Microcirculation Flashcards
Define blood flow rate and state the equation
Blood flow rate – Volume of blood passing through a vessel per unit time
Blood flow rate is ………………… proportional to pressure gradient and ……………….. proportional to vascular resistance
Blood flow rate is directly proportional to pressure gradient and inversely proportional to vascular resistance
List 3 factors that affect resistance
Blood Viscosity
Vessel Radius - the narrower the vessel the more resistance there is
Vessel Length
What is the only way for the body to change resistance and why?
Resistance is directly proportional to 1/r4 (r = radius)
Generally blood viscosity and vessel length is fairly CONSTANT
So the only way for the body to change resistance is to change the vessel radius
To SUMMARISE, blood flow is determined by pressure difference in the vascular bed and vascular resistanc
Pressure Difference i.e.
Heart Contraction vs
Frictional Loss- causes a loss of pressure
What are the major resistance vessels?
MAP=Mean Arterial pressure
Arterioles
Blood pressure generated by the heart does NOT change much as you go through the arteries
The normal blood pressure is called Mean Arterial Pressure (MAP) - in any artery in the body the blood pressure is around MAP
The changes in pressure occur across the arterioles
So pressure A is always MAP and pressure B will be influenced by how dilated the vessels are
As the blood goes through the capillary bed you want it to pass relatively slowly to allow exchange of nutrients - so this system allows the blood to slow down
As pressure A is always mean arterial pressure and the pressure at the end of the capillaries is usually venous blood pressure which is more or less 0 mmHg
Write the equation down which tells you the Blood flow of an organ
Why in the equation below does change in pressure equal mean arterial pressure?
Blood pressure generated by the heart does NOT change much as you go through the arteries
The normal blood pressure is called Mean Arterial Pressure (MAP) - in any artery in the body the blood pressure is around MAP
The changes in pressure occur across the arterioles
So pressure A is always MAP and pressure B will be influenced by how dilated the vessels are
As the blood goes through the capillary bed you want it to pass relatively slowly to allow exchange of nutrients - so this system allows the blood to slow down
As pressure A is always mean arterial pressure and the pressure at the end of the capillaries is usually venous blood pressure which is more or less 0 mmHg, the blood flow rate equation can be rearranged to:
This can be applied to any tissue in the body
As the pressure going in is around MAP and the pressure going out is pretty much 0 mmHg ( becuase veins have pretty much 0 resistance), the pressure change in the blood passing through an organ is usually the same - around MAP
So, the major determinant of the blood flow in the body is mainly the resistance of the arterioles in the organ
What factor determines blood flow in different organs?
Resistance of the organs becuase MAP is mor or less the same through out the body
r=radius
Fill in the diagram below to show what happens whyen you have vasoconstriction and vasodilation?
What is vascular tone?
Arteriolar smooth muscle usually displays a state of partial constriction - this is VASCULAR TONE
They have to be partially constricted because then there’s a way of changing the blood flow up or down by constricting or dilating
The radii of arterioles are adjusted independently to accomplish TWO functions
What are these functions?
Match blood flow to the metabolic needs of specific tissues - this is regulated by local intrinsic control (independent of nerves and hormones - the tissue determines how much blood it needs)
Help regulate arterial blood pressure - regulated by extrinsic controls
If a muscle becomes more active - metabolism increases and oxygen consumption increases
This change is detected in the tissues and sends a signal to the arterioles to vasodilate
The arterioles are responding to the local conditions
What is this process called?
ACTIVE HYPEREMIA
Define Active hyperemia
Active Hyperemia = an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue
If O<span>2 usage levels increase (organ requires more O2) than this stimulates the release of vasodilators from the endothelial cells</span>
Give an example of how arterioles respond to their physical environment?
Tissues will respond to a change in temperature
If blood temperature decreases, the microcirculation will detect this change and make the arteriolar smooth muscle contract so that less blood reaches the surface and so less heat is radiated away
When you excercise some parts of the body do not need as much blood supply. Excercise causes an increase in blood pressure, so the tissues with a lower metabolic need undergo myoyogenic vasoconstriction which means that the blood supply decreases so the skeletal muscle, lungs and heart tissue get more blood. The arterioles sense the stretch from the increase BP- Autoregulation
GFR= Glomerular filtration rate
Cardiac output: The amount of blood the heart pumps through the circulatory system in a minute.
Total peripheral resistance = the sum of the resistance of all arterioles in the body
Average pressure in the arteries is MAP
Average pressure in the veins is next to nothing
So the pressure difference is considered to be MAP
Regulation of arterial blood pressure by changing the radii of the arterioles can be influenced by NEURAL or HORMONAL pathways
Neural regulation of arterial blood pressure
Regulated by the …………………… …………………. ……………….. in the MEDULLA
To increase blood pressure, ……………………. will take place thus reducing the blood flow to organs
There are ………………………….. which facilitate constriction and dilation
Name the two types of adrenoreceptors and state what happens when they are activated?
