Microcirculation Flashcards
What is meant by microcirculation?
Every tissue in the body has their own microcirculation - main arteries deliver blood to different regions of the body, but arterioles branch off the arteries, which then pass on the blood to the capillaries (in which exchange takes place), then the blood drains back into the venules, which merge into the veins that return the venous blood to the heart
What is the overall aim of the CVS? And why?
What is the blood flow rate?
What is Darcy’s law rearranged to form ‘flow rate = ‘?
Adequate blood flow through capillaries - as that is where exchange takes place
Volume of blood passing through a vessel per unit time (dictates how much blood gets to a particular tissue at any given moment)
Flow rate = pressure gradient / resistance
How does blood get through the arterioles to the capillaries? How is this measured?
How can more blood be delivered to specific tissue?
Via pressure gradient = Pressure A (pressure at the start of the arteriole) - Pressure B (pressure at the end of the arteriole)
Increase pressure in the arteries (Pressure A) to increase pressure gradient
What hinders blood flow?
What are the 3 things that impact resistance?
What is the equation that links these 3 factors to resistance?
Which of these has a major determinant on resistance (i.e. which one can change quickly)?
Resistance - friction between the blood flow and stationary vascular walls
Blood vessel length, blood vessel radius and blood viscosity
*look at image*
Radius - halving the radius decreases flow by 16x
Fill in the table below on what happens to the pressure, resistance and flow when:
a) there is high BP?
b) there is arteriolar vasoconstriction?
Vasoconstriction = more difficult for blood to flow through due to resistance, which decreases flow
High BP = greater pressure = greater flow
Which part of the circulation system has the greatest impact on resistance?
How does flow rate increase?
Arterioles to capillary - as it is where there is the greatest drop in pressure
Greater pressure gradient
How does pressure vary across the whole organ? Does the pressure gradient change across different tissues?
What is the main determinant of blood flow that can change? So why can there be different blood flows to different organs?
Pressure gradient across the tissue is the same due to venule pressure being v. low - the venule pressure is almost nothing. So the pressure gradient aross all tissues is MAP (mean arterial pressure) subtract almost nothing.
Therefore, the only real determinant of blood flow that can change is resistance.
As blood flow is different to different organs, it must be due to resistance as that is the only thing that can really change
How is resistance controlled in the arterioles?
At arteriole smooth muscle relaxation - why does it display partial constriction?
Contraction and relaxation of the vessels controls the radius, which in turns controls resistance, which in turn controls flow. This mechanism is called vasoconstriction (reduced flow) and vasodilation (increased flow)
Partial constriction = vascular tone; if the vessel needs to be able to do both, dilate and contract, then it must be partially constricted. One is lost if it is fully dilated or fully constricted
What are the 2 ways by which radii of vessels can be altered?
- Blood flow matches the metabolic needs of specific tissues. Response to metabolic change: Respond to local environment (independent of the brain) - happening via local mechanisms e.g. if being exercised more, greater oxygen use and more waste being produced, the chemoreceptors detect this chemical change, which directly causes smooth muscle constriction or relaxation. Increased metabolic activity that leads to increase blood flow to that organ is called active hyperaemia
- Helps regulate systemic artrial pressure. Respond to the external control i.e. the brain / nerves e.g. physical factors such as a drop in blood temperature (generally relevant for peripheral vessels), physical effect causes vasoconstriction of the arterioles = directs blood elsewhere to stop reduction in blood temperature. Or raise in BP causes vasodilation to reduce BP - this is called myogenic autoregulation (coming from the muscles itself - regulating in response to its own stretch)
Which concept is applied to iceing an injury?
Ice-ing an inflammed injury causes vasoconstriction, so blood flow is directed elsewhere = reduce inflammation / swellingof injury by decreasing blood flow to that area
During exercise, how do BP and vessel size change?
When BP increases, pressure gradient goes up causing increased flow rate through all arterioles
Increased pressure = dilated vessel; but if the tissue does not require the blood, the arteriole actively constricts to reduce blood flow to the area
This is required so BP does not plummet / drop suddenly, and increased blood supply only goes to the areas that require it
During exercise, how do the skeletal muscle arterioles differ from the small intestine arterioles?
Skeletal muscle arterioles = active hyperaemia
Small intestine arterioles = myogenic vasoconstriction as these tissues do not require the increased blood supply
What is the equation for flow across the whole circulation? (Hint: Cardiac output = ?)
What is the BP equation (previous equation rearranged)?
Cardiac output is the blood flow within the whole circulation and so across the whole body
CO = MAP / total peripheral resistance
MAP = CO x total peripheral resistance
TPR is a measure of total arteriolar constriction
MAP = systolic BP - diastolic BP
How does the brain influence microciculation and blood flow?
Would this decrease blood flow to specific organs?
If significant blood is lost from trauma, this is detected by the brain. How does it react?
