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Renal phys > Renal Transport Mechanisms > Flashcards

Renal Transport Mechanisms Flashcards

1
Q

Urine formation

A

= GFR - tubular reabsorption + tubular secretion

-TUBULAR REABSORPTION is more vital for determining excretion rate

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2
Q

Filtration

A

= GFR x plasma concentration

-this calculation only works if the substance is freely filtered (NOT bound to plasma protein; i.e., glucose)

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3
Q

Tubular Reabsorption

A

-glomerular filtration and tubular reabsorption of substances are large compared to excretion

  • HIGHLY SELECTIVE; glomerular filtration is nonselective
    1) kidneys regulate the excretion of substances independently
    2) kidneys excrete substances at different rates (based on needs; allows for precise control)

-both active and passive

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4
Q

Steps for resorption of a substance

A

1) transported across the tubular epithelial membrane
2) through the interstitial fluid
3) through the peritubular capillary membrane
4) into the BLOOD

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5
Q

Transcellular route

A

through the cells membranes (tubular reabsorption)

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6
Q

Paracellular route

A

between the cells through the tight junctions (tubular reabsorption)

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7
Q

Water and solutes can travel through the tubular epithelium by way of:

A

1) transcellular route
2) paracellular route
3) through medullary interstitium fluid
4) through the peritubular capillary walls into the blood (ULTRAFILTRATION; mediated by hydrostatic pressure and osmotic flow)

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8
Q

Tubular reabsorption - Active Transport

A

-the movement of a solute against an electrical gradient

  • PRIMARY
  • SECONDARY
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9
Q

Primary active transport (tubular reabsorption)

A
  • coupled with HYDROLYSIS of ATp

- i.e., sodium-potassium ATPase pump

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10
Q

Secondary active transport (tubular reabsorption)

A
  • coupled indirectly to the energy source (i.e., ion gradient)
  • ex, reabsorption of GLUCOSE by the renal tubule
  • energy liberated during downhill transport of one substance allows for the uphill transport of another substance in the opposite direction
  • 2 OR MORE substances transported across a membrane by a carrier molecule
    1) one substance diffused down it’s electrochemical gradient
    2) the energy released drives another substance against it’s electrochemical gradient

Proximal tubule –> luminal membrane (by SGLT 2 or 1) –> peritubular capillary (PASSIVE UPTAKE; by BULK FLOW)

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11
Q

Tubular reabsorption - Water

A
  • water is ALWAYS absorbed by passive mechanism

- OSMOSIS

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12
Q

Tubular reabsorption - Sodium (from proximal tubular membrane to inside cell)

A
  • first step
    1) DIFFUSION across the luminal membrane into the cell
    2) high Na concentration (tubular lumen) –> low Na concentration (inside cell)

*Na will move to the NEGATIVELY CHARGED CELL

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13
Q

Tubular reabsorption - Sodium (from cell into medullary interstitium)

A
  • second step
    1) the membrane on the basolateral surface has a sodium-potassium ATPase system (hydrolysis of ATP)
    2) ACTIVE TRANSPORT of Na and K from inside the cell to the interstitium

*causes the -70 millivolt negative charge in the cell

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14
Q

Tubular reabsorption - Sodium (from intercellular fluid into peritubular capillary)

A
  • PASSIVE process

- Na, water, and other substances are reabsorber by ULTRAFILTRATION (driven by the osmotic gradient)

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15
Q

Sodium Glucose co-transporters (SGLT2 and SGLT1)

A

-part of secondary active transport in tubular reabsorption

  • located in BRUSH BORDER
  • transport glucose into the cell AGAINST a concentration gradient
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16
Q

SGLT2

A
  • located in the early part of the proximal tubule

- transports 90% of the filtered glucose for reabsorption

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17
Q

SGLT1

A
  • located in the later part of the proximal tubule

- transports the remaining 10% of filtered glucose for reabsorption

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18
Q

Counter transport

A

secretion of a substance into the tubule by SECONDARY active transport

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19
Q

Tubular reabsorption - Pinocytosis

A
  • ACTIVE transport mechanism for the reabsorption of proteins and amino acids
    1) protein attaches to BRUSH BORDER of the luminal mambrane
    2) causes INVAGINATION of cell wall, completely surrounding protein
    3) PINCHES off into the cell
    4) protein is broken down
    5) AAs reabsorbed into the interstitial fluid through the BASOLATERAL MEMBRANE
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20
Q

Tubular reabsorption - Transport Maximum

A
  • the limit to the rate at which a solute can be transported during active reabsorption or sectretion
  • i.e., GLUCOSE
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21
Q

Gradient Time Transport

A

-some substances that are PASSIVELY absorbed do NOT demonstrate a transport maximum (sometimes active, i.e., Na)

  • rate of transport depends on:
    1) electrochemical gradient
    2) permeability of membrane to the substance
    3) time the fluid containing the substance remains in the tubule
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22
Q

