Unit 5 - Excretory System

Osmoregulation

Maintenance and selective retention/excretion of salt and water from the body

What are the 3 main body fluids in animals?

Circulatory, interstitial, intracellular

What are the 2 main routes of water transport in epithelial cells?

Transcellular and paracellular

Transcellular transport

Movement through the cells across membranes; involves Na+/K+ ATPase, Ca2+ ATPase, ion channels, electroneutral cotransporters and transporters

Paracellular transport

Movement between cells; involves leaky vs tight epithelia

Osmotic pressure

amount of pressure exerted by solutes needed to stop the movement of water by osmosis

Water moves from low solute concentration (__ ___ _____) to high solute concentration (__ ___ _____) = ______ ______ (________ __ _____ ____)

high water potential, low water potential, osmotic gradient, difference in osmotic pressure

What transports water during osmosis?

Aquaporins

Osmolarity

the measure of solute concentration (number of osmoles per litre); 1 mol of glucose = 1 osmol, 1 mol NaCl = 2 osmol

Osmolality

number of solutes per kilogram (osmol/kg)

Osmolytes

inorganic ions and organic molecules like glucose and proteins

Isotonic conditions

intracellular fluid = 300 mOsm, solution = 300 mOsm; no net movement of water, no change in cell volume

Hypotonic conditions

intracellular fluid = 300 mOsm, solution = 200 mOsm; water diffuses into cell, cell swells

Hypertonic conditions

intracellular fluid = 300 mOsm, solution = 400 mOsm; water diffuses out of cells, cell shrinks/crenulate

How are solutes classified?

Based on their effects on macromolecules

Perturbing

solute classification; disrupt molecular function; ions, charged amino acids

Compatible

solute classification; little effect on macromolecular function; polyols and uncharged amino acids

Counteracting

solute classification; disrupt molecular functions on their own; counteract disruptive effects of other solutes when employed in combination; urea

What is a critical problem faced by all cells?

Maintenance of constant volume with extracellular and intracellular osmotic perturbations

Why does most regulation occur in extracellular fluid?

Because large changes in the levels (gain/loss) of inorganic ions are incompatible with long-term normal protein function

Osmoregulators

osmotic pressure of body fluids is homeostatically regulated and usually different from the external and usually different from the external environment; cells and tissues can’t cope with changes in extracellular osmolarity and ion concentration

Osmoconformer

Body fluids and cells are equal in osmotic pressure to the environment, not actively controlled; high degree of cellular osmotic tolerance

How do osmoconformers deal with high extracellular osmolarities?

By increasing intracellular osmolarities with compatible osmolytes to maintain cell volume

What does osmoregulation look like in freshwater fish?

Water = <5 mOsm, and body fluids = ~300 mOsm; gain majority of salts with food, drink little water; water (passive) and salt (active) uptake at gills; some water gain and salt loss across the skin; excrete a very dilute urine (little salt loss)

What does osmoregulation look like in marine bony fish?

Water = 1000 mOsm, body fluids = ~400mOsm; majority of water is drunk, gain some salts with food; water (passive) loss and salt (active) secretion at gills; some water loss and salt gain across the skin; excrete very concentrated urine (salt-heavy, little water)

What are the two types of marine environments?

Stenohaline and euryhaline

Stenohaline environment

narrow range of environment conditions (deep sea), mainly find stenohaline conformers which are restricted to a narrow range of salinity and cannot regulate own osmolytes to compensate

Euryhaline environment

large changes in environment conditions (tide pools); mainly find euryhaline osmoconformers, which are tolerant to changes in salinity and regulate organic osmolytes in own cells

Osmoregulator control systems

Control systems at interface of epithelia and environment

Osmoconformer control systems

control systems at cell/intracellular fluid

What is a perk of being an osmoconformer?

Less energetically expensive than osmoregulation

How do osmoconformer maintain osmotic pressure in their intracellular fluid?

Via compatible and/or counteracting organic osmolytes, and cellular function is maintained

Excretion

closely tied to osmoregulation; removal of H+ ions, toxic nitrogenous waste from metabolism, and water

Nitrogen waste

Metabolism of proteins and nucleic acids, produces ammonia (NH3)

What are the problems and advantages with NH3 waste?

Highly toxic, must be excreted, but it’s very soluble in water and can be excreted with large volumes of water

What do the different strategies of nitrogenous waste depend on?

Water availability, energy conversion, and toxicity

Which of the nitrogen conversion paths is the most energetically expensive?

Uric acid

Which of the nitrogen conversion paths requires the most water?

