Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism, while excretion is the removal from the body of the waste products of metabolic pathways, such as urea and .
Nitrogenous waste varies by species: aquatic animals excrete ammonia (), mammals excrete urea, and birds/reptiles excrete uric acid () to conserve water.
The Malpighian tubule system in insects carries out osmoregulation and removal of nitrogenous wastes by actively transporting ions into the tubules, causing water to follow via osmosis from the hemolymph.
Ultrafiltration occurs in the glomerulus where high blood pressure forces water and small solutes through the basement membrane and podocytes into the Bowman’s capsule, while proteins and blood cells remain in the capillaries.
The Proximal Convoluted Tubule (PCT) is the site of selective reabsorption; it contains microvilli and many mitochondria to facilitate the active transport of all glucose (), amino acids, and approximately of ions and .
The Loop of Henle maintains hypertonic conditions in the medulla. The descending limb is permeable to but not to , while the ascending limb is impermeable to and actively pumps out and ions.
Antidiuretic Hormone (ADH) controls the permeability of the collecting duct. In the presence of ADH, aquaporins are inserted into the membrane, allowing to be reabsorbed into the hypertonic medulla, resulting in concentrated urine.
The length of the Loop of Henle is positively correlated with the need for water conservation; desert animals (e.g., Kangaroo rats) have exceptionally long loops to create a steeper osmotic gradient in the medulla.
📐Formulae
(Net Filtration Pressure)
(Water Potential equation relevant to osmosis in the nephron)
💡Examples
Problem 1:
Calculate the Net Filtration Pressure () in a glomerulus where the Glomerular Hydrostatic Pressure () is mmHg, the Blood Colloid Osmotic Pressure () is mmHg, and the Capsular Hydrostatic Pressure () is mmHg.
Solution:
Explanation:
The net pressure driving fluid out of the capillaries into the Bowman's capsule is the difference between the outward hydrostatic pressure and the sum of the inward osmotic and hydrostatic pressures.
Problem 2:
Explain the movement of water in the descending limb of the Loop of Henle relative to the solute concentration of the medulla.
Solution:
The descending limb is permeable to but impermeable to solutes. As the filtrate moves down into the medulla where the concentration of and is high (hypertonic), moves out of the tubule via osmosis.
Explanation:
Because the interstitial fluid of the medulla has a lower water potential () than the filtrate, water moves down its concentration gradient into the vasa recta capillaries.