Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmoregulation and excretion The physiological systems of animals operate in a fluid environment The relative concentrations of water and solutes in this environment must be maintained within fairly narrow limits
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Freshwater animals s how adaptations that reduce water uptake and conserve solutes Desert and marine animals face desiccating environments w ith the potential to quickly deplete the body water Figure 44.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmoregulation regulates solute concentrations and balances the gain and loss of water Excretion gets rid of metabolic wastes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmoregulation Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment. 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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmotic Challenges Osmoconformers, which are only marine animals are isoosmotic with [at the same concentration as] their surroundings and do not regulate their osmolarity. Osmoregulators expend energy to control water uptake and loss in a hyperosmotic [more concentrated] or hypoosmotic [less concentrated] environment.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Marine Animals Most marine invertebrates are osmoconformers Most marine vertebrates and some invertebrates are osmoregulators
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Marine bony fishes are hypoosmotic to sea water a nd lose water by osmosis and gain salt by both diffusion and from food they eat These fishes balance water loss b y drinking seawater and actively excrete salt through their gills. They produce little urine. Figure 44.3a Gain of water and salt ions from food and by drinking seawater Osmotic water loss through gills and other parts of body surface Excretion of salt ions from gills Excretion of salt ions and small amounts of water in scanty urine from kidneys (a) Osmoregulation in a saltwater fish
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Freshwater Animals Freshwater animals constantly take in water from their hypoosmotic environment They lose salts by diffusion
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Freshwater animals maintain water balance b y excreting large amounts of dilute urine Salts lost by diffusion a re replaced by foods and uptake across the gills Figure 44.3b Uptake of water and some ions in food Osmotic water gain through gills and other parts of body surface Uptake of salt ions by gills Excretion of large amounts of water in dilute urine from kidneys (b) Osmoregulation in a freshwater fish
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Land animals Land animals manage their water budgets by drinking and eating moist foods and by using metabolic water. Animals adapted to deserts produce highly concentrated urine and use a wide range of behavioral adaptations to avoid the heat (burrowing, nocturnal activity)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 44.5 Water balance in a human (2,500 mL/day = 100%) Water balance in a kangaroo rat (2 mL/day = 100%) Ingested in food (0.2) Ingested in food (750) Ingested in liquid (1,500) Derived from metabolism (250) Derived from metabolism (1.8) Water gain Feces (0.9) Urine (0.45) Evaporation (1.46) Feces (100) Urine (1,500) Evaporation (900) Water loss
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Benefits of insulation Desert animals also get major water savings from simple anatomical features Knut and Bodil Schmidt-Nielsen et al. observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals’ skin remained more than 30°C cooler. The Schmidt-Nielsens reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they compared the water loss rates of unclipped and clipped camels. Removing the fur of a camel increased the rate of water loss through sweating by up to 50%.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Control group (Unclipped fur) Experimental group (Clipped fur) Water lost per day (L/100 kg body mass)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transport Epithelia Transport epithelia are specialized cells that regulate solute movement. The epithelia form a layer of cells across whose selectively permeable membranes solutes must flow. These epithelia are essential components of osmotic regulation and metabolic waste disposal and usually are arranged into complex tubular networks.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An example of transport epithelia is found in the salt glands of marine birds which remove excess sodium chloride from the blood. In birds such as petrels, gulls and albatrosses transport epithelia arranged in tubes in a salt gland actively secrete salt from the blood into collecting ducts [a countercurrent system is used to maximize salt concentration] that drain to the nostrils where the salt solution is shed.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nasal salt gland Nostril with salt secretions Lumen of secretory tubule NaCl Blood flow Secretory cell of transport epithelium Central duct Direction of salt movement Transport epithelium Secretory tubule Capillary Vein Artery (a) An albatross’s salt glands empty via a duct into the nostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist. (b) One of several thousand secretory tubules in a salt- excreting gland. Each tubule is lined by a transport epithelium surrounded by capillaries, and drains into a central duct. (c) The secretory cells actively transport salt from the blood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentration gradient of salt in the tubule (aqua), this countercurrent system enhances salt transfer from the blood to the lumen of the tubule.