salt and water balance and nitrogen excretion 40

Salt and Water Balance and Nitrogen Excretion

40

Chapter 40 Salt and Water Balance and Nitrogen Excretion

Key Concepts

• 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid

• 40.2 Excretory Systems Eliminate Nitrogenous Wastes

• 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion

Chapter 40 Salt and Water Balance and Nitrogen Excretion

Key Concepts

• 40.4 The Mammalian Kidney Produces Concentrated Urine

• 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition

Chapter 40 Opening Question

How do excretory systems of animals maintain homeostasis of the interstitial fluid in the face of extreme challenges?

Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid

Excretory systems control volume, concentration, and composition of the extracellular fluid and excrete wastes.

Four excretory functions:

• Regulate fluid volume in the body

• Regulate solute concentrations, or osmolarity, of extracellular fluid

• Maintain individual solutes

• Eliminate nitrogenous wastes

Urine is the liquid waste product.


The osmolarity of a solution is the number of osmoles of active solutes per liter of solvent.

The osmolarity of the extracellular fluid must be maintained for cellular water balance.

If the osmolarity of the extracellular fluid is different than the cytoplasm, water will move into or out of the cells via osmosis, and cells may be damaged.


Animals in different environments face different osmolarity problems.

On land, salt and water must be conserved.

Terrestrial animals are osmoregulators and actively regulate the osmolarity of their extracellular fluid.

Freshwater animals have to conserve salts but excrete excess water, so are also osmoregulators.


Marine animals are exposed to the high osmolarity of the ocean:

Osmoconformers equilibrate their osmolarity with seawater.

Artemia (brine shrimp) can survive in varied environmental osmolarities:

• In high osmolarity, Cl– is actively transported out through the gills, Na+ ions follow.

• In low osmolarity, the transport of Cl– is reversed.

Figure 40.1 Osmoconformity Has Limits


Most animals are ionic regulators, conserving some ions and excreting others to maintain ionic composition of extracellular fluid.

Ionic conformers allow their ionic composition, as well as their osmolarity, to match the environment.

Commonly regulated ions are Na+, Cl–, K+, Ca2+, H+, and HCO3

– (bicarbonate).


H+ concentration, or pH, is closely regulated—important to protein structure and function.

A buffer is a substance that can absorb or release hydrogen ions.

The major buffer in blood is bicarbonate (HCO3

–), which is formed from CO2.

Lungs eliminate CO2 from blood, and kidneys reabsorb HCO3

– and excrete H+ to maintain pH.

Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes

Animals must eliminate metabolic waste products:

• Carbohydrates and fat end up as water and CO2 and are easily excreted

• Proteins and nucleic acids contain nitrogen, so metabolism produces nitrogenous waste

Ammonia (NH3) is the most common nitrogenous waste.


Ammonia is soluble in water—aquatic animals who secrete NH3 through gills are ammonotelic.

Animals must convert NH3 to urea or uric acid.

Ureotelic animals mostly excrete urea. It is water-soluble but results in large water loss.

Uricotelic animals mostly excrete uric acid. It is insoluble in water and precipitates out of the urine with little water loss.

Figure 40.2 Waste Products of Metabolism


Most species secrete more than one nitrogenous waste.

Humans are ureotelic but also excrete:

• Uric acid—from metabolism of nucleic acids and caffeine

• Ammonia—regulates pH of extracellular fluid by buffering urine

Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion

In most systems urine is produced by filtering extracellular fluid.

The resulting fluid is a filtrate—similar to blood plasma.

Filtrate flows through tubules and is modified by reabsorption or secretion of solutes.

The modified filtrate is excreted as urine.


Annelids have segmented bodies, with a coelom in each segment.

In earthworms, blood pumped under pressure causes blood to filter across capillary walls.

Water, small molecules, and some waste products enter the coelom.


Each earthworm segment contains a pair of metanephridia.

A metanephridium begins as a nephrostome, or opening, which leads into a tubule.

The tubule ends in a nephridiopore.

Fluid enters through the nephrostomes, and tubule cells reabsorb or secrete molecules into it.