Regulated by the cardiovascular control centre in the MEDULLA
To increase blood pressure, vasoconstriction will take place thus reducing the blood flow to organs
There are ADRENORECEPTORS which facilitate constriction and dilation
Alpha = CONSTRICTION
Beta = Dilation
By activating these receptors, the brain can control constriction and dilation
The brain can stimulate the release of various hormones that have an effect on blood pressure
The brain can mimic the sympathetic nervous system to control the heart - by stimulating production of adrenaline and noradrenaline
The brain can control blood pressure by action on arterioles - by stimulating production of Vasopressin and Angiotensin II
Summary
Name the three main types of capillaries
Continuous
Fenestrated
Discontinuous
Capillary Density is important because the more metabolically demanding a tissue is, the………….. the capillary density
Skeletal muscle, myocardium/brain and lungs have a ……………… ……………. …………….
The myocardium and brain need a …………….. …………… …………… because they are particularly vulnerable to ……………
Adipose tissue, on the other hand, is poorly perfused because there is nothing that the adipose tissue does that needs a rapid response
Skeletal muscle has a …………. ……………… ………….. but a large number of these capillaries are SHUT OFF at rest
It is the …………… ……………which shuts off the arterioles/capillaries
Capillary Density is important because the more metabolically demanding a tissue is, the greater the capillary density
Skeletal muscle, myocardium/brain and lungs have a HIGH CAPILLARY DENSITY
The myocardium and brain need a high capillary density because they are particularly vulnerable to hypoxia
Adipose tissue, on the other hand, is poorly perfused because there is nothing that the adipose tissue does that needs a rapid response
Skeletal muscle has a high capillary density but a large number of these capillaries are SHUT OFF at rest
It is the PRECAPILLARY SPHINCTER which shuts off the arterioles/capillaries
MORE CAPILLARY DENSITY MORE BRANCHING
What type of capillary is found in the brain, lungs, skeletal and smooth muscle?
What type of capillary is found in the kidneys, villi of smooth intestine, Choriod plexus of the brain and endocrine glands?
What type of capillary is found in the red bone marrow, spleen , liver and anterior pituitary gland?
What type of capillary form a continuous tube except for intercellular clefts?
What tyoe of capillary has a plasma membrane that has fenestrations or pores?
What type of capillart has large fenestrations and an incomplete basement membrane?
Continuous
A vast number of capillaries have a CONTINUOUS structure
There are small water-filled gap junctions between the endothelial cells which allows the passage of electrolytes and other small molecules
MOST substances move through the endothelial cells
Fenestrated
These are leakier capillaries
They have slightly bigger holes called FENESTRAE which allow larger substances to pass through
Discontinuous
There are LARGE holes in the capillary
This is particularly important in the bone marrow where white cells have to get into the blood
What is bulk flow?
Bulk Flow = a volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
There are TWO main pressures which affect the movement of fluid in and out of the capillary:
Name these pressures
There are TWO main pressures which affect the movement of fluid in and out of the capillary:
Hydrostatic Pressure
Oncotic Pressure
These are STARLING FORCES
Oncotic pressure is an osmotic force due to protein in the capillary drawing water back in
The arteriolar end of the capillary has the highest pressure
As these are resistance vessels - as the blood moves through the capillaries and comes to the venular end, the pressure will DROP considerably
Oncotic pressure is THE SAME the whole way because the plasma protein concentration doesn’t really change
Depending on the balance of the two Starling forces you get movement of fluid in or out
Arteriolar end - hydrostatic pressure > oncotic pressure = fluid leaves the capillary
Venular end - oncotic pressure > hydrostatic pressure = fluid enters the capillary
Notice that there is 9 mmHg leaving the capillary and 8 mmHg re-entering the capillary so there is some fluid that doesn’t return to the capillary
This extra fluid is drained by the lymphatic system
Summary of Capillary
Circulation of the blood forms a closed loop but the lymphatic system is NOT a closed loop
You have lymphatic capillaries which are BLIND ENDED
The 1 mmHg net loss of fluid is drained by the lymphatics
There are valves in the lymphatic vessels which means that you can NOT get backwards flow
The movement of the fluid is driven by lymphatic pressure (e.g. muscle pressure, lungs inflating and deflating)
Lymphatic system also has an important immune surveillance role
The lymph then drains into the:
Thoracic Duct
Right Lymphatic Duct
Right Subclavian Vein
Left Subclavian Vein
3L of fluid is returned to the circulatory system by the lymphatic system per day
Oedema
Rate of production of fluid > Rate of removal of fluid = OEDEMA
Can be caused by parasitic blockage of lymph nodes
For example filaria can cause ELEPHANTIASIS
Lymphatics Summary
