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Physiological concepts > Wk6 - cell reabsorption > Flashcards

Wk6 - cell reabsorption Flashcards

1
Q

Where does most reabsorption occur within the nephron?

A

PCT

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2
Q

The following refers to the tubular cells of the kidney:

a. ) Is tubular resorption a selective process?
b. ) Do tubule cells have a high or low reabsorptive capacity?

A

a. ) Is tubular resorption a selective process - tubular reabsorption is a highly selective process.
b. ) Do tubule cells have a high or low reabsorptive capacity - tubule cell reabsorptive capacity depends on whether or not the substance is required by the body. For substances required, capacity is high. For substances not required, reabsorptive capacity is low - meaning the substance is mostly left in the filtrate and thus exits the body.

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3
Q

In some circumstances, substances which are required by the body (and thus usually all reabsorbed) appear in the urine. Why does this occur?

A

As the substance is present in the body to excess.

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4
Q

Detail the thickness of the epithelia of the nephron.

A

One cell thick (simple).

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5
Q

What lies in close proximity to the kidney tubules?

A

the peritubular capillary network.

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6
Q

The following refers to renal cells:

a. ) Are the junctions of renal cells loose or tight?
b. ) What lies below the junctions of renal cells?

A

a. ) Are the junctions of renal cells loose or tight - tight
b. ) What lies below the junctions of renal cells - Lateral Intercellular Spaces (LIS).

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7
Q

What are the 2 mechanisms which occur for tubular reabsorption?

A
  1. Passive reabsorption - all steps are passive.

2. Active reabsorption - where at least one step requires energy

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8
Q

In general, how is the following substance transported through the kidney tubules:
Solutes.

A

Solutes are reabsorbed by active transport (which may be primary or secondary), or passive mechanisms.

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9
Q

In general, how is the following substance transported through the kidney tubules:
Water.

A

Always passive, via osmotic gradients (the process of transport being osmosis).

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10
Q

Substances can be transported through the nephron by two different routes. What are these routes known as?

A
  1. Transepithelial (transcellular) - across the cell

2. Paracellular - between the cells; through LIS

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11
Q

Detail some general steps involved with kidney tubule reabsorption.

A
  1. Na+ reabsorbed by active transport
  2. Electrochemical gradient drives anion reabsorption
  3. Solute reabsorption increases osmotic gradient causing water to be reabsorbed by osmosis.
  4. Concentration of other solutes still within filtrate increases due to decreased filtrate volume (following osmosis). Permeable solutes are then reabsorbed by diffusion.
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12
Q

In general, how is Na+ reabsorbed in the nephron?

A

Active transport.

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13
Q

Provide an example of transcellular reabsorption in the nephron.

A

Na+ transport in PCT:

  1. NaK pump (basolateral membrane) establishes Na+ gradient (low Na+ in tubule cell).
  2. Sodium diffuses down its electrochemical gradient from the filtrate, pass the apical membrane and into the tubule cell.
  3. Sodium actively transported across basolateral membrane by NaK pump, as it is going against its gradient (toward where it is high in concentration, the interstitial fluid).
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14
Q

Provide an example of paracellular reabsorption in the nephron.

A

Ca+2, Mg+2, K+ in PCT:
1. Water moves down its osmotic gradient from the filtrate, through LIS, into the interstitial fluid.
2. The water contains solutes (calcium, magnesium, potassium) which are thus reabsorbed with it.
This process is known as SOLVENT DRAG.

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15
Q

How much water is reabsorbed in the PCT?

A

67% of water in filtrate.

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16
Q

What solutes are reabsorbed within the PCT?

A
  • Na+
  • Cl-
  • K+
  • glucose (100%)
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17
Q

NaCl reabsorbtion is divided into 2 phases. Briefly, what do these entitle?

A
  1. Reabsorption of Na together with glucose, AA, PO4(3-), lactate and HCO3-
  2. Reabsorption of Na with Cl-
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18
Q

Detail the processes occurring during the first phase of NaCl reabsoption.

A
  1. Na+ uptake across apical membrane via symport with organic solutes (lactate, glucose, AA).
  2. Na+ uptake across apical membrane via antiport with H+.
  3. Carbonic anhydrase within the tubule cell converts CO2 and H2O to H+ and HCO3-. The H+ is used for antiport with Na+, whereas the HCO3- is reabsorbed into the blood.
  4. Na+ passes across basolateral membrane via NaK pump
  5. Organic solutes (lactate, glucose, AA) are reabsorbed into the blood via facilitated diffusion
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19
Q

How long does it take for the organic substances (glucose, AA, lactate) which are involved with NaCl reabsorption to be cleared from the filtrate? (note: answer is relative)

A

All these substances are reabsorbed during the first half of the PCT, so a short period of time.

