Microcirculation

Question 1

What is meant by microcirculation?

Answer 1

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

Question 2

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 = ‘?

Answer 2

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

Question 3

How does blood get through the arterioles to the capillaries? How is this measured?

How can more blood be delivered to specific tissue?

Answer 3

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

Question 4

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)?

Answer 4

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

Question 5

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?

Answer 5

Vasoconstriction = more difficult for blood to flow through due to resistance, which decreases flow

High BP = greater pressure = greater flow

Question 6

Which part of the circulation system has the greatest impact on resistance?

How does flow rate increase?

Answer 6

Arterioles to capillary - as it is where there is the greatest drop in pressure

Greater pressure gradient

Question 7

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?

Answer 7

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

Question 8

How is resistance controlled in the arterioles?

At arteriole smooth muscle relaxation - why does it display partial constriction?

Answer 8

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

Question 9

What are the 2 ways by which radii of vessels can be altered?

Answer 9

1. 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
2. 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)

Question 10

Which concept is applied to iceing an injury?

Answer 10

Ice-ing an inflammed injury causes vasoconstriction, so blood flow is directed elsewhere = reduce inflammation / swellingof injury by decreasing blood flow to that area

Question 11

During exercise, how do BP and vessel size change?

Answer 11

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

Question 12

During exercise, how do the skeletal muscle arterioles differ from the small intestine arterioles?

Answer 12

Skeletal muscle arterioles = active hyperaemia

Small intestine arterioles = myogenic vasoconstriction as these tissues do not require the increased blood supply

Question 13

What is the equation for flow across the whole circulation? (Hint: Cardiac output = ?)

What is the BP equation (previous equation rearranged)?

Answer 13

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

Question 14

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?

Answer 14

1. 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

2. Via endocrine system = hormonal controls e.g. ADH, Angiotension II, Adrenaline etc. Angiotensin II = most powerful vasocontrictor in the body

Question 15

Why are arterioles important?

What is the function of the capillaries? Why are the capillaries important?

What is the structure of the capillaries?

Answer 15

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

Question 16

Why is capillary density important?

Why don’t all tissues have the same capillary density?

Answer 16

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)

Question 17

What is Fick’s law?

Answer 17

Minimise diffusion distance and maximise surface area

Rate of diffusion = [surface area x concentration difference] / wall thickness

Question 18

Why do the lungs have the largest capillary density?

Answer 18

Not fully due to increased metabolic activity, also to have as many capillaries near the alveoli as possible for maximum gas exchange

Question 19

What is the capacity and flow in skeletal muscle?

Answer 19

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

Question 20

What are the 3 types of capillary structures?

What is the structure of continuous capillaries? How do substances cross continuous capillary walls?

Answer 20

Endothelial cells = thin, one cell thick, cells line in a row in 3 ways:

1. 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

Question 21

What are fenestrated capillaries? How do substances diffuse through these capillaries?

What are discontinuous capillaries?

Answer 21

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

Question 22

Which type of capillary structure is found in the blood brain barrier?

Answer 22

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

Question 23

What is Bulk flow? How does bulk flow work?

Answer 23

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

Question 24

How is the balance between the hydrostatic and oncotic pressure established?

Answer 24

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

Question 25

How does hydrostatic and oncotic pressure change across a capillary?

Answer 25

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

Question 26

What is the significance of the fact that ultrafiltration is more effective that reabsorption?

How is that issue fixed?

Answer 26

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

Question 27

Why is residual volume important?

Why is tidal volume important?

Why is vital capacity important?

Why is expiratory volume least used clinically?

Answer 27

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

Question 28

Why is volume expired slightly larger than the volume inspired?

Answer 28

Inspired air = colder = smaller volume

Expired air = warmed up inside the lungs and is also wetter = greater volume

Question 29

How does the oxygen dissociation curve change during exercise?

Answer 29

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