Glomerular Filtration Flashcards
What is the renal function?
Process the plasma portion of the blood by removing substances from it and, in a few cases, by adding substances to it
What are the functions of the kidneys?
Regulation of water, inorganic ion balance, and acid-base balance
Removal of metabolic waste products from the blood and their excretion in the urine
Removal of foreign chemicals from the blood and their excretion in the urine
Gluconeogenesis
Which hormones and enzymes are produced by the kidneys?
Erythropoietin
Renin
Conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D
What is the purpose of erythropoietin?
Controls erythrocyte production
What is the function of renin?
Controls formation of angiotensin, which influences blood pressure and sodium balance
What is the function of 1,25-dihydroxyvitamin D?
Influences calcium balance
What are the three basic renal processes?
Glomerular filtration, tubular reabsorption and tubular secretion
Synthesis and catbolism of certain substances
What does the excretion of a substance equal to?
The amount filtered plus the amount secreted minus the amount reabsorbed
What % of the plasma enters the glomerulus and is filtered?
20%
What happebs ti the remaining 80% of the plasma?
Enters the glomerulus but is NOT filtered and leaves through the efferent arterioles and into the peritubular capillaries
What is secretion?
The movement of substances from peritubular to tubular or epithelial cells into the lumen of the tubule
What is the pathway of the plasma once entering the renal artery?
Renal artery, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries
What is glomerular filtration?
Nondiscriminant filtration of protein-free plasms from the glomerulus into Bowman’s capsule
What is excretion?
Removal of substances in the urine
What is reabsorption?
Selective movement of filtered substances from the tubular lumen into the peritubular capillaries
Where does glomerular filtration occur?
From the glomerular capillary into the Bowman’s capsule
Where does tubular secretion occur?
From the peritubular capillary to Bowman’s capsule
Where does tubular reabsorption occur?
From the Bowman’s space into the vein
What is the equation for the amount excreted?
Amount filtered + amount secreted - amount reabsorbed
Do the renal processes apply to all substances?
No, for instance, 100% of glucose and amino acids are reabsorbed because they are needed in the body
What happens to the toxins in the kidneys?
Secreted and not reabsorbed, excreted from the body
What are the three layers that make up the glomerular membrane?
The glomerular capillary wall
The basement membrane (acellular membrane)
The inner layer of Bowman’s capsule
What is the function of the three layers ?
A fine molecular sieve that retains the blood cells and plasma proteins but permits H2O and solutes of small molecular dimension to filter through
What is the space between the podocytes?
Filtration slit
What does the glomerular filtarte contain?
All plasma substances other than protein (and substances bound to proteins) in virtually the same concentration as in plasma
What is the force that drives the glomerular filtration?
Hydrostatic pressue in glomerular capillaries
What are the forces that oppose the glomerular filtration?
Hydrostatic pressure in Bowman’s space and the osmotic force due to the proteins in the glomerular capillary plasma
What is the net difference favoring filtration?
Net glomerular filtration pressure
Why the osmotic pressure in Bowman’s capsule not considered?
It is 0, thus it does not count. The reason for that is the fact that osmotic pressure is determined by proteins and proteins are NOT filtered
What is the glomerular filtration rate? (GFR)
The volume of fluid filtered from the glomeruli into Bowman’s space per unit time
Why are the glomeruli of the kidney much more efficient than the other capillary beds in the body?
Filtration membrane is a large surface area and very permeable to water and solutes
Glomerular pressure is higher so they produce 180L/day vs 3 to 4L/day formed by other capillaries
What are the forces that favour filtration?
Glomerular capillary blood pressure
Net foltration pressure
What is the pressure inside the kidney glomeruli?
About 55mmHg
What are the forces that oppose filtration?
Plasma colloid osmotic pressure
Bowmanm’s hydrostatic pressure
Which forces normally do not vary much and also cannot be regulated?
Plasma colloid osmotic pressure and Bowman’s capsule hydrostatic pressire
Which pressure can be controlled to adjust the GFR to suit the body’s needs?
Glomerular capillary hydrostatic pressure, more blood, higher pressure, imncreased GFR
What are the two major control mechanisms that regulate GFR?
Autoregulation
Extrinsic sympathetic control
What is the aim of the autoregulation mechanism?
Aims to prevent spontaneous changes in GFR
WHat is the purpsoe of extrinsic sympathetic control?
AIms to have long-term regulation of arterial blood pressure
What are the two mechanisms that contribute to autoregulation of the GFR?
Myogenic
Tubuloglomerular
What is the myogenic mechanism like?
Respondes to changes in pressure within the nephron’s vascular component, myogenic will contract or relax depending on the blood flow
What is the tubuloglomerular mechanism like?
