BIOL #20: Osmoregulation & Excretory Systems

Question 1

Osmoregulation & Excretion

Answer 1

Maintaining the fluid environment of cells, tissues, and organs requires that organisms keep the relative concentrations of water and solutes within fairly narrow limits (i.e. homeostasis)
- Cells require precise concentrations of electrolytes (Na+, Cl –, K+, and Ca2+) to function normally (e.g. muscle movement, neuron signaling)

Osmoregulation is the process by which animals control solute concentrations and balance water gain and loss.

Animals must also rid the body of nitrogenous (nitrogen-containing) molecules that are toxic byproducts of breaking down proteins and nucleic acids.
- Excretion is the process that rids the body of nitrogenous metabolites and other metabolic waste products.

Water balance, electrolyte balance, and excretion of waste products are tightly integrated processes.

Question 2

Evolution of Osmoregulatory Strategies

Answer 2

A number of strategies for water and solute control have arisen during evolution, reflecting the challenges presented by different environments:

Animals in arid (dry) environments (e.g. lizards) must conserve water.

Marine animals (e.g. birds, fish) must also conserve water and contend with eliminating salt.

Freshwater animals (e.g. fish) must conserve solutes, such as salts, because they contend with an environment that threatens to flood and dilute their body fluids.

Question 3

Osmosis

Answer 3

The process of osmoregulation is based largely on the controlled movement of solutes between internal fluids and the external environment.

Because solute movement always results in the movement of water by osmosis, the net effect is to regulate both solutes and water.
- Osmosis is the diffusion of water through a selectively permeable membrane from areas of higher water concentration to areas of lower water concentration.

Question 4

Osmolarity

Answer 4

Osmolarity is the total solute concentration of a solution, measured as moles of solute per liter of solution.

Comparison of osmolarity between different solutions predicts the direction of osmosis:

• Isoosmotic solutions are solutions with equal concentrations of solutes (no net movement of water)
• A hyperosmotic solution has a higher solute concentration than a hypoosmotic solution – water will move by osmosis from a hypoosmotic solution to a hyperosmotic one.

Question 5

Osmoconformer

Answer 5

be isoosmotic with the surrounding environment.

All osmoconformers are marine animals.
- However, not all marine animals are osmoconformers.

Organisms such as sponges and jellyfish are osmoconformers.

• Theses animals do not need to osmoregulate because seawater is a fairly constant ionic and osmotic environment and nearly matches the electrolyte concentrations found within these animals – relative to seawater, their tissues are isoosmotic.
• Osmoconformers may require certain concentrations of specific solutes and will actively transport only these solutes to maintain homeostasis.

Question 6

Osmoregulator

Answer 6

osmoregulator, i.e. control the internal osmolarity independent of the surrounding environment.

Question 7

Marine Osmoregulators

Answer 7

Osmoregulation allows some marine animals, such as marine vertebrates, to maintain different internal osmolarity than seawater.

For instance, marine fish actively regulate osmolarity inside their bodies to achieve homeostasis.
- Their tissues are hypoosmotic relative to salt water (the solution inside the body cells contains fewer solutes than the solution outside).

Marine fish live in a strongly dehydrating environment and are under osmotic stress because they lose water and gain electrolytes (e.g. salt) – therefore they must continually take in water to counteract water loss.

Marine osmoregulators must continually take in (drink) water to counteract water loss and lose little water in the urine (produce little urine).

Question 8

Marine Osmoregulatory Adaptations

Answer 8

Marine osmoregulators must continually take in (drink) water to counteract water loss and lose little water in the urine (produce little urine).

Marine osmoregulators must also continually discard excess solutes.

To rid the body of the excess salts taken in, marine fish typically use both their gills and kidneys:

• The gills of marine fish have specialized chloride cells that actively transport chloride ions (Cl -) out of the body, which causes sodium ions (Na+) and potassium ions (K+) to follow passively via diffusion.
• Special kidney adaptations allow for excess calcium, magnesium, and sulfate ions to be excreted via the kidneys with only a small loss of water.

Question 9

Freshwater Osmoregulators

Answer 9

Osmoregulation allows freshwater animals to maintain different internal osmolarity than freshwater.

