Microcirculation Flashcards
What is the microcirculation?
This is a branch of the vascular system that enters a tissue. Diffusion of substances occurs through the capillaries
What is blood flow and how do you calculate it?
The blood flow rate is the volume of blood passing through a vessel per unit time.
The main aim of the CVS is adequate blood flow through the capillaries.
You can apply Darcy’s Law:
Flow rate= pressure gradient/ vascular resistance
This means that the flow rate is directly proportional to pressure gradient and inversely proportional to vascular resistance
What does the pressure gradient refer to?
The pressure gradient is the pressure difference between 2 areas.
Blood will always flow from an area of high pressure to an area of low pressure.
An increase in the pressure difference increases the flow rate.
What does resistance to blood flow refer to?
Resistance is hinderance to blood flow due to friction between moving fluid and stationary vascular walls.
Which factors affect resistance?
- Blood viscocity
- Vessel radius- the narrower the vessel, the greater the resistance
- Vessel length
Factors 1 and 3 tend to stay the same so therefore the resistance depends mainly on the vessel radius:
R= 1-r^4
How will blood flow be affected if the:
- BP increases
- Ateriolar vasoconstriction
- The flow will increase as there is a greater pressure difference
- The flow will decrease as the radius and therefore resistance to blood flow is greater
What are the standard values for MAP and pressure in the capillaries?
MAP- 93 mmHg
Capillaries- 37 mmHg
Applying the blood flow equation to the an organ
The pressure difference is really important for the blood the reach the capillary bed. Instead of taking Pressure B as the middle of the capillaries (i.e. 37mmHg), we take it at the end of the capillaries. This is the venous blood pressure which is around 0 mmHg.
Pressure A remains MAP
Therefore the pressure difference is equal to MAP
What does vasoconstriction and vasodilation mean for the radius, resistance and flow of the vessles?
Vasoconstriction- low radius, greater resistance and reduced flow
Vasodilation- greater radius, reduced resistance and increased flow.
In normal conditions, out arteriolar smooth muscle displaus a state of partial constriction- this is vascular tone
The radii of the arterioles are adjusted independently to accomplish 2 functions- what are they?
1) Match blood flow to the metabolic need of specific tissues. This is regulated by local intrinsic controls that are independent of nervous/ endocrine stimulation
2) Help regulate systemic arterial blood pressure. This is regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated.
Arterioles changing due their chemical environment
They do this to match the blood flow to the metabolic needs of specific tissues.
Increased metabolism and oxygen consumption leads to vessel vasodilation. The arterioles are responding to their local conditions.
This is known as active hyperaemia (increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue)
Arterioles change due to their physical environment
Again, to match the metabolic needs of specific tissues.
For instance, tissues will change in response to temperature.
If blood temperature decreases, there is a greater stretch (distension), making the smooth muscle contract. Less blood can reach the surface. This is known as myogenic autoregulation.
The picture shows the difference between myogenic autoregulation and myogenic reaction
Linking equation for blood flow and cardiac output
With substitution:
Cardiac output= MAP / TPR (total peripheral pressure)
Or, MAP= CO x TPR
TPR is the total sum of the resistance in all the arterioles in the body.
Describe neural regulation of arteriole blood pressure
Regulating BP.
There is a cardiovascular control centre in the medulla of the brain. To increase the blood pressure, vasoconstriction will take place, reducing blood flow to organs.
Adrenoreceptors- faciliate constriction and dilation
Alpha- constriction
Beta- dilation
Hormonal control of arterial blood pressure
Release the following (look at pic) hormones that have an effect on bp. By stimualting production of vasopressin and angiotensin
The brain can stimulate release adrenalin and noradrenaline to mimic the sympathetic nervous system
All effects lead to vasoconstriction.
What are the different types of capillaries?
Continuous- small water filled gap junctions between the endothelial cells which allow electrolytes and small molecules through.
Fenestrated- leaker capillaries, slightly bigger holes which allow bigger substances to pass through
Discontinuous- have have huge holes- important in the bone marrow where white cells have to get into the blood
Describe fluid movement in the capillaries from arteriole to venule end
Arteriole end- the HP is greater than the oncotic pressure. This is ultrafiltration.
Venule end- Huge drop in the hydrostatic pressure and no change in the oncotic pressure. This is reabsorption.
Delta 9 pushing out but only delta 8 pushing in. Therefore you are always losing a little bit of fluid from your blood system in the capillaries. The lymphatic system will restore this lost fluid.
What is the lymphatic system?
System to restore fluid to blood. It is also has an important surveillance role.
It is not a closed loop system, however, there are some lymphatic capillaries that are blind ended. There are small openings in the lymphatic capillaries.
There are valves in the lymphatic system which means that flow is unidirectional.
Drainage in the lymphatic system
Thoracic area
Right Lymphatic duct
Right subclavian vein
Left subclavian vein
Are the points where the lymphatic fluid drains into the venous system.
