Capillaries I Flashcards
What is a solute and give some examples
A solute is any substance dissolved in the body fluid - oxygen, carbon dioxide, nutrients, amino acids, electrolytes, ions etc
What are the 2 barriers to solute transport
- Cell membrane
- Capillary membrane
Explain how metabolism facilitates the movement of solutes and fluids
- A H2O solution containing electrolytes, oxygen, glucose, amino acids, hormones, immune response etc for energy growth and repair
- These are used in metabolism and then a H2O solution containing metabolic end products re-enters circulation from the cells
- Cell metabolism is continually generating concentration gradients that facilitate this movement
Describe the structure and function of the cell membrane
Structure -
- Consists of two layers of amphipathic phospholipids
- Phosphate head is polar (hydrophilic)
- Fatty acid tails is non-polar (hydrophobic)
- Form bilayers in solution
Function -
- 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
State the two types of transport and describe them
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
Active transport
- Movement of molecules AGAINST a gradient
- Requires energy (uses ATP) - eg. ATP-dependent pumps, endocytosis, exocytosis
State and describe the types of passive transport
- Diffusion
Concentration gradient eg. O2 uptake from lungs into blood - Convection
Pressure gradient eg. blood flow from heart to blood vessels - Osmosis
Osmotic pressure gradient eg. water uptake by cells - Electrochemical flux
Electrical and concentration gradient eg. ion flow across cell membranes
Describe the structure of capillaries
- They connect terminal arterioles to venules - extension of inner lining of arterioles
- They have the smallest diameter of the blood vessels with an 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
Why is passive diffusion over 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:
t = x^2/2D
D = diffusion coefficient for molecule within the medium
What are the properties of solutes and membranes that affect transport
Properties of the solute
- Concentration gradient
- Size of the solute
- Lipid solubility of solute (lipophilic, lipophobic nature)
Properties of the membrane
- Membrane thickness/composition
- Aqueous pores in the membrane
- Carrier-mediated transport
- Active transport mechanisms
Describe Ficks law including the equation
- It describes how much of a substance is transported per unit time
- Solute movement is mass per unit time determined by 4 factors
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 the 3 types of capillaries - where they are found and how their structure differs
Continuous capillaries
* Moderate permeability; tight gaps between neighbouring 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
State 3 properties of the capillary that also impact solute transport
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
- The glycocalyx is very dynamic can be broken down and remade as required. The cells can regulate this and these processes are currently an active research area.
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’
Explain how a porous membrane interferes with diffusion
- A porous membrane interferes with the diffusion of lipid insoluble solutes in many ways
- It 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
- All the factors affecting diffusion go into one term - permeability
Describe how ficks 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 going to be a passive process down the concentration gradient
- Filtration is through pores, gaps and fenestrations so a fluid moves solutes ( e.g. glucose)
- The concentration of solutes like glucose in the filtrate are much lower than our consumption per day, so diffusion is the dominant transport route
Define filtration
Filtration is the 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
During metabolism more solute is used up thereby increasing the concentration difference. Also, metabolism increases blood flow - increased O2 delivery, controlled by the arterioles - Recruitment of capillaries
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 - Blood flow
More blood brings more solutes. Increased blood volume means less time for equilibration to occur across capillaries; eg. in flow limited difussion 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
