Microcirulation and Oedema Flashcards
what are the micro vessels in the microcirculation system
- Terminal arterioles
- Capillary
- Venules
what is the function of the capillaries
- Site of exchange of nutrients and waste productions between the circulation and interstitial fluid that surrounds the cells
what is another name of a capillary
exchange vessels
describe what vessels lead into what vessels in microcirculation
arterioles lead into terminal arterioles which lead into capillaries which lead into venues
what is the diameter of arterioles
10-200um
what is the diameter of terminal arterioles
10-40um
what is the diameter of capillaries
5-8um
what is the diameter of venues
10-200um
what is the structure of the arterioles
- endothelium
- muscular walls
- sympathetic nerves
what is the structure of terminal arterioles
smooth muscle
few nerves
what do precapillary sphincters do
control blood flow into the capillaries, controls what capillaries have blood in them
what is arteriovenous anastomoses
- This is a connection between two vessels (arteriole and venule)- blood flows through the arteriole into the venule
describe the structure of the capillary
- Composed of endothelial cells that are held together by tight junctions surrounded by a basement membrane upon which the endothelium rests
what is the difference between the capillary and arterioles
- No smooth muscle in the capillary – this differentiates capillaries from arterioles
how many types of capillaries are there
3
what are the three types of capillaries
- Continuous capillary
- Fenestrated capillary
- Sinusoidal/discontinuous capillary
describe the structure of the continuous capillary
- Least permeable but most widely distributed
- Have a sealed endothelium and only allow small molecules like water and ions to diffuse through
- Continuous capillaries have tight junctions and can be further divided into two subtypes
- The basement membrane is continuous and there are no holes in it
what are the two subtypes fo the continuous capillary
- Those with numerous transport vesicles that are primarily found in skeletal muscles, lungs, gonads and skin
- Those with few vesicles that are primarily found in the central nervous system
describe the structure of the fenestrated capillary
- Have small circular pores in the endothelial cells (60-80 nm in diameter).
- usually have a continuous (closed) basal lamina.
- Pores Permit relatively free passage of salts and water from plasma to the tissues therefore they are more permeable than continuous capillaries
- Found in tissues that are specialized for bulk fluid exchange.
- Are primarily located in the exocrine glands, intestines, pancreas, and glomeruli of kidney.
describe the structure of the sinusoidal/discontinous capillary
- Discontinuous capillaries (sinusoidal) have the highest permeability.
- Contain special fenestrated capillaries that have larger openings (30-40 μm in diameter) in the endothelium to allow red and white blood cells (7.5μm - 25μm diameter) and various serum proteins to pass, a process that is aided by a discontinuous basal lamina.
- These capillaries lack pinocytotic vesicles and gaps may be present in cell junctions permitting leakage between endothelial cells.
- Discontinuous capillaries are primarily located in the liver, spleen, bone marrow.
- Blood cells can pass through and so can large proteins,
- They are found in places where you want large proteins and cells to go out of the capillaries such as in the liver, spleen and bone marrow
where is the fenestrated capillary found
exocrine glands, intestines, pancreas, and glomeruli of kidney.
where is the discontinuous capillary found
liver, spleen and bone marrow
what are the 4 main routes for movement across a capillary endothelial cell
- Diffusion through membrane
- Movement through intercellular clefts
- Movement through fenestrations
- Transport via vesicles or caveolae, these undergo endocytosis
what law determines diffusion
Ficks law
How do you work out the amount moved by diffusion
surface area x concentration gradient x diffusion coefficient
what does the surface area and diffusion distance of a capillary depend upon
– depends on the density of the capillaries, if the density of the capillaries is higher larger surface area for exchange and a relatively short distance between each capillary
what does the concentration gradient of diffusion depend upon
- The net rate of diffusion of a substance through any membrane is proportional to the concentration difference between the two sides of the membrane.
- Oxygen: concentration in the plasma is greater than that in the interstitial fluid thus causing net movement from blood towards the tissues.
- Converse true for Carbon Dioxide.
what does diffusion depend upon
- surface area
- concentration radeitn
- diffusion coefficient
describe the diffusion coefficient for small lipid molecules
- Small lipid soluble molecules can diffuse through the lipid bilayer (plasma membrane of the endothelial cells).
- Higher diffusion coefficient
- Oxygen, carbon dioxide, anaesthetics.
- They can therefore cross the entire surface area of the capillary.
describe how larger molecules move through the capillary
- via extracellular pathways
- use paracellular transporter, this is the transfer of substance across an epithelium by passing through the intracellular space between cells
describe the diffusion coefficient for lipid insoluble and larger molecules
- Small lipid insoluble ions/molecules and larger molecules cannot easily cross the cell membranes
- Passage is confined to water filled channels in between the cells (extracellular pathways) and through the pores that are present.