Senses changes in salt levels in the fluid flowing through the nephrons tubular component, will send signals through adenosine to contract or relax afferent arterioles
What is the MAP range in which autoregulation of GFR occurs in?
80 to 180mmHg
Why is at
autoregulation important?
Unintentional shifts in GFR could lead to dangerous imbalances of fluid, electrolytes, and wastes
What do dramatic changes in MAP cause?
Glomerular capillary pressure and GFR to decrease if less than 80mmHg or increase if greater than 180mmHg
What is the tubuloglomerular feedback mechanism like?
Afferent or efferent will contract or dilate based on the volumes of salt
Which mechanism can override the autoregulation mechanism?
Strong sympathetic nervous system activation
For example: severe hemorrhage
What will happen to MAP in the case of a severe hemorrhage?
It will be reduced, activated the sympathetic nervous system via the baroreceptor reflex –> renal sympathetic nerves constrict the afferent arteriole, decreasing GFR
Does the parasympathetic nervous system exert any influence on the kidneys?
NO
What happens to afferent/efferent to decrease the GFR?
Constriction of afferent or dilation of efferent, decrease in blood flow, decrease in hydrostatic pressure –> decrease in GFR
What happens to afferent/efferent to increase the GFR?
Dilation of afferent and constriction of efferent, increase in blood flow, increase in hydrostatic pressure –> increase in GFR
What is the effect of simultaneous constriction and dilation of both sets of arterioles?
Levels of GFR remain unchanged because of opposing effects
What other factor can influence the GFR?
Changes in the filtration coefficient (permeability)
What kind of input do the mesangial cells that surround the glomerular capillaries receive?
Neural and humoral
What happens if there is contraction of the mesangial cells by sympathetic stimulation?
Decreases the surface area of the capillaries, which causes decrease in GFR
What do podocytes also possess?
Actinlike filaments
What is the effect of contraction/relaxation of the actinlike filaments?
Contraction –> decrease
Relaxation –> inrease the number of filtration slits open in the inner membrane of the Bowman’s capsule by changing shapes and proximities of the secondary foot processes
What is the relation between the number of slits and permeability?
More slits –> greater permeability
At the average net filtration pressure and permeability, what % of plasma is filtered?
20% of plasma that enters the kidney is converted into glomerular filtrate
What is the average GFR and total renal plasma flow?
125mL/min –> GFR
625mL/min –> renal plasma flow
What is the percentage of plasma in the blood?
55% of blood is plasma
So, what is the total blood flow through the kidneys?
1140mL/min
What percentage of the cardiac output is supplied to the kidneys?
22%
What is the total cardiac output volume, if 1140 mL/min is 22%?
5000mL/min
What is the purpose of the majority of blood supply to the kidneys?
It does not supply the renal tissue, but it is adjusted and purified by the kidneys, kidneys perform regulatory and excretory function on huge volumes of plasma
What is the reason kidneys perform regulatory and excretory functions on plasma?
To maintain internal fluid environment stability
How do you calculate the total amount of any protein or non-protein-bound substance filtered into Bowman’s capsule?
By multiplying the GFR by the plasma concentration of the substance –> known as filtered load
What is the purpose of comparing filtered load to the amount of substance excreted?
Detect whether the substance undergoes net tubular reabsorption or net secretion
What is the indication that tubular reabsorption of a substance has occurred?
The filtered load is greater than the amount excreted
What does filtered load < amount excreted mean?
Net secretion of a substance has taken place
What is renal clearance?
The volume of plasma from which that substance is completely removed by the kidneys per unit time
Why is renal clearance important?
A useful way of qualifying renal functions in terms of clearance
How do you calculate the renal clearance?
Divide the mass of the substance excreted in the urine per unit of time by the plasma concentration of the substance itself
What is the formula of renal clearance?
C = UV/P
C –> clearance rate of substance
U –> urine concentration of substance
V –> urine volume per unit time
P –> plasma concentration of substance
What is the unit for clearance rate of substance?
mL/min
What is the unit for urine concentration of substance?
Quantity/mL urine
What is the unit for urine volume per unit time?
mL/min
What is the unit for plasma concentration of the substance?
Quantity/mL plasma
How can GFR be MEASURED?
Means of inulin clearance
Why can GFR be measured by inulin clearance?
Inulin is freely filtred and neither absorbed nor secreted
How can GFR be ESTIMATED?
Creatinine clearance
Why can GFR be estimated by creatinine clearance?
Creatinine is freely filtered but also secreted in small amounts
What is creatinine?
Metabolite of muscle creatine
What is the effect of slightly secreted amounts of creatinine on GFR?
Slightly overestimated GFR
When is the creatinine clearance test used?
In clinical practice, there is a specific formula that accounts for age, weight and gender
