Chapter 51
Salt and Water Balance and Nitrogen Excretion
Salt and Water Balance and Nitrogen Excretion
Tissue Fluid and Water Balance
Excreting Nitrogenous Wastes
The Diverse Excretory Systems of Invertebrates
Vertebrate Excretory Systems
The Mammalian Excretory System
Regulation of Kidney Functions
Tissue Fluid and Water Balance
The extracellular environment of the simplest marine animals is seawater.
More complex marine animals have extracellular fluids (tissue fluids) isolated from, but similar to, seawater.
Most marine vertebrates, all freshwater vertebrates, and terrestrial animals maintain extracellular fluids that differ substantially from seawater.
The extracellular fluids influence the water balance of the cells.
Tissue Fluid and Water Balance
Osmolarity of a solution is the moles of osmotically active particles per liter of solvent.
A 1 molar solution of glucose equals 1 osmolar. A 1 molar solution of NaCl is 2 osmolar.
High extracellular osmolarity dehydrates cells; low extracellular osmolarity causes them to swell.
To achieve water balance, animals must be able to maintain the osmolarity of their tissue fluid within an appropriate range.
Tissue Fluid and Water Balance
Excretory organs control osmolarity and volume of tissue fluid by excreting excess solutes and retaining others that are valuable or in short supply.
Terrestrial organisms also use excretory organs to eliminate waste products of nitrogen metabolism. The output is called urine.
Organisms have diverse excretory systems depending on the environments in which they live.
In all cases, water moves by simple __________.
Tissue Fluid and Water Balance
The basics of excretory system function are the same for many animals.
The excretory organs filter tissue fluid to produce a filtrate that does not contain cells or large molecules such as proteins.
This is often done on blood plasma driven across capillary walls by blood pressure.
The filtrate is then further processed, usually in a series of tubules, where active secretion and absorption change the composition of the filtrate.
Tissue Fluid and Water Balance
Some aquatic animals are osmoconformers; the osmolarity of their tissue fluids equilibrates with the external environment.
Marine invertebrates are __________, within limits.
Animals could not survive if their tissue fluid had the osmolarity of fresh water or highly concentrated seawater.
Tissue Fluid and Water Balance
Osmoregulators maintain constant osmolarity in tissue fluids..
To osmoregulate in fresh water, solutes must be retained and excess water excreted. These animals produce large amounts of dilute urine.
To osmoregulate in salt water, water must be conserved and excess solutes excreted. These animals produce small amounts of urine and have ways of excreting salts.
Tissue Fluid and Water Balance
Even animals that osmoconform over a range of environmental osmolarities must osmoregulate in extreme environments.
Artemia, the brine shrimp, can survive in concentrated salt water with osmolarities as high as 2,500 mosm/l.
They maintain tissue fluid osmolarities well below that and are called hypoosmotic regulators.
Artemia actively transport of Cl out of gill membranes and Na+ ions follow.
In dilute seawater environments they reverse transport of Cl across the gills and are called hyperosmotic regulators.
Tissue Fluid and Water Balance
Osmoconformers can also be ionic conformers, allowing the ionic composition, as well as the osmolarity, to match the environment.
But most osmoconformers are ionic regulators and employ an __________ transport mechanism to alter tissue fluid ionic composition.
Tissue Fluid and Water Balance
Terrestrial environments present entirely different problems. Most terrestrial organisms must conserve water.
Terrestrial animals get salts from food. Most plants are low in sodium, so herbivores must conserve Na+ or get it from other sources such as salt licks.
Birds that feed on marine animals must get rid of excess salt. __________ salt glands excrete concentrated salt solutions.
Excreting Nitrogenous Wastes
Fats and carbohydrates break down into water and CO2, which are easily eliminated.
Proteins and nucleic acids contain nitrogen. Breakdown of these produces nitrogenous waste.
Ammonia is the most common nitrogenous waste product and is __________ toxic.
It must be quickly eliminated or converted into less toxic molecules such as urea and uric acid.
Excreting Nitrogenous Wastes
Aquatic animals continuously excrete ammonia.
It is very soluble in water and diffuses rapidly across gill membranes.
These animals are called ammonotelic.
Excreting Nitrogenous Wastes
In mammals, as little as 5 mg/100 ml of ammonia in the blood would be lethal.