The tissues of freshwater fish are hyperosmotic relative to the surrounding water (the solution inside the body cells contains more solutes than the solution outside).

Freshwater animals are under osmotic stress because they gain water and lose salt, therefore they must continually discharge excess water.

Freshwater osmoregulators solve the problem of water balance by drinking very little water and excreting large amounts of very dilute urine.

Question 10

Freshwater Osmoregulatory Adaptations

Answer 10

Freshwater osmoregulators solve the problem of water balance by drinking very little water and excreting large amounts of very dilute urine.

Freshwater osmoregulators must also continually take in solutes because they cannot tolerate solute levels as low as freshwater.

To increase salt intake, freshwater fish typically use their gills:
- The gills of freshwater fish have specialized chloride cells that actively transport chloride ions (Cl -) into the body, which causes sodium ions (Na+) and potassium ions (K+) to follow passively via diffusion.

Question 11

Osmoregulatory Adaptations of Migrating Fish

Answer 11

Salmon, sea bass and other fish that migrate between freshwater and seawater undergo dramatic physiological (hormonal) changes that affect their osmoregulatory capabilities.

When in rivers and streams, these fish osmoregulate like other freshwater fish, bringing solutes into their body with chloride cells and producing large amounts of dilute urine.

When they migrate to the ocean they discard solutes from their body with chloride cells and produce small amounts of urine to decrease water loss.

Question 12

Terrestrial Osmoregulation

Answer 12

Land animals constantly lose water to the environment, just as many marine animals do, but they lose it by evaporation (e.g. sweating, panting) rather than osmosis.

Land animals also lose water when they produce urine.

Terrestrial animals are subject to a dehydrating environment and must continually take in water to counteract water loss, and/or use mechanisms to conserve water internally.

Question 13

Terrestrial Osmoregulatory Adaptations

Answer 13

Terrestrial animals are subject to a dehydrating environment and must continually take in water to counteract water loss and/or use mechanisms to conserve water internally.

Body coverings that prevent dehydration:

• Waxy, chitinous layers of insect exoskeletons (cuticle)
• Shells of land snails
• Layers of dead, keratinized skin cells that cover most terrestrial vertebrates, including humans.

Behavioral modifications:

• Insects can close tracheal openings to prevent respiratory water loss.
• Some organisms opportunistically take in more water-laden food to maintain water balance in arid environments.

Question 14

Nitrogenous Wastes

Answer 14

Ammonia (NH3) is a by-product of catabolic reactions (breakdown of proteins and nucleic acids).

Ammonia is very toxic in part because its ion, ammonium (NH4+) interferes with the oxidative phosphorylation stage of cellular respiration.

Some animals excrete ammonia directly but many animals expend energy to convert it to a less toxic compound prior to excretion.

Animals excrete nitrogenous waste as ammonia, urea, or uric acid.

Question 15

Ammonia

Answer 15

Ammonia excretion is most common in aquatic animals.

• Because ammonia is very toxic it can only be tolerated in low concentrations and diluting it is typically only possible in an aquatic environment.
• Ammonia is highly soluble so it easily passes through membranes into surrounding water – in small aquatic organisms it can be lost across the whole body surface, in freshwater fish it is typically lost across the gill epithelium with only a minor amount lost via the kidneys.

Question 16

Urea

Answer 16

Most terrestrial animals and many marine osmoregulators do not have access to enough water to dilute and excrete toxic ammonia, instead producing urea.

Urea is soluble in water.

The main advantage of producing urea is its low toxicity – animals can transport urea in the circulatory system, and store it safely at fairly high concentrations, which helps to prevent water loss.

The main disadvantage of urea is its energy cost – energy must be expended to produce urea from ammonia.

In vertebrates, urea is produced by the liver as a combination of carbon dioxide and ammonia.

Question 17

Uric Acid

Answer 17

Insects, land snails, and many reptiles, including birds, excrete uric acid, which is relatively nontoxic.

Uric acid does not readily dissolve in water, meaning that it can be excreted as a semisolid paste with very little water loss, which is a great advantage for organisms with little access to water.

The main disadvantage of uric acid is its energy cost – producing uric acid is more energetically expensive than producing urea.

What are bird droppings?
- A mixture of white uric acid and brown feces

Question 18

Why Do Nitrogenous Waste Vary among Species?

