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Y1 Fundamentals of Life Science > L9: Fluid Compartments > Flashcards

L9: Fluid Compartments Flashcards

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Biology 101 flashcards
1
Q

Define a fluid

A

A fluid is a substance that deforms under a shear stress. In physiology, the important fluids are those in which water, or a fat/lipid, are the solvent.

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2
Q

What are the key fluid compartments in the body?

A
  • Intracellular Fluid: Water inside cells
  • Interstitial Fluid: Water between cells
  • Fat
  • Plasma: Liquid component of blood
  • Transcellular Fluid: Special compartments separated from the extracellular fluid by epithelial membranes (e.g. CSF (brain), aqueous humor (eye), peritoneal fluid (abdominal cavity), synovial fluid (joints)
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3
Q

What is total body water for a 70kg person?

A

42 L

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4
Q

How does total body water content differ between men and women, and with age?

A
  • Proportionally greater in men that women
  • Reduces with age
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5
Q

What are the volumes of body fluid per compartment in a 70kg person?

A
  • Intracellular - 28 L
  • *Plasma - 3 L
  • *Interstitial fluid - 10 L
  • Transcellular fluid - 1L
  • these make up extracellular fluid
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6
Q

What property do drugs and substances have, related to where they are stored?

A
  • Very lipophilic
  • Accumulate in fats
  • Transported in the circulation with a carrier, then across the interstitial space to exchange into fats
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7
Q

Describe the peritoneal space

A

Can greatly expand (used therapeutically during peritoneal dialysis)

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8
Q

Describe the cerebospinal fluid

A

Protected by the blood-brain barrier (endothelial cells joined by tight junctions, with a role for glia

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9
Q

How is cell transport regulated?

A
  • Facilitated diffusion
  • Requires aquaporins - a protein to allow water to pass through the plasma membrane bilayer
  • Requires a driving force: osmosis (not hydrostatic)
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10
Q

What is meant by facilitated diffusion?

A

Water molecules moving by Brownian motion involving the action of proteins

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11
Q

What is hydrostatic pressure?

A
  • The force per unit area in a fluid
  • Normally generated in our body by the heart
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12
Q

Why is it that hydrostatic pressure increasing outside the cell doesn’t drive water in?

A
  • The hydrostatic pressure on the outside will equal the hydrostatic pressure on the inside
  • Water is compressible so any changes in pressure to a cell will not cause a change in volume to the cell
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13
Q

How can water movement be changed across the cell?

A
  • Change the driving force (changing the osmotic pressure by changing the concentration of solutes)
  • Change the expression of aquaporins (affect the rate of change/resistance of flow, but not the equilibrium position)
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14
Q

How does changing the number of aquaporins affect the rate of change/flow?

A
  • Fewer aquaporins over the surface means water will not be able to equilibrate as quickly
  • Slower rate of change but equilibrium will ultimately reach the same position just slower
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15
Q

How can solute concentrations change in the long term?

A
  • Change the concentration of small molecules through metabolic process (e.g. amino acids from proteins, betaine from glycine; glucose from glycogen, sorbitol from glucose)
  • This doesn’t change the concentration per unit mass, but changed the concentration per mole
  • The number of solute molecules inside the cell increases, which increases the osmotic pressure, causing water to move into the cell by osmosis
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16
Q

What can a cell do if it becomes too small or is shrinking in size?

A
  • Metabolise solutes within the cell to increase the solute concentration
    -Causes water to move into the cell by osmosis to lower the solute concentration
  • The cell expands
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17
Q

How can solute concentrations change in the short term?

A

Change the influx of ions and small molecules through volume-regulated anion channels, amd stretch-activated cation channels
- stretch-activated cation channels: in response to swelling, let Nat and Cal+ in to trigger cellular responses. For example, several different types of TRP channels.

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18
Q

What are volume-regulated anion channels (VRACS)?

A
  • Activated by cell stretch/swell
  • Cell becomes too large these channels sense the increasing tension within the plasma membrane
  • Open a chloride channel (the intracellular space/membrane potential is negatively charged and chloride is
    -vely charged)
  • Chloride is repelled and will leave the cell
  • Lower osmotic pressure in the cell, so water will move out by osmosis and the cell will shrink
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19
Q

What are stretch-activated cation channels?

A
  • Activated by cell stretch
  • Open a potassium channel which leaves the cell
  • Calcium ions move into the cell - activates other signalling pathways to regulate volume
  • Sodium ion move in and cause depolarisation that can change other processes within the plasma membrane to regulate volume
    E.g. several different types of TRP channels
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20
Q

What is plasma?

A
  • This is the fluid component of the blood, and usually represents about 55% of the blood by volume
  • The rest of the volume is occupied by cells
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21
Q

What is haematocrit?

A

A measure of the proportion of the blood occupied by cells (usually around 45%)

22
Q

How can blood be separated?

A

Whole blood can be separated by centrifugation

23
Q

How can total body water be measured?

A
  • Label with special water
  • Special water is deuterium (hydrogen with mass no. of 2) or tritium (hydrogen with mass no. of 3)
  • These replace the normal isotope of hydrogen with a mass no. of 1
  • Inject a small amount of labelled water, allow equilibration within the body, and measure the proportion of water molecules that are labelled in a blood sample
  • Measure the level of dilution to estimate the total water volume of the body
24
Q

How can plasma volume be measured?

