Microcirculation Flashcards
What is the overall aim of the cardiovascular system?
Adequate blood flow through the capillaries
What is the definition of the blood flow rate?
The volume of blood passing through a vessel per unit of time
What is the equation which describes the change in pressure in a fluid circuit?
Change in pressure (between A and B) =
Flow rate (Q) x Resistance (R)
What is the pressure gradient?
The difference in pressure between Point A and Point B
(P when blood enters the 1st order arteriole - P when blood enters capillaries)
What affect will increases in flow rate have on the pressure gradient?
It will increase the pressure gradient
Flow rate is proportional to…
Pressure gradient
Flow rate is inversely proportional to…
Resistance
What is the definition of resistance?
Hindrance to blood flow due to friction between moving fluid and stationary vascular walls
What is the equation for the resistance experienced in a vessel?
What part of the resistance equation is the most variable and can change from minute to minute?
The radius of the vessel
Why is having a pressure difference in the organs essential?
Otherwise blood would not reach tissue capillary beds
During vasoconstriction, what happens to the pressure gradient, radius, resistance and flow across the vessel?
No change to pressure gradient
Radius decreases
Resistance increases
Flow to capillaries decreases
During vasodilation, what happens to the radius, resistance and flow across the vessel?
No change to pressure gradient
Radius increases
Resistance decreases
Flow to capillaries increases
What does increasing BP do to pressure gradient, resistance of the vessel and flow to capillaries?
Increases pressure gradient
No change to resistance
Increases flow to capillaries
What is meant by vascular tone?
The arteriolar smooth muscle normally displays a state of partial constriction
Why is vascular tone important?
If the smooth muscle is in a contracted state, then the arteriole wall can contract or dilate - if it was fully dilated at rest, it could only constrict, not dilate further
What does vascular tone affect?
Radius of the vessel
What is meant by active hyperaemia?
When the blood flow to a specific tissue is matched to its needs and is regulated by chemical changes
What chemical changes can drive active hyperaemia?
Increased oxygen usage and increased metabolites
What affect does active hyperaemia have on the arteries?
Causes them to vasodilate
What is myogenic autoregulation?
When the blood flow to a particular organ is matched by its needs, and is dependent on physical stimuli
What physical stimuli could trigger myogenic autoregulation?
- A drop in blood temperature
- High blood pressure stretching artery wall
What effect does myogenic autoregulation have on the arteries?
causes the arteries to constrict
Through what mechanism are the radii of arteries adjusted after a big meal?
Through active hyperaemia in the gut
What is the mean arteriole pressure for 120/80 mmHg?
MAP = DBP + 1/3(PP)
MAP = 80 + 1/3(40)
93 mmHg
What happens to the flow of blood through an arteriole following a rapid increase?
- When the flow increases, so does the stretch on the arterioles
- Therefore, this physical stimuli is detected by the arterioles, which then response through myogenic auto-regulation to bring the flow back to normal
What is laminar blood flow like?
- Velocity of blood is constant at any one point and blood flows in layers
- Blood flows fastest at the centre of the lumen
What is turbulent blood flow like?
- Blood flows eratically, forming eddys, and is prone to pooling
- Associated with pathophysiological changes to blood vessel endothelium
What is turbulent blood flow more likely to cause?
Activate clotting factors and produce a thrombus
How is turbulent flow used to measure blood pressure?
Slow deflation of the cuff causes turbulent flow which can be heard with a stethoscope
What is the skeletal muscle arteriolar response to exercise?
Active hyperaemia
What is the small intestine arteriolar response to exercise?
Myogenic vasoconstriction
The radii of the arterioles are