Kidney Function and Osmoregulation

Osmoregulation in Fish

• Freshwater fish have adaptations to reduce water uptake and conserve solutes.
• Desert and marine animals face environments that quickly deplete body water.
• Osmoregulation regulates solute concentrations and balances water gain and loss.
• Excretion removes metabolic wastes.
• Osmoregulation relies on controlled solute movement between internal fluids and the external environment.
• Osmoregulators expend energy to control water uptake and loss in hyperosmotic or hypoosmotic environments.

Saltwater Fish Osmoregulation

• Saltwater fish gain water and salt ions from food and drinking seawater.
• They lose water osmotically through gills and body surfaces.
• They excrete salt ions from their gills.
• They excrete small amounts of water in scanty urine from their kidneys.

Freshwater Fish Osmoregulation

• Freshwater fish take in water and some ions from food.
• They gain water osmotically through their gills and body surfaces.
• They uptake salt ions through their gills.
• They excrete large amounts of water in dilute urine from their kidneys.

Effects of Osmoregulation

• Impacts internal pH, metabolite concentration, and waste management.
• Affects the composition of internal body fluids (e.g., hemolymph, interstitial fluid).
• Maintains cytoplasmic composition within cells.

Salt Excreting Glands

• Marine birds and reptiles (unable to produce concentrated urine) have extrarenal salt excretion pathways.
• Birds use nasal glands to release salt secretions into nasal passages.
• Sea turtles use modified tear ducts to secrete salt into their eye orbits.

Osmoconformers vs. Osmoregulators

• Osmoconformers have body fluids that are isotonic to their environment; no active adjustment of internal osmolarity.
• Osmoregulators have hypotonic body fluids that gain water from the environment, continuously eliminating excess water.
• Osmoregulators have hypertonic body fluids, losing water to the environment, and continuously taking in excess water.
• Osmoregulators expend energy to maintain osmotic balance (5% to 30% of metabolic rate).

Nitrogenous Wastes

• Animals excreting ammonia as a waste product require substantial water.
• Ammonia is released across the body surface or through gills.
• The liver of mammals and most adult amphibians convert ammonia to less toxic urea.
• The circulatory system carries urea to the kidneys for excretion.
• Insects, land snails, and most reptiles excrete uric acid, which is insoluble in water and excreted as a paste, conserving water.

Excretory Processes

• Most excretory systems produce urine by refining a filtrate derived from body fluids.
• Key functions include filtration (pressure-filtering of body fluids), reabsorption (reclaiming valuable solutes), secretion (adding toxins and solutes from body fluids to the filtrate), and excretion (removing the filtrate from the system).

Kidneys and Nephrons

• Nephrons are the functional units of vertebrate kidneys, involved in excretion and osmoregulation.
• The mammalian excretory system centers on paired kidneys that regulate water balance and salt.
• Each kidney receives blood from a renal artery and drains into a renal vein.
• Urine exits kidneys through ureters and drains into the urinary bladder.

Nephron Structure and Function

• Mammalian kidneys have two regions: an outer renal cortex and an inner renal medulla.
• Nephrons consist of a long tubule and glomerulus (ball of capillaries).

Blood Filtration

• Filtration occurs in the glomerulus, forcing fluid from the blood into Bowman's capsule.
• The filtrate in Bowman's capsule mirrors the solute concentration of blood plasma.

Filtrate Pathway

• The filtrate travels through the proximal tubule, loop of Henle, and distal tubule.
• Fluid from multiple nephrons enters a collecting duct.

From Filtrate to Urine

• As filtrate passes through the nephron and collecting duct, it becomes urine.
• Secretion and reabsorption alter the filtrate's composition and volume in the proximal tubule.
• Water reabsorption occurs as filtrate moves into the descending loop of Henle's limb.

Loop of Henle and Urine Concentration

• In the ascending limb of the loop of Henle, salt diffuses from the permeable tubule into the interstitial fluid.
• The distal tubule regulates the concentrations of potassium (K+) and sodium chloride (NaCl).
• The collecting duct carries filtrate through the medulla to the renal pelvis, reabsorbing NaCl.
• The mammalian kidney conserves water by concentrating urine more than body fluids.

Solute Gradients and Water Conservation

• Loops of Henle and collecting ducts create osmotic gradients responsible for urine concentration.
• NaCl and urea contribute to the interstitial fluid's osmolarity, triggering water reabsorption and urine concentration.

Urine Concentration and Collecting Duct

• Collecting ducts travel through osmolarity gradients, increasing water excretion via osmosis.
• Urea diffuses from the collecting duct into the medulla.
• Urea and sodium chloride establish osmotic gradients, allowing the kidneys to produce urine hyperosmotic to blood.

Regulation of Kidney Function

• Urine osmolarity is controlled by nervous and hormonal mechanisms regulating water and salt reabsorption.
• Antidiuretic hormone (ADH) increases water reabsorption in distal tubules.
• Aldosterone regulates salt reabsorption in the kidneys.

Explore the key roles of the distal tubule, collecting ducts, and loops of Henle in kidney function. This quiz covers mechanisms of water conservation and osmoregulation in mammals, including adaptations in various species. Test your knowledge on how these processes maintain internal fluid composition.