Terrestrial animals convert ammonia into __________ or uric acid.
Ureotelic animals, including mammals, amphibians, and cartilaginous fishes, excrete urea as their principal nitrogenous waste.
Urea is water-soluble. Mammals have evolved excretory systems to conserve water by concentrating urea.
Cartilaginous fishes keep their body fluids __________ to sea water by retaining high concentrations of urea.
Excreting Nitrogenous Wastes
Uricotelic animals excrete nitrogenous waste as uric acid.
Insects, reptiles, birds, and some amphibians are uricotelic.
Uric acid is water-insoluble and is excreted as a __________ (the whitish material in bird droppings).
Uric acid excretion prevents water loss during removal of nitrogenous waste.
Excreting Nitrogenous Wastes
Most species produce more than one nitrogenous waste.
Humans are ureotelic but also excrete some ammonia and uric acid in their urine.
Different developmental stages may have different forms of nitrogen excretion. For example, tadpoles excrete ammonia; adult frogs excrete urea.
Some frogs and toads in arid regions excrete uric acid.
There seems to be considerable evolutionary flexibility in nitrogenous waste excretion.
The Diverse Excretory Systems of Invertebrates
Freshwater flatworms, such as Planaria, excrete water by means of a network of tubules that run throughout their bodies.
Each tubule ends in a __________ cell, which is a tubule with cilia beating inside.
The flame cell and tubule together form a protonephridium.
Tissue fluid enters tubules, and beating cilia cause this fluid to flow toward the excretory pore of the worm. The fluid is modified as it flows through the tubules.
The fluid that leaves is less concentrated, so ions are conserved and water is excreted.
The Diverse Excretory Systems of Invertebrates
Annelids have a closed circulatory system and develop a blood pressure, which is used to filter blood.
The blood is filtered through the capillary walls, and the filtrate accumulates in the coelom.
The coelomic fluid is then processed by metanephridia, paired structures in each body segment.
Each metanephridium begins in one segment as a ciliated, funnel-like opening called a nephrostome.
The Diverse Excretory Systems of Invertebrates
Coelomic fluid enters the metanephridia through nephrostomes, which lead to a tubule in the next segment.
Resorption and secretion occur in the tubules and dilute, hypotonic urine leaves the animal through nephridiopores.
The Diverse Excretory Systems of Invertebrates
Insects have a system of tubules called __________ tubules which attach to the gut and are closed at the other end. They project throughout the body cavity.
The tubules actively transport uric acid, K+, and Na+ into the tubules, and water follows.
Muscle fibers help move the tubule contents toward the hindgut.
The Diverse Excretory Systems of Invertebrates
In the hindgut, processing causes uric acid precipitation.
Hindgut and rectum epithelial cells transport Na+ and K+ back into the tissue fluid.
Water follows the resorbed ions.
Vertebrate Excretory Systems
The kidney is the major excretory organ of vertebrates.
The functional unit of the vertebrate kidney is the __________.
Each human kidney has about a million nephrons.
The nephron probably evolved to excrete excess water. But adaptations of different vertebrate groups have allowed them to exploit environments in which water must be conserved and salts excreted.
Vertebrate Excretory Systems
Marine bony fishes cannot produce urine more concentrated than their body fluid.
They prevent excessive loss of water by producing very little urine.
The fishes take in a lot of salt with their food. Some ions (such as Mg2+) are not absorbed by the gut, and others (such as Cl) are actively excreted from the gill membranes and renal tubules.
Nitrogenous wastes are excreted as ammonia.
Vertebrate Excretory Systems
Cartilaginous fishes are osmoconformers but not ionic conformers. These fishes convert nitrogenous wastes to urea and trimethylamine oxide.
They retain these compounds in their tissues and as a result, their tissues have an osmolarity close to that of seawater.
Sharks and rays excrete extra salt primarily through a salt-secreting __________ gland.
Vertebrate Excretory Systems
Most amphibians produce dilute urine and conserve salts just as freshwater fishes do.
Some have adapted to habitats that require water conservation by having reduced permeability of their skin to water, burrowing in the ground, and entering estivation during dry periods.
Estivation is a state of very low metabolic activity.
Before estivation, they store dilute urine in a large bladder and use it as a source of water during estivation.
