Osmoregulation and excretion Overview A balancing act Osmoregulation

A balancing act • Freshwater animals – Show adaptations that reduce water uptake and conserve solutes • Desert and marine animals face desiccating environments – With the potential to quickly deplete the body water Figure 44. 1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Osmoregulation • Osmosis • Osmotic challenges • Adaptations for osmoregulation Copyright © 2005 Pearson

Osmosis • Osmosis is the movement of water across a selectively permeable membrane – Osmolarity: total solute concentration • Solutions can be – Isoosmotic: solutions have the same osmolarity – Hypoosmotic: dilute solution – Hyperosmotic: solution with higher osmolarity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Most animals are said to be stenohaline – And cannot tolerate substantial changes in external osmolarity • Euryhaline animals – Can survive large fluctuations in external osmolarity Figure 44. 2 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 – And lose water by osmosis and gain salt by both diffusion and from food they eat • These fishes balance water loss – By drinking seawater Gain of water and salt ions from food and by drinking seawater Excretion of salt ions from gills Figure 44. 3 a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmotic water loss through gills and other parts of body surface Excretion of salt ions and small amounts of water in scanty urine from kidneys (a) Osmoregulation in a saltwater fish

Freshwater Animals • Freshwater animals – Constantly take in water from their hypoosmotic environment

• Freshwater animals maintain water balance – By excreting large amounts of dilute urine • Salts lost by diffusion – Are replaced by foods and uptake across the gills 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 Figure 44. 3 b (b) Osmoregulation in a freshwater fish Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Animals That Live in Temporary Waters • Some aquatic invertebrates living in temporary ponds – Can lose almost all their body water and survive in a dormant state • This adaptation is called anhydrobiosis 100 µm Figure 44. 4 a, b (a) Hydrated tardigrade Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Dehydrated tardigrade

Land Animals • Land animals manage their water budgets – By drinking and eating moist foods and by using metabolic water Water balance in a kangaroo rat (2 m. L/day = 100%) Water balance in a human (2, 500 m. L/day = 100%) Ingested in food (750) Ingested in food (0. 2) Ingested in liquid (1, 500) Water gain Derived from metabolism (250) Derived from metabolism (1. 8) Feces (0. 9) Water loss Figure 44. 5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Urine (0. 45) Evaporation (1. 46) Feces (100) Urine (1, 500) Evaporation (900)

Excretion • Forms of nitrogenous waste • Excretory processes • Excretory systems Copyright ©

• 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Among the most important wastes – Are the nitrogenous breakdown products of proteins and nucleic acids Nucleic acids Proteins Nitrogenous bases Amino acids –NH 2 Amino groups Many reptiles Most aquatic Mammals, most (including animals, including amphibians, sharks, birds), insects, most bony fishes some bony fishes land snails O NH 3 Figure 44. 8 O C Ammonia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings HN NH 2 Urea O C C H N C C N N H H Uric acid C O

Forms of Nitrogenous Wastes • Different animals – Excrete nitrogenous wastes in different forms

Ammonia • Animals that excrete nitrogenous wastes as ammonia – Need access to lots of water – Release it 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

The Influence of Evolution and Environment on Nitrogenous Wastes • The kinds of nitrogenous wastes excreted – Depend on an animal’s evolutionary history and habitat • The amount of nitrogenous waste produced – Is coupled to the animal’s energy budget Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Excretory processes • Diverse excretory systems are variations on a tubular theme • Excretory systems – Regulate solute movement between internal fluids and the external environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Most excretory systems – Produce urine by refining a filtrate derived from body fluids Capillary Filtrate Excretory tubule 1 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. 2 Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids. 3 Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule. Urine Figure 44. 9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4 Excretion. The filtrate leaves the system and the body.

• 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

Survey of Excretory Systems • The systems that perform basic excretory functions – Vary widely among animal groups – Are generally built on a complex network of tubules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Protonephridia: Flame-Bulb Systems • A protonephridium – Is a network of dead-end tubules lacking internal openings Nucleus of cap cell Cilia Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock) Tubule cell Flame bulb Protonephridia (tubules) Figure 44. 10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tubule Nephridiopore in body wall

• The tubules branch throughout the body – And the smallest branches are capped by a cellular unit called a flame bulb • These tubules excrete a dilute fluid – And function in osmoregulation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Metanephridia • Each segment of an earthworm – Has a pair of open-ended metanephridia Coelom Capillary network Bladder Collecting tubule Nephridiopore Figure 44. 11 Nephrostome Metanephridia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Metanephridia consist of tubules – That collect coelomic fluid and produce dilute