Sodium and Gradient Time Transport

A

-maximum transport capacity of basolateral sodium potassium ATPase pump is greater than the actual rate of sodium reabsorption

  • Na leaks back into the TUBULAR LUMEN through the epithelial TIGHT JUNCTIONS
  • rate of leakage depends on:
    1) permeability of tight junction
    2) rate of bulk flow reabsorption into the peritubular capillaries

*the greater the concentration of Na in the proximal tubule, the greater the reabsorption rate (gradient time transport principles)

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23
Q

Passive water absorption is coupled with sodium reabsorption

A
  • Na reabsorption causes OSMOSIS of water in/between highly permeable cells (proximal tubule)
  • the osmosis causes a SOLVENT DRAG (movement of water carries solutes with it)
  • the osmosis can only occur across the tubular membrane is it is permeable to WATER (no matter how large the osmotic gradient is)
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24
Q

Tubular reabsorption - Chloride

A
  • PASSIVE transport out of the negatively charged TUBULAR LUMEN
  • as water moves from the tubular lumen, the chloride concentration INCREASES causing a PASSIVE transport
  • secondary active transport - co-transport of Cl and Na across tubular membrane
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25
Q

Tubular reabsorption - Urea

A
  • as water is reabsorbed, urea concentration INCREASES - causing PASSIVE reabsorption
  • urea transporters are also found in the MEDULLARY COLLECTING DUCT
  • the tubule is NOT very permeable to urea (allows for large amounts to be excreted)
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26
Q

Tubular reabsorption - Creatine

A
  • **can NOT cross the tubular membrane

- almost all creating filtered at the glomerulus is excreted

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27
Q

___% of filtered water and Na+ are reabsorbed in the _______, before reaching the _______. Why?

A

65%; proximal tubule; loop of Henle

1) highly active and passive reabsorption
2) highly metabolic epithelial cells
3) large surface area (brush border, basolateral surface)

28
Q

What is the primary way to reabsorb sodium, chloride, and water during primary tubular reabsorption?

A

Sodium-potassium ATPase pump

29
Q

What substances would you find in the FIRST half of the proximal tubule?

A
  • sodium
  • glucose
  • amino acids
  • water
30
Q

What substances would you find in the second half of the proximal tubule?

A
  • LOW concentrations of glucose and amino acids

- HIGH concentrations of chloride

31
Q

T/F: there is a constant osmolarity along the proximal tubule

A

True

32
Q

Proximal tubular secretion

A
  • organic acids and bases
  • drugs and toxins
  • substances are filtered in the glomerulus and sexreted into the tubules (rapidly)
33
Q

3 parts of the loop of Henle

A

1) THIN descending segment
2) THIN ascending segment
3) THICK ascending segment

34
Q

Thin descending segment of the loop of Henle

A
  • allows for SIMPLE DIFFUSION
  • THIN epithelial membrane
  • NO brush border
  • FEW mitochondria
  • highly permeable to water; 20% of water absorbed here
35
Q

Thin ascending segment of the loop of Henle

A
  • thin walls, NO brush border
  • site of PASSIVE NaCl reabsorption
  • driven by reabsorption of water from thin descending segment
36
Q

Thick ascending segment of the loop of Henle

A
  • THICK epithelial cells
  • brush border
  • highly metabolic activity
  • ACTIVE reabsorption of Na, Cl, and K (~25% absorbed here)
  • also, calcium, HCO3, and Mg absorbed here
  • Na-K ATPase pump HIGHLY ACTIVE here; maintains low intracellular Na concentration
  • 1-sodium, 2-chloride, 1-potassium co-transporter (in luminal membrane)
  • Na-H counter-transport mechanism (Na reabsorption, H secretion)
  • paracellular reabsorption

*IMPERMEABLE to water!!

37
Q

Parts of the Distal Tubule

A

1) Macula densa

2) Diluting segment

38
Q

Macula densa of distal tubule

A
  • part of the juxtaglomerular complex

- provides feedback and control of GFR

39
Q

Diluting segment of distal tubule`

A
  • ACTIVELY reabsorbs sodium, potassium, and chloride

- IMPERMEABLE to water and urea

40
Q

Early distal tubule

A
  • 5% of filtered load of Na+ is reabsorbed here
  • Na-Cl co-transporter moves NaCl from tubular lumen into the cell
  • Na-K ATPase pump transports Na+ across basolateral membrane
41
Q

Late distal tubule and cortical collecting tubule

A
  • composed of:
    1) Principal cells
    2) Intercalated cells
  • impermeable to UREA
  • permeability of these segments to WATER is dependent on ADH (vasopressin)
    1) HIGH ADH = high permeability
    2) LOW ADH = low permeability
  • controls the dilution and concentration of urine
42
Q

Reabsorption of sodium/secretion of potassium in distal tubule and cortical collecting tubule controlled by hormone ______

