52 Reabsorption and secretion Flashcards
Myogenic mechanism and macula densa feedback
Renal artery pressure autoregulatory mechanisms
Increase in arterial BP leads to increased stretching of blood vessels. This leads to increased intracellular Ca++ levels in the cells of the blood vessels. Ca++ promotes vascular resistance, which inhibits blood flow and GFR
Myogenic feedback
Depending on how much sodium is filtered in the glomerulus, the macula densa regulates renin (hormone) release. Renin increases the levels of angiotensin II, which increases the resistance of the efferent arterioles, increasing pressure in the glomerular capillaries and increasing GFR.
Tuboglomerular feedback (macula densa feedback)
o In the case of increase in BP, the macula densa decreases the afferent arteriolar resistance, decreasing BP in the glomerulus and decreasing GFR
Tuboglomerular feedback (macula densa feedback)
T/F. Both secretion and reabsorption can occur by either of two pathways: paracellular and transcellular
True
T/F. Tubular secretion occurs if the filtered load is higher than the excreted load
False. Tubular reabsorption occurs if the filtered load is higher than the excreted load
GFRPx - UxV = filtered load (mg/min) - excretion rate (mg/min) = ?
Reabsorption rate (mg/min)
Transport maximum (Tm) and gradient time mechanisms
Mechanisms of active reabsorption in the proximal tubule
Mainly for Na+ reabsorption. It is always 67% (in a normal person) of the filtered load. Characteristics of carriers: Low affinity, Not easily saturated, High back leak
Gradient-time mechanisms (active reabsorption)
Used for all other reabsorbed solutes (i.e. glucose, amino acids, ketone bodies, calcium, phosphate). Characteristics of carriers: High affinity, Easily saturated, Low back leak
Transport maximum (Tm) mechanism (active reabsorption)
All reabsorptive processes have a limit on how fast they can occur, either b/c the substance leaks back into the lumen (___) or b/c the transporters are saturated (___)
gradient-time systems, transport maximum systems
The difference in plasma glucose concentration b/t the beginning of excretion and the point of maximum filtration. This difference exists b/c the nephrons are not homogeneous, and some become saturated before others. Therefore, excretion will occur before the known saturation point of the transporters
The splay
What happens to the Tm as you lose nephrons?
It decreases b/c you have less ability to reabsorb glucose (or other solutes)
T/F. Na+ is always reabsorbed with glucose and amino acids (co-transporter mechanism), as well as with other electrolytes. Na+ travels passively along its concentration gradient.
True. The sodium-potassium ATPase establishes the Na+ concentration gradient responsible for this diffusion
We always reabsorb equal amounts of Na+ and water in the PT of the kidneys
Isosmotic reabsorption
T/F. Sodium reabsorption leads to passive Cl- reabsorption and water reabsorption in the collecting ducts
Fasle. Sodium reabsorption leads to passive Cl-, urea, and water reabsorption in the PT
___ occurs if the filtered load is lower than the excreted load
Tubular secretion
Clearance of ___ is the marker for eRPF (b/c it is freely filtered and secreted)
PAH
Excretion rate (mg/min) – Filtered rate (mg/min) = UXV – GFRPx = ?
Secretion rate (mg/min)
ClearancePAH = Urine PAH concentration * flow/(plasma PAH concentration) = UPAH*((V)/(Ppah)) = ?
eRPF (effective renal plasma flow)
1) Glomerulotubular balance (Peritubular physical forces)
2) Hormones
3) Sympathetic NS
4) Arterial pressure (pressure natriuresis)
5) Osmotic factors
Mechanisms of regulation of tubular reabsorption
Simply means that as filter load increases, reabsorption increases. Regulated by peritubular physical forces (Kf, oncotic pressure of the capillaries, hydrostatic pressure of capillaries)
Glomerotubular balance
T/F. In the sympathetic response, the GFR/RPF ratio increases b/c RPF decreases much more than GFR
True
T/F. Increased oncotic pressure of peritubular capillaries decreases reabsorption in peritubular capillaries
False. Decreased oncotic pressure (as well as Kf) leads to decreased reabsorption
T/F. Increased hydrostatic pressure of the peritubular capillaries decreases reabsorption
True. This is the same as increasing arterial pressure
Preferentially constricts the afferent arteriole, decreasing RPF more than GFR, thus increasing the FF, which increases π in the peritubular capillaries and increases reabsorption.