Figure 40.3 Metanephridia in Earthworms


The insect excretory system consists of Malpighian tubules—blind-ended tubules that open into the gut.

Tubule cells actively transport uric acid, K+, and Na+ into the tubules.

Water follows the solutes and moves contents toward the gut.

Water and ions are recovered in the hindgut, and uric acid and other waste are excreted.

Figure 40.4 Malpighian Tubules in Insects (Part 1)

Figure 40.4 Malpighian Tubules in Insects (Part 2)


Vertebrates are well-adapted to excrete excess water.

Kidney—the main excretory organ

Nephron—the main functional unit of the kidney, consisting of a renal tubule and the surrounding blood vessels

Nephrons filter large volumes of blood and achieve bulk reabsorption.

Figure 40.5 The Vertebrate Nephron


A nephron begins with Bowman’s capsule, which encloses the glomerulus.

Blood enters through the afferent arteriole and leaves through the efferent arteriole.

The glomerulus is highly permeable to water, ions, and small molecules, but impermeable to cells and large molecules.

Figure 40.6 A Tour of the Nephron (Part 1)

Figure 40.6 A Tour of the Nephron (Part 2)


Filtration occurs when blood pressure drives water and solutes through fenestrations in glomerular capillaries.

Filtration slits in Bowman’s capsule are formed by podocytes—specialized cells with projections that wrap around capillaries.

Glomerular filtration rate is the rate of the filtered fluid entering the capsule.


The filtrate entering the capsule is similar to blood plasma—its composition is adjusted as it passes along the renal tubule

Peritubular capillaries transport substances to and from the renal tubules.


Marine bony fishes must conserve water in their high osmolarity environment.

They minimize water loss by producing very little urine.

Some ions are not absorbed in their gut—NaCl is excreted through the gills.


Cartilaginous fishes convert nitrogenous wastes to compounds and retain large amounts in the extracellular fluid.

The fluid is similar in osmolarity to seawater so water is not lost by osmosis to the environment.


Amphibians in dry environments reduce permeability of their skin to water.

Estivation is a state of low metabolic activity and low water turnover.

Some frogs fill a large bladder with dilute urine before estivation and gradually reabsorb it into the blood.


Reptiles are amniotes and have three major adaptations that allow them to exist outside of water:

• Amniotic reproduction, shelled eggs

• Scaled epidermis that retards water loss

• Excretion of nitrogenous wastes as uric acid, with little water loss


Mammals also have adapted to conserve water:

• Skin covering to reduce water loss

• Amniotic reproduction

• Evolution of a kidney to produce concentrated urine

Concept 40.4 The Mammalian Kidney Produces Concentrated Urine

Mammals have kidneys that filter blood and produce urine.

In each nephron, there is a specialized feature: the loop of Henle.

Ion transport in this region creates an area of high osmolarity, so that water is able to be reabsorbed from the urine.

This concentrates the urine and reduces water loss.


Ureter—a duct from the kidney that leads to the urinary bladder

Urethra—a tube for urine excretion leading from the urinary bladder, where urine is stored, to the outside of the body

The ureter, renal artery, and renal vein enter the kidney on the concave side.

Figure 40.7 The Human Excretory System (Part 1)


Kidneys have an outer cortex that covers the inner medulla.

The glomeruli and Bowman’s capsules are in the cortex.

Proximal convoluted tubules—the initial, twisted segments of the renal tubules, located in the cortex


The renal tubule descends into the medulla and forms the loop of Henle, which is important for urine concentration.

After forming the loop, the tubule returns to the cortex.

The loop of Henle leads to the distal convoluted tubule.

The distal convoluted tubules join the collecting duct in the cortex.

Collecting ducts empty into the pelvis, which drains into the ureter.


The vasa recta is a network of peritubular capillaries parallel to the loops of Henle and the collecting duct.

Blood plasma that does not enter Bowman’s capsule goes via the efferent artery to the peritubular capillaries.

These play an important role in secretion and reabsorption and in maintaining the high-osmolarity region.


The proximal convoluted tubule (PCT) is responsible for the reabsorption of water and solutes—osmolarity does not change.