- Depends on the permeability characteristics of the capillary wall.
- In continuous endothelia (blood brain barrier) passage of these molecules will be very limited
- However, tissues that have greater capillary permeability (fenestrations) facilitate movement of larger molecules such as plasma proteins.
what is movement across a capillary endothelium determined by
determined by hydrostatic pressure and oncotic pressure
what are the 4 pressures that determine movement of substance across an endothelium
hydrostatic pressure of the capillary,
hydrostatic pressure of the interstitial fluid,
oncotic pressure of the capillary
oncotic pressure of the interstitial fluid
describe hydrostatic pressure of the capillary
- pressure reduces as it flows through the capillary bed
- filtration at arterial end and reabsorption at the Venus end
what is the hydrostatic pressure of interstitial fluid
0 mmHg
what is the oncotic pressure in the capillaries generated by
plasma proteins
what is the value of the capillary colloid oncotic pressure
26mmHg
what is the value of the oncotic pressure of the interstitial fluid
1 mmHg
- it is called the interstitial fluid colloid omsotic
- should not get a lot of proteins around in the interstitial fluid
How do you work out the net filtration pressure
(HPc-HPif)-(OPc-OPif)
what happens if the net filtration pressure is positive
- If net filtration pressure is positive like it is at the arterial end water is forced out of the capillary
what happens if the net filtration pressure is negative
- If the net filtration is negative like it is at the Venus end water is pulled back into the capillary
what happens when you alter the hydrostatic or osmotic pressure
• Altering either the hydrostatic or osmotic pressure will disturb the fluid balance across the capillary wall.
how much of plasma passes through the capillaries a day
- 4000 L
How much of the plasma is filtered
- 0.1-0.2% is filtered
- this means that 4-8 litres of fluid moves from the capillaries to the interstitial fluid every day
- Some fluid is reabsorbed but most of it drains into the specialised vessels which is the lymphatic system
Describe the structure of the lymphatic system
- Specialised vessels made up of an endothelium with large intercellular gaps surrounded by permeable basement membrane.
- End as blind sacs within tissues.
- Also contain one-way valves that ensure lymph travels away from tissues.
where does reabsorption of the lymphatic system happen
- the fluid within the lymph enters the subclavian vein or water reabsorption into the lymph nodes
what is oedema
Increased volume in the interstitial compartment leads to tissue swelling
what are the two types of oedema
systemic and peripheral
where does systemic oedema occur and what is it due to
occurs in the lower regions of the body for example the ankles- this is due to the venous pressure in the legs being elevated during prolonged standing
How can you distinguish oedema from inflammation
- This can be distinguished by applying firm pressure to the affected area – it does not bounce back immediately, this is pitting oedema and it distinguishes it from inflammation which does bounce back immediately
what are the factors causing oedema
- Increased capillary hydrostatic – forces more fluid out
- Decreased plasma oncotic pressure – reduces drawing of water into the capillires
- Increased capillary permeability
- Lymphatic obstruction
what does heart failure need to do
- means heart is unable to pump blood around the body
- breathlessness after activity or at rest
- feeling tired most of the time and finding exercise exhausting
- swollen ankles and legs
how does increased capillary hydrostatic pressure cause oedema
- two forces are balanced between driving fluid into the interstitial space and driving fluid into capillary
- if the venous pressure becomes elevated by gravitational forces in heart failure or with venous obstruction then the capillary hydrostatic pressure will be effected
- this reduces reabsorption of fluid into the capillaries
what is kwashiorkor
decreased plasma oncotic pressurepressrue
what does kwashiorkor due to
due to
- Severe malnutrition
- Deficiency in dietary protein
- Lack of proteins in blood and tissues
- Reduced OPc
- pitting edema
- Water retention in the gut
what is the cause of kwashiorkor leading to oedema
- If the plasma oncotic pressure decreases as occurs with hypoproteinaemia during malnutrition or liver disease there is a decrease in the oncotic pressure
- Reduce reabsorption of fluid into het capillaries
how does increased capillary permeability lead to oedema
- Increased capillary permeability - allows water to flow more freely and reduces the oncotic pressure difference by allowing protein to leave the vessel more easily.
- E.g. Vascular damage (burns, trauma, inflammation)
how does lymphatic obstruction cause oedema
• Obstruction of lymph drainage
- (e.g. fibrosis or filiariasis – elephantiasis, a worm infestation in lymphatic system)
• Tissue injury
• Inflammation of lymph vessels
How do you treat oedema
- Treatment of oedema itself by drugs that promote the loss of sodium and water.
- Diuretics either osmotic diuretics (increase water excretion) or loop diuretics (increase sodium excretion, natriuresis).
