214-216. Renal Function Flashcards
What are the functions of the kidney?
- Removes metabolic waste (urea, acids)
- Controls volume of extracellular fluid (close link to BP)
- Maintains optimal concentrations of vital solutes in the extracellular fluid (electrolyte balance) - Na, K, H, Ca, Mg, Cl, Phos
- Other functions: Vit D activation, erythropoeitin production
What % of body weight is made up of water?
60%
What are the 3 fluid compartments of the body?
Intracellular space
Interstitial space
Intravascular space
What % of water is intracellular?
40%
What % of water in the body is extracellular (intravascular and interstitial)?
20%
What determines fluid volume at the cellular barrier?
Osmotic (oncotic) forces
Selectively permeable membrane
What determines fluid volume at the vascular barrier?
Hydrostatic pressure and osmotic (oncotic) forces
Define hydrostatic pressure
The pressure exerted by a fluid due to the force of gravity
On which side is hydrostatic pressure greater? The arterial side or the venous side?
The arterial side
On which side is oncotic pressure greater than hydrostatic pressure; the arterial side or the venous side?
Venous side
Are cations positively charged or negatively charged?
Positively charged
Are anions positively charged or negatively charged?
Negatively charged
What is the extracellular fluid composition and volume dependent on?
Salt intake - hunger, food availability
Water intake - thirst, water availability
Salt and water losses - sweat, gut
THE KIDNEYS
Around what % of cardiac output do the kidneys receive?
20%
What are the 3 basic processes of the nephron?
Glomerular filtration
Tubular reabsorption
Tubular secretion
What is the role of glomerular filtration in the kidney?
Filters the blood into tubule forming the primitive urine (glomerular filtrate)
What is the role of tubular reabsorption in the kidney?
Selective absorption of substances from tubule to blood
What is the role of tubular secretion in the kidney?
Secretion of substances from blood to tubular fluid
What are the 4 main structures of the glomerulus?
Podocytes
Bowman’s capsule
Capillary
Glomerular filtration barrier
What three specialised structures of the glomerulus allows for extracellular fluid to be filtered and leave the body?
Podocyte foot processes
Specialised capillary endothelium
Collagen based glomerular basement membrane
What 3 things are 100% filtered by the glomerulus?
Water
Glucose
Urea
What is the average normal GFR per minute and how many litres are filtered a day?
100ml/min
144L/day
What aquaporin channels are present on the proximal convoluted tubule which allows for reabsorption of water?
Aquaporin I
On the thin descending limb of the loop of Henle, how does its structure make it suited to reabsorb water?
Loose junctions, and aquaporin I channels
What is the functional significance of not having aquaporin channels in the thick ascending limb of the loop of Henle, as well as its tight junctions?
Prevents the movement of water along the concentration gradient to prevent excess fluid build up
In the collecting duct, what aquaporin channels are present on:
a) The apical membrane
b) The basolateral membrane?
a) Aquaporin II
b) Aquaporin IV
What direction does potassium move in?
K+ exits the blood and enters the tubule
In the proximal convuluted tubule, what is present on the apical membrane and what is present on the basolateral membrane which allows for effective sodium and water reabsorption?
Apical membrane - aquaporin channels on microvillae
Basolateral membrane - ATPase transporters which promote Na+ movement into the blood (and thus water) while removing K+ from the blood and into the tubule
List some of the mediators involved in the control of the presence and activity of membrane channels
Angiotensin II
ADH
Aldosterone
Parathyroid hormone
Approximately what % of sodium chloride is reabsorbed in the proximal tubule?
70%
What two substances are nearly fully absorbed in the proximal tubule?
Amino acids and glucose
What causes increased solute concentration in the loop of Henle?
Water leaves the descending loop
What causes decreased solute concentration in the loop of Henle?
Na+ is transported out of the ascending loop
What causes the concentration in response to reduced sensed volume and increased serum osmolality?
ADH on the collecting ducts
Allows for: reabsorption of water
Vasoconstriction
Thirst
What is the role of baroreceptors in the regulation of fluid balance?
Detects changes minute by minute and responds via the sympathetic nervous system
Where are baroreceptors present?
Afferent arteriole of kidney
Aortic arch
Carotid sinus
Left ventricle
Vasoconstriction at which point protects the extracellular fluid volume?
Afferent arteriole
What is the role of the juxta-glomerular apparatus?
Maintains the GFR in the face of increased or decreased blood flow to the kidney
What is the role of the macula densa in the juxta-glomerular apparatus?
Senses tubular flow
How does the juxta-glomerular apparatus respond when there is increased tubular flow?
Increased tubular flow sensed by macula densa
Macula densa release adenosine
Adenosine causes afferent arteriolar constriction
How does the juxta-glomerular apparatus respond when there is decreased tubular flow?
Reduced tubular flow sensed by macula densa
Granular cells produce renin
What can caused reduced sensed volume in the kidneys (circulatory failure)? (4)
Vasodilation
Pump failure
Extracellular fluid depletion
Haemorrhage
What is the response to reduced inflow?
Macula densa cells secrete renin, which then stimulates the production of angiotensin II; causing an increase in water and sodium reabsorption in the kidney, vasoconstriction of efferent arteriole and aldosterone production
Reduced inflow sensed by systemic baroreceptors causes the increase in ADH which stimulates water reabsorption in the collecting ducts and the feeling of thirst
What are the effects caused by the kidneys’ response to reduced sensed volume?
Oliguria
Concentrated urine
Low urine sodium concentration
High urine potassium secretion
What are the two mechanisms that baroreceptors respond by?
Sympathetic nervous system and natriuretic peptides (ANP, BNP)
What is the role of natriuretic peptides?
Produced in response to increased kidney volume - opposes effects of angiotensin II on kidney
‘Pressure natriuresis’
