Capillaries I: Solute Exchange Flashcards
What is a solute and give some examples?
Substance dissolved in body fluid - e.g oxygen, carbon dioxide, nutrients, electrolytes
What are the 2 barriers to solute transport?
- Cell membrane
- Capillary membrane
Describe the structure of the cell membrane
- Consists of two layers of amphipathic phospholipids
- Phosphate head is polar (hydrophilic)
- Fatty acid tails is non-polar (hydrophobic)
- Form bilayers in solution
Describe the function of the cell membrane
- Provide support and protection
- Cell-to-cell recognition – eg. immune system
- Controls what enters or leaves the cell – e.g. ion movement in nerves
- Regulates cell function – eg. Insulin-mediated glucose uptake
Describe passive transport
- Movement of molecules DOWN a gradient (high to low) - concentration / Pressure / Osmotic / Electrical
- Does not require energy
- Simple (O2/CO2) OR facilitated (ions, glucose)
- Molecules move randomly
Describe active transport
- Movement of molecules AGAINST a gradient
- Requires energy (uses ATP) - eg. ATP-dependent pumps, endocytosis, exocytosis
What gradient does diffusion use and give an example?
Concentration gradient eg. O2 uptake from lungs into blood
What gradient does convection use and give an example?
Pressure gradient eg. blood flow from heart to blood vessels
What gradient does osmosis use and give an example?
Osmotic pressure gradient eg. water uptake by cells
What gradient does electrochemical flux use and give an example?
Electrical and concentration gradient eg. ion flow across cell membranes
Describe the structure of capillaries
- Connect terminal arterioles to venules - extension of inner lining of arterioles
- Smallest diameter of the blood vessels
- Endothelium that is one cell thick and semi permeable
Where are capillaries found?
Found near every cell in the body but higher density in highly active tissue (muscles, liver, heart, kidney, brain etc)
What does solute exchange rely on?
Passive diffusion
What does fluid exchange rely on?
Pressure gradients
State briefly what the rate of solute transport depends on
- Properties of passive diffusion - eg. concentration, rate, distance
- Properties of solutes and membranes - eg. Fick’s law
- Properties of capillaries - affect movement
What are the properties of solutes that affect transport?
- Concentration gradient
- Size of the solute
- Lipid solubility of solute (lipophilic, lipophobic nature)
What are the properties of membranes that affect transport?
- Membrane thickness/composition
- Aqueous pores in the membrane
- Carrier-mediated transport
- Active transport mechanisms
Describe Fick’s law
- Describes how much of a substance is transported per unit time
- Solute movement is mass per unit time determined by 4 factors
Outline the equation for Fick’s Law
Js = - DA x (change in C / x)
D = Diffusion coefficient of solute – ease of movement through solvent
A = Area
ΔC = concentration gradient (C1-C2)
x = distance (between C1 and C2)
It has a negative value = flowing ‘down’ a concentration gradient
Describe continuous capillaries.
- Moderate permeability; tight gaps between neighbouring cells; constant basement membrane
- Goes to blood-brain barrier, muscle, skin, fat, connective tissue
Describe fenestrated capillaries.
- High water permeability, fenestration structures, modest disruption of membrane
- Found at ‘high water turnover’ tissues eg. salivary glands, kidney, synovial joints, gut mucosa
Describe discontinuous capillaries
- Very large fenestration structures, disrupted membrane
- Found in structures when movement of cells is required; RBCs in liver, spleen, bone marrow
State three properties of the capillaries that impact solute transport.
- Intercellular cleft
- Caveolae & vesicles
- Glycocalyx
Describe the intercellular cleft
10-20 nm wide
- allows solutes and fluids to move through between the cells
Describe the caveolae and vesicles
Large pore system
- for the movement of proteins across the membrane e.g by exocytosis - moving proteins from inside the vessel into the interstitial fluid
Describe the glycocalyx
- Covers endothelium, negatively charged material, blocks solute permeation and access to transport mechanisms, highly regulated
- The glycocalyx is very dynamic can be broken down and remade as required.
Define permeability
Rate of solute transfer by diffusion across unit area of membrane per unit concentration difference
Explain how a porous membrane interferes with diffusion
- Decreases the area for diffusion , there is increase path length through the membrane, and there is restricted diffusion in the pore which produces hydrostatic issues
Describe how Fick’s law is different for a porous membrane
Js = - P Am ΔC
Where:
Js = Rate of solute transport
P = Permeability - involving pore size,
length, diffusion coefficient
Am = Surface area of capillary involved
in transport
∆C = Concentration gradient
Explain whether diffusion or filtration is more prominent as a transport method
- Diffusion is a passive process down the concentration gradient
- Filtration is through pores, gaps, fenestrations so moves solutes like glucose
- The concentration of solutes like glucose in the filtrate are much lower than consumption of glucose, so diffusion is the dominant transport route
Define filtration
Bulk movement of fluid (water and solutes) - can move through gaps between endothelial cells or through fenestrations
Describe the 3 ways that the diffusion rate is controlled
Fall in interstitial concentration
Recruitment of capillaries
Blood flow
What is the effect of a fall in interstitial concentration?
During metabolism more solute is used up thereby increasing the concentration difference. Also, metabolism increases blood flow - increased O2 delivery, controlled by the arterioles
What is the effect of capillary recruitment?
Dilation of arterioles leads to increased number of capillaries perfused which increases total surface area A for diffusion (Fick’s law). Also this shortens
diffusion distance between capillary and cell
What is the effect of blood flow?
More blood brings more solutes. Increased blood volume means less time for equilibration to occur across capillaries.
eg. in flow limited diffusion where there is slow flow in a long capillary oxygen and carbon dioxide equilibrate over proximal section (no concentration gradient) and there is little diffusion the rest of the vessel
What 2 things does metabolism of cells create?
- A need for nutrient and gaseous exchange
- Concentration gradients that facilitate this movement
What is the main issue facing metabolism?
Membranes that act as a barrier to movement
What type of particles can pass through the membrane?
- Non-polar molecules are able to pass through
- Polar molecules are unable to pass through due to the hydrophobic nature of the membrane. They rely on transporters.
What are the layers of a capillary?
Cellular membrane, single layer of endothelial cells, a cellular basement membrane made of connective tissue
What is the function of capillaries?
- Solute exchange
- Fluid exchange
- Regulation of plasma and interstitial fluid
When can flow limited diffusion occur and what can it lead to?
- In sepsis when blood pressure and flow are low
- Can lead to ischaemia
