Microcirculation Flashcards
What does every organ have
Its own microcirculation
What is meant by the microcirculation
It is the branch of the arterial system that enters the tissues
Describe the arrangement of the microcirculation
▪ The blood flows through the arteries which branch off into the first order arterioles → terminal arterioles → capillaries.
▪ Capillaries carry out exchange functions and then the blood flows → post-capillary venules (pericytic) → venules → veins.
What is the overall aim of the CVS
Overall aim of CVS = Adequate blood flow
through the capillaries
What is meant by blood flow rate
Blood flow rate – Volume of blood passing through a vessel per unit time
How do we calculate pressure differences in the microcirculation
Pressure A (start of arteriole) - Pressure B (leaving the terminal arteriole) Greater difference= Greater flow rate
What is meant by resistance in the blood vessels
‘Hindrance to blood flow due to friction between moving fluid and stationary vascular walls’
How is Cardiac Output related to MAP and TPR
CO= MAP/TPR
What is meant by TPR
▪ TPR is the sum of resistance of all arterioles in the body.
What is the resistance dependant on
o Blood viscosity. o Vessel length. o Vessel radius (MOST IMPORTANT – Poiseuille’s Law).
Halving the radius decreases the flow 16 times
What is the effect of increasing blood pressure
Increases change in pressure
Therefore increasing flow
What is the effect of arteriolar vasoconstriction
Increased resistance
Therefore decreased flow
What is the velocity of blood flow inversely proportional to
The velocity of blood flow is inversely related to the total cross-sectional area. The branching nature of the circulatory system means that the total cross-sectional area of the capillaries is much greater than that of arterioles. This substantially reduces the velocity in the capillaries, allowing diffusion to take place.
In a single vascular bed, what is blood flow determined by. How does this compare to that of the whole vasculature
In a single vascular bed, the blood flow is determined by the arterial pressure - venous pressure (perfusion pressure) divided by the resistance to flow in the vascular bed.
As a whole, total blood flow= CO
And C0=MAP/TPR
What is the major determinant of blood flow in the body
This usually means the major determinant of the blood flow in the body is the resistance of the arterioles in the organ (radii). Arteriolar radius is controlled by numerous factors.
Describe the differences in resistance of the capillaries and arterioles
Total resistance in the vasculature is greatest in the arterioles, through a combination of their length and reduced radius without a significant change in their cross-sectional area.
Capillary resistance is less than arteriolar resistance because capillaries are shorter, large numbers of capillaries occur in parallel, and they have single-file flow as opposed to laminar flow. However, due to their smaller radius, a given length of capillary will have a greater resistance for the same length of arteriole.
What is the effect of blood viscosity on resistance
According to Poiseuille’s Law, resistance is proportional to viscosity.
Viscosity is influenced by plasma and cellular components of blood, but the haematocrit (percentage of red blood cells in the blood volume) is the main determinant
Describe the effect of the haematocrit on viscosity
An elevated haematocrit increases the carriage of oxygen, but increases viscosity (more friction of red cells with blood vessel wall) impeding flow and increasing cardiac work. Polycythaemia describes an increased haematocrit and it can be caused by a physiological adaptation to chronic hypoxia or a myeloproliferative disease increasing red cell production. Sever dehydration where blood volume is reduced without any changes in red cell number is also a cause
Describe plasma viscosity
Plasma viscosity is determined by the level of plasma proteins and is increased in conditions such as myeloma, in which there is increased production of immunoglobulin. In addition to increasing plasma viscosity, this increase in plasma proteins also promotes the aggregation of red blood cells, further increasing viscosity. The vessel diameter and rate of flow also influences red cell aggregation, and as a result, viscosity is different at different points of the vascular system.
What is pressure A and pressure B normally
▪ Blood entering the arterioles has a Mean Arterial Pressure (MAP) of 93mmHg. ▪ Blood leaving the arterioles has a MAP of 37mmHg (but depends on tissue bed)
When talking about flow to an organ, why do we assume that the change in pressure is equal to the MAP
Average pressure in arteries is MAP and in veins is ~0 so change in pressure is regarded as MAP.