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nitrogenous wastes An animal’s nitrogenous wastes reflect its phylogeny and habitat The type and quantity of an animal’s waste products may have a large impact on its water balance Among the most important wastes are the nitrogenous breakdown products of proteins and nucleic acids. Different animals excrete nitrogenous wastes in different forms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Proteins Nucleic acids Amino acids Nitrogenous bases –NH 2 Amino groups Most aquatic animals, including most bony fishes Mammals, most amphibians, sharks, some bony fishes Many reptiles (including birds), insects, land snails Ammonia Urea Uric acid NH3NH3 NH2NH2 NH2NH2 O C C C N C O N H H C O N C HNHN O H
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ammonia Animals such as fish that excrete nitrogenous wastes as ammonia need access to lots of water. Ammonia is released across the whole body surface or through the gills.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Urea The liver of mammals and most adult amphibians converts ammonia to less toxic urea Urea is carried to the kidneys, concentrated and excreted with a minimal loss of water
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Uric Acid Insects, land snails, and many reptiles, including birds excrete uric acid as their major nitrogenous waste Uric acid is largely insoluble in water and can be secreted as a paste with little water loss.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Excretory Systems Diverse excretory systems are variations on a tubular theme Most excretory systems produce urine by refining a filtrate derived from body fluids
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 44.9 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. Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids. Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule. Excretion. The filtrate leaves the system and the body. Capillary Excretory tubule Filtrate Urine
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vertebrate Kidneys Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation. The mammalian excretory system centers on paired kidneys which are also the principal site of water balance and salt regulation.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Each kidney is supplied with blood by a renal artery and drained by a renal vein. Posterior vena cava Renal artery and vein Aorta Ureter Urinary bladder Urethra (a) Excretory organs and major associated blood vessels Kidney
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Urine exits each kidney through a duct called the ureter. Both ureters drain into a common urinary bladder. Urine is voided from the bladder via a duct called the urethra.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Kidney structure Ureter Section of kidney from a rat Renal medulla Renal cortex Renal pelvis Figure 44.13b Structure and Function of the Nephron and Associated Structures The mammalian kidney has two distinct regions:an outer renal cortex and an inner renal medulla
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The nephron is the functional unit of the vertebrate kidney. It consists of a single long tubule and a ball of capillaries called the glomerulus.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 44.13c, d Juxta- medullary nephron Cortical nephron Collecting duct To renal pelvis Renal cortex Renal medulla 20 µm Afferent arteriole from renal artery Glomerulus Bowman’s capsule Proximal tubule Peritubular capillaries SEM Efferent arteriole from glomerulus Branch of renal vein Descending limb Ascending limb Loop of Henle Distal tubule Collecting duct (c) Nephron Vasa recta (d) Filtrate and blood flow
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Filtration of the Blood Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of the Bowman’s capsule. Filtration of small molecules is nonselective and the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma. The filtrate contains water, urea, salts and other small molecules.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pathway of the Filtrate From the Bowman’s capsule, the filtrate passes through three regions of the nephron the proximal tubule, the loop of Henle, and the distal tubule. [proximal means closest to and distal furthest from in the case the Bowman’s capsule]. Fluid from several nephrons flows into a collecting duct.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Proximal tubule Filtrate H 2 O Salts (NaCl and others) HCO 3 – H + Urea Glucose; amino acids Some drugs Key Active transport Passive transport CORTEX OUTER MEDULLA INNER MEDULLA Descending limb of loop of Henle Thick segment of ascending limb Thin segment of ascending limb Collecting duct NaCl Distal tubule NaClNutrients Urea H2OH2O NaCl H2OH2O H2OH2O HCO 3 K+K+ H+H+ NH 3 HCO 3 K+K+ H+H+ H2OH2O Figure 44.14
