Homeostasis Chapter 30

Homeostasis Homeostasis refers to maintaining internal stability within an organism and returning to a particular stable state after a fluctuation.

Homeostasis Changes to the internal environment come from: Metabolic activities require a supply of materials (oxygen, nutrients, salts, etc) that must be replenished. Waste products are produced that must be expelled.

Homeostasis Systems within an organism function in an integrated way to maintain a constant internal environment around a setpoint. Small deviations in pH, temperature, osmotic pressure, glucose levels, & oxygen levels activate physiological mechanisms to return that variable to its setpoint. Negative feedback

Osmoregulation & Excretion Osmoregulation regulates solute concentrations and balances the gain and loss of water. Excretion gets rid of metabolic wastes.

Osmosis Cells require a balance between osmotic gain and loss of water. Water uptake and loss are balanced by various mechanisms of osmoregulation in different environments.

Osmosis Osmosis is the movement of water across a selectively permeable membrane. If two solutions that are separated by a membrane differ in their osmolarity, water will cross the membrane to bring the osmolarity into balance (equal solute concentrations on both sides).

Osmotic Challenges Osmoconformers, which are only marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity. Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment.

Osmotic Regulation Most marine invertebrates are osmotic conformers – their bodies have the same salt concentration as the seawater. The sea is highly stable, so most marine invertebrates are not exposed to osmotic fluctuations. These organisms are restricted to a narrow range of salinity – stenohaline. Marine spider crab

Osmotic Regulation Conditions along the coasts and in estuaries are often more variable than the open ocean. Animals must be able to handle large, often abrupt changes in salinity. Euryhaline animals can survive a wide range of salinity changes by using osmotic regulation. Hyperosmotic regulator (body fluids saltier than water) Shore crab.

Osmotic Regulation The problem of dilution is solved by pumping out the excess water as dilute urine. The problem of salt loss is compensated for by salt secreting cells in the gills the actively remove ions from the water and move them into the blood. Requires energy.

Osmotic Regulation - Freshwater Freshwater animals face an even more extreme osmotic difference than those that inhabit estuaries.

Osmotic Regulation - Freshwater Freshwater fishes have skin covered with scales and mucous to keep excess water out. Water that enters the body is pumped out by the kidney as very dilute urine. Salt absorbing cells in the gills transport salt ions into the blood.

Osmotic Regulation - Freshwater Invertebrates and amphibians also solve these problems in a similar way. Amphibians actively absorb salt from the water through their skin.

Osmotic Regulation – Marine Marine bony fishes are hypoosmotic regulators. Maintain salt concentration at 1/3 that of seawater. Marine fishes drink seawater to replace water lost by diffusion. Excess salt is carried to the gills where salt-secreting cells transport it out to the sea. More ions voided in feces or urine.

Osmotic Regulation – Marine Sharks and rays retain urea (a metabolic waste usually excreted in the urine) in their tissues and blood. This makes osmolarity of the shark’s blood equal to that of seawater, so water balance is not a problem.

Osmotic Regulation – Terrestrial Terrestrial animals lose water by evaporation from respiratory and body surfaces, excretion (urine), and elimination (feces). Water is replaced by drinking water, water in food, and retaining metabolic water.

Osmotic Regulation – Terrestrial The end-product of protein metabolism is ammonia, which is highly toxic. Fishes can excrete ammonia directly because there is plenty of water to wash it away.

Osmotic Regulation – Terrestrial Terrestrial animals must convert ammonia to uric acid. Semi-solid urine – little water loss. In birds & reptiles, the wastes of developing embryos are stored as harmless solid crystals.

Osmotic Regulation – Terrestrial Marine birds and turtles have a salt gland capable of excreting highly concentrated salt solution.

Excretory Processes Most excretory systems produce urine by refining a filtrate derived from body fluids (blood, hemolymph, or coelomic fluid).

Excretory Processes Key functions of most excretory systems are: Filtration, pressure-filtering of body fluids producing a filtrate. Reabsorption, reclaiming valuable solutes from the filtrate. Secretion, addition of toxins and other solutes from the body fluids to the filtrate. Excretion, the filtrate leaves the system.

Invertebrate Excretory Structures Contractile vacuoles are found in protozoans and freshwater sponges. An organ of water balance – expels excess water gained by osmosis.

Invertebrate Excretory Structures The most common type of invertebrate excretory organ is the nephridium. The simplest arrangement is the protonephridium of acoelomates and some pseudocoelomates. Fluid enters through flame cells, moves through the tubules, water and metabolites are recovered and wastes are excreted through pores that open along the body surface. Highly branched due to lack of circulatory system.

Invertebrate Excretory Structures The metanephridium is an open system found in annelids, molluscs, and some smaller phyla. Tubules are open at both ends. Water enters through the ciliated, funnel shaped nephrostome. The metanephridium is surrounded by blood vessels that assist in reclaiming water and valuable solutes.

Invertebrate Excretory Structures In arthropods, antennal glands are an advanced form of the nephridial organ. No open nephrostomes, hydrostatic pressure of the blood forms an ultrafiltrate in the end sac. In the tubule, selective resorption of some salts and active secretion of others occurs.

Invertebrate Excretory Structures Insects and spiders have Malpighian tubules that are closed and lack an arterial supply. Salts (especially potassium) are secreted into the tubules from the hemolymph (blood). Water & other solutes (including uric acid) follow. Water & potassium are reabsorbed. Uric acid is expelled in feces.

Vertebrate Kidneys Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation.

