Osmoregulation and Renal Physiology - 1 and 2 Flashcards
What is osmosis?
The flow of water across a partially permeable membrane .
What determines osmosis?
The osmolarity of the solution
What happens if osmosis is not controlled?
It can lead to problems e.g. oedema or swelling
How does osmosis affect cell volume?
Cells swell or shrink depending on the osmolarity (tonicity) of the bathing solution
What happens in a hypotonic bathing solution?
There is a net water movement in => cell swells
Increased cell volume - cell may burst
What happens in a hypertonic bathing solution?
Net water movement out the cell => cell shrinks
Reduced cell volume
What happens in a isotonic bathing solution?
An equal amount of water moves in and out of the cell
There is no change in cell volume
How is extracellular (ECF) and intracellular fluid (ICF) separated?
Separated by a semi-permeable cell membrane.
How is the osmolarity of the extracellular fluid (ECF) maintained?
By homeostatic mechanisms
What is intracellular fluid (ICF) osmolarity dependent on?
the extracellular fluid (ECF) osmolarity
What is homeostasis?
The maintenance of a constant internal environment.
- a dynamic equilibrium
- variables change but are continually
corrected and kept within very narrow limits - occurs by negative feedback
What is one of the key principles in physiology?
Maintaining the balance of solutes and removal of waste products.
What is a key organ in homeostasis?
= the kidney
How is body fluid osmolarity maintained?
Body fluid osmolarity is a controlled variable. The kidney is a key organ in the regulation of body fluid osmolarity.
Anytime there is a stress correcting mechanisms come into play and it is returned to the optimum environment.
What factors can affect water balance?
Normal body fluid ~ 60% of total body weight
affected by :
- dry vs humid environment
- physiological factors
- dry vs wet food
Water in = water out
What are sources of water intake for the body?
Ingestion of water in drinks and within food
What are sources of water output for the body? (Excretory organs)
- Skin - Sweat (water, salts, urea)
- Lungs - CO2, water
- Gastrointestinal tract - Water, Bile
- Kidneys - Urine (metabolites, (including urea), water, salts, drugs)
Which sources of water secretion are controlled and which are not?
Water loss in urine is tightly controlled by the kidney.
Excretion via skin, lungs, GI tract is not controlled.
What are the 2 main mechanisms in controlling body water osmolarity?
Urine concentration and thirst
What is the main organ in regulating body fluid osmolarity?
Kidney
What are the functions of the kidney? (renal functions)
- Regulation of water balance
- Regulation of salt balance - Na+, K+
-Removal of metabolic waste products -
Urea/uric acid - Removal of foreign substances - drugs or
metabolites - Regulation of pH
- Hormone production
What makes up the urinary tract?
Kidney (cortex, medulla, capsule, renal artery,
renal vein)
Ureter
Urethra
What is the ureter?
The tube that carries urine and waste products out of the kidney to the bladder.
What is the nephron?
The key functional unit of the kidney - filtration
What % of cardiac output does renal blood supply make up?
Approx 20% of cardiac output
How does water remain in the capillaries even with a concentration gradient?
Oncotic pressure opposes osmosis to keep water in the capillaries
What are the 4 main processes in the formation of urine?
- Filtration of plasma
- Reabsorption of water
- Reabsorption of solutes
- Secretion of solutes
What is glomerular filtration?
The filtration of plasma - fluid is filtered from capillary to Bowmans capsule across filtration barrier
How is the rate of glomerular filtration controled?
Rate is due to the balance of hydrostatic pressure and oncotic pressure in the glomerular capillaries and Bowmans space
What is oncotic pressure in the nephron produced by?
The oncotic pressure is the osmotic pressure produced by large molecules that are not filtered
What is the equation for glomerular filtration rate?
Glomerular filtration rate =
Pcap - ([pi-3.14]cap+ Pbc)
glomerular filtration = capillary pressure – (oncotic pressure + bowmans capsule pressure)
Pcap = hydrostatic pressure in capillary
Pbc = hydrostatic pressure in Bowman’s capsule
3.14 cap = oncotic pressure in capillary
What is the composition of the filtrate from glomerular filtration?