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20
Q

Outline the processes occurring during the second phase of NaCl reabsoption (through what ‘pathways’ does this second phase occur)?

A

NaCl ionized (Na+ and Cl-), reabsorbed by both transcellular and paracellular pathways.

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21
Q

Detail the processes occurring during the second phase of NaCl reabsoption, regarding the paracellular pathway.

A
  1. NaCl paracellular transport occurs due to a rise in tubular fluid [Cl-].
  2. Cl- moves through LIS, past the interstitial spaces and into the blood.
  3. Cl- movement creates transepithelial voltage, which causes Na+ to follow (unlike charges attract).
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22
Q

Detail the processes occurring during the second phase of NaCl reabsoption, regarding the transcellular pathway.

A

Transcellular second phase NaCl transport relies upon the parallel operation of Na-H and Cl-Base antiporters.

  1. Na+ moves across apical membrane, as H+ moves out (antiport).
  2. Cl- moves across apical membrane, as a base moves out (antiport).
  3. The hydrogen and base which moved out recombine in the filtrate (HBase), re-enter the cell, to subsequently break apart into H + Base so that they can once again be used in antiport.
  4. Na is pumped across basolateral membrane via NaK pump (active, against gradient).
  5. Cl- moves across basolateral membrane in symport with K+.
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23
Q

Transcellular second phase NaCl transport relies upon the parallel operation of what?

A

Na-H & and Cl-Base antiporters.

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24
Q

What part of the kindey tubule cells does water pass through?

A

Both the basolateral membrane and the LIS.

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25
Q

What does the movement of water into the LIS induce?

A

Solutes move into the LIS, setting up a concenctration gradient for water within the tubule cell to follow (the LIS becomes hyperosmotic).

Within the water is more solutes, which are brought with it (solvent drag).

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26
Q

Are glucose or AA usually present in urine?

A

No.

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27
Q

Glucose and amino acids are reabsorbed by the same mechanism. Explain the steps involved with the resorption of glucose or AA.

A
  1. Glucose is in low concentration within tubule fluid, whereas Na is high. Na concentration within the cell is maintained at a low level due to the operation of the NaK pump.
  2. Glucose is cotransported with Na across the apical membrane (active).
  3. Glucose passively diffuses across the basolateral border, down its concentration gradient (facilitated diffusion).
    (note: ‘glucose’ above refers to either glucose or AA)
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28
Q

What is the apical membrane AKA?

A

The luminal border.

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29
Q

Carriers of glucose and AA are said to be ‘substance specific’ - what is the significance of this?

A

A limited number of these transporters/ carriers exist across the apical membrane of the tubule. This imposes an upper limit on the quantity of the substance which can be actively transported into the tubule cell, per given unit of time.

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30
Q

Describe the relative abundance of the following chemcial:

Glucose in the lumen of nephron tubule.

A

low concentration

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31
Q

Describe the relative abundance of the following chemcial:

Water in the lumen of nephron tubule.

A

highly abundant.

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32
Q

Describe the relative abundance of the following chemcial:

Sodium in the lumen of nephron tubule.

A

high concentration

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33
Q

Describe the relative abundance of the following chemcial:

Sodium in the tubule cell.

A

low concentration.

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34
Q

Describe the relative abundance of the following chemcial:

Glucose in the tubule cell.

A

high concentration

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35
Q

Describe the relative abundance of the following chemcial:

Glucose in the lumen of nephron tubule.

A

low concentration.

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36
Q

Describe the relative abundance of the following chemcial:

Glucose in the interstitial fluid.

A

low concentration.

37
Q

Describe the relative abundance of the following chemcial:

Sodium in the interstitial fluid.

A

high concentration.

38
Q

The following questions relate to glucose and AA resorption.

a. ) When does maximum rate of transport occur?
b. ) What is this point known as?
c. ) What happens when substances are in excess of this maximum rate of transport?

A

a. ) When does maximum rate of transport occur - when all substance specific carrier molecules are occupied.
b. ) What is this point known as - the Transport maximum (Tm)
c. ) What happens when substances are in excess of this maximum rate of transport - these substances remain in the filtrate, to be passed out of the body as urine.

39
Q

What substances display a Tm?

A

ALL actively transported substances, except for Na.

40
Q

What is the normal plasma concentration of glucose?

ie. [glucose] within the blood

A

100 mg per 100ml of blood

ie. 1mg of glucose per ml of blood

41
Q

What is normal GFR?

A

125ml per minute

42
Q

What is the equation for ‘filtered load’?

A

filtered load = plasma concentration x GFR

43
Q

What is the Tm for glucose?

