Renal Physiology Flashcards
Fluid compartments of the body
- Blood plasma: fluid component of the blood
- Interstitial fluid: fluid surrounding cells
- Intracellular fluid: fluid within cells
1&2 separated by endothelium
2&3 separated by cell membrane
Why is it necessary to have control over water and electrolyte balance?
No known species has a completely passive relationship between their internal and external environment. Water and ion regulation is important to prevent unwanted loss or gain of those substances.
Osmolarity
Solute concentration (number of osmoles of solute per litre) based on both penetrating and non penetrating solutes
Tonicity
Refers to solute concentration based only on penetrating solutes (particles that pass through membrane)
How does osmotic pressure affect water movement?
Water moves from low to high osmotic pressure (areas of low solute to high solute)
Which fluid compartments have similar ion concentrations and which do not?
Blood plasma and interstitial fluid have same concentration. Intracellular fluid has different concentration.
Osmotic regulators
Show constant blood osmotic pressure (maintains constant fluid composition regardless of the external environment)
Ex. Shrimp
Osmotic conformers
Show blood osmotic pressure that mimics the external environment (changes fluid composition to equal that of the environment)
Ex. Mussel
Challenges to freshwater regulators
Environment is hypo-osmotic
Therefore, water is constantly being brought in to the internal environment due to osmosis
Solution: Get rid of excess water to prevent ion loss
Challenges to marine regulators
External environment is hyper-osmotic
Therefore, water is constantly being drawn out to the external environment
Solution: Retain more water to prevent ion loading
Special case: Elasmobranchs and osmoregulation
Elasmobranchs produce high levels of inorganic solute to meet the same osmotic pressure as the external environment. These solutes are not identical to the ones in the external environment but they create a similar solute concentration.
Special case: Salmon
Salmon are born in fresh water and migrate to sea water. To return to fresh water for the breeding season salmon spend time in brackish water which is an intermediate between fresh and sea water.
In the transition process:
- Kidney function change to make more dilute urine in freshwater
- Gills take up ions in fresh and lose them in sea
How do gills help osmoregulation in freshwater environments?
Gills have a high surface area and permeability which benefits gas exchange. This is counter productive for water-salt balance.
High surface area = increase water uptake through osmosis
High permeability = increase ion loss or loading
Solution to gills
Active transport of ions into the gills coupled with copious amount of dilute urine
Osmoregulatory gill cells
Pavement cells: occupy 90% of the gill epithelium and is responsible for oxygen uptake
Mitochondria rich cells: uptake Na, Cl, Ca; partial under hormonal control; density and type depends on conditions
How do marine birds and reptiles deal with a hyper osmotic environment ?
Thick and less permeable integument (skin)
Salt glands
Salt glands in birds
Nasal
Ducts transfer ions to the nostrils located in the beak (replaces function of gills)
Salt glands in reptiles
Lingual or tongue
Nasal
Optical
How salt glands work?
When osmoreceptors near the heart and brain detect high blood osmotic pressure, the parasympathetic system releases AcH. This induces Cl and K gated channels in the mitochondria rich cells. The build up of negative charge inside the mitochondria rich cells attract Na to the apical membrane for excretion.
Humidic Animals
Restricted to humid and water rich environments
(Ie. earthworms, slugs, etc.)
Highest integument permeability
Xeric animals
Animals that live in dry, water poor places
(Ie. mammals, birds, etc.)
Lower integument permeability via thin lipid layers
Temperature and water loss through respiration
- Air entering body increases in temperature
- Warmer moist air travels down tubes until matched to the temperature of the body
- During exhale, air moves back out the same tube where it’s temperature is cooled to reduce the water content of the leaving air.
Evaporative water loss
Depends on body size and phylogenetic group.
Water loss decreases with increasing body size.
Excretory Water Loss
2nd most abundant source of water loss
Required to remove waste from body
Composition of urine can be modified to reduce water loss or ion loss.
When is urine concentration modified?
During times of drought
During times of water loading
Represented by U/P ratio
U/P ratio
Osmotic pressure of urine (divide) osmotic pressure of plasma.
