Osmoregulation Rachel Lee Victoria Trinh and Excretion

An Introduction An albatross relies on osmoregulation – process by which animals control solute concentrations and balance water gain and loss Breakdown of nitrogenous molecules releases ammonia System for excretion and osmoregulation are linked

44.1 Balancing uptake and loss of water and solutes the rates of water uptake and loss must balance. Water enters and leaves cells by osmosis, the movement of water across a selectively permeable membrane. Osmosis occurs whenever two solutions separated by a membrane differ in osmotic pressure, or osmolarity (moles of solute per liter of solution). The unit of measurement of osmolarity is milliosmoles per liter (mosm/L).

Osmoregulators expend energy to control their internal osmolarity; osmoconformers are isoosmotic with their surroundings. There are two basic solutions to the problem of balancing water gain with water loss. One—available only to marine animals—is to be isoosmotic to the surroundings as an osmoconformer osmoconformers often live in water that has a very stable composition and have a very constant internal osmolarity. In contrast, an osmoregulator is an animal that must control its internal osmolarity because its body fluids are not isoosmotic with the outside environment. Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity and are said to be stenohaline. In contrast, euryhaline animals can survive large fluctuations in external osmolarity.

Dehydration dooms most animals, but some aquatic invertebrates living in temporary ponds and films of water around soil particles can lose almost all their body water and survive in a dormant state, called anhydrobiosis, when their habitats dry up. Anhydrobiotic animals must have adaptations that keep their cell membranes intact. The threat of dehydration is perhaps the largest regulatory problem confronting terrestrial plants and animals.

Water balance and waste disposal depend on transport epithelia. In most animals, osmotic regulation and metabolic waste disposal depend on the ability of a layer or layers of transport epithelium to move specific solutes in controlled amounts in specific directions. In most animals, transport epithelia are arranged into complex tubular networks with extensive surface area.

44.2 An animal’s nitrogenous wastes reflect its phylogeny and habitat When proteins and nucleic acids are broken apart, enzymes remove nitrogen in form of ammonia Animals excrete nitrogenous wastes as ammonia, urea, or uric acid

Metabolic wastes must be dissolved in water when they are removed from the body, the type and quantity of waste products may have a large impact on water balance. Nitrogenous breakdown products of proteins and nucleic acids are among the most important wastes in terms of their effect on osmoregulation. Animals that excrete nitrogenous wastes as ammonia need access to lots of water. Mammals, most adult amphibians, sharks, and some marine bony fishes and turtles excrete mainly urea. Urea is synthesized in the liver by combining ammonia with carbon dioxide and is excreted by the kidneys. The main advantage of urea is its low toxicity, about 100,000 times less than that of ammonia. The main disadvantage of urea is that animals must expend energy to produce it from ammonia.

Land snails, insects, birds, and many reptiles excrete uric acid as the main nitrogenous waste. Like urea, uric acid is relatively nontoxic. Uric acid is largely insoluble in water and can be excreted as a semisolid paste with very little water loss. While saving even more water than urea, it is even more energetically expensive to produce.

44.3 Hydrostatic pressure drives a process of filtration Filtrate: solution formed by water and small solutes that crosses the membrane Reabsorption recovers useful molecules and water from the filtrate and returns them to body fluids Nonessential solutes are left in filtrate or added to it by selective secretion Flatworms, rotifers, and some annelids have excretory systems called protonephridia: network of dead-end tubules connected to external openings Flame bulbs cap the branches of each protonephridium Each flame bulb has a tuft of cilia projecting into the tubule

Most annelids have metanephrdia: excretory organs that open internally to the coelom Metanephridia are found in pairs in each segment of a worm, and are immersed in coelomic fluid Insects and other terrestrial arthropods have Malpighian tubules, which remove nitrogenous wastes and also function in osmoregulation The transport epithelium that lines the tubules secretes solutes from the hemolympth into the lumen of the tubule Water follows solutes into tubule, and fluid passes into rectum, and nitrogenous wastes are excreted

Excretory system of mammals center on kidneys Each kidney is supplied with blood by renal artery and drained by renal vein Urine exits each kidney through ureter Both ureters drain into urinary bladder, and urine is expelled through urethra Kidneys have an outer renal cortex and inner renal medulla Nephron weaves back and forth across cortex and medulla ; it’s the functional unit of the kidney ->nephron consists of long tubule and ball of capillarries called glomerulus Blind end of tubule is Bowman’s capsule

Pathway of the Filtrate Filtrate passes into proximal tube, the 1 st of 3 major regions of nephron -> through loop of Henle (2 nd ) -> through distal tubule (last) -> collecting duct -> renal pelvis -> ureter 85% of nephrons in kidneys are cortical nephrons, which have short loops of Henle and are almost entirely confined to the renal cortex 15% are juxtamedullary nephrons, which have loops that extend deeply into the renal medulla

44.4 Nephron is organized for stepwise processing of blood filtrate Filtration: The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule. Reabsorbtion: The transport epithelium reclaims valuable substances from the filtrate and returns then to the body fluids Secretion: Other substances such as toxins and excess ions are extracted from the body fluids and added to the contents of the excretory tubule Excretion: The altered filtrate (urine) leaves the system

In the human kidney, most of the nephrons are cortical nephrons, these are in the renal cortex. The rest are juxtamedullary nephrons, with long loops of Henle that extend into the renal medulla Capillaries called afferent arterioles are associated with the nephrons, and as they leave the glomerulus, the capillaries converge into an efferent arteriole. This vessel subdivides again to form peritubular capillaries which surround the proximal and ditstal tubules. There are Five Main Steps in the Transformation of Blood Filtrate to Urine 1.) In the proximal tubule, secretion and reabsorbtion changes the volume and composition of the filtrate. The pH of body fluids is controlled and bicarbonate is absorbed as are NaCl and water. 2.) In the descending loop of Henle, reabsorbtion of water continues 3.) In the ascending loop of Henle, the filtrate loses salt without giving up water and becomes more dilute. 4.) In the distal tubule, K+ and NaCl levels are regulated, as is filtrate pH 5.) The collection duct carries the filtrate though the medulla to the renal pelvis and the filtrate becomes more concentrated by the movement of salt.

44.5 Hormonal circuits link kidney function, H2O balance, and blood pressure Mammalian kidneys can adjust the concentration of their urine depending on their environment Antidiuretic Hormone is produced in the hypothalamus of the brain, stored in the pituitary gland. Osmoreceptor cells (located in the hypothalamus) monitor the osmolarity of blood and regulate the release of the posterior pituitary gland. Makes the collecting ducts more permeable to water, resulting in a more concentrated urine. Another regulatory mechanism is the renin-angiotensin- aldosterone system (RAAS).

Involves a specialized tissue (juxtaglomerular apparatus (JGA)), located near the afferent arteriole that supplies blood to the glomerulus. When blood pressure or volume drops, the JGA releases renin. Renin initaiates chemical reactions that cleave a plasma protein called angiotensinogen, yielding angiotensin II. Angiotensin II acts as a hormone and causes arterioles to constrict, raising blood pressure. Thiss also causes the adrenal glands to release aldosterone. Aldosterone causes the kidneys to reabsorb nor Na+, increasing the retension of water and blood volume and pressure. (drugs that block angiotensin II production are used to treat hyper tension, or chronic high blood pressure) ADH and RAAS may seem similar, but they aren’t ADH is a response to blood osmolarity, RAAS responds to drops in blood pressure and volume

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