Malpighian Tubules • In insects and other terrestrial arthropods, malpighian tubules – Remove nitrogenous wastes from hemolymph and function in osmoregulation Digestive tract Rectum Intestine Hindgut Malpighian Midgut tubules (stomach) Salt, water, and nitrogenous wastes Feces and urine Anus Malpighian tubule Rectum Figure 44. 12 HEMOLYMPH Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reabsorption of H 2 O, ions, and valuable organic molecules

• Insects produce a relatively dry waste matter – An important adaptation to

Vertebrate Kidneys • Kidneys, the excretory organs of vertebrates – Function in both excretion and osmoregulation – They are compact structures containing highly organized tubules – Vertebrate excretory system includes a dense network of capillaries associated with tubules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mammalian kidney • 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 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 Kidney Aorta Ureter Urinary bladder Urethra Figure 44. 13 a (a) Excretory organs and major associated blood vessels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Urine exits each kidney – Through a duct called the ureter •

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 Renal cortex Renal pelvis Ureter Figure 44. 13 b Section of kidney from a rat (b) Kidney structure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The nephron, the functional unit of the vertebrate kidney – Consists of a single long tubule and a ball of capillaries called the glomerulus Juxta. Cortical medullary nephron Afferent arteriole from renal artery Glomerulus Bowman’s capsule Proximal tubule Renal cortex Peritubular capillaries Collecting duct To renal pelvis 20 µm Renal medulla SEM Efferent arteriole from glomerulus Loop of Henle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Collecting duct Branch of renal vein Descending limb Ascending limb Figure 44. 13 c, d (c) Nephron Distal tubule (d) Filtrate and blood flow Vasa recta

Filtration of the Blood • Filtration occurs as blood pressure – Forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Pathway of the Filtrate • From Bowman’s capsule, the filtrate passes through three regions of the nephron – The proximal tubule, the loop of Henle, and the distal tubule • Fluid from several nephrons – Flows into a collecting duct Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

From Blood Filtrate to Urine: A Closer Look • Filtrate becomes urine – As it flows through the mammalian nephron and collecting duct 1 Proximal tubule Na. Cl Nutrients HCO 3 H 2 O K+ H+ NH 3 4 Distal tubule Na. Cl H 2 O HCO 3 K+ H+ CORTEX Filtrate H 2 O Salts (Na. Cl and others) HCO 3– H+ Urea Glucose; amino acids Some drugs 2 Descending limb of loop of Henle 3 Thick segment of ascending limb Na. Cl H 2 O OUTER MEDULLA Na. Cl 3 Thin segment of ascending limb Key Active transport Passive transport 5 Collecting duct Urea Na. Cl INNER MEDULLA Figure 44. 14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings H 2 O

• 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• 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 Na. Cl concentration of body fluids • The collecting duct – Carries the filtrate through the medulla to the renal pelvis and reabsorbs Na. Cl Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Two solutes, Na. Cl and urea, contribute to the osmolarity of the interstitial fluid – Which causes the reabsorption of water in the kidney and concentrates the urine 300 Osmolarity of interstitial fluid (mosm/L) 300 100 CORTEX Active transport Passive transport OUTER MEDULLA Na. Cl H 2 O 400 H 2 O INNER MEDULLA H 2 O 200 Na. Cl 600 Na. Cl 900 Na. Cl 300 H 2 O 400 600 H 2 O 400 Na. Cl H 2 O Na. Cl 300 H 2 O Urea 700 900 H 2 O Urea 1200 Figure 44. 15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Diverse adaptations of the vertebrate kidney have evolved in different environments • The form and function of nephrons in various vertebrate classes – Are related primarily to the requirements for osmoregulation in the animal’s habitat Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Exploring environmental adaptations of the vertebrate kidney MAMMALS Bannertail Kangaroo rat (Dipodomys spectabilis) Beaver (Castor canadensis) BIRDS AND OTHER REPTILES Roadrunner (Geococcyx californianus) Desert iguana (Dipsosaurus dorsalis) FRESHWATER FISHES AND AMPHIBIANS MARINE BONY FISHES Rainbow trout (Oncorrhynchus mykiss) Figure 44. 18 Frog (Rana temporaria) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Northern bluefin tuna (Thunnus thynnus)