Basic Tubular Transport Mechanisms Flashcards
When excretion is less than filtration…
Reabsorption= Filtration- Excretion
Filtration= GFR x Plasma concentration
Excretion= Urine concentration x Urine flow rate
NOTE: This means that there is no net secretion
When excretion is greater than filtration…
Secretion= Excretion- Filtration
Filtration= GFR X plasma concentration
Excretion= Urine concentration x Urine flow rate
Glucose transport maximum
NOTE: Some substances have a maximum rate of tubular transport due to saturation of carriers, limited ATP, etc
EX: Glucose, amino acids, phosphate, sulfate
_____________ is the tubular load at which transport maximum is exceeded in some nephrons.
Threshold
NOTE: This is not exactly the same as the transport maximum of the whole kidney becuase some nephrons have lower transport maximums than others
Changes in concentration in proximal tubule
Transport in thick ascending loop of Henle
Which loop diueretics inhibit the Na-K-2Cl transporter of the thick limb of the loop of henle?
Furosemide
Ethacrynic acid
Bumetanide
Which diuretics inhibit the Na-Cl channels of the early distal tubule?
Thiazide diueretics
Which component of the distale tubule is functionally simular to the thick ascending loop?
Early Distal Tubule
- Not permeable to water
- Active reabsorption of Na+, Cl-, K+, Mg++
- Contains macula densa
What relationship do the early distal tubules, late distal tubules, and collecting duct have with water and urea, respectively?
Early distal tubule
- Not permeable to water
- Not very permeable to urea
Late distal tubule
- Permeability to H2O depends on ADH
- Not very permeable to urea
Collecting duct
- Permeability to H2O depends on ADH
- Not very permeable to urea
Which cells secrete K+?
Principle cells
Aldosterone antagonists
Spironolacone
Eplerenone
Na+ channel blockers
Amiloride
Triamterene
Normal renal tubular Na+ reabsorption
If water is reabsorbed to a greater extent thatn the solute, the solute will become _______ (more/less) concentrated in the tubule.
More
Ex: Creatinine, inulin
If water is reabsorbed to a lesser extent than the solute, the solute will become ________ (more/less) concentrated in the tubule.
Glucose, amino acids
Changes of concentration substances in the renal tubules
Which hormones regulate tubular reabsoption?
Aldosterion
Angiotensin II
ADH
ANF
PTH
Normal urine concentration
1.2
NOTE: It is important for tubular reabsorption to increase when GFR increases to maintain a normal urine concentration.
How is peritubular capillary reabsorption calculated?
Reabsorption= Net Reabsorption pressure x Kf
NOTE: An increase in peritubular oncotic pressure increases reabsorption, while an increase in peritubular hydrostatic pressure leads to decrease in reabsorption.
Which factors determine peritubular capillary hydrostatic pressure?
- Arterial pressure- Directly related
- Increase in afferent resistance- Indirectly related
- Increase in efferent resistance- Indirectly related
What factors determine peritubular capillary colloid osmotic pressure?
-
Plasma protein
- An increase in plasma protein concentration, leads to an increase in arterial oncotic pressure, which leads to an increase in peritubular capillary oncotic pressure
-
Filtration fraction
- An increase filtration fraction leads to an increase in peritubular capillary oncotic pressure
Factors that can influence peritubular capillary reabsorption
What actions does aldosterone have on late distal, cortical and medullary collecting tubules?
- Increases Na+ reabsorption- principal cells
- Increases K+ secretion- principal cells
- Increases H+ secretion- Intercalated cells
Conn’s syndrome
Primary aldosteronism
- Excess aldosterone
- Na+ retention
- Hypokalemia
- Alkalosis
- Hypertension
Addison’s disease
- Aldosterone deficiency
- Na+ wasting
- Hyperkalemia
- Hypotension
What factors increase aldosterone secretion?
- Angiotensin II
- Increased K+
- Adrenocorticotrophic hormone (ACTH)
Which factors decrease aldosterone secretion?
