16.10 - EXERCISES

The Glomerular Filtration Rate (GFR) is a measure of how well the kidneys are filtering blood. It indicates the volume of blood filtered by the glomeruli in the kidneys per minute, providing an important assessment of kidney function. A normal GFR is about 125 milliliters per minute in a healthy adult.

The autoregulation of GFR is the body's ability to maintain a constant GFR despite fluctuations in blood pressure. This is primarily accomplished through mechanisms involving the juxtaglomerular apparatus (JGA). When blood pressure drops, JGA cells release renin, which leads to changes that can increase the GFR back to normal levels.

This exercise tests comprehension of key concepts about kidney function:

(a) True - Micturition, or urination, is indeed regulated by a reflex action facilitated by nerve signals.
(b) False - ADH actually promotes water reabsorption, leading to more concentrated urine, not hypotonic urine.
(c) True - The Bowman’s capsule allows the passage of protein-free fluid, effectively filtering out larger molecules like proteins.
(d) True - The loop of Henle is essential in concentrating urine due to its unique permeability properties.
(e) True - Glucose is actively reabsorbed in the proximal convoluted tubule to prevent its loss in urine.

The counter current mechanism involves the movement of fluid in opposite directions in the tubules of the nephron and the blood vessels surrounding them. This arrangement helps maintain an osmotic gradient in the kidney, effectively concentrating urine. As the filtrate moves down the loop of Henle, water is reabsorbed and the filtrate becomes more concentrated; when it moves up, electrolytes are reabsorbed, diluting the filtrate.

Apart from the kidneys, the liver, lungs, and skin contribute to excretion. The liver filters toxins and produces bile which is excreted with digestive waste. The lungs expel carbon dioxide and water vapor. The skin helps eliminate waste through sweat, which contains salts and urea. Together, these organs ensure efficient waste removal from the body.

Micturition is the process of urination. When the bladder fills with urine, stretch receptors signal the nervous system, prompting a reflex to contract the bladder muscles and relax sphincters, allowing urine to be expelled through the urethra. This is a controlled and coordinated response involving both involuntary and voluntary muscle actions.

This exercise matches terms related to excretion to their appropriate definitions: (a → iii), (b → v), (c → iv), (d → i), (d → ii). Understanding these connections reinforces knowledge of key physiological principles in excretory functions.

Osmoregulation refers to the process by which the body maintains the balance of water and salts in its fluids. This involves regulating concentrations of solutes and water in blood and tissues, ensuring cells remain hydrated but not bursting from excess water. The kidneys and hormones like ADH play crucial roles in this regulatory process.

Terrestrial animals primarily excrete urea (ureotelic) or uric acid (uricotelic) rather than ammonia because urea and uric acid are less toxic and require less water for excretion. Ammonotelic animals, like many aquatic species, can afford to excrete ammonia directly because they have plenty of water to dilute it. This adaptation helps conserve water in terrestrial environments where it is more limited.

The juxtaglomerular apparatus (JGA) is significant as it monitors and regulates blood pressure and GFR. By detecting changes in blood volume and pressure, it can release hormones like renin to adjust kidney function accordingly, ensuring the body maintains homeostasis through effective filtration.

In this exercise, students are asked to recall specific excretory structures: (a) Flame cells are found in flatworms. (b) Cortical portions are called renal columns. (c) The loop of capillary is known as vasa recta. Each structure contributes to the organism's excretory function and reflects evolutionary adaptations.

Filling in these gaps reinforces critical knowledge:
(a) The ascending limb of Henle’s loop is impermeable to water, whereas the descending limb is permeable to it.
(b) The hormone ADH facilitates water reabsorption.
(c) Dialysis fluid lacks nitrogenous wastes.
(d) The average urea excretion is about 25-30 grams per day. Each of these points underpins an essential aspect of renal physiology and functioning.