- Filtration produces ultrafiltrate
- Small molecules and ions in almost exactly
same concentration as plasma - No proteins or red blood cells
What is the average Glomerular filtration rate (GFR) ?
In a 70 kg man
GFR = 180 litres / day
Is a high or low filtration rate needed for normal kidney function?
a high rate of filtration is essential to normal functioning of the kidney
Where is the proximal tubule located?
Located after the glomerulus
What are the 4 roles of the proximal tubule?
- Filtration of plasma
- Reabsorption of water
- Reabsorption of solutes
- Secretion of solutes
–> approx 70% of filtrate is reabsorbed
–> some substances are secreted into tubule
What type of transport does most reabsorption happen by?
Most reabsorption is via secondary active transport
How does most reabsorption happen via secondary active transport?
- Driven by ATPase (Na+ K+ ATPase) in basal
and baso-lateral membranes - Ionic gradient drives movement of ions
across apical membrane - of two types - Symport (co-transport) and antiport (counter
transport)
What is a symport?
Co-transport
What is an antiport?
Counter transport
How does the reabsorption of glucose take place?
Reabsorption occurs against the concentration gradient - dependent on Na+ gradient
Where does the reabsorption of glucose take place?
In the proximal tubule
–> Co-transport of Na+ and glucose
Where is the filtered water reabsorbed?
In the proximal tubule
(60-70% of filtered water reabsorbed)
How is water reabsorbed?
- Reabsorption is passive
- Water follows movement of solutes out of
tubule - osmotic effect - Osmotic effect is aided by build up of solutes
in clefts (basolateral spaces) between tubule
cells
What are 3 types of secretory mechanisms?
Organic anions (acids) - e.g. bile salts, penicilin => Out of capillary lumen–>into tubular lumen
Organic cations (bases) e.g. adrenaline, morphine
=> Out of capillary lumen–>into tubular lumen
Na+ = out of tubular lumen –> into capillary lumen
What are the mechanisms that allow urine to be concentrated to a higher osmolarity than the plasma osmolarity?
- The loop of Henle is essential for formation
of concentrated urine - Maximum urine concentration is related to
length of loop of Henle
What effect does a longer loop of Henle have on urine concentration?
A longer loop of Henle increases urine concentration
What does a counter current multiplier within the loop of Henle lead to?
Counter current multiplier - leads to a hypertonic medulla
What is the osmolarity of the proximal tubule?
Isotonic
What is the osmolarity of the distal tubule?
Hypotonic
Where is the collecting duct located?
Runs parallel to the ascending limb of the loop of Henle, with the flow in the opposite direction
What is the function of the collecting duct?
Collects urine from nephrons.
Collecting duct is impermeable to water
Permeability of collecting duct to water increases in presence of Antidiuretic hormone (ADH)
Water exits the collecting duct due to high osmolarity in medulla - urine becomes more concentrated - water retained in the body
How is ADH (antidiuretic hormone (vasopressin)) release controlled?
- Release stimulated by changes in plasma
osmolarity - Plasma osmolarity detected by
osmoreceptors - ADH released
Where is ADH released?
from posterior pituitary gland
At normal plasma osmolarity is ADH present in the blood?
Yes - at normal plasma osmolarity there is ADH present in the blood
What happens to ADH release when the body is in water deprivation?
- Water deprivation increases body fluid
osmolarity - ADH release is increased
=> Decrease in water excretion
What happens to ADH release when the body has an excess water intake?
- Excess water intake lowers body fluid
osmolarity - ADH release is decreased
=> Increase in water excretion
What hormones influence the regulation of body fluid?
- Aldosterone
- Renin-Angiotensin system
- Atrial Natriuretic peptide
Where is aldosterone produced?
In the Adrenal Cortex
Why is aldosterone produced?
Produced in response to high plasma K+
What is the function of aldosterone?
In the kidney aldosterone stimulates:
- Na+ reabsorption
- K+ secretion
What is micturition?
The process of emptying urine from the urinary bladder
How is urine transported to the bldder?
Transported along ureters - by peristaltic contractions of smooth muscle
How does the body allow urine to leave the body?
When the bladder is full the detrusor muscle contracts to expel urine via the urethra.
The internal and external sphincters open allowing urine to flow into the urethra.
