capillaries: solute exchange Flashcards
Describe the structure of the cell membrane
Lipid bilayer
Phosphate head is polar (hydrophilic)
Fatty acid tail is non-polar (hydrophobic)
Name 4 functions of the cell membrane
- Provides support and protection
- Cell-to-cell recognition (eg- immune system)
- Controls what enters and exits the cells (eg- ion movement in nerves)
- Regulates cell function (eg- insulin-mediated glucose uptake)
What are the 2 types of transport in the body
- Passive transport
- The movement of molecules down a gradient
- Does not require energy
- Can be simple (O2 + CO2) or facilitated (ions + glucose)
2. Active transport - The movement of molecules against a gradient
- Requires energy (uses ATP)
What are the 4 passive transport processes?
- Diffusion
- Has a concentration gradient
- For example uptake of oxygen from the lungs—> blood
2. Convection - Has a pressure gradient
- For example blood flow from the heart —> blood vessels
3. Osmosis - Has an osmotic pressure gradient
- For example uptake of water by cells
4. Electrochemical flux - Has an electrical and concentration gradient
For example ion flow through the action potential in a nerve
Where does solute and fluid exchange take place?
It takes place in the capillaries
These vessels connect arterioles to venules; they’re an extension of the inner linings of the arterioles
They are one cell thick and are semi permeable
Name some properties of capillaries
· Smallest diameter out of the blood vessels
· One cell thick
· Semi permeable
Where are capillaries found?
Found near every cell in the body but higher density in highly active tissue (muscle, liver, heart, kidney, brain etc)
How does solute exchange occur in capillaries?
Solute exchange via passive diffusion for example oxygen, glucose, amino acids, hormones and drugs
Fluid exchange along pressure gradients
What affects the rate of solute transport?
- Properties of passive diffusion
Rate
Distance
Concentration - Properties of solutes and membrane
Ficks law - Properties of capillaries affect the movement
List the properties of passive diffusion
• DOES NOT require energy (ATP)
• Movement from a high —> low concentration gradient
• Efficiently transports lipid-soluble solutes over very short distances
Only short distances because the time taken (t) for a molecule to move a net distance (x) in one direction increases with the distance squared ( t= x^2/2D)
List some properties of solutes and membranes that affect transport
1. Solutes • Concentration gradient • Size of solute • Lipid solubility of the solute (lipophilic or lipophobic) 2. Membranes • Membrane thickness and composition • Aqueous pores in the membrane • Carrier-mediated transport Active transport mechanisms
What is Fick’s law and what does it state?
It describes diffusion (for example: the movement of a molecule down the concentration gradient)
It states that the solute movement can be determined by four factors:
1. The diffusion coefficient of the solute (the ease of movement through the solvent)
2. The area
3. The concentration gradient
4. The distance
List the three types of capillaries and describe them
- Continuous capillaries
• Moderate permeability with tight gaps between neighbouring cells
• Have a constant basement membrane
• Found in the blood-brain barrier, muscle, skin, fat and connective tissue- Fenestrated capillaries
• High water permeability with fenestration structures throughout the cells (hence a moderate disruption of the membrane)
• Found in high water turnover tissues, such as salivary glands, kidneys, synovial joints, anterior eye, choroid plexus (spinal fluid), and gut mucosa - Discontinuous capillaries
• Very large fenestration structures throughout, and a disrupted basement membrane
Found where the movement of RBC’s is required such as the liver, spleen and bone marrow
- Fenestrated capillaries
List some properties of capillaries that affect transport
- Intracellular cleft
- 10-20 nm wide
- Allows solutes and fluids to move through between the cells
2. Caveolae and vesicles - Large pore system
- For the movement of proteins across the membrane
3. Glycocalyx - A negatively charged material that covers the endothelium, blocking solute permeation and access to transport mechanisms
- Its highly regulated
Glycocalyx is very dynamic and can be broken down and remade as required
What is permeability and what is its relation with diffusion?
Permeability is the rate of solute transfer by diffusion across a unit area of membrane per unit concentration difference (how freely a solute crosses a membrane)
A porous membrane interferes with the diffusion of lipid insoluble solutes in multiple ways:
1. A reduction in the area for diffusion
2. An increased path length through the membrane
3. Restricted diffusion in the pore produces hydrostatic issues
What is the dominant route of transport; diffusion or filtration?
diffusion
Describe the modification of Fick’s law for a porous membrane
Js = rate of solute transport P = permeability; involving pore size, length, diffusion coefficient Am = surface area of capillary involved in transport ΔC= change in concentration
Describe the mechanisms of transport of glucose around the body
Filtration transport only accounts for 2% glucose transport
98% of glucose transport into interstitial space via passive diffusion (via GLUT transporter system)
Name 3 things that increase diffusion rate
1. Increased blood flow Increases solutes in capillaries 2. Fall in interstitial concentration Increases the concentration difference 3. Recruitment of capillaries Dilation of arterioles leads to an increased number of capillaries perfused
What is the importance of fluid exchange?