Vertebrate Excretory Systems
Reptiles occupy habitats ranging from aquatic to extremely hot and dry.
Three major adaptations allowed reptiles to exploit dry environments:
Internal fertilization and eggs with shells that retard water loss
Scaly, dry skin that retards water loss
Excretion of nitrogenous wastes as uric acid solids, losing little water in the process
Vertebrate Excretory Systems
Birds also have internal fertilization, shelled eggs, and excrete uric acid as the nitrogenous waste product.
Some birds can produce a urine that is more concentrated than their tissue fluids.
Vertebrate Excretory Systems
Nephrons have three main parts:
The glomerulus is a ball of capillaries that filters plasma.
The renal tubules receive and modify filtrate.
Peritubular capillaries carry substances to and from the renal tubules.
Vertebrate Excretory Systems
The two capillary beds of the nephronthe glomerulus and the peritubular capillarieslie in series between the arteriole and the venule.
An afferent arteriole supplies blood under pressure to the glomerulus; the blood leaves through an efferent arteriole.
The renal tubule begins with Bowmans capsule which encloses the glomerulus.
Cells of the capsule that come into direct contact with the glomerular capillaries are called podocytes. They have fine projections that wrap around and cover the capillaries.
Vertebrate Excretory Systems
The glomerulus filters the blood to produce a fluid that lacks cells and large molecules (renal filtrate).
The walls of the capillaries, the basal lamina of the capillary endothelium, and the podocytes of Bowmans capsule all participate in filtration.
The force that drives filtration in the glomerulus is the pressure of the arterial blood.
Vertebrate Excretory Systems
The composition of fluid entering the nephron is similar to that of blood plasma, except it lacks the plasma proteins.
As the fluid moves down the renal tubule, it is concentrated and altered to form urine.
The cells of the tubule control the composition of the urine by actively secreting and resorbing specific molecules.
The Mammalian Excretory System
Humans have two kidneys, which filter blood, process the filtrate into urine, and release the urine into a duct called the ureter.
The ureter of each kidney leads to the urinary bladder, where urine is stored until it is excreted through the urethra.
Two sphincter muscles surrounding the base of the urethra control the __________ of urination.
One is smooth muscle, controlled by the autonomic nervous system. When the bladder is full, a spinal reflex relaxes this sphincter.
The other sphincter is skeletal muscle and is under voluntary [we hope] control.
The Mammalian Excretory System
The ureter, renal artery, and renal vein enter the kidney on its concave side.
Subunits of the kidney called renal pyramids include all the functional units.
The renal pyramids make up the medulla of the kidney and are surrounded by tissue called the cortex.
Each kidney has about a million nephrons, which are its basic functional units.
All glomeruli are located in the cortex.
The Mammalian Excretory System
The first section of a renal tubule (closest to the glomerulus) is called the proximal convoluted tubule. These lie in the cortex.
The renal tubule then dives into the medulla and makes a loop called the loop of Henle.
The tubule of the loop of Henle makes a hairpin turn within the medulla and continues back up to the cortex.
When it reaches the cortex again it is called the distal convoluted tubule.
Distal convoluted tubules join to form collecting ducts.
The Mammalian Excretory System
The organization of blood vessels in the kidney parallels the organization of the nephrons.
Arterioles branch from the renal artery and travel into the cortex.
An afferent arteriole carries blood to each glomerulus. The efferent arteriole also gives rise to the peritubular capillaries.
Most peritubular capillaries are in the cortex, but a few run down into the medulla, parallel to the loop of Henle and the collecting ducts.
In the medulla region, these capillaries form the vasa recta.
The Mammalian Excretory System
Most of the water and solutes filtered in the glomerulus are __________.
The kidneys filter 12% of the blood they receive, which is about 180 liters per day.
About 2 3 liters of urine are produced, which is only 1 2% of the volume filtered.
The Mammalian Excretory System
The proximal convoluted tubule has specialized cuboidal cells with thousands of microvilli which greatly increase the surface area for resorption of ions and molecules.
The cells have many mitochondria to produce the ATP needed to operate the transport systems.
They actively transport Na+ and other solutes, such as glucose and amino acids, out of the tubule lumen, and water follows by osmosis.
Almost all glucose and amino acid molecules are resorbed.