A

Aldosterone

43
Q

Principal cells

A
  • sodium reabsorption

- potassium secretion

44
Q

Intercalated cells

A
  • hydrogen ion secretion

- HCO3 and K reabsorption

45
Q

ADH (vasopressin)

A
  • increases water REABSORPTION in response to dehydration
  • without ADH, the DISTAL TUBULE and COLLECTING DUCTS are NOT very permeable to water
  • without ADH, we would have dilute urine (plays a role in concentration of urine)

*Conserves WATER

46
Q

Glomerulotubular Balance

A
  • the ability of the tubules to increase reabsorption in response to increased tubular inflow
  • the rate of absorption INCREASES but the % of the filtrate reabsorbed remains constant
  • second line of defense when there is an increase in systemic pressure
47
Q

What is the normal rate of capillary reabsorption?

A

124 ml/min

48
Q

Net Reabsorptive Forces

A
  • peritubular capillary and renal interstitial fluid physical forces
  • the sum of HYDROSTATIC and COLLOID OSMOTIC FORCES that favor or oppose reabsorption across the peritubular capillaries
49
Q

Forces that FAVOR reabsorption

A
  • Plasma colloid osmotic pressure (32mmHg)

- Hydrostatic pressure in the interstitium (6mmHg)

50
Q

Forces that OPPOSE reabsorption

A
  • Peritubular hydrostatic pressure (13mmHg)

- Osmotic pressure of the renal interstitium (15mmHg)

51
Q

What happens if we RAISE peritubular capillary hydrostatic pressure?

A

-caused by an increase in arterial pressure

  • DECREASE in reabsorption rate
  • because it OPPOSES reabsorption
52
Q

What happens if we LOWER peritubular capillary hydrostatic pressure?

A
  • caused by an increase in resistance of either afferent or efferent arterioles
  • INCREASE in reabsorption rate
53
Q

What happens if we RAISE the colloid osmotic pressure of the plasma in the capillaries?

A
  • happens when there is an increase in protein concentration in the plasma
  • INCREASE capillary reabsorption
54
Q

What happens when peritubular capillary pressure is REDUCED?

A
  • INCREASE in interstitial fluid hydrostatic pressure
  • water and solutes leak back into tubular lumen
  • REDUCES the rate of net reabsorption
55
Q

What happens when peritubular capillary pressure is INCREASED?

A
  • DECREASE in interstitial fluid hydrostatic pressure
  • RAISES interstitial osmotic pressure
  • back leak is decresed
  • INCREASED rate of net reabsorption
56
Q

Pressure Natriuresis

A

Na+ excretion in urine

57
Q

Pressure Diuresis

A

water excretion to regulate BP

58
Q

What happens to the excretion of sodium and water when there is an increase in arterial pressure?

A

BOTH increase in excretion

-reduction of angiotensin II leads to decreased tubular sodium reabsorption

59
Q

Aldosterone

A

-secreted by the ZONA GLOMERULOSA CELLS of the adrenal cortex

  • regulates Na+ reabsorption and K+ secretion
    1) found in principal cells of cortical collecting tubule
    2) stimulates Na+-K+ ATPase pump on basolateral side of tubule membrane
    3) INCREASES permeability of Na+ on luminal side
60
Q

Aldosterone is released in response to:

A

1) increased extracellular K+ concentration

2) increased Angiotensin II levels

61
Q

Atrial Natriuretic Peptide (ANP)

A
  • released in cardiac atria when distended (i.e., congestive heart failure)
  • counters renin-angiotensin system
  • INHIBITS renin secretion
  • INHIBITS reabsorption of sodium and water
  • INCREASES urinary secretion
  • helps return blood volume to normal
62
Q

Parathyroid hormone

A

-INCREASES calcium reabsorption (distal tubule, loop of Henle)

  • INHIBITS phosphate reabsorption in prox tubule
  • STIMULATES magnesium reabsorption in loop of Henle
63
Q

Angiotensin II

A
  • INCREASES sodium and water reabsorption
  • increase when there is
    1) low blood pressure
    2) low extracellular volume
  • STIMULATES aldosterone secretion (inc Na+ reabs)
  • CONSTRICTS efferent arterioles
    1) reduces peritubular capillary pressure (^ reabs)
    2) decreases renal blood flow (^ reabs)
64
Q

Angiotensin II DIRECTLY stimulates sodium reabsorption in:

A
  • proximal tubule
  • loop of Henle
  • distal tubule
  • collecting tubules

stimulates:

1) Na+-K+ ATPase pump
2) Na+-H+ exchange
3) Na+-HCO3 co-transport

65
Q

SNS activation

A
  • DECREASE in sodium and water EXCRETION
  • severe stimulation = const of renal arterioles; dec GFR
  • low stimulation = inc Na+ reabsorption in:
    1) distal tubule
    2) THICK ascending limb of loop of Henle

-INCREASES renin release and angiotensin II formation

Renal phys (8 decks)

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