An introduction to kidneys and body fluid Flashcards
what does osmosis determine?
the movement of water between ICFV and ECFV
why might we want to prevent large fluid shifts between ICFV and ECFV?
too much water into cells will make them burst, causing tissue damage
too much water out causes them to shrink massively
ECFV
The ECFV consists of two sub-compartments:
Plasma (3L) - usually 20% of ECV
Interstitial fluid (11L)
Starling forces determine fluid and solute movement between plasma and interstitial fluid
how much of the body weight is ICV?
40%
what is osmolarity?
the total concentration of osmotically active solutes
what is the major determinant of ECV osmolarity and why?
sodium, because it is the principal electrolyte of ECF
Why is the control of body fluids important ?
Cell structure and function
-large shifts between ECFV and ICFV will disrupt tissue structure and function
Tissue perfusion
-depends on balance between circulating volume (plasma) and interstitial volume
what determine fluid and solute movement between plasma and interstitial fluid?
Starling forces
what do salt and water balance depend on?
Osmoregulation: maintain osmotic equilibrium between ICFV and ECFV
Volume regulation: maintain adequate ECFV to support plasma volume
why do we control ECFV osmolarity?
in order to maintain osmotic equilibrium between ICFV and ECFV
how can plasma osmolarity be estimated?
2[Na] + 2[K] + [glucose] + [urea]
(Why do we double sodium and potassium but not glucose and urea? Balance the charges, if you have a certain amount of sodium there is positive charge which needs to be balanced out with anions (most of the anions will be made up of chloride, but it also includes bicarbonate and other molecules). Glucose is not charged so isn’t doubled
give a difference between interstitial fluid and plasma fluid?
plasma proteins (cannot cross the plasma membrane)
how does the body achieve osmoregulation?
by adding or removing water (not sodium) through controlling by controlling ADH levels
when plasma osmolarity rises how do the kidneys respond?
by producing small volume of concentrated urine (water retention)
a rise in plasma osmolarity indicates what?
that solute concentration has gone up (so more water is needed)
what does volume regulation refer to?
the control of the circulating (plasma) volume
what are changes in plasma volume detected by?
stretch and pressure receptors in the CVS
what is a fall in blood volume opposed by?
sodium retention - water follows osmotically, restoring volume
(although the total amount of body sodium may be increased, osmolarity is barely changed because the retained sodium brings water with it)
function of the kidney?
salt and water balance
- Osmoregulation
- Volume regulation
- Acid-base balance
- Regulation of electrolytes balance (eg potassium, calcium, phosphate)
- Removal of metabolic waste products from blood
- Removal of foreign chemicals in the blood (e.g. drugs)
- Regulation of red blood cell production (erythropoietin)
- endocrine organ (EPO, renin, Vit D)
what is urine production?
a by-product of kidney function
urinary tract
important for temporary storage, then removes urine from the body
what is a nephron?
functional unit of the kidney -consists of special blood vessels and elaborate tubules (tiny tubes) -where urine production begins -consists of several major structures: Blood vessels The glomerulus Bowman capsule The renal tubule
what are the outer and inner sections of the kidney called?
outer - cortex
inner - medulla
explain how the kidney (and nephrons) are supplied with blood
The whole kidney is supplied by a major artery called the renal artery
This subdivides many times into fine arterioles supplying each nephron, the afferent arteriole
Drained by the renal vein, which is supplied by venules coming away from each nephron, joining together into larger veins which merge into the renal vein.
In between there are complex capillary networks
what does the collecting duct form?
the ureter, which drains into the bladder.
what does a kidney tubule begin with?
Bowman’s capsule
The Four Basic Processes
of Urine Formation
- Glomerular Filtration
- Tubular Reabsorption
- Tubular Secretion
- Excretion of water and solutes in the urine
Glomerular Filtration
Hydrostatic pressure in the afferent arteriole forces fluids and solutes through the glomerular capillary membrane
Small molecules pass readily- large ones (proteins) and cells cannot pass
This leads to a plasma ultra filtrate in the Bowman’s capsule
what can be used as an index of GFR?
Plasma creatinine
- breakdown product of creatine phosphate in muscle
- excreted unchanged
what is Glomerular filtration rate (GFR)?
the amount of filtrate kidneys produce each minute
-averages 125 ml/min (approx 20% of renal plasma flow)
when is GFR reduced?
renal failure
Reabsorption
many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries
importance of secretion
eliminating undesirable substances (urea/ uric acid)
getting rid of excess potassium ions
controlling blood pH
what determines that amount of solute present in the urine?
the amount filtered out of the plasma MINUS the amount reabsorbed
the control of water balance (osmoregulation) is based on the control of?
the osmolality of the ECFV
how are changes in ECF osmolality sensed?
via sensory cells in the hypothalamus, and effectors ( in this case, the kidneys) are then needed to adjust the volume of water excreted or contained
how is total body water balance calculated?
input vs output
what should total body water balance be?
zero - otherwise there will be a change in osmolality
insensible water loss
evaporation of water through the skin and the lungs
give an example of uncontrollable water loss?
water loss via the GI tract
Physiological response to water restriction (thirsty, but no water available to drink)
- Plasma osmolality rises
- Response is increased secretion of hormone, ADH/vasopressin
Results in:
Decreased urine volume
Increased urine osmolality
Physiological response to increase in water intake
Increase in water absorption through GIT
Plasma osmolality falls
Response is reduced secretion of ADH
Results in:
Urine volume increases
Urine osmolality decreases
what is a good indicator of ADH status?
osmolarity of the urine
what happens when you drink TOO much water?
- can be fatal
- you will start to dilate the ECF, reducing the osmolarity
- decreased osmolarity of ECF but ICF will have the same as normal, so water will move into the cells
- shifts of fluid from EC to IC causes brain damage
why must sodium intake and excretion be balances?
to maintain a constant ECF volume
how do you calculate [Total body Na+]?
intake minus elimination (elimination is through sweat, diarrhoea and vomit)
how do we expand the ECF volume?
sodium reabsorption in kidneys is increased, and water follows osmotically (so concentration is constant)
what detectors detect changes in blood pressure/volume?
changes in pressure and volume are sensed in the CVS by baroreceptors, atrial stretch receptors
Sodium retaining system
The Renin-angiotensin-aldosterone system (RAAS)
Sodium eliminating pathways
Cardiac natriuretic peptides (ANP)
sodium eliminating pathways depend on ANP’s which are released from cells in the atria in response to increased stretch due to increased volume.
a decrease in ECFV means what in terms of these sodium pathways?
decrease in ANP pathway, increase in RAAS pathway - overall decrease in sodium excretion
example of when there in a decrease in ECFV?
haemorrhage
increase in amount of body sodium means what?
increase in ECV
how does the kidney act as an endocrine organ?
produces EPO, renin and Vitamin D
