Capillaries I Flashcards
What 2 things does metabolism of cells create?
- Metabolism creates a need for nutrient and gaseous exchange, need to bring in oxygen, amino acids etc and remove carbon dioxide and urea.
- The metabolism going on in cells is generating concentration gradients that facilitate this movement
What is the issue with metabolism
The transport of nutrients and gas exchange across the barriers… membranes are a barrier to movement
Describe the basic 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.
What are the roles 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
- They are incredibly important in allowing us to recognise different kinds of cells, regulating what enters or leaves, allow hormones and neurotransmitters to work…..providing support and protection.
Describe the basic principles of diffusion, passive and active
Passive diffusion:
- Movement of molecules down a concentration gradient.
- Does not require energy
- Conc/ Osmotic/ Pressure/ electrical
- Simple e.g CO2/ O2 or facilitated (ions and glucose)
Active Transport:
- Movement of molecules against a concentration gradient
- Requires energy (ATP)
- ATP- dependant pumps, exocytosis and endocytosis
Give and describe some examples of passive transport
Diffusion:
- Concentration gradient
- O2 uptake from the lungs
- Dissolved solutes
Convection:
- Pressure gradient
- Movement of blood from heart to blood vessel
Osmosis
- Osmotic pressure gradient
- e.g uptake of water into cells
Electrochemical flux:
- Electrical and concentration gradient
- Ion flow across membranes
Give a brief overview of Solute and fluid exchange occur at capillaries.
What is solute/ fluid exchange and describe the capillaries
Solute exchange relies on diffusion, for example glucose transport
Fluid exchange is the movement of water which contains molecules, for example glucose
• Connect terminal arterioles to venules - extension of inner lining of arterioles.
• Smallest diameter blood vessels endothelium, 1 cell thick, semi-permeable.
• Found near every cell in the body but higher density in highly active tissue (muscles, liver, heart, kidney, brain etc)
• Solute exchange (passive diffusion): electrolytes, O2, glucose, amino acids, hormones, drugs
• Fluid exchange along pressure gradients. Regulation of plasma and interstitial fluid volumes (next lecture).
What are the properties of passive diffusion
Concentration, rate and distance
What are the properties of solutes and membranes
Ficks Law
Do the properties of capillaries affect movement?
Yes
What do these factors form when you combine them?
Concept of permeability - how easily substances can pass through
Why does diffusion work for only short distances?
Time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared.
D = diffusion coefficient for molecule within the medium eg. D for O2 in water and O2 in air are different
Describe the properties of the solute that affect transport
Properties of the solute
• Concentration gradient
• Size of the solute
• Lipid solubility of solute (lipophilic, lipophobic nature)
Describe the properties of membranes that affect transport
Properties of the membrane • Membrane thickness/composition • Aqueous pores in the membrane • Carrier-mediated transport • Active transport mechanisms
What does Ficks Law state?
State the equation
How much solute is moved per unit time = solute movement = mass per unit time (Js)
On image
Has a negative value because it is flowing down a concentration gradient
What are the 3 types of capillaries and describe them
Continuous capillaries
• Moderate permeability; tight gaps between neighboring cells; constant basement membrane
Blood-brain barrier
muscle, skin, fat, connective tissue
Fenestrated capillaries
• High water permeability, fenestration structures, modest disruption of membrane
‘High water turnover’ tissues eg. salivary glands, kidney, synovial joints, anterior eye, choroid plexus (cerebrospinal fluid), gut mucosa
Discontinuous capillaries
Very large fenestration structures, disrupted membrane
When movement of cells is required; RBCs in liver, spleen, bone marrow
What is the size of the intracellular cleft?
10-20nm wide
Describe the Caveolae & vesicle system
Large pore system.
There are also Caveolae and vesicles. This is where substances can be taken up on one side of the membrane, taken up and moved to the other side….so you have endocytosis followed by exocytosis and that will move the solute from the lumen across into the interstitial space…also related to large ores which are selective
Describe Glycocalyx
Covers endothelium, negatively charged material, blocks solute permeation and access to transport mechanisms, highly regulated
But it is a charged carbohydrate medium. It is like a negatively charged carbohydrate mesh that sits on the top of the endothelial cells next to the lumen. It acts like an extra barrier…it is very dynamic can be broken down and remade as required. So the cells can regulate this and it is on the the cutting edge of research, through your carees we will understand more about the glycocalyx.
Define permeability
Permeability is the rate of solute transfer by diffusion across unit area of membrane per unit concentration difference ie. ’how freely a solute crosses a membrane’
How does a porous membrane interferes with the diffusion of lipid insoluble solute in multiple ways?
- reduction in area for diffusion (A),
- increased path length through membrane (x),
- restricted diffusion in pore produces hydrostatic issues (D)
What is the equation for the rate of solute transport?
Js= -PAmAC
Highlight some different routes and transport
- During inflammation, some big gaps can form between cells and so large lipophobic proteins such as plasma proteins
- Transcellular diffusion for lipophilic molecules.
- Small lipophobic molecules may move via intercellular channels or fenestrations
- Water can move by a variety of means including specific water channels.
- Glut transporters are on the lumen side and interstitial space.
Which is the dominant route of transport diffusion or filtration?
Diffusion is going to be passive down its concentration gradient whereas filtration is going to be through pores, gaps and fenestrations etc, so as fluid moves through dissolved glucose goes with it.
Example - glucose transport from blood plasma to tissues…
• Glucose concentration in plasma is 1 g / litre
• Total volume of plasma filtrate flowing into tissues per day = 8 litres
• Maximum filtration of glucose = 8 g / day
BUT glucose consumption of human adult is 400 g / day
Therefore filtration transport only accounts for 2% glucose transport
98% of glucose transport into interstitial space via passive diffusion – via GLUT transporter system
Describe Fall in interstitial concentration
During metabolism, more solute is used up increasing the concentration gradient. Metabolism also increases blood flow - increased O2 delivery controlled by arterioles
Describe the recruitment of capillaries
Dilation of arterioles leads to an increased number of capillaries perfused which increases total cross-sectional area for diffusion. This also shortens distance between capillary and cell
Describe Blood Flow
More blood brings more solutes. Increased blood volume means less time for equilibration to occur across capillaries.
e.g Flow limited diffusion where there is a slow flow in a capillary. O2/ CO2 equilibrate over a proximal section and then there is little diffusion over the rest of the vessel. Sometimes occurs in sepsis where the pressure and flow are low and can lead to ischemia
