Flashcards in Renal Structure and Function Deck (49)
Where do the renal arteries originate?
They're a branch off the abdominal aorta
What are the ureters?
Tube carrying urine to the bladder from the kidneys
What is the urethra?
The tube carrying urine from the bladder to outside of the body (excretion)
Describe the passage of the renal artery into the kidney
Renal arteries pass into the interlobar vessels and then divide into the small arcuate arteries in the renal cortex; these arteries terminate in a little clump of capillaries called a glomerulus
What is each capillary glomerulus in the kidney surrounded by?
Back of tissue known as the Bowman's capsule
How do efferent arterioles compare to the afferent arterioles at the glomerulus?
The afferent arterioles are wider than the efferent arterioles
What special type of cell covers the glomerulus (capillaries) on the capsular side and what is its function?
Cells called podocytes which have slits in them which form the filtration mechanism
Describe the flow of renal filtrate in the nephron
Glomerulus --> Bowman's capsule, PCT, loop of Henle --> DCT --> collecting duct
Why does only ~20% of the plasma from the blood leave the glomerulus in the afferent arteriole?
Too much filtration (plasma loss) would make the blood too viscous for flow in the efferent arteriole due to excessive haematocrit concentration.
How is GFR regulated and maintained?
By the balance of constriction in the smooth muscle of the afferent and efferent arterioles; this maintains constant GFR despite changes in systemic blood pressure.
How does the constrictor tone differ between the afferent and efferent arterioles and what is the consequence of this?
Constrictor tone in efferent arterioles is higher than in the afferents to produce a filtration pressure in the glomerulus
How can the afferent and efferent arterioles cause a reduction in GFR?
If afferents constrict and efferents relax --> lowered filtration pressure --> slower GFR
How can the afferent and efferent arterioles cause a increase in GFR?
If afferents relax and efferents constrict --> increased filtration pressure --> increased GFR
What controls the balance of constriction in the afferent and efferent arteriolar smooth muscle?
Juxtaglomerular apparatus, a structure where the distal tube folds back and contacts the glomerulus at the point where the afferent and efferent arterioles enter.
What are macula densa cells?
Part of the juxtaglomerular apparatus; they line the DCT where the tubule contacts the entry of the afferent and efferent arterioles. These cells sense sodium concentrations in the DCT fluid
How and why does renal flow affect sodium reabsorption?
In the PCT sodium is removed at a relatively constant rate, so if the flow of fluid through the PCT is low, more sodium is reabsorbed to cause more water to enter the blood. If the flow is high through the PCT than the sodium concentration increases to draw water into the urine, as the individual has sufficient water in their blood.
How do the macula densa cells respond if the sodium concentration is too low in the PCT?
This signals that GFR is too low (potentially due to low filtration pressure, meaning that more sodium can be reabsorbed), causing chemical factors to be released from the macula densa which cause constriction of the efferent arteriole to increase GFR to normal (by increasing filtration pressure). In addition, renin is also released from the juxtaglomerular cells simultaneously which diffuses into the blood.
Outline the RAAS system
Renin released into the blood travels to liver veins where it cleaves the protein angiotensin to release angiotensin I which then travels to the lungs where it is converted to angiotensin II by ACE (angiotensin converting enzyme). Angiotensin II then acts on the adrenal cortex to release aldosterone which increases sodium retention in the DCT.
Explain how the RAAS system is regulated by negative feedback
Works to maintain steady sodium levels in the blood; If blood sodium is too low, it’s sensed by a low sodium in the DCT which causes the juxtaglomerular cells to release renin which causes a pathway which increases sodium retention in the kidney.
What is meant by 'renal clearance'
The effective volume of plasma completely ‘cleared’ of a substance per minute
What range of values can renal clearance take?
Zero (no plasma completely cleared of substance in a minute) up to the renal plasma flow (all substances in the plasma cleared into the urine)
How may renal flow and GFR be affected by kidney damage?
Generally GFR with decrease but renal flow may remain normal
How do you calculate clearance?
(urine concentration x urine flow) / plasma concentration
Outline how you could measure renal clearance
Measure the concentration of the substance in the plasma, then collect urine for a fixed period to get the urine flow (ml/min) and then measure the concentration of the substance in the urine
What is GFR?
Glomerular filtration rate, auto-regulated in a healthy individual
What is the physical pressure ordinarily in the glomerular capillaries?
What is the net filtration pressure ordinarily in the glomerular capillaries?
Name two methods that can be used to measure GFR
Creatinine clearance and use of inulin (gold standard)
What is inulin?