PCT cells actively transport Na+, glucose, and amino acids.

Water follows the transport of solutes.

Next steps in urine processing:

• Reabsorb salts, leaving urea

• Set up conditions for hypertonic urine production


Concentration of urine is due to a countercurrent multiplier mechanism in the loops of Henle.

Tubule fluid flows in opposite directions in the two limbs of a loop of Henle.

The loops increase osmolarity of extracellular fluid in a graduated way.

Figure 40.8 Concentrating the Urine


Loop of Henle segments:

• Thick ascending limb—actively transports Na+ (Cl– follows) and raises its concentration in the interstitial fluid

• Thin descending limb—loses water to the neighboring interstitial fluid with high Na+ and Cl– concentration


• Thin ascending limb—receives concentrated fluid from descending limb and allows diffusion of Na+ and Cl– into the interstitial fluid

Fluid reaching the distal collecting duct is less concentrated—solutes in the medulla create a concentration gradient.


The concentration gradient is preserved by the vasa recta.

Blood flowing down the descending limb loses water and gains solutes.

Concentrated blood flowing up the ascending limb gains water and loses solutes—water is thus returned to the bloodstream.


Water reabsorption and fine-tuning of ionic composition begins in the distal convoluted tubule.

Fluid that leaves the tubule and flows into the collecting duct as urine has different solute composition than blood plasma.


In collecting duct the major solute in tubular fluid is urea.

Fluid flows down collecting duct and loses water to interstitial fluid because of concentration gradient established by loops of Henle.

Some urea also diffuses and adds to osmotic force—recycling this urea contributes to urine concentration.


Renal failure results in:

• Salt and water retention (high blood pressure)

• Urea retention (uremic poisoning)

• Decreasing pH (acidosis)

Dialysis treatment passes blood through membrane channels bathed in a plasma-like solution to remove wastes.

Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition

A constant glomerular filtration rate (GFR) requires blood supplied to the kidneys under adequate pressure.

Autoregulatory mechanisms ensure blood supply and blood pressure.

Hormones released by other organs also help regulate the kidneys.


If GFR begins to fall, the first autoregulatory response is dilation of afferent renal arterioles—increases glomerular blood pressure.

Kidney releases renin if GFR still falls, this activates angiotensin.


Angiotensin:

• Constricts efferent renal arterioles

• Constricts peripheral blood vessels to raise blood pressure in the body

• Stimulates release of aldosterone to increase Na+ uptake

• Stimulates thirst to increase water ingestion to raise blood volume and pressure

Figure 40.9 Renin-Angiotensin-Aldosterone System Helps Regulate GFR


The hypothalamus can stimulate release of antidiuretic hormone (ADH, also called vasopressin).

ADH increases the permeability of membranes to water.

Osmoreceptors that detect a rise in blood osmolarity will stimulate ADH release.


ADH causes aquaporins, or water channels, to be inserted in the membranes of cells in the collecting duct.

More water is reabsorbed and urine is more concentrated.

Alcohol inhibits release of ADH—excessive alcohol intake can cause substantial dehydration.


Atrial muscle fibers release atrial natriuretic peptide (ANP) when blood volume in the atria increases

ANP decreases the reabsorption of Na+ in the kidney.

Increased loss of Na+ and water decreases blood volume and pressure.

Figure 40.10 ADH Induces Insertion of Aquaporins into Plasma Membranes (Part 1)

Figure 40.10 ADH Induces Insertion of Aquaporins into Plasma Membranes (Part 2)

Answer to Opening Question

Excretory systems include active transport of Na+, often with Cl–.

Ion transport creates osmotic concentration gradients that move water across membranes.

Depending on an animal’s environment, it will direct the absorption or secretion of solutes.

Sea birds ingest salt water and use a salt-secreting organ with special transporters that take up NaCl and secrete it.

Figure 40.11 Salt Excretion in a Marine Bird

salt and water balance and nitrogen excretion 40

Documents

extracellular fluid

excretory systems of

fluid volume

blood composition slide

excretory functions

high osmolarity

low osmolarity

limits slide