The peritubular capillaries take up the water and solutes.
The Mammalian Excretory System
Humans can produce urine that is four times more concentrated than their blood. Some mammals produce urine even more concentrated.
The loop of __________ functions as a countercurrent multiplier system to increase the solute potential of the surrounding tissue fluid.
The tubule fluid in the descending limb of the loop flows in the opposite direction from that of the ascending limb.
The system creates a solute concentration gradient in the renal medulla.
The Mammalian Excretory System
The segments of the loop of Henle are anatomically and functionally different.
Cells of the descending limb are flat, with no microvilli, and not specialized for transport.
Part way up the ascending limb, cells become specialized for active transport. These cells are cuboidal and have lots of mitochondria.
Therefore, the loop of Henle is divided into the thin descending limb, the thin ascending limb, and the thick ascending limb.
The Mammalian Excretory System
The thick ascending limb __________ resorbs Cl and Na+ follows __________. This limb is not permeable to water, so the surrounding tissue becomes more concentrated with NaCl.
The thin descending limb is permeable to water, but not to NaCl, so water is withdrawn osmotically here. Fluid in the descending limb becomes more concentrated as it nears the turn.
The thin ascending limb is not permeable to water but is permeable to NaCl, which moves out of the tubule.
Fluid reaching the distal convoluted tubule is more dilute than blood plasma, and a concentration gradient is set up in the medulla.
The Mammalian Excretory System
When the fluid enters the collecting duct, it is the same concentration as blood plasma, but the composition is different.
The major solute in the duct is now urea. As fluid flows down the collecting duct, it loses water osmotically.
Some urea also leaks out of the collecting duct to the surrounding tissue, which increases the osmotic potential of the tissue.
The urea diffuses back to the loop of Henle. The recycling of urea contributes to the ability to concentrate urine.
The Mammalian Excretory System
Animals such as the desert gerbil that can concentrate urine to a high degree have very long loops of Henle, which increases the concentration gradient in the surrounding tissues.
The desert gerbil gets all of its water from the food it eats.
The Mammalian Excretory System
The kidneys regulate the pH of blood. Blood pH is critical because it influences the structure and function of proteins.
A buffer is a substance that can absorb or release H+ ions. Blood is buffered by bicarbonate ions released from carbonic acid.
CO2 + H2O H2CO3 H+ + HCO3
If excess H+ is added, the reaction goes to the left and H+ is removed.
If H+ are removed, the reaction goes to the right and more H+ are produced.
The Mammalian Excretory System
The kidneys control the levels of H+ and HCO3 in the blood.
The renal tubules secrete H+ into the tubule fluid, and resorb HCO3 .
Regulation of Kidney Functions
Autoregulatory mechanisms compensate for decreased cardiac output or decreased blood pressure so that glomerular filtration rate (GFR) stays high.
One mechanism for maintaining pressure is dilation of afferent renal arterioles when blood pressure drops.
If this is still inadequate to keep GFR from falling, kidneys release an enzyme called renin, which activates a circulating hormone called angiotensin.
Regulation of Kidney Functions
Angiotensin has several effects:
It causes the efferent renal arterioles to constrict, increasing blood pressure in the glomerular capillaries.
It causes all peripheral blood vessels to constrict.
It stimulates the adrenal cortex to release the hormone aldosterone, which stimulates greater sodium resorption by the kidney. This increases water retention and circulatory plasma volume.
Regulation of Kidney Functions
Loss of blood volume causes stretch receptors in the aorta and carotid arteries to trigger the hypothalamus to release antidiuretic hormone (ADH).
ADH increases permeability of the collecting ducts of the kidneys to water.
This increases water resorption and the concentration of the urine. Increased water resorption increases blood pressure.
ADH increases water permeability by stimulating the cellular production of aquaporins, membrane proteins that form water channels.
Regulation of Kidney Functions
The atria of the heart release atrial natriuretic peptide (ANP) when blood pressure gets abnormally high.
When this hormone reaches the kidney, it decreases the resorption of sodium. This causes an increased loss of sodium and water, which lowers blood volume and pressure.
Animation 51.1 The Mammalian Kidney
Video 51.1 Lysis of a salt-water Paramecium in a freshwater environment
Video 51.2 Endoscopic view of kidneys, ureters, and bladder