Answer 18

Type of waste production:
- Correlates with evolutionary history
- Correlates with the habitat that a species occupies, particularly the availability of water.
+ Example:
* Terrestrial turtles, which typically live in dry habitats, excrete mainly uric acid
* Aquatic turtles excrete both urea and ammonia
- Correlates with energy budgets:
+ Endotherms, especially carnivores, typically eat more protein and produce more nitrogenous waste, requiring a nontoxic but energy-efficient way of disposing waste – favoring urea over uric acid.

Question 19

Transport Epithelia

Answer 19

In most animals, osmoregulation and metabolic waste disposal rely on transport epithelia – one or more layers of epithelial cells specialized for moving particular solutes in controlled amounts and in specific directions.

Transport epithelia are typically arranged into complex tubular networks with extensive surface areas.

The coordinated function of transport epithelia in maintaining water balance and metabolic waste disposal is evident in the excretory system of insects, as well as the vertebrate kidney.

Question 20

Excretory Systems

Answer 20

Excretory systems dispose of metabolic wastes and control body fluid composition.

Many animals produce a fluid waste called urine via four basic steps:

1) Filtration
2) Reabsorption
3) Secretion
4) Excretion

Question 21

Excretory Processes

Answer 21

Filtration: body fluid (blood or hemolymph) is brought into contact with selectively permeable membranes of a transport epithelium.
- The filtrate is made up of the water and small solutes (salts, sugars, amino acids, and nitrogenous waste) that can passively move through the membrane, large molecules (such as proteins) remain in the body fluid.

Reabsorption: A selective process that occurs along the transport epithelia that recovers useful molecules (e.g. glucose, salts, vitamins, hormones, and amino acids and water), returning them to body fluids.

Secretion: Addition of nonessential solutes and waste from the body fluid to the filtrate via active transport.

Excretion: the filtrate (e.g. urine) leaves the body

Question 22

Insects: Malpighian Tubules

Answer 22

Insects and other terrestrial arthropods have organs called Malpighian tubules that remove nitrogenous waste and function in osmoregulation.

Water follows the solutes into the tubule by osmosis, and the fluid then passes into the rectum, where most solutes are pumped back into the hemolymph and water reabsorption by osmosis follows.

The nitrogenous waste is excreted mainly as uric acid.

This system allows for very efficient water conservation, making the insect excretory system a key adaptation contributing to success of these animals in terrestrial habitats.

Question 23

Vertebrates: Kidneys

Answer 23

In vertebrates, a specialized organ, the kidney, functions in both osmoregulation and excretion.

Kidneys consist of numerous tubules arranged in a highly organized manner.

The kidney tubules are closely associated with networks of capillaries to allow for passage of material from the circulatory fluid (blood) to the transport epithelium and vice versa.

Question 24

The Mammalian Excretory System: Overview

Answer 24

Kidneys are the organs that function in creating and concentrating urine.

Urine produced by each kidney exits through a duct called a ureter.

Each ureter drains into a sac called the urinary bladder.

Urine is expelled from the urinary bladder through a tube called the urethra upon urination.

Question 25

The Mammalian Excretory System: Kidney

Answer 25

Each kidney has an outer renal cortex and an inner renal medulla.
- Triangular sections in the medulla are called renal pyramids

The kidney tissue is supplied by blood from the renal artery and filtered blood is drained by the renal vein.

Within the cortex and medulla lie tightly packed excretory tubules and associated blood vessels.

The inner renal pelvis collects urine from the excretory tubules to be passed on to the ureters and urinary bladder.

Question 26

The Mammalian Excretory System: Nephron

Answer 26

The nephrons, which are the functional units of the kidney, weave back and forth across the renal cortex and medulla.

Cortical nephrons only reach a short distance into the medulla
- Make up 85% of kidney nephrons

Juxamedullary nephrons extend from the cortex deep into the medulla
- This nephron type is essential for producing urine that is hyperosmotic to body fluids (high in solutes, low in water), which is key for water conservation.

Question 27

The Mammalian Excretory System: Filtrate Formation

Answer 27

Filtrate is formed when blood pressure forces fluid from the blood in the glomerulus (cluster of capillaries) into the lumen of the Bowman’s capsule (beginning of transport epithelium).

