Haemodynamics & Microcirculation Flashcards
How do calculate mean arterial pressure (MAP)?
Heart rate x Total peripheral resistance x Stroke volume
HR x SV x TPR = MAP
What does Darcy’s Law say?
Flow (in the steady state) is linearly proportional to the pressure difference between two points
What is the difference between fluid flow and fluid velocity?
Fluid flow= volume/time
Fluid velocity= distance/time
How do you calculate mean velocity?
Flow / total cross sectional area
What happens when total cross sectional volume is increased as blood enters the microcirculation?
Mean velocity falls progressively
What factor is never altered in the vascular system?
TOTAL FLOW
It remains equal to the cardiac output at each level of the vascular system
What is the pressure gradient in the vascular system effectively equal to?
Mean arterial pressure
How and why is a pressure gradient created in the systemic circuit?
As aortic pressure is 90mmHg and pressure in the vena cava is 0mmHg. This creates a large pressure gradient
The pressure gradient is the overall driving force pushing blood through the systemic circuit
How many patterns of flow are there and what are they called?
There are 3 types:
- Laminar flow
- Turbulent flow
- Single-file flow
Where does laminar flow occur?
Occurs in normal arteries and veins
Where does turbulent flow occur?
Occurs in the ventricles and sometimes, in ascending aorta of healthy subjects
Where does single-file flow occur?
Occurs in the capillaries
What determines mean arterial pressure?
Cardiac output and total peripheral resistance
State the formula for resistance
R= (8n.L)/(π.r^4)
R= Resistance n= Fluid viscosity L= Tube length r= Radius
What would you so if you want to increase resistance?
Increase fluid viscosity and tube length
What would you so if you want to decrease resistance?
Increase radius
State the expression for flow through a tube
Q= (P1-P2)x ((π.r^4)/(8n.L))
P1== pressure one
What is flow extremely sensitive to?
Radius
E.g. a fall in radius o=from 1cm to 0.01cm will increase resistance by a factor of 10^8
(This is why the arterioles are the main site of resistance)
Collectively what type of circuit are the arteries, arterioles, capillaries, venules and veins arranged in?
Series
What are vessels of the same type arranged in?
What are the two exceptions to this?
Parallel
Exception:
Pulmonary trunk and aorta
If series units are added to a circuit what happens to the total resistance?
Total resistance increases
If parallel units are added to a circuit what happens to the total resistance?
Total resistance decreases
State the formula for compliance
Change in volume/ distending pressure
What is distending pressure?
Pressure inside minus the pressure outside (ie transmural pressure)
Define compliance of a vessel
The change in volume per unit change in distending pressure
Which blood vessel type has the highest compliance and why?
Veins as they are thin walled and easily stretched
Veins having a high compliance means what?
That they can accommodate a large increase in blood volume in response to a small increase in blood pressure
(I.e. They are good at storing volume)
Veins act as V_______ R_______
Volume reservoirs
Arteries act as P_______ R_______
Pressure reservoirs
What does distending pressure acting on vessel walls do?
Stretches out the vessel walls
What causes vessel walls to rupture
When the distending force acting non the vessel wall is not balanced by forced within the vessel walls
State the Law of LaPlace
T= (Pt.r)/u)
Pt= Transmural pressure
r= Radius
u (mu)= Wall thickness
State the relationship between internal pressure and wall tension and vessel radius
Wall tension increases with internal pressure and vessel radius
State the relationship between wall tension and wall thickness
As wall thickness increases wall tension decreases
Where are transmural pressure and radius large?
In large arteries
This means walls of the arteries need to be thick in order to compensate
Where are transmural pressure and radius small?
In capillaries
This is why they have thin walls
Where is transmural pressure low but radius large?
In the veins
But as vessel walls are thin significant tension is gathered here
Which vessel is the most likely to rupture?
Elastic arteries like the aorta
What is microcirculation?
It is defines as the circulation of blood through the smallest vessels like the arterioles, capillaries and venules
Where is density highest in the body?
In metabolically active tissues such as skeletal muscles
Capillaries form an I______ N______
Interconnecting network
What does blood flow in the capillaries depend on and why?
As blood flow through the capillaries is not uniform is depends primarily on the contractile state of the arteriolar smooth muscle
What are changes in blood flow induced by?
- By changes in diameter of the arterioles
2. Altering the contraction of precapillary sphincters
What is auto regulation?
