Capillaries I & II Flashcards
How does metabolism affect solute exchange?
Metabolism creates concentration gradients to transport solutes and fluids for gaseous and nutrient exchange
What is the fate of O₂ , glucose, urea and CO₂ in the body?
O₂ and glucose are transported around the body
CO₂ and urea are removed
Describe the structure of cell membranes?
- consist of 2 layers of ampiphatic phospholipids
- polar phosphate heads (hydrophilic)
- non polar fatty acid tails (hydrophobic)
- form bilayers in solution
Outline the functions of cell membranes
- providing support and function
- cell to cell recognition (immune system)
- controls what enters / leaves cells
- regulates cell function (e.g. insulin mediated glucose
uptake)
How does the membrane allow us to differentiate between cells?
The various membrane functions allow us to differentiate between cells
How do membranes act as barriers to transport?
it’s difficult to get solutes across the hydrophobic centres of the membrane
How does the structure of membranes aid transporting functions?
- contains transmembrane proteins that span across the
membrane and facilitate transport and signalling - carbohydrates anchor proteins and orient them
These complicated structures are a barrier and aid
movement across the membrane
What are the different types of transport?
- passive
- active
What is passive transport?
Random movement of molecules down a gradient of:
- concentration
- pressure
- osmotic
- electrical
Passive - doesn’t require ATP
Works effectively over short distances - t is proportional to distance^2
Describe the features of passive diffusion?
Doesn’t require energy
Simple diffusion O₂ /CO₂
facilitated diffusion is via specific channels or transmembrane molecules (ions/glucose via GLUT)
What is active transport?
The movement of molecules against a gradient requiring energy (ATP)
- ATP dependent pumps, endocytosis + exocytosis
What are the different passive transport processes?
Diffusion
- conc. gradients
e. g. uptake of O₂ from lungs -> blood
Convection
- pressure gradient
e. g. blood flow from heart -> vessels
Osmosis
- osmotic pressure graident
e. g. H₂O uptake in cells
Electrochemical flux
- electrical + concentration gradient
e. g. ion flow during action potentials
Describe the structure of capillaries
Capillaries are made of endothelial cells - 1 cell thick
semi-permeable and amenable to certain types of diffusion over short distances
Where in the body are capillaries found?
Found near every cell in the body, but higher density in highly active tissues (muscle, liver, heart, kidney, brain etc.)
What is solute exchange?
Passive diffusion of O₂ , glucose, amino acids, hormones, drugs etc. between substances
What is the role of fluid exchange?
occurs along pressure gradients and regulates plasma and interstitial fluid volumes
Where does most exchange of solutes occur in the body?
Most exchange occurs at capillaries
Outline the vessel network solutes pass through
arteries branch into arterioles -> capillaries -> venules -> veins towards the heart
What is the role of capillaries in the network of vessels?
capillaries connect terminal arterioles to venules
- extension of inner lining of arterioles
What is the rate of solute transport dependent on?
Depends on properties of:
- passive diffusion (conc. , rate & diffusion)
- solutes + membranes (Fick’s Law)
- Capillaries
combining all of these provides concept of permeability
What is permeability?
How easy it is for the solute to cross the membrane
Describe the movement of molecules during passive diffusion?
Molecules move at random (due to random thermal motion) from an area of high to low concentration, eventually reaching equilibrium
When is diffusion most effective?
Great for lipid-soluble solutes over short distances e.g. O₂ / CO₂
What is the relationship between time and diffusion rate?
t ∝d² time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with distance squared (d²)
Whys is simple diffusion not applicable for whole body movement?
Movement across a capillary membrane will happen in fractions of a second, but around the body may take years
What are the solute properties that affect transport?
- membrane thickness / composition
- aqueous pores in the membrane
- carrier mediated transport
- active transport mechanisms
How does solute concentration affect diffusion?
[high solute] = faster diffusion
How does solute size affect diffusion rate?
The bigger the solute, the slower the transport across the membrane
How does charge / hydrophobicity affect diffusion?
Lipophilic solutes get across very easily but glucose would struggle as it’s large and charged.
What is the role of membrane aqueous pores?
Membrane aqueous pores act as carriers or active transport mechanisms
What is Fick’s Law?
Js = -DA ΔC / x
D - Diffusion coefficient for solute in solvent
A - area available for diffusion
ΔC - concentration difference
X - distance
(always a negative value)
What are the different types of capillaries?
Continuous capillaries
Fenestrated capillaries
Discontinuous capillaries
Describe the properties of continuous capillaries?
- Tight junctions between the endothelial cells
- very poor permeability as no space to move out from
lumen into interstitial tissue. - Constant basement membrane, which all the cells
adhere to has- no breaks in it. - Useful in blood brain barrier, doesn’t non-selectively
allow things into the brain, but all entry is regulated.
What controls solute diffusion rates?
Permeability is the rate of solute transfer by diffusion across unit are of membrane per unit concentration difference - it’s a measure of how freely a solute crosses a membrane
Js = -PAm ΔC
Or
Js / Am = - PΔC
What is Starling’s Principle of solute exchange?
Fluid movement across the capillary wall is proportional to pressure difference across the capillary wall
Balance between pressures that favour filtration (hydrostatic - increased arterial BP) and reabsorption (osmotic pressure due to plasma proteins, as long as they can’t pass through)
What are fenestrated capillaries?
Fenestrated structures are protein structures that act as a sieve for small molecules to pass through.
Describe the features of fenestrated capillaries
- More permeable due to fenestrated structures (holes in
the endothelial cells) - Permeable to water & small solutes (not bigger)
- Good where a lot of water is required e.g. salivary
glands - lots of water leaves the capillaries into the
interstitial space to make saliva. - Present in all areas where fluid moves out of plasma.
What are the features of Discontinuous capillaries?
- Allow everything through
- Have very large fenestrations and disrupted basement
membrane - Incredibly leaky so large molecules & some cells
(rbc & immune cells) move out
Where are the different capillaries distributed in the body?
The 3 distinct types of capillaries, found in different parts of the body, depending on the specific functions of cells and tissues.
Describe the anatomy of capillaries
The capillary anatomy is a single layer of endothelial cells forming the endothelium.
What other capillary wall structural features influence solute transfer?
- Intercellular cleft - 10-20 nm wide
- Caveolae & vesicles - large pore system
- Glycocalyx - covers endothelium, negatively charged
material, blocks solute permeation and access to
transport mechanisms, highly regulated
Describe the role of intercellular clefts in capillaries
Intercellular clefts are in between adjacent cells, not linked by tight junctions. This allows solutes and fluids to move between the cells,aiding permeability.
What is the role of Caveolae & vesicles?
Caveolae and vesicles is where substances are taken up on one side of the membrane, and moved to the other side => endocytosis & exocytosis
Explain the role of endocytosis & exocytosis in solute exchange at the capillaries
Endocytosis followed by exocytosis which will move the solute from the lumen across into the interstitial space…also related to selective large pores