chapter: 'Each capillary cluster in the kidney is associated with the cup-shaped end of a coiled tube called Bowman's capsule that collects the filtrate.' and 'Some substances in the initial filtrate, such as glucose, amino acids, salts and a major amount of water, are selectively re-absorbed as the urine flows along the tube... Normally, in a healthy adult, the initial filtrate in the kidneys is about 180 L daily. However, the volume actually excreted is only a litre or two a day, because the remaining filtrate is reabsorbed in the kidney tubules.' Also: 'The biological process involved in the removal of these harmful metabolic wastes from the body is called excretion.'

(a) Bowman's capsule structure and function: It is the cup-shaped end of the coiled tubule of a nephron, wrapped around a cluster of very thin-walled blood capillaries called the glomerulus. As blood passes through these capillaries under pressure, water and small dissolved solutes (including wastes, glucose, amino acids and salts) are filtered out of the blood into the capsule, forming the initial filtrate that then flows into the rest of the nephron tubule.

(b) Why only 1-2 L is excreted despite 180 L filtered: As the filtrate flows along the nephron tubule, useful substances — a major amount of water, glucose, amino acids, and salts — are selectively reabsorbed back into the surrounding blood capillaries depending on how much excess water and dissolved waste the body needs to get rid of. Only the concentrated waste-containing fluid that remains after this extensive reabsorption is passed on as urine, which is why the final excreted volume (1-2 L) is a tiny fraction of the initial filtrate (180 L).

(c) Excretion and its necessity: Excretion is the biological process by which harmful metabolic waste products (mainly nitrogenous wastes like urea and uric acid, generated from the breakdown of proteins and other compounds) are removed from the body. It is necessary for every living organism because these waste products, if allowed to accumulate inside cells and tissues, would become toxic and disrupt the ordered molecular processes (maintenance functions) that are essential for the organism to stay alive.

OR

(c) Similarities between lungs and kidneys: (1) Both possess a filtration/exchange unit built around a cluster of very thin-walled blood capillaries providing a large surface area — alveoli in the lungs and the glomerulus enclosed by Bowman's capsule in the kidney's nephron. (2) Both organs function to remove waste products generated by the body's metabolism from the blood — the lungs remove the gaseous waste carbon dioxide by diffusion at the alveoli, while the kidneys remove dissolved nitrogenous wastes such as urea by filtration at the nephrons.