A polysaccharide that is completely filtered from the plasma and not reabsorbed
Describe how you can use inulin to measure GFR
Inulin is completely filtered from the plasma and not reabsorbed so you can measure the plasma concentration and then the urine concentration to determine GFR
Describe how you can use creatinine clearance to measure GFR
Creatinine is produced naturally by the body as it’s a breakdown product of phosphocreatine which is freely filtered by the glomerulus but also actively secreted by the peritubular capillaries in small amounts. The secretion of creatinine by peritubular capillaries means that creatinine clearance overestimates actual GFR by 10-20%, but is approximate enough to be valuable considering its ease of measuring
Describe how you can use clearance to measure renal plasma flow
If all of a particular substance is filtered out of the blood and all of the material in the efferent arteriolar blood is secreted into the urine, then the renal venous blood will have none of this substance/material in it. If this is the case then clearance will equal renal plasma flow for this substance. PAH is such a substance, and therefore to measure RPF, PAH is infused until a steady concentration in arterial blood is reached. Thereafter, urine is collected for 24 hours and urine flow and PAH concentration measured.
What is the consequence of no ADH present in the kidney?
There is no reabsorption of water in the nephron --> dilute urine
What is the consequence of high ADH levels in the kidney?
Reabsorption of water from the nephron --> urine becomes as concentrated as the renal medullary fluid
Define 'water diuresis'
Due to drinking too much water, blood becomes dilute causing ADH release to be inhibited resulting from a high volume of dilute urine – this condition is called diabetes insipidus.
Define 'osmotic diuresis'
Sugars are normally completely reabsorbed in PCT, if they’re not due to excess glucose in the blood, the glucose passing through the collecting duct provides an osmotic force to pull water into the urine, creating a high volume of sugary urine – this is called diabetes mellitus
Explain the function of the ascending loop of Henle in the reabsorption of water
In epithelia of ascending loop of Henle there are ATP-dependent potassium channels (ROMK) that transport K+ out of the cells and into the nephron lumen, creating a positive voltage. There are also Na-K-Cl cotransporters (NKCC2) which work to draw in these molecules from the nephron lumen . The positive luminal potential formed by the K+ efflux causes the increased uptake of electrolytes by NKCC2, the Na+/K+ ATPase pump allows sodium to be thereafter pumped out into the interstitial fluid and then blood, and potassium and chloride also leave the epithelia into the interstitial fluid passively. This creates a negative concentration gradient outside of the Loop of Henle which draws more water out of the descending loop --> less dilute urine
How does urea contribute to the formation of concentrated urine?
Urea is actively pumped into the interstitial fluid from the collecting ducts by urea transporting proteins in the ducts in order to increase solute concentration in the renal medulla, to facilitate more water loss from the loop of Henle and collecting duct too
What is the function of furosemide?
Acts by inhibiting NKCC2 and by blocking this transport it abolishes the high concentration of solutes in the renal medulla and prevents the formation of concentrated urine
What is the function of loop diuretics?
Diuretics which block the transport of sodium and chloride out of the loop of Henle in order to abolish the high concentration of solutes in the renal medulla and therefore prevent the formation of concentrated urine
What is a countercurrent multiplier?
A mechanism of urine concentration involving the process of pumping out salt into the extracellular fluid around the loop of Henle
What is the countercurrent exchange mechanism?
This is where the vasa recta flow in the opposite direction to the fluid movement in the loop of Henle in order to preserve the concentration gradient despite a blood flow through the vasa recta capillaries
Describe the role of atrial stretch receptors
Sensory nerve fibres found in the tissue of the right atrium (few in left) --> act as stretch receptors --> when venous return increases become more stretched, signal volume of blood returning to heart per minute --> information carried up vagus nerve to brainstem --> integrated to obtain information on total blood volume --> stretch of these sensors decreases BP by inhibiting sympathetic output
When is BNP produced by the body?
Cardiac muscle can release brain-derived natriuretic peptide (BNP) but these levels are very low in healthy person as BNP only released in quantity when the ventricles are overstretched in heart failure.
Describe the role of the hormonal volume receptors in the tissue of the right atrium and inferior vena cava
Specialised muscle cells which act as stretch receptors --> in response to stretch --> release atrial natriuretic peptide (ANP) which decreases the sodium reabsorption in DCT of nephron --> reduced water re-uptake --> increased water loss in urine --> reduced blood volume --> decrease BP
Which substance opposes the action of aldosterone?
Where are osmoreceptors located?
These are located in the hypothalamus; paraventricular and supraoptic nuclei
How do osmoreceptors react to hypo-osmolarity (decreased serum solute concentration)?
Trigger renin release from kidney (sympathetic fibres which stimulate this release directly) -->increases sodium reabsorption in the DCT (which also causes more water reabsorption) --> water reabsorption is counter-acted by osmoreceptors inhibiting ADH release from the pituitary gland --> more water can be lost in urine --> increase in the plasma sodium concentration