- Atrial natrueretic factor
- Increased Na+ concentration
NOTE: ANP increases glomerular filtration rate and glomerular permeability. ANP also inhibits the effect of aldosterone on the mesangial cells.
How does aldosterone function to increase Na+ and water reabsorption?
- Stimulates aldosterone secretion
- Directly increases Na+ reabsorption
- Contricts efferent arterioles
- Decreases peritubular capillary hydrostatic pressure
- Increases filtration fraction, which increases pertubular colloid osmotic pressure
ACE inhibitors
Captopril
Benazipril
Ramipril
Angiotension II antagonists
Losartan
Candesartin
Irbesartan
Renin inhibitors
Aliskirin
NOTE: Renin inhibitors decrease aldosterone, directly inhibit Na+ reabsorption, and decrease efferent arteriolar resistance
ADH is secreted by _______
Posterior pituatary
NOTE: ADH is synthesized in the manocellular neurons of the hypothalamus
Mechanism of action of ADH in distal and collecting tubules
Feedback control of extracellular fluid osmolarity by ADH
Excess ADH secretion can lead to ..
Decreased plasma osmolarity
Hyponatremia
What condition can lead to insufficient release of ADH?
“Central” Diabetes Insipidus
- Increased plamsa osmolarity
- Hypernatremia
- Excess thirst
How does atrial natriuretic peptide increase Na+ excretion?
- Secreted by cardiac atria in response to stretch (increased blood volume)
- Directly inhibits Na+ reabsorption
- Inhibits renin release and aldosterone formation
- Increases GFR
- Helps to minimize blood volume expansion
PTH action on Ca++
- Increases Ca++ reabsorption by kidneys
- Increases Ca++ reabsorption by gut
- Decrease phosphate reabsorption
- Helps to increase extracellular Ca++
NOTE: PTH is released by parathyroids in response to decreased extracellular Ca++
How does the sympathetic nervous system increase Na+ reabsorption?
- Directly stimulates Na+ reabsorpion
- Stimulates renin release
- Decreases GFR and renal blood flow
- Only at high levels of sympathetic stimulation
How does increased arterial pressure decrease Na+ reabsorption?
- Increase peritubular capillary hydrostatic pressure
- Decreased renin and aldosterone
- Increased release of intrarenal natriuretic factors
- Prostaglandins
- EDRF
What effect does osmosis have on reabsorption?
- Increasing the amount of unreabsorbed solutes in the tubules decreases water reabsorption
-
Diabetes mellitus
- Unreabsorbed glucose in the tubules causes diueresis and water loss
- Osmotic diuretics (mannitol)
-
Diabetes mellitus
NOTE: Water is reabsorbed only by osmosis
Conn’s syndrome
Primary aldosterone excess
Glucocorticoid remediable aldosteronism
Excess aldosterone secretion due to abnormal contraol of aldosterione synthase by ACTH (genetic)
Renin secreting tumor
Excess Angiotensin II formation
Inappropriate ADH syndrome
Excess ADH
Liddle’s syndrome
Excess activity of amiloride sensitive Na+ channel (genetic)
Diabetes Insipidus
- Decreased water reabsorption
- Hypernatremia
- Nephrogenic
- Lack of ADH
Addison’s disease
- Decreased Na+ reabsorption
- Decreased K+ secretion
- Lack of aldosterone
Bartter’s syndrome
- Decreased Na+, Ca++, HCO3- reabsorption
- Hypotension
- Decreased activity of Na-K-2Cl transporter in loop of Henle
*
Gitleman’s Syndrome
- Decreased NaCl reabsorption
- Hypotension
- Decreased activity of NaCl co-transporter in distal tubule
Fanconi syndrome
- Generalized decrease in reabsorption
- Often in proximal tubules
- Causes:
- Genetic
- heavy metal damage
- Drugs (tetracyclines)
- Multiple myeloma
- Tubular necrosis (ischemia)
Renal tubular acidosis
- Decreased H+ secretion
- Increased HCO3- excretion
- Acidosis
- Causes:
- Genetic
- Renal injury
Assessing Kidney Function