Why is this pressure difference important
Without this pressure difference blood would not reach tissue capillary beds
What is the consequence of the change in blood pressure being equal to MAP
Blood flow to any tissue depends on the resistance of that organ (as blood pressure is the same)
What are the effects of arteriolar vasoconstriction
Decreased radius
Increased resistance
Decreased flow
What are the effects of arteriolar vasodilation
Increased radius
Decreased resistance
Increased flow
What causes vasoconstriction and vasodilation
Vasoconstriction- Smooth muscle contraction
Vasodilation- Smooth muscle relaxation
What is the ‘resting state’ of the arterioles
Arteriolar smooth muscle normally displays a state of partial constriction – this is called vascular tone
This allows the smooth muscle to both increase and decrease blood flow accordingly, it needs to be in a ‘halfway’ state.
Describe the functions of the changes in radii of the arterioles
Coordinated constriction or dilatation of a large proportion of arterioles will alter the TPR and thus arterial blood pressure
Constriction or dilatation of arterioles in single organs allows the distribution of the cardiac output to the different organs to be regulated. Dilatation of the arterioles in a particular organ will decrease resistance and increase blood flow to that organ.
Constriction or dilatation of arterioles influences the hydrostatic forces in the capillaries and thus has an effect on fluid filtration. Constriction of arterioles increases resistance and causes a greater pressure drop, reducing the pressure in the distal vessels, including the capillaries.
Describe how matching blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs) is regulated
Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation
This regulation may be chemically driven, or physically driven.
Describe how regulating systemic arterial blood pressure is regulated
Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated
This regulation can be neural or hormonal
Describe how vasodilation of the arterioles can be regulated chemically
The muscle becomes more metabolically active. Increased production of metabolites and increased oxygen usage.
These metabolites diffuse into the tunica media of the arterioles and cause relaxation of the vascular smooth muscle and thus vasodilation.
vasodilation occurs to to INTRINSIC responses. This is known as ACTIVE HYPERAEMIA.
To increase blood flow to that tissue bed, vasodilation of arteries and arterioles upstream also needs to occur. This is achieved by flow-mediated vasodilation in which the shear force on the endothelial cells stimulates them to release factors that cause relaxation of adjacent smooth muscle cells (increased flow increases shear force)
What is meant by active hyperaemia
Active Hyperaemia – an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue.
Describe the physical factors that can lead to a change in radii of arterioles to alter the blood flow to that tissue to match metabolic needs
the tissue responds to local changes in temperature → vasoconstriction so less heat is lost in the blood flow (as less blood is flowing past the cold area).
The increased blood pressure increases the stretch (distension) of the blood vessel wall.
This is called myogenic autoregulation.
What is meant by myogenic autoregulation
raised BP increases stretch on cells which stimulates myogenic vasoconstriction
This decreases flow and increases resistance. This occurs in response to increased perfusion pressure when the tissues don’t require an increased flow.
Describe the neural regulation of vasoconstriction and vasodilation
Sympathetic vasoconstrictor nerves innervate the smooth muscle of arterioles and veins. In arterioles, a basal activity of these nerves is responsible for vessel tone at rest. The neurotransmitter involved is noradrenalin which acts on alpha 1 receptors causing vasoconstriction.
A decrease in sympathetic nervous activity has the opposite effect, causing vasodilation
Describe the parasympathetic vasodilator nerves
Innervate the blood vessels of the: Skin Genitalia Salivary glands Pancreas GI mucosa The effect of these nerves on TPR is small because of their limited innervation Under normal circumstances in most vascular beds, it is decreases sympathetic activity acting on alpha 1 adrenoreceptors that brings about vasodilation
Where are these neurons found
Cardiovascular control centre in the medulla
When is it important to vasoconstrict
In times of blood loss, vasoconstrict to increase TPR and thus MAP, to maintain MAP. Prolonged vasoconstriction has negative consequences by reducing flow to the tissues (especially the brain).
Describe the hormonal control on MAP
the brain can mimic the SNS to control the heart by stimulating production of A and NE. i. Different hormones such as vasopressin, angiotensin II, adrenaline and noradrenaline can all help regulate arterial BP, by increasing vasoconstriction.
NE and A bind to alpha 1 receptors on smooth muscle, resulting in vasoconstriction.
What is the priority of the hormonal system in times of need
To preserve flow to the brain
What is the purpose of capillary exchange
The purpose of capillary exchange is the delivery of metabolic substrates to the cells of the organism [which is the ultimate function of the CVS
Describe the dimensions of the capillaries
The capillaries are around 7 microm wide and the wall is 1 microm thick.