Question 28

The Mammalian Excretory System: Processing

Answer 28

Processing occurs as the filtrate passes through three major regions of the nephron:

1) Proximal tubule
2) Loop of Henle
3) Distal tubule

A collecting duct receives processed filtrate from many nephrons and transports the filtrate to the renal pelvis.

Question 29

The Mammalian Excretory System: Blood vessels

Answer 29

Blood enters the glomerulus from the afferent arteriole and exits to the efferent arteriole.

The blood vessels become peritubular capillaries around the proximal and distal tubules and the vasa recta around the Loop of Henle before reaching the renal vein.

The close association between the tubules and blood vessels allows for reabsorption of molecules back into the blood and secretion of molecules into the filtrate.

Question 30

Original Filtrate

Answer 30

The original filtrate that enters the nephron from the glomerulus typically consists of:

H2O
Salts (NaCl and others)
HCO3- (bicarbonate)
H+
Urea
Glucose
Amino acids
Some drugs

Question 31

Nephron: Reabsorption & Secretion

Answer 31

Proximal tubule:

• H2O, Salts (NaCl and others), HCO3- (bicarbonate), other ions are brought back into the blood stream (reabsorption).
• Active transport of salt allows water (and other ions) to follow passively.

Urea is rarely reabsorbed.

More H+, plus NH3 (ammonia), are secreted from the blood into the kidney tubules – H+ acts as a buffer, creating ammonium ions (NH4+) in the filtrate. However, most nitrogenous waste in the filtrate is in the form of urea.

Question 32

Loop of Henie

Answer 32

Loop of Henle (Descending limb):
- Reabsorption of H2O due to large concentration of aquaporins (water channels) in the transport epithelium.

The interstitial fluid is hyperosmotic (high in solutes) in this region, driving water out of the tubules.

This part of the Loop of Henle has few channels for salts and small solutes.

Loop of Henle (Ascending limb):
- Reabsorption of ions but not H2O due to large concentration of ion channels but not aquaporins (water channels) in the transport epithelium.

NaCl moves out passively and then actively as the concentration of NaCl in the interstitial fluid increases – this is a very energetically expensive step of filtration.

Movement of NaCl out into the interstitial fluid helps set up the concentration gradient (solute gradient) for the passive movement of water out of the tubule in the descending limb – much of this salt remains in the kidney tissue and does not enter the surrounding blood vessels so this gradient is not depleted.

Question 33

Distal Tubule

Answer 33

Distal Tubule:

• Plays a key role in regulating the K+ and NaCl concentration in body fluids.
• Water is also passively reabsorbed in this region.
• Contributes to pH regulation by controlling secretion of H+ and reabsorption of HCO3- (bicarbonate).

Question 34

Collecting Duct

Answer 34

Collecting duct:
- Reabsorption of solutes and water very important in this region and is highly regulated.

Hormonal control of permeability (water) and transport (solutes) determines the extent to which urine becomes concentrated –

• More aquaporins are activated when the kidneys need to conserve water.
• Salts are actively reabsorbed and aquaporins shut down to produce dilute urine.

Question 35

Kidney Function: Hormonal Regulation

Answer 35

In mammals, the volume and osmolarity of urine is adjusted according to water and salt balance and rate urea production in the body.

A combination of nervous and hormonal controls manages the osmoregulatory function of the mammalian kidney.

The antidiuretic hormone (ADH), also called vassopressin, is key to osmoregulation.

ADH is produced in the hypothalamus of the brain and stored in the posterior pituitary gland. Osmoreceptor cells in the hypothalamus monitor osmolarity of the blood and regulate release of ADH.

Question 36

Kidney Function: ADH

Answer 36

After eating salty foods or losing water through sweating, blood osmolarity rises (high solute concentration) and more ADH is released into the bloodstream.

ADH makes the kidney transport epithelium in the collecting ducts more permeable to water, resulting in more water being absorbed back into the body to return blood osmolarity back to the set point.
- More aquaporins are activated

High water intake would have the opposite effect, stopping the hypothalamus from releasing ADH so that more water is lost in the urine and blood osmolarity is not too low.
- Fewer aquaporins are functional