Freshwater animals show adaptations that reduce water uptake and conserve solutes Desert and marine animals face desiccating environments that can quickly deplete body water Osmoregulation regulates solute concentrations and balances the gain and loss of water Excretion gets rid of metabolic wastes

Osmoregulation balances the uptake and loss of water and solutes Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment

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 Osmoregulation in a saltwater fish Excretion of salt ions and small amounts of water in scanty urine from kidneys

Excretion of large amounts of water in dilute urine from kidneys Osmotic water gain through gills and other parts of body surface Osmoregulation in a freshwater fish Uptake of salt ions by gills Uptake of water and some ions in food

Nostril with salt secretions Nasal salt gland

Nitrogenous bases Nucleic acids Amino acids Proteins —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 AmmoniaUreaUric acid

Ammonia Animals that excrete nitrogenous wastes as ammonia need lots of water They release ammonia across the whole body surface or through gills

Urea The liver of mammals and most adult amphibians converts ammonia to less toxic urea The circulatory system carries urea to the kidneys, where it is excreted

Uric Acid Insects, land snails, and many reptiles, including birds, mainly excrete uric acid Uric acid is largely insoluble in water and can be secreted as a paste with little water loss

Excretory Processes Most excretory systems produce urine by refining a filtrate derived from body fluids Key functions of most excretory systems: – Filtration: pressure-filtering of body fluids – Reabsorption: reclaiming valuable solutes – Secretion: adding toxins and other solutes from the body fluids to the filtrate – Excretion: removing the filtrate from the system

Nephrons and associated blood vessels are the functional unit of the mammalian kidney 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 Each kidney is supplied with blood by a renal artery and drained by a renal vein Urine exits each kidney through a duct called the ureter Both ureters drain into a common urinary bladder

Excretory organs and major associated blood vessels Renal medulla Renal cortex Renal pelvis Section of kidney from a rat Kidney structure Ureter Kidney Glomerulus Bowman’s capsule Proximal tubule Peritubular capillaries Afferent arteriole from renal artery Efferent arteriole from glomerulus Distal tubule Collecting duct SEM 20 µm Branch of renal vein Filtrate and blood flow Vasa recta Descending limb Ascending limb Loop of Henle Renal medulla Nephron To renal pelvis Renal cortex Collecting duct Juxta- medullary nephron Cortical nephron Posterior vena cava Renal artery and vein Aorta Ureter Urinary bladder Urethra

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 The nephron 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 The filtrate in Bowman’s capsule mirrors the concentration of solutes in blood plasma

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

From Blood Filtrate to Urine: A Closer Look Filtrate becomes urine as it flows through the mammalian nephron and collecting duct Secretion and reabsorption in the proximal tubule change the filtrate’s volume and composition Reabsorption of water occurs as filtrate moves into the descending limb of the loop of Henle

In the ascending limb of the loop of Henle, salt diffuses from the permeable tubule into the interstitial fluid. The distal tubule regulates the K + and NaCl concentrations of body fluids. The collecting duct carries filtrate through the medulla to the renal pelvis and reabsorbs NaCl. The mammalian kidney conserves water by producing urine that is much more concentrated than body fluids.

Filtrate H 2 O Salts (NaCl and others) HCO 3 – H + Urea Glucose; amino acids Some drugs Key Active transport Passive transport INNER MEDULLA OUTER MEDULLA NaCl H2OH2O CORTEX Descending limb of loop of Henle Proximal tubule NaCl Nutrients HCO 3 – H+H+ K+K+ NH 3 H2OH2O Distal tubule NaCl HCO 3 – H+H+ K+K+ H2OH2O Thick segment of ascending limb NaCl Thin segment of ascending limb Collecting duct Urea H2OH2O

Solute Gradients and Water Conservation The action and precise arrangement of the loops of Henle and collecting ducts are largely responsible for the osmotic gradient that concentrates the urine. NaCl and urea contribute to the osmolarity of the interstitial fluid, which causes reabsorption of water in the kidney and concentrates the urine.

INNER MEDULLA OUTER MEDULLA CORTEX Osmolarity of interstitial fluid (mosm/L) NaCl Urea H2OH2O Active transport Passive transport H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O NaCl Urea H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O

The collecting duct conducts filtrate through the osmolarity gradient, and more water exits the filtrate by osmosis 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 Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney. Aldosterone is a hormone that regulates salt reabsorption in the kidney.