It is the intricacy adjustment of blood flow to a tissue or specific vascular bed such that the flow meets the local requirement at any given point in time
What are the 2 classifications intrinsic control mechanisms can have?
Metabolic or myogenic
Metabolic and myogenic are classification of what?
Intrinsic control mechanisms
Is there a relationship between blood flow and rate of metabolism? If so what is it?
There is a clear relationship between the rate of metabolism and the rate of blood flow.
As one increases so does the other
List some factors involved in metabolic auto regulation
Oxygen
Products of metabolism like carbon dioxide, H+,K+ and adenosine
Substances synthesised within the vascular endothelium
Describe how oxygen is a factor in metabolic auto regulation
(Hint: negative feedback)
When the metabolic rate of tissues increases oxygen consumption increases beyond sustainability
Oxygen concentration in tissues decreases resulting in a local hypoxia
To counteract this:
1. Vasoconstriction increases
2. Resistance decreases
3. Blood flow increases
4. Oxygen delivery increases
So oxygen concentration in tissues increases
Describe how products of metabolism are a factor in metabolic auto regulation
(Also give examples)
E.g. CO2, H+, K+ and adenosine
These products diffuse from the surrounding tissue and cause relaxation of the vascular smooth muscle so less oxygen consumed
Describe how substances synthesised within the vascular endothelium are a factor in metabolic auto regulation
(Also give examples)
E.g. Prostacyclin and nitric oxide
These substances also diffuse into the adjacent smooth muscle where they usually mediate vasodilation
Again this helps ensure oxygen concentration in tissues remains high
Give an Endothelium-Derived Relaxing factor
Nitric oxide
What is an Endothelium-Derived Relaxing factor (EDRF)
A substance produces in the endothelium casing reaction of vascular smooth muscle
The discovery of nitric oxide being an Endothelium-Derived Relaxing factor has allowed for what…
The discovery has allowed the mode of action of drugs such as nitroglycerin to be more clearly understood
An isolated perfused organ displays what?
An intrinsic abilty to maintain a constant rate of blood flow through its vascular bed over a wide range of perfusion pressures
Give the formula to calculate flow
Flow= pressure gradient / resistance
What is the mechanism of the myogenic response to reduce flow rate?
- Perfusion pressure increases increasing flow rate
- Arteriolar smooth muscle stretches more
- Constriction increases
- Resistance increases
So flow rate decreases
This is negative feedback
What does the myogenic reposes (to reduce flow rate) allow?
This response keeps tissue perfusion fairly constant in the face of most variations in systemic arterial blood pressure
When may long term auto regulation develop?
If the nutritional and/or oxygen demands of a tissue chronically exceed delivery, long term auto regulation may develop over a period of weeks to months
Chronic exposure to high altitudes elicits a similar response in the body
How is long term auto regulation achieved
Due to an increase in the number of microcirculatory vessels supplying blood to the tissue
And due to the enlargement of existing vessels
What provokes long term auto regulation?
This reposes is provoked in the heart by the gradual partial occlusion of a coronary vessel
How do lipophilic solutes enter/leave the capillaries?
The transcellular route
Hydrophilic solutes can cross through the I______ C_____ which have a diameter of…?
Intercellular Clefts
Diameter of approx 60A
What can not pass through the clefts?
Water Ions Small organic solutes Albumin Other plasma proteins
Where is capillary hydrostatic pressure greatest?
At the arterial end of the capillary
Where is capillary hydrostatic pressure lowest?
At the venous end
How does hydrostatic pressure arise?
Hydrostatic pressures result essentially from the capillary blood pressure
How does osmotic pressure arise?
Osmotic pressures result from the presence of large, non-diffusible molecules. Mainly these are confined to the plasma
Where does the net osmotic pressure gradient favour fluid absorption?
By the capillaries
Does the osmotic gradient vary over the capillaries
The gradient does not vary significantly over the length of the capillary.
What is the net fluid balance?
Net movement of fluid out of the circulation at the arterial capillary end and net movement of fluid into the circulation at the venous capillary end
What is the net loss of fluid from the circulation? And how and why is the fluid lost returned to the body?
1.5mL/min
If unchecked this rate of fluid loss would completely empty the vascular system of plasma in 24hrs
Returning his fluid to the circulation is done by the lymphatic system
Define perfusion?
The passage of blood, a blood substitute, or other fluid through the blood vessels or other natural channels in an organ or tissue.