Why is it important that the capillaries are branches
To ensure that every cell in the body is relatively close to a capillary to deliver metabolites to every cell to allow them to function. Why capillary density is important.
Why do highly metabolically active cells have
A denser capillary network.
Skeletal muscle = 100cm2/g
Myocardium/brain = 500cm2/g
Lung = 3500cm2/g
Describe Fick’s law
▪ The capillaries are around 7m wide and the wall is 1m thick. ▪ The capillaries are ideally suited to enhance diffusion as described by Fick’s Law (minimise diffusion distance and time, maximise surface area).
Describe pre-capillary sphincters
Blood flow through these capillaries is controlled by small rings of smooth muscle called ‘pre-capillary’ sphincters. Important in skeletal muscle to prevent large volumes of blood being wasted.
Skeletal muscle shuts off the majority of its capillaries via the precapillary sphincters (~10% flow) in absence of exercise
Describe continuous capillaries
Endothelial cells arranged in a line, separated by small water filled gap junctions.
The majority of capillaries: ▪ Small water-filled gap junctions to allow passage of electrolytes. ▪ Most substances move THROUGH cells.
Describe fenestrated capillaries
Have holes called fenestrae (~80nM wide). Allows filtering of the blood. Bigger things than normal to leave.
Describe discontinuous capillaries
Large holes which is used in places like the bone marrow for passage of leukocytes. Also in liver, to allow easy access for metabolites.
Describe the blood brain barrier.
In the brain, there is a continuous formation with VERY tight gap junctions. Allows for a much tighter control regarding what enters the brain tissue.
Describe the role of P-Glycoprotein
Transporter protein that removes invading substances that enter the brain, such as drugs. Provides more protection.
What do certain parts of the brain have
Not all capillaries in the brain have a tight junction, these have a normal continuous structure, need to access the vasculature more easily.
What is meant by bulk flow
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
Describe the starling forces that regulate fluid movement in the capillaries
o Hydrostatic Pressure – from the heart. o Oncotic Pressure – from the osmotic pressure
Describe Starling’s hypothesis
“…there must be a balance between the hydrostatic pressure of the blood in the capillaries and the osmotic attraction of the blood for the surrounding fluids. “
” …and whereas capillary pressure determines transudation, the osmotic pressure of the proteins of the serum determines absorption.”
When does ultrafiltration occur
If pressure inside the capillary > in the IF - Ultrafiltration.
Net ultrafiltration occurs at the arterial end
When does reabsorption occur
If inward driving pressures > outward pressures across the capillary – Reabsorption
Net reabsorption occurs at the venous end
What is the significance of the fact that ultrafiltration is more effective than reabsorption?
∆1 – Net loss
Role of Lymphatic system
To return this fluid lost, otherwise blood volume and hence BP would decrease throughout the day
Describe the components of lymph vessels
▪ Lymphatic vessels are BLIND ENDED (not a closed loop). ▪ Valves exist to prevent backflow.
What is the function of the lymphatic system
Fluid, proteins and fat globules not reabsorbed into capillaries are brought back into the vascular system through the lymphatic system.
▪ 3L of fluid is returned to the circulatory system/day.
Describe the network of the lymphatic system
There is a network of lymphatic capillaries, ducts and lymph nodes which unit to form the thoracic duct, which drains the collected fluid into the left subclavian vein.
Lymph drains into the: o Thoracic Duct. o Right Lymphatic Duct. o Right Subclavian Vein. o Left Subclavian Vein.
Describe the mechanics of the lymphatic system
The is no pump (heart) to induce flow!
Movement of fluid is driven by lymphatic pressures such as muscle pressures and thoracic cavity pressures, as well as the intestinal peristalsis. Extrinsic propulsions.
What drives the IF into the lymph capillaries
The hydrostatic pressure of the IF
Describe oedema
If rate of production > rate of drainage
then OEDEMA ensures
This can be caused by parasitic blockage of the lymph nodes or removal of the lymph nodes. o E.G. Elephantiasis
Tissue damage can make the capillaries leakier.
Describe how substances can be transported across the capillaries
Lipid soluble- endothelium
Small, water soluble- gap junction
Big, water soluble- vesicular transport (active)