U/P = 1: urine is isosmotic to plasma
U/P > 1: urine is hyper osmotic to plasma
U/P < 1: urine is hypo osmotic to plasma
Podocytes
Specialized endothelial cells contain end feet that cross over each other and contact the basement membrane of the glomerulus. The crossovers form fenestrations that allow water, waste, and glucose to pass but prevent the permeability of large proteins.
Hydrostatic pressure of blood outside Bowman’s Capsule
Created by systole, it is the driving force that directs content of blood towards the lumen of the Bowman’s Capsule. This force exceeds the colloid pressure and hydrostatic pressure of the lumen.
Also known as filtration pressure
Primary urine
The aqueous solution that is first introduced into the kidney tubules. That is the filtrate entering the Bowman’s capsule.
Colloid osmotic pressure
The pressure generated due to impermeability of blood proteins. This makes blood hyper osmotic compared to the filtrate which causes some water to be drawn back to the blood from the capsular fluid.
Glomerular filtration rate
The rate of production of primary urine
About 120mL/min
Full blood volume is filtered every 30 mins
How does glomerular filtration rate alter?
Disease
Chronic hypertension or diabetes mellitus
Increased thickness of basement membrane
Damaged capillaries in glomerulus
Normal conditions: Change blood pressure or vary number of filtrating nephrons
Loops of Henle
Long in mammals
Short in non mammals
Composed of a descending and ascending limb (longer limb = greater urine concentration; associated with thicker medulla)
Loop of Henle: Descending thin segment
Highly permeable to water
Moderately permeable to most solutes
Loop of Henle: Ascending thin segment
Impermeable to water
Moderately permeable to most solutes
Loop of Henle: Ascending thick segment
Impermeable to water
Active transport of Na/Cl
Osmotic pressures in the Loop of Henle
Single effect: initial change in pressure due to active transport
Countercurrent multiplication: osmotic pressure multiplied due to fluid moving in opposite directions
Osmotic pressure gradient
Osmotic pressure increases towards the inner medulla
Antidiuresis
The process of concentrating urine as it moves through the collecting duct. The concentration gradient generated from the loop of Henle allows water to move out of the collecting duct as urine travels through it.
Diuresis
The process of maintain dilute urine by decreasing the permeability of the collecting duct to water.
Hormonal control of the collecting duct
- ADH or vasopressin
ADH
Antidiuretic hormone
Released when blood plasma is low (dehydration) to alter permeability of collecting duct
Change in blood pressure is detected by osmoreceptors and baroreceptors
Baroreceptors
Located in pulmonary venous system, cardiac atria, aortic arch, carotid sinus
Osmoreceptors
Located in hypothalamus
Aquaporin 2 release
Aquaporin channels inserted into the wall of collecting duct in the presence of ADH.
ADH stimulates the release of the channels from aquaporin water channel containing vesicles.
Aquaporin 2 retracted
Decrease in ADH levels cause the retraction of aquaporin 2 channels from the membrane back into free floating vesicles.
Aquaporin 2 structure
6 transmembrane domains
5 inter helical loops
Pore is found between 2 and 5 loop
Urea
Produced by oxidation of amino acids not used in protein synthesis
Or
Produced by conversion of ammonia from nitrogenous metabolism
Permeability of urea
Freely passes into Bowman’s capsule but is only permeable in the thin segment of loop of Henle and the collecting duct of the inner renal medulla
Urea transporter protein
Facilities diffusion of urea from collecting duct into interstitial fluid which contributes to osmotic gradient.
Upregulated by ADH
How does urea leave the body?
Urea that enters the interstitial space is facilitated back into the loop of Henle until it is excreted out.
The vasa recta
Parallels the nephron but has slow blood flow
Does not impede osmotic pressure gradients
Descending limb absorb solute and release water
Ascending limb absorb water and release solute
Methods of water Retention
- Large body size (less surface area to mass)
- Modulate body temp with greater range (large in camels, limited in humans)
- Allow body temp to rise to decrease absorption of heat
- Keep brain cool via cool blood to brain via nasal passage
- Reduce water lost in waste
- Special physical attributes (ie. camel fur acts as shield)
- Behavioural adaptation (ie. create shade)
Camels
Extreme temperature adapted
High dehydration tolerance
Can tolerate 30-40% loss of body weight
Drink copious amount of water
