GFR and filtration Flashcards
What’s the renal corpuscle?
the bowman capsule (podocytes) and the glomerulus (fenestrated endothelial lining and basement membrane)
What capillaries are in the glomerulus?
fenestrated capillaries (with little holes in)
What is the point of fenestrated capillary endothelium?
allows passing of electrolytes, small proteins, H2O and nutrients but doesn’t allow proteins, WBC, platelets etc
What is the supply and drainage of the glomerulus?
Afferent arteriole supplies it and efferent arteriole drains it
What is the normal functions of the kidney?
REEM
- Regulation - controls conc of substances in ECF
- Excretion - excretes waste products
- Endocrine - synthesis of renin, EPO and prostaglandins
- Metabolism - active form of Vitamin D3, catabolism of insulin, PTH calcitonin
What is important about the glomerular basement membrane?
it is lined with negatively charged glycoproteins so repels -ve charged anions however, attracts cations like Na+, Ca2+, Mg2+, K+
What is the predominant ions in ECF?
+ve is Na+
-ve is Cl-
(k+ and bicarbonate are also important)
What is the electrolyte composition in the intracellular fluid?
high K+, low Na+
What is the electrolyte composition in the extracellular fluid?
low K+ high Na+
How is a high constant filtration pressure maintained in the glomeruli?
the diameter of the afferent arteriole is slighter greater than the efferent arteriole so the hydrostatic pressure of the blood inside the glomerulus is increased due to this diameter difference - this helps to force certain components out of glomerular capilarries
What components of the blood are forced out of the glomerular capillaries?
-most of water
-most of salts
most of glucose
most of urea
What components of the filtrate are reabsorbed?
-most of water
-all glucose and AA
-all bicarbonate
most Cl- and Na+
What are podocytes?
-specialised type of epithelia that is critical for the selectivity of glomerular filtration
What is the space between each podocyte?
filtration slit - fixed width adding to the filtering capacity
How do podocytes contact the glomerular capillaries basement membrane?
through foot processes (the space in between each foot process is the slit diaphragm)
What is the bowman capsule?
layer of epithelial cells that surrounds the glomerular capillaries
What is bowman space?
anything filtered through is collected here
What is mesangial cells?
- basement membrane like matrix in which the glomerular capillaries are embedded and which provides them structural support
- maintains the matrix
What is the normal GFR per day?
(90-120mL/min/1.73m2) - the kidneys filter about 140-180 L of blood per day
How much blood from the renal artery is filtered at any one time?
20%
- 80% that arrives leaves by the efferent arteriole
How can you get proteinuria?
-in many disease processes, the -ve charge on the GBM is lost so you get proteins more readily filtered
What is the juxtoglomerular apparatus?
-well defined endocrine structure consisting of the macula dense and extra glomerular mesangial cells
What do juxtaglomerular cells do?
produce renin in response to low GFR and low blood pressure
-renin is secreted when the NaCl conc in the filtrate decreases
What do the macula densa cells do?
- line the DCT
- they detect changes in ions as is sensitive to changes in NaCl conc and controls renin release
- chemoreceptors
How are juxtaglomerular cells and extraglomerular mesangial cells connected?
gap junctions
Why do the kidneys require a high constant blood flow?
-not only to maintain pressure but reabsorption is metabolically demanding so the kidneys needs oxygen and glucose to maintain their activity
What is the definition of the GFR?
the amount of filtrate that is produced from the blood flow per minute
What does GFR depend on?
- age
- gender
- size of individual
- size of kidneys
- pregnancy
What do size of kidneys have to do with GFR?
- the amount of filtrate is determined by the number of nephrons in each kidney
- women have smaller kidneys so less nephrons so lower GFR typically
What is the GFR of babies and why?
- nephron development not finished until 35th week of development so premature and LBW babes have lower nephron number
- GFR at birth 20ml/min/1.73m2
- GFR reaches normal at about 18 months
What is the GFR of elderly people and why?
GFR decreases as you get older because you get loss of functioning nephrons so the kidneys compensates by increasing its size
-GFR starts declining about 30 yrs old at rate of 6-7ml/min/decade
How does compensatory kidney hypertrophy work?
The renal cortex reduces its size so the kidney compensates by increasing the size of the renal medulla but as there is no glomerulus in the medulla, GFR still decreases
What happens to GFR in pregnancy?
- GFR will increase to about 130-180ml/min
- kidney size increases due to increased fluid volume
- dont get more nephrons as nephrogenesis doesn’t occur after birth
- back to pre-pregnancy levels about 6 months post-partum
What is the pressure that pushes plasma contents out of the capillary?
glomerular hydrostatic pressure (about 50mmHg)
What is oncotic pressure?
- pressure exerted by the plasma proteins that is trying to keep the water (albumin) in the blood and prevent it leaving into bowman space
- about 25mmHg
What is the hydrostatic pressure in the bowman capsule?
pressure exerted in the bowman capsule due to increase in volume of the bowman space
- tries to push filtrate back into the glomerulus
- about 15mmHg
So what is the net filtration pressure?
-all the pressures added together
50-15-25 = 10mmHg
What is the relationship between the pressure in the glomerulus and blood pressure?
glomerular pressure is proportional to the blood pressure applied in the kidney
When can auto regulation of the kidney be used?
-it is able to maintain GFR when blood pressure is within the physiological limits (80-180mmHg)
What are the 2 auto regulation mechanisms?
- intrinsic (myogenic and tuboglomerular feedback)
- extrinsic (RAAS and sympathetic nervous system)
When will extrinsic mechanisms occur?
-whenever systolic bp is less than 80mmHg