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Proximal tubule Selective secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate from what is initially produced in the Bowman’s capsule. In the proximal tubule hydrogen ions (H + ) and ammonia (NH 3 ) are secreted into the tubule as are drugs and toxins. Nutrients (e.g. glucose and amino acids) are reabsorbed as are potassium (K + ), NaCl and water.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Descending limb of loop of Henle Reabsorption of water out of the filtrate continues as the filtrate moves into the descending limb of the loop of Henle, which is very permeable to water, but not very permeable to salts. NaCl concentration in the urine thus increases.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ascending Limb of Loop of Henle The ascending limb of the Loop of Henle in contrast to the descending loop is permeable to salt, but not to water. Thus, as filtrate travels through the ascending limb of the loop of Henle, salt diffuses out of the permeable tubule into the interstitial fluid. First the movement of salt is passive, but in the upper section of the ascending loop salt is actively pumped out into the interstitial fluid.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ascending Limb of Loop of Henle The movement of salt out of the ascending loop of Henle contributes to the development of an osmolarity gradient that is important to the kidney’s ability to produce a concentrated urine as we will see later.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Distal tubule The distal tubule p lays a key role in regulating the K + and NaCl concentration of body fluids by regulating the K + secreted and NaCl reabsorbed.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Collecting Duct The collecting duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water conservation ability of the kidney 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Solute Gradients and Water Conservation In a mammalian kidney, the cooperative action and arrangement of the loops of Henle and the collecting ducts produce an osmotic gradient along the length of the loop of Henle in the kidney. Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid w hich causes the reabsorption of water in the kidney and concentrates the urine
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O NaCl Active transport Passive transport OUTER MEDULLA INNER MEDULLA CORTEX H2OH2O Urea H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Osmolarity of interstitial fluid (mosm/L) 300
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water passing down the descending loop of Henle meets more and more concentrated interstitial fluid and so water passively moves out of the tubule into the kidney. Because the ascending loop of Henle is not permeable to water, water does not flow back into the filtrate as it ascends. However, salts are actively pumped out to maintain the concentration gradient.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Collecting duct The collecting duct, is also permeable to water but not salt. The collecting duct carries the filtrate through the kidney’s osmolarity gradient, and more water exits the filtrate by osmosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Collecting duct Urea also diffuses out of the collecting duct as it traverses the inner medulla Urea and NaCl together form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Because of the countercurrent arrangement of the loop of Henle, the longer the loop the more urine can be concentrated. Thus, desert rodents have very long loops of Henle, which enable them to produce urine much more concentrated than humans can produce.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The South American vampire bat, which feeds on blood h as a unique excretory system in which its kidneys offload much of the water absorbed from a meal by excreting large amounts of dilute urine up to 24% of bodyweight per hour. Figure 44.17
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Regulation of Kidney Function The osmolarity of the urine – Is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys Antidiuretic hormone (ADH) – Increases water reabsorption in the distal tubules and collecting ducts of the kidney
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmoreceptors in hypothalamus Drinking reduces blood osmolarity to set point H 2 O reab- sorption helps prevent further osmolarity increase STIMULUS: The release of ADH is triggered when osmo- receptor cells in the hypothalamus detect an increase in the osmolarity of the blood Homeostasis: Blood osmolarity Hypothalamus ADH Pituitary gland Increased permeability Thirst Collecting duct Distal tubule (a) Antidiuretic hormone (ADH) enhances fluid retention by making the kidneys reclaim more water.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RAAS system The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis. The juxtoglomerular apparatus located around arterioles supplying glomeruli detect drop in blood pressure. Releases enzyme renin. Renin stimulates production of angiotensin II which decreases blood flow to kidney and stimulates proximal tubules to reabsorb more water na dNaCl.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Increased Na + and H 2 O reab- sorption in distal tubules Homeostasis: Blood pressure, volume STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (such as due to dehydration or loss of blood) Aldosterone Adrenal gland Angiotensin II Angiotensinogen Renin production Renin Arteriole constriction Distal tubule JGA (b) The renin-angiotensin-aldosterone system (RAAS) leads to an increase in blood volume and pressure.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ANF Another hormone, atrial natriuretic factor (ANF) opposes the RAAS. Walls of the atria of the heart release ANF in response to increase in blood volume and pressure. It inhibits release of renin.