Vertebrate Kidneys Nephrons and associated blood vessels are the functional unit of the mammalian kidney. The mammalian excretory system centers on paired kidneys which are also the principal site of water balance and salt regulation.

Vertebrate Kidneys Each kidney is supplied with blood by a renal artery and drained by a renal vein.

Vertebrate Kidneys Urine exits each kidney through a duct called the ureter. Both ureters drain into a common urinary bladder.

Structure and Function of the Nephron and Associated Structures The mammalian kidney has two distinct regions: An outer renal cortex An inner renal medulla (b) Kidney structure Ureter Section of kidney from a rat Renal medulla cortex pelvis

Structure and Function of the Nephron and Associated Structures The nephron, the functional unit of the vertebrate kidney consists of a single long tubule and a ball of capillaries called the glomerulus.

Filtration of the Blood Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule.

Pathway of the Filtrate From Bowman’s capsule, the filtrate passes through three regions of the nephron: Proximal tubule Loop of Henle Distal tubule Fluid from several nephrons flows into a collecting duct.

From Blood Filtrate to Urine: A Closer Look Filtrate becomes urine as it flows through the mammalian nephron and collecting duct. The composition of the filtrate is modified through tubular reabsorption and secretion. Changes in the total osmotic concentration of urine through regulation of water excretion.

From Blood Filtrate to Urine: A Closer Look Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate. Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle.

From Blood Filtrate to Urine: A Closer Look As filtrate travels through the ascending limb of the loop of Henle salt diffuses out of the permeable tubule into the interstitial fluid. The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids. The collecting duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl.

Conserving Water The mammalian kidney’s ability to conserve water is a key terrestrial adaptation. The mammalian kidney can produce urine much more concentrated than body fluids, thus conserving water.

Solute Gradients and Water Conservation In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradient that concentrates the urine.

Solute Gradients and Water Conservation The collecting duct, permeable to water but not salt conducts the filtrate through the kidney’s osmolarity gradient, and more water exits the filtrate by osmosis.

Solute Gradients and Water Conservation Urea diffuses out of the collecting duct as it traverses the inner medulla. Urea and NaCl form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood.

Regulation of Kidney Function The osmolarity of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys.

Regulation of Kidney Function Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney.

Temperature Regulation Animals must keep their bodies within a range of temperatures that allows for normal cell function. Each enzyme has an optimum temperature. Too low and metabolism slows. Too high and metabolic reactions become unbalanced. Enzymes may be destroyed.

Temperature Regulation Poikilothermic animals’ body temperatures fluctuate with environmental temperatures. Homeothermic animals’ body temperatures are constant.

Temperature Regulation All animals produce heat from cellular metabolism, but in most this heat is lost quickly. Ectotherms – lose metabolic heat quickly, so body temperature is determined by the environment. Body temp may be regulated environmentally. Endotherms – retain metabolic heat and can maintain a constant internal body temperature.

Ectothermic Temperature Regulation Many ectotherms regulate body temperature behaviorally. Basking to increase temperature. Shelter in shade or coolness of a burrow to decrease temperature.

Ectothermic Temperature Regulation Most ectotherms can also adjust their metabolic rates to the environmental temperature. Activity levels can remain unchanged over a wider range of temperatures.

Endothermic Temperature Regulation Constant temperature in endotherms is maintained by a delicate balance between heat production and heat loss. Heat is produced by the animal’s metabolism. Producing heat requires energy – supplied by food. Endotherms must eat more in cold weather.

Endothermic Temperature Regulation If an animal is too cool, it can generate heat by increasing muscular activity (exercise or shivering). Heat is retained through insulation. If an animal is too warm it decreases heat production and increases heat loss.

Adaptations for Hot Environments Small desert mammals are mostly fossorial (living underground) or nocturnal. Burrows are cool and moist. Adaptations to derive water from metabolism and produce concentrated urine & dry feces.

Adaptations for Hot Environments Larger desert mammals (camels, desert antelopes) have different adaptations. Glossy, pallid color reflects sunlight. Fat tissue is concentrated in a hump, rather than being evenly distributed in an insulating layer. Sweating and panting are ways of dumping heat.

Adaptations for Cold Environments In cold environments, mammals reduce heat loss by having a thick insulating layer of fat, fur, or both. Heat production is increased. Extremities are allowed to cool. Heat loss is prevented through countercurrent heat exchange.

Adaptations for Cold Environments Small mammals are not as well insulated. Many avoid direct exposure to the cold by living in tunnels under the snow. Subnivean environment. This is where food is located.

Adaptive Hypothermia Endothermy is energetically expensive. Ectotherms can survive weeks without eating. Endotherms must always have energy supplies.

Adaptive Hypothermia Some very small mammals & birds (bats or hummingbirds) maintain high body temperatures when active, but allow temperatures to drop when sleeping. Daily torpor

Adaptive Hypothermia Hibernation is a way to solve the problem of low temperatures and the scarcity of food. True hibernators store fat, then enter hibernation gradually. Metabolism & body slows to a fraction of normal. Body temperature decreases. Shivering helps increase temperatures when they are waking up.

Adaptive Hypothermia Other mammals, such as bears, badgers, raccoons and opossums enter a state of prolonged sleep, but body temperature does not decrease.

Adaptive Hypothermia Adverse conditions can also occur during the summer. Drought, high temperatures. Some animals enter a state of dormancy called estivation. Breathing rates and metabolism decrease. African lungfish, desert tortoise, pigmy mouse, ground squirrels.