A

375 mg per minute

44
Q

To what plasma concentration must glucose rise, in order for in to overcome its Tm?

A

> 300mg/ 100ml

as this 3 fold concentration would mean the filtered load reaches 375mg/ 100ml, which is the Tm for glucose

45
Q

Explain what is meant by the following general term(s): Renal threshold.

A

The plasma concentration of a substance which must be reached in order for it to reach its Transport Maximum (Tm).

Simply put, the concentration of substance (in the blood) where it is in such excess it can no longer be 100% reabsorbed, and is consequently lost in the urine (whereas under normal conditions, it is not).

46
Q

t/f: the kidneys regulate blood glucose concentration

A

False, the kidneys do not regulate blood glucose as such. This is because what the kidneys do is simply reabsorb glucose to it’s fullest extent, and allow the rest to pass out the body. The term ‘regulate’ implies the active ability to increase or decrease the [glucose] which the kidneys can not do. However, the liver certainly can!

47
Q

Provide some examples of substances which display a Tm.

A
  • glucose
  • AA
  • lactic acid
  • water soluble vitamins
48
Q

What proportion of general absorption of nutrients occurs in the small intestine?

A

90%

49
Q

Describe the basic path which nutrients take to be absorbed into the body.

A
  1. Lumen of gut
  2. mucosa
  3. blood capillaries
  4. hepatic portal vein (HPV)
  5. liver
50
Q

Explain how monosaccharides are absorbed into the mucosal cell.
(note: consider individual types of monosaccharides)

A

Glucose & galactose - sodium symporter (secondary active transport)
Fructose - facilitated diffusion

51
Q

Explain how monosaccharides are transported out of the mucosal cell, into the bloodstream.

A

All types by facilitated diffusion.

52
Q

Explain how AA’s, as well as di/tripeptides are absorbed into the mucosal cell.

A

AA - active transport OR secondary active transport with sodium.
Di/tripeptides - secondary active transport with H+.

53
Q

Explain how AA’s, as well as di/tripeptides are transported out of the mucosal cell, into the bloodstream.

A

All types by passive diffusion.

54
Q

On what structure is the capillary which receives nutrients from the mucosal cell(s)?

A

a villus.

55
Q

AA’s, as well as di/tripeptides move into the mucosal cell from the lumen of the gut tube via active means. However larger molecules, small peptides, also cross the mucosa. How do they do this?

A

Endocytosis.

56
Q

Absorption of lipids may depend on whether the lipid is long, or short chain. How many hydrocarbons permits a lipid to be ‘short’ or ‘long’ in chain?

A

Short chain = <10-12 hydrocarbons
Long chain = >12 hydrocarbons

(note: this is a weird way to write it (thanks phil), but what is being said is that short chain lipids can be anywhere between less then 10 and 12 hydrocarbons in length).

57
Q

How are long chain lipids absorbed from the lumen of the GIT?

A
  1. Long chain lipids emulsified by bile salts (mechanical digestion; means chemical structure unchanged but physically seperated into smaller droplets)
  2. Lipases convert these lipids into free fatty acids (FFA) and glycerol.
  3. FFA and glycerol transferred to mucosa as micelles, and then enter mucosal cell by simple diffusion.
58
Q

How are short chain lipids absorbed from the lumen of the GIT? How are these lipids then passed into the blood?

A

Enter directly into mucosal cells by simple diffusion, and enter blood by simple diffusion.

59
Q

t/f: larger lipids exist only within micelles.

A

true

60
Q

How are bile salts processed, post emulsifying lipids in the GIT?

A

Bile salts are reabsorbed & reformed into bile in the liver (recycled)

61
Q

How do fat soluble vitamins enter cells?

A

Directly enter mucosal cells with fatty acids.

62
Q

What occurs with to long chain lipids within the mucosal cell?

A

The FFA + glycerol recombine to form triglycerides. These form fat droplets, which combine with other substances (such as cholesterol) in the ER to form chylomicrons. Then, exocytosis of chylomicrons occurs, they diffuse into lacteals (lymph) -> thoracic duct (lymph) -> enter venous system at junction of left jugular and left subclavian vein -> into circulatory system -> to liver.

63
Q

Briefly, in what state are fats absorbed into the mucosal cell?

A

as FFA + glycerol

64
Q

Electrolytes are present within the alimentary canal. What is the source of these electrolytes?

A

GI secretions + ingested foods/ liquids.

65
Q

In general, how do electrolytes enter mucosal cells?

A

Electrolytes enter mucosal cells via diffusion or secondary active transport.

66
Q

Electrolytes enter mucosal cells via diffusion or secondary active transport. Specifically, how does sodium and potassium enter mucosal cells?