A
  • Labelled proteins injected intravascularly e.g. Evan’s Blue
  • This binds and stains the plasma proteins
  • Allow equilibration
  • Measure the concentration of Evan’s Blue
  • The more dilute the Evan’s Blue, the larger the volume of plasma
25
Q

How can the volume of extracellular fluid be measured?

A
  • Thiosulfate/thiocyanate, insulin (polysaccharide) or Cl- (with mass no. 36)
  • Inject some and allow equilibration
  • Measure the concentration of injected substance
  • The more dilute the large the volume of extracellular fluid
26
Q

How does the concentration of Na+ differ between extracellular and intracellular environment? What is the function?

A
  • Higher in extracellular
  • Regulate membrane potential
27
Q

How does the concentration of K+ differ between extracellular and intracellular environment? What is the function?

A
  • Higher in intracellular
  • Regulates membrane potenital
28
Q

How does the concentration of Ca2+ differ between extracellular and intracellular environment?

A
  • Higher in extracellular
  • Only half of extracellular calcium is present as free Ca2+ ions
29
Q

How does the concentration of Cl- differ between extracellular and intracellular environment?

A
  • Higher in extracellular
30
Q

How does the concentration of HCO3- differ between extracellular and intracellular environment?

A
  • Higher in extracellular
31
Q

How much protein is in the plasma? What protein is most prevalent?

A
32
Q

What is an osmole?

A

A measure of the number of moles that a compound dissociates into when dissolved in solution. It is very useful in renal physiology for measuring osmotic forces

33
Q

Give an example of using osmoles

A
  • 100 mmol of NaCl, yields 200 mOsm in solution, because it dissociates into Nat and Cl.
  • 2.4 mmol of CaCk yields 7.2 mOsm in solution, as each molecule dissociates into 1 Ca2+ and 2 Cl- ions.
34
Q

What is osmolarity?

A
  • The number of osmoles per unit volumes of the solution
  • e.g. making a standard solution
  • Units shown
35
Q

What is osmolality?

A
  • The number of osmoles per unit mass of the solvent
  • E.g. add a kilo of water at a fixed mass of the solvent
  • Units shown
36
Q

How does osmolality differ to osmolarity at physiological ranges?

A

In physiological ranges, the difference is very small, as the density of water is close to 1kg.l’., and the difference between the volume of the solvent and the volume of the solution is very small

37
Q

What is the relationship between osmotic pressure and osmolality?

A

Osmotic pressure is proportional to the osmolality rather than the osmolarity

38
Q

How does osmotic pressure occur?

A
  • At the interface between two solutions
  • Molecules exchange because of diffusion
  • If the concentration of any species is different on either side of the interface there will be a net movement of molecules from one side the membrane to the other.
  • In water, the force (per unit area) required to oppose such a new movement is called the ‘osmotic pressure’
39
Q

What is the interface between solutions in biological tissues?

A

Semi-permeable membrane usually in the plasma membrane

40
Q

Describe the relationship between water potential and osmotic pressure

A
  • Osmotic pressure is the inverse of water potential
  • High water potential means a low osmotic pressure
  • Low water potential means a high osmotic pressure
41
Q

What is an alternative definition for osmotic pressure?

A
  • The amount of pressure required to oppose osmosis
  • In this configuration, the osmotic pressure is equal to the hydrostatic pressure generated
  • This is density of the solution x acceleration due to gravity x height
42
Q

Where is the osmotic pressure very high?

A

Plasma

43
Q

How can osmotic pressure be estimated?

A
44
Q

What value is osmotic pressure around?

A

7.1 atm

45
Q

What is significant about osmotic pressure?

A
  • Very high
  • Indicates how difficult it is to prevent flow across a semipermeable membrane using hydrostatic pressure
46
Q

What is means by isosmotic?

A

Two solutions share the same osmolality

47
Q

What is means by isotonic?

A

Applying the solution to cells (traditionally red blood cells) will not cause net fluid movement - water moves in both directions at an equal rate

48
Q

Can a solution be isosmotic and not isotonic?

A
  • Yes the intracellular fluid can be isosmotic but not isotonic
  • Example is the urea
49
Q

Why is urea isosmotic and not isotonic?

A
  • Urea crosses relatively freely across the plasma membrane - down a concentration gradient
  • An isosmotic solution of urea, if applied to cells, is not isotonic because urea will enter the cells, increasing the intracellular osmotic pressure
  • This encourages water to enter the cells as urea solute concentration is higher in the cells and osmotic pressure is higher in the cells
  • This causes cell swelling and ultimately rupture
  • red is urea in diagram
50
Q

How does movement across capillaries work?

A
  • Capillary walls create a barrier to diffusion like the plasma membrane does - a semipermeable membrane
  • The net movement of water will be a balance between hydrostatic force and osmotic pressures
  • Across capillary membranes, the ions (which are small) are in equilibrium, so the main osmotic forces are due to the proteins
  • The protein-mediated force is sometimes called the oncotic pressure
51
Q

What is a clinical example of the importance of osmotic pressure?

A
  • Albumin concentrations fall as liver does not produce it
  • Causes osmotic pressure to fall in circulation
  • Water cannot stay in circulation so leaks out into the abdominal cavity - causes ascites
  • Liver failure, protein malnutrition and renal failure
  • Ascites is a secondary effect of liver failure

Y1 Fundamentals of Life Science (17 decks)

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