It also directly and indirectly promotes reabsorption of Na+
Sympathetic NS
increase in arterial pressure leads to decrease in Na+ reabsorption
Arterial pressure (pressure natiuresis)
T/F. Water is reabsorbed only by osmosis. Thus, decreasing the amount of unreabsorbed solutes in the tubules decreases water reabsorption (and increases excretion)
False. Water is reabsorbed only by osmosis. Thus, increasing the amount of unreabsorbed solutes in the tubules decreases water reabsorption (and increases excretion)
In this case, plasma glucose concentration is very high. Therefore filtration increases past the saturation point of the reabsorption transporters. At this point, glucose is being excreted, increasing the osmolarity of the urine; which promotes secretion of water and inhibits reabsorption of water
Diabetes mellitus
T/F. The major factor controlling what is being reabsorbed is Na+. We always have about the same amount of Na+ being reabsorbed ~67%. Na+ has glucose and amino acid co-transporters. Cl-, bicarbonate, potassium, and water are also reabsorbed w/ Na+
True
T/F. H+, organic acids, and bases are all secreted into the proximal tubule (mostly occurs via channels and transporters)
True
T/F. Creatinine and urea decrease in TF/P concentration b/c they are being reabsorbed into the proximal tubule
False. Creatinine and urea increase in TF/P concentration b/c they are being secreted into the proximal tubule
This portion of the nephron is very permeable to water. The only transport that occurs here is water reabsorption, which leads to increased osmolarity of the filtrate
Descending loop of henle
This portion of the nephron is NOT permable to water. Here, reabsorption of Na+, Cl-, K+, Ca++, HCO3-, and Mg++ occurs. Secretion of H+ occurs.
Thick ascending loop of henle
T/F. B/c of reabsorption of electrolytes and not water, the filtrate in the thick ascending loop of henle is hypo-osmotic
True
T/F. Transport of solutes in the thick ascending loop of Henle is electroneutral
False. Transport of solutes in the thick ascending loop of Henle is electrogenic (generates electrical gradient) b/c of the K+ leak channels
This portion of the nephron (electroneutral) Transports Na+ and Cl- into the tubular cells via a co-transporter (Cl- leaks into the renal interstitial fluid). Not permeable to water (aka diluting segment). Contains the macula densa
Early distal tubule. Functionally similar to thick ascending loop. Active reabsorption of Na+, Cl-, Mg++ (and K+, dependent of K+)
Permeability to water in these portions of the nephron depends on ADH (if ADH is present, aquaporins are inserted into the tubular cells, rendering them permeable to water)
Early and late distal tubules and collecting tubules
These cells reabsorb Na+ and Cl-, and secrete K+ (activated by aldosterone)
Principal cells (of collecting tubules and DT)
These cells secrete H+ and reabsorb K+ (via antiporter) depending on the needs of the body
Intercalated cells (of collecting tubules and DT)
T/F. Na+ channel (in epithelium) is the major sodium transporter (for reabsorption)
True
This portion of the nephron is capable of Na+, Cl- reabsorption. Reabsorbs water (if ADH is present), this concentrates urine. Urea is reabsorbed. Bicarbonate is reabsorbed. H+ is secreted (increasingly in the case of acidosis)
Transport characteristics of medullary collecting ducts
What are the only physiological methods of altering GFR?
Constricting or dilating the afferent and/or efferent arterioles
The efferent arterioles become the ___ capillaries. These capillaries then become the interlobular veins
Peritubular capillaries
