Lecture 16: Tubular Reabsorption and Secretion Flashcards
Paracellular route of reabsorption
Transport of filtrate between cells; diffusion
Transcellular route of reabsorption
Transport of filtrate through cells; transporters and channels involved
Aquaporins
Specific channel for water transport which can be removed or inserted onto membrane, regulating H2O reabsorption
Aquaporin-1
Widespread, including in renal tubules
Aquaporin-2
Present in apical membranes of collecting tubule cells, CONTROLLED BY ADH
Aquaporin-3
Present in basolateral membranes of collecting tubule cells
ATPases
Establish ionic gradients across nephron cell membranes which drive reabsorption or secretion of many solutes; SECONDARY active transport; Na+ gradient powers movement (symporters, antipoers)
ENaC channel
Found in apical membrane of nephron cells, closed by amiloride, opened by several hormones
Facilitated transport
Transport occurring through channels or uniporters; example: glucose transport
Active transport
Transport directly coupled to an energy source; example: Na+/K+ ATPase pump, Ca2+ ATPase pump, H+ATPase pump, etc.
Secondary active transport
Transport coupled indirectly to an energy source; example: Ion gradient, Na+/glucose cotransporters
SGLT2
Reabsorbs 90% of glucose in early proximal tubule (sodium glucose cotransporter on brush border of PCT cells)
SGLT1
Reabsorbs 10% of glucose in late proximal tubule (sodium glucose cotransporter on brush border of PCT cells)
What substances are actively secreted into the renal tubules?
Creatinie, para-aminohippuric acid
Transport maximum
There is a limit to the rate at which the solute can be transported due to saturation of a specific transport system
Transport maximum for glucose
375 mg/min; if we exceed this amount, all glucose will be excreted in urine
Solvent drag
Solutes dragged along with solvent regardless of gradient due to osmotic movement of water
Which of the following in L/day represents the GFR in the average adult human?
A. 3
B. 125
C. 180
D. 360
C. 180
An increase in which of the following factors will result in a decrease in glomerular hydrostatic pressure?
A. Arterial pressure B. Afferent arteriolar resistance C. Efferent arteriolar resistance D. Both B and C E. All of the above
B. Afferent arteriolar resistance
Almost 90% of glucose reabsorption occurs in the early proximal tubule. Which of the following mechanisms is responsible for moving glucose from the tubular lumen into the cytoplasmic compartment?
A. Primary active transport via glucose ATPase pump
B. Secondary active transport via Na+/glucose cotransporter
C. Secondary active transport via Na+/glucose antiporter
D. Diffusion due to concentration differences between tubular fluid and cytoplasm
B. Secondary Active Transport via a sodium/glucose cotransporter
Transport characteristics of proximal tubule
highly metabolic, many mitochondria, extensive brush borders (lumen), extensive channels (interstitial)
What substances are reabsorbed in proximal tubule?
65% filtered sodium, chloride, bicarbonate, and potassium; all filtered glucose and amino acids
What substances are secreted into lumen by proximal tubule?
H+, organic acids, bases
Transport characteristics of thin descending limb of Loop of Henle
Highly permeable to H2O, moderately permeable to most solutes (urea, sodium)
What substances are reabsorbed by the thin descending segment of the Loop of Henle?
20% of filtered H2O
Thin ascending segment of Loop of Henle
IMPERMEABLE TO WATER
Thick ascending segment of Loop of Henle
Na+/K+ ATPase pump in basolateral membranes, Na+/K+/Cl- cotransporter, slight leak of K+ into lumen; IMPERMEABLE TO WATER
Distal tubule
Contains macula densa, reabsorbs most ions but is IMPERMEABLE to WATER and UREA (diluting segment)
Principal cells
Reabsorb Na+, H2O from lumen; secrete K+ into lumen using Na+/K+ ATPase pump (late distal/collecting ducts)
Intercalated cells
Reabsorb K+ (HCO3-?) from lumen; secrete H+ into lumen via H_ ATPase transporter
Medullary collecting duct
Cuboidal epithelium, smooth surface with few mitochondria; ADH controls permeability to H2O; permeable to urea via transporters; can secrete H+ against large concentration gradient (acid/base control)
What substances are reabsorbed in medullary collecting duct?
Sodium, Chloride, H2O in presence of ADH, Urea, Bicarbonate
Aldosterone
Source: adrenal cortex
Function: increases Na+ reabsorption, stimulates K+ secretion
Site of action: principal cells of cortical collecting ducts
Stimulus for secretion: increased extracellular K+, increased levels angiotensin II
Addison’s Disease
Occurs due to the absence of aldosterone, results in marked loss of sodium and accumulation of potassium
Conn’s Syndrome
Hypersecretion of aldosterone (few symptoms)
Angiotensin II
Function: increased Na+, H2O reabsorption, returns blood pressure and extracellular volume toward normal
Effects: stimulates aldosterone secretion, constricts efferent arterioles, stimulates Na+ reabsorption in proximal tubules
ADH
Source: posterior pituitary
Function: increases H2O reabsorption
Effects: binds to V2 receptors in late distal tubules, collecting tubules/ducts; increases formation of cAMP stimulating movement of aquaporin-2 proteins
Atrial Natriuretic Peptide (ANP)
Source: cardiac atrial cells in response to distension
Function: inhibits reabsorption of Na+, H2O
Parathyroid Hormone (PTH)
Source: Parathyroid glands
Function: increases Ca2+ reabsorption
Renal clearance
Volume of plasma that is completely cleared of the substance by the kidneys per unit time
Cs = (Us x V)/Ps
Inulin
Polysaccharide that is not produced in body; used to measure GFR via renal clearance (125 ml/min = normal)