A 
NaK pumps (secondary active transport). 
This is 'secondary' as the transport of sodium across the luminal border itself is not actually active, its a passive movement of sodium down its concentration gradient. The energy expenditure (why it is active) comes from the operation of the NaK pump across the basolateral surface, which sets and maintains the gradient for sodium to enter - that is, it uses energy to pump sodium out of the mucosal cell and keep that intracellular [Na] low. This is what then allows sodium to flow passively across the luminal border. So its active, but not directly, its SECONDARY ACTIVE TRANSPORT BIH
67
Q

Electrolytes enter mucosal cells via diffusion or secondary active transport. Specifically, how do chloride, iodide and nitrate enter mucosal cells?

A

passive diffusion.

68
Q

Electrolytes enter mucosal cells via diffusion or secondary active transport. Specifically, how do iron, magnesium and phosphate enter mucosal cells?

A

active transport.

69
Q

What does calcium absorption require?

A

vitamin D & parathyroid hormone (PTH).

70
Q

Provide some examples of fat soluble vitamins.

A

Vitamins K, A, D and E

remember: “KADE is fat”

71
Q

Provide some examples of water soluble vitamins.

A

Vitamins B1-B12, C and H.

72
Q

How are fat soluble vitamins absorbed?

A

Travel in micelles, absorbed by simple diffusion.

73
Q

How are water soluble vitamins absorbed?

A

Absorbed by diffusion.

74
Q

What must vitamin B12 combine with before it is transported into cells? What is this transportation process known as?

A

B12 must combine with an intrinsic factor before it is able to enter cells through a process called receptor mediated endocytosis.

75
Q

How many litres of fluid are secreted into the GI tract per day?

A

9L/ day

76
Q

9L of water is secreted into the GI tract per day. How much of this is reabsorbed by the small intestine?

A

8L

77
Q

9L of water is secreted into the GI tract per day, of which 8L is then reabsorbed by the small intestine. What percentage of that single remaining litre of water is reabsorbed by the large intestine?

A

90%

78
Q

How is water absorbed from the GIT?

A

Osmosis through cell walls (using transport proteins, as water obviously cannot pass through cell membranes). Then, water passes into vascular capillaries inside the villi.

79
Q

The liver produces large quantities of bile which is stored and concentrated between meals. What organ is responsible for this storage?

A

the gall bladder.

80
Q

How does the gall bladder (GB) concentrate the bile?

A

the GB epithelium extracts salts and water from the stored bile, reducing its volume and thus making it more concentrated.

81
Q

What is the capacity of the GB?

A

15-60ml.

82
Q

How much bile is secreted by the GB per day?

A

0.8-1L / day

83
Q

What is the colour, and pH of bile?

A

Bile is yellow-green in colour, and has a pH between 7.6 and 8.6.

84
Q

What are the components of bile?

A
  • water
  • cholesterol
  • bile salts (Na+ and K+)
  • bilirubin (bile pigment, derived from Hb molecule)
85
Q

The bile pigment bilirubin is derived from haemoglobin molecules. Briefly detail how Hb is broken down to provide bilirubin.

A

Hb is broken down into globin (a reusable protein) and haem. Haem is broken down into iron and bilirubin. The bilirubin is the bile pigment, found in bile.

86
Q

What is the “standing osmotic gradient mechanism” for fluid reabsorption?

A

A mechanism used for water reabsorption, a good example of which is found in the GB (as the GB has a high rate of water resorption).

  1. During fluid resorption in the GB, the LIS between epithelial cells enlarge.
  2. Na+ is actively pumped into the LIS.
  3. NaK pumps are dense near the apical end of the LIS channel. Thus, the fluid near the apical end is hypertonic, and induces an “osmotic flow” toward the basolateral end, where the fluid is less hypertonic (near isotonic infact).
  4. Cl- and HCO3- follow Na+ to maintain electrical neutrality
  5. greater levels of solutes within the LIS further increases the osmotic gradient, causing more fluid to move there.
  6. Water then crosses the basement membrane and is carried away by the blood.
87
Q

What is the evidence to suggest that LIS on mucosal cells are a major route of fluid flow, during fluid reabsorption?

A

During fluid resorption in the GB, the LIS between epithelial cells enlarge.
Conversely, when fluid transport is blocked, these LIS almost disappear.
Collectively, these observations suggest that the LIS are a major route of fluid flow during fluid resportion.

88
Q

Why is it that chylomicrons are taken up by the lymphatic system (via a villus), and not absorbed into the circulatory system like most other nutrients?

A

As chylomicrons are too large of structures, they are unable to enter the capillaries.

Physiological concepts (12 decks)

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