Fluid exchange is important for normal physiological function, for example, we need H2O for chemical reactions
Fluid re-absorption from the tissues to the blood can maintain circulation during a haemorrhage
Abnormalities in fluid exchange can lead to oedema/swelling
What is the cause of lymphoedema?
Causes by removal of lymph nodes OR lymph node damage due to radiotherapy for testicular cancer
Briefly describe the fluid movement at the capillary wall in terms of hydraulic and oncotic pressure
The capillary wall is semi permeable
Fluid moves across the membrane into the interstitial space due to blood flow, which exerts a hydraulic pressure
Large molecules cannot pass through the membrane so they exert an osmotic pressure (oncotic pressure); creating a suction force to move fluid into the capillary
Fluid movement across capillary walls depend on the balance between hydraulic and oncotic pressure across the capillary walls
What are the 4 pressures that determine filtration rate?
- Pc: capillary blood pressure
- Pi: interstitial fluid pressure
- Πp: plasma proteins
- Πi: interstitial proteins
How do we calculate effective osmotic pressure?
Reflection coefficient x potential osmotic pressure
What is the reflection coefficient for plasma proteins and what does this show?
0.9
This means that 10% of plasma proteins are conducted across the capillary wall into the interstitial space
What does Starling’s law normally favour in capillaries?
Favours filtration
There are factors that favour filtration (Πi and Pc) and factors that favor reabsorption (Πp) however the balance is tipped more towards filtration. This is balanced by OTHER factors
Describe how Pc and Πi change along the length of the capillary
Πi stays the same along a well perfused capillary
Pc is higher at the arterial end of the capillary and decreases towards the venule end of the capillary
Along the capillary at around Pc=25 mmHg tissue fluid leaves the capillary into surrounding cells to form excess fluid which is returned to the CVS as lymph via the lymphatic system
How do you calculate the net filtration along a capillary?
P = (Pc-Pi) - sigma(Πp-Πi)
This is because in normal conditions (well perfused capillaries) Lp and A are constant so are not taken into account in the equation
Describe the levels of filtration along the capillary
More filtration at arterial end
Less filtration at venule end due to pressure drop
How does the lymphatic circulation relate to fluid exchange in capillaries?
Lymphatic system returns excess tissue fluid back into the CVS
Lymphatic system also contains immune cells at the lymph nodes
How is lymph fluid carried to the CVS?
Lymph vessels has valves and smooth muscle; the smooth muscle contraction is what allows the lymph to flow
Surrounding skeletal muscle contraction and relaxation also contributes to lymph flow
What 3 things does the overall control of extracellular fluid balance depend on?
Capillary filtration
Capillary reabsorption
Lymphatic system
Starling’s factors determine changes in fluid balance in:
Circulation
Interstitial fluid
Lymphatic system
What is hypovolemia?
Low blood volume
Describe what happens in the capillaries during hypovolemia (for example due to a haemorrhage)
Drop in CO = drop in BP = Pc reduced = improper capillary filtration
In this case, you would transfuse interstitial fluid into the blood. This would be life preserving as it would:
- Support CVP
- Increase CO
- Increase BP
- Greater blood flow
What is oedema and its causes?
Oedema is an excess of fluid within the interstitial space. This indicates an imbalance between filtration, reabsorption and lymph function
Increased Pc gives rise to oedema such as in a DVT (deep vein thrombosis)
Oedema can then cause:
- Increased capillary pressure (Pc)
- Decreased plasma protein oncotic pressure (Πp)
- Inflammatory response
- Lymphatic problems
Give 3 clinical scenarios of what can occur due to increased capillary pressure (Pc)
Dependent oedema (standing for long periods of time)
DVT
Cardiac failure
What happens during a DVT?
Thrombosis occurs resulting in prevention of venous return = increased venous pressure which results in back up of pressure = increased Pc = increased filtration
What are some causes of low protein oedema?
Malnutrition or malabsorption: not enough protein to make plasma proteins to generate oncotic pressure
Nephrotic syndrome: urinary protein loss
Liver disease: not enough albumin production
Describe how someone with low protein intake can develop oedema
Reduced protein concentration = reduced oncotic pressure
This causes fluid efflux from capillaries into the interstitial fluid resulting in oedema
This is also known as kwashiorkor
Describe how an inflammatory response can lead to oedema
Inflammatory response triggered results in a LARGE increase in capillary permeability
Increased Lp = increased protein permeability (Πi) due to decreased sigma
Increased Πi = less reabsorption taking place = oedema
Name some lymphatic problems and describe them
Elephantitis: nematode infestation migrate to lymphatic system and reproduce to BLOCK lymph drainage
Lymphoedema: caused by surgical testicular cancer treatment; removal if lymphatics causing swelling to leg