- Via nervous system = CVS control centre in the medulla
Yes
Heavily constrict vessels in many areas to maintain BP = less blood supplying the tissues (increasing oxygen debt). Brain sarcrifices blood flow to almost everywhere in the body to supply the brain
- Via endocrine system = hormonal controls e.g. ADH, Angiotension II, Adrenaline etc. Angiotensin II = most powerful vasocontrictor in the body
Why are arterioles important?
What is the function of the capillaries? Why are the capillaries important?
What is the structure of the capillaries?
For matching blood flow to the tissues
Capillary exchange takes place in the capillaries - important as metabolic substrates are delivered to the cells, and some waste products carried back
Structure is adapted to its function: Diameter = extremely narrow (about the width of a RBC)
Cells lining the cpaillary wall = incredibly thin
Capillaries = highly branched (many capillaries interconnecting so no cells are too far from the blood supply)
Small diffusion distance between the capillary and the cell tissue
Why is capillary density important?
Why don’t all tissues have the same capillary density?
To minimise diffusion distance and maximise surface area for exchange
The more metabolically active the tissue is, the more capillaries there are - due to increased demand in oxygen, glucose, removal of waste products, etc.
e.g. the brain has a very high capillary density, and skeletal muscle (when exercising utilises lots of oxygen - but limited flow at rest)
What is Fick’s law?
Minimise diffusion distance and maximise surface area
Rate of diffusion = [surface area x concentration difference] / wall thickness
Why do the lungs have the largest capillary density?
Not fully due to increased metabolic activity, also to have as many capillaries near the alveoli as possible for maximum gas exchange
What is the capacity and flow in skeletal muscle?
Huge capacity, but at rest is heavily constricted to limit flow as the skeletal muscles cannot house all the blood in it’s huge capacity (as the other tissues would be starved of blood flow or there would be too much stress on the heart)
Beginning exercise = 30 fold increase
What are the 3 types of capillary structures?
What is the structure of continuous capillaries? How do substances cross continuous capillary walls?
Endothelial cells = thin, one cell thick, cells line in a row in 3 ways:
- Continuous, 2. Fenestrated, 3. Discontinuous
Between the endothelial cells, there are small water-filled gaps at the junctions
Therefore molecules must be small enough to diffuse through via the gaps, diffuse across the endothelial cells (if lipid soluble), or the endothelial cells must contian proteins to transport molecules across
What are fenestrated capillaries? How do substances diffuse through these capillaries?
What are discontinuous capillaries?
Contain many fenestrae - bigger holes within the capillary (in comparison to continuous), which allow for larger substances to diffuse out to the tissue space
Large junctions and tight junctions mixed e.g. in bone marrow to allow for WBCs, and liver as the liver deals with metabolism so large molecules needs to pass through
Which type of capillary structure is found in the blood brain barrier?
Continuous structure with no gap junctions = protects the brain from harmful substances
Allows brain to have tight control over what can get through into the brain
Lipid soluble = always enters the brain, other substances need to be allowed into the brain e.g. via channel proteins
What is Bulk flow? How does bulk flow work?
A certain volume of protein-free plasma filters out of the capillary, mixes with the surrounding intertstitial tissue and is reabsorbed
Some plasma / fluid is forced out of the capillary due to high hydrostatic pressure, oncotic (osmotic) pressure pulls the fluid back into the capillaries, as there is no protein in the interstitial fluid and many proteins in the capillary. If the fluid was not pulled back into the capillaries, there would be constant blood volume loss
oncotic = protein related
How is the balance between the hydrostatic and oncotic pressure established?
Starling’s theory - Must be a balance between the hydrostatic force pushing out and oncotic force drawing it back in
No balance = too much fluid lost from the blood
How does hydrostatic and oncotic pressure change across a capillary?
Hydrostatic pressure is much greater at the start of the capillary than at the end
Oncotic pressure does not change across the capillary as the protein content in the capillary remains the same
Therefore at the start of the capillary, the net movement is fluid pushed out as hydrostatic pressure exceeds the oncotic pressure. Towards the end of the capillary, the oncotic pressure exceeds the hydrostatic pressure, so the net flow of fluid is back into the capillary
What is the significance of the fact that ultrafiltration is more effective that reabsorption?
How is that issue fixed?
There is a slight net loss in the fluid as not all the fluid pushed out is reabsorbed
Via the lymphatic system - fluid returns into the blood via this system
All of which allows for BP to be maintained
Why is residual volume important?
Why is tidal volume important?
Why is vital capacity important?
Why is expiratory volume least used clinically?
Shows how much of the lung is not being used
Tidal volume = steady, regular breathing = important for ventilator settings
Vital capacity = important clinically as low vital capacity = indicative of underlying obstructive lung diseases
Exhalation always changes depending on many different factors - therefore, not utilised clinically
Why is volume expired slightly larger than the volume inspired?
Inspired air = colder = smaller volume
Expired air = warmed up inside the lungs and is also wetter = greater volume
How does the oxygen dissociation curve change during exercise?
Shifts to the right - as exercising produce more CO2 which increases the acidity of the blood, hypercapnia = increases unloading of oxygen from the RBCs so more O2 is provided to the respiring tissue