Ammonia (NH3)

Ammoniotelic animals

aquatic animals that excrete ammonia directly into the water

Ammonia

very soluble in water and toxic in low concentrations; easily permeates membranes

Ureotelic animals

animals that create urea from ammonia and excrete it

Urea

less toxic than ammonia, tolerated in more concentrated form, uses less water to excrete but uses more energy to create

Uricotelic animals

animals that create uric acid from ammonia and excrete it; birds, land snails, insects and many reptiles

Ammonia excretion in invertebrates

diffuses out of body surface into surrounding water

Ammonia excretion in fish

excreted out of gills and from kidneys (to a minor degree)

Uric acid

adaptation for limited water; 1000 less soluble, precipitates, non toxic but requires the most energy to produce

Why is it important that uric acid can be stored?

Important for vertebrates that develop in shelled eggs

Examples of animals that have the ability to form all 3 types of nitrogen excretion

South American tree frogs use uric acid to conserve water during dry season, American cockroach uses ammonia excretion when in damp environment but can store uric acid during the dry season. and mammal urine contains small amounts of uric acid, but urea dominates

Animals that use nitrogen waste as an osmolyte

cartilaginous fish (sharks and rays) are osmoconformers but use urea to increase tissue osmolarity

Why is it important that cartilaginous fish use urea as an osmolyte

Increasing tissue osmolarity makes them more concentrated than the sea water, which helps prevent water loss in marine environments

Why is high concentration of urea bad and how is it counteracted?

can denature proteins, but effects are counteracted by methylamines (TMAO)

Osmoregulatory organs

Specialized internal organs (kidneys), external surfaces, gut, salt glands

What are the specialized kidneys based on class?

Kidneys - vertebrates

protonephrones - flatworms

metanephrones - annelids

malphighian tubules - insects

Kidneys

internal organ mainly involved with osmoregulation, common architectural and physiological principles; regulates water and inorganic solute levels, removes nitrogenous wastes

Regions of mammalian kidney

renal cortex, renal medulla, renal pelvis

Renal cortex

Outer portion of kidney

Renal medulla

Inner portion of kidney

Medulla is _______ into _____ ________ in larger mammals

divided, renal pyramids

Renal pelvis

Drainage area in center of the kidney

Nephron

Smallest functional unit of the kidney, responsible for the formation of urine; 1 million within human kidney; consists of a tubule and associated with vascular component

Renal cortex components

glomerulus, bowman’s capsule, afferent/efferent arteriole, proximal/distal convoluted tubule

Renal medulla composition

descending/ascending loop of Henle, peritubular capillaries

What tissue-component spans the renal cortex and medulla?

the collecting duct

Four basic processes of nephron

glomerular filtration, tubular reabsorption, tubular secretion, excretion

Glomerular filtration

non-selective filtering of blood into tubule forming the primary urine

What is the functional unit of glomerular filtration

Renal corpuscle

Glomerulus

ball of capillaries with large pores (fenestrations = very leaky); high permeability (~400x greater than other capillaries); podocytes; glomerular filtration

What forms the filtration structure in glomerulus?

Podocytes

Bowman’s capsule

cup-like structure surrounding glomerulus; receives the filtrate; glomerular filtration

What is glomerular filtration?

First step of urine production where plasma is filtered from blood into the capsule

What type of filtration occurs in the glomerulus?

Ultrafiltration

What drives glomerular filtration?

High hydrostatic pressure in capillaries pushes fluid into the capsule

What passes through vs. stays in blood during glomerular filtration?

Water, small solutes, and waste are filtered, cells and plasma protein are retained in capillaries

Forces involved in glomerular filtration

Blood pressure within glomerulus, protein concentration in blood within glomerulus, and hydrostatic pressure of liquid in Bowman’s capsule

Which forces oppose filtration in glomerular filtration?

High concentration of proteins in blood within the glomerulus and hydrostatic pressure of liquid in Bowman’s capsule

Which forces favor filtration in glomerular filtration?

Blood pressure in glomerulus from capillaries and net filtration pressure

Glomerular filtration rate

About 125mL per min, 7.5L per hour, 180L per day, entire plasma volume filtered every 45 mins

How much of the filtrate is reabsorbed by the nephron tubules?

About 99%

Which portion of the kidney handles tubular reabsorption and secretion?

Proximal convoluted tubule in the renal cortex

Major function of the proximal convoluted tubule

Reabsorption of all useful components (glucose, inorganic ions, most of the water) and secretion of harmful waste

All tubular exchange with blood is done by?

A single layer of renal epithelial cells

What are the two pathways of exchange in the proximal convoluted tubule?

Transcellular (through cells) or paracellular (between cells)

What drives reabsorption in the proximal convoluted tubule?

Na⁺/K⁺ ATPase creates a Na⁺ gradient that powers secondary active transport (glucose, amino acids) and indirectly drives Cl⁻ and water reabsorption

What transport mechanisms are used in the proximal convoluted tubule?

Na/K ATPase pumps (active), Cl- reabsorption (passive), aquaporins (water)

What occurs during reabsorption? (statistically)

65% of filtered water is reabsorbed, 67% of filtered sodium, potassium, and chloride is reabsorbed, all glucose and amino acids

What occurs during secretion?

Variable proton secretion for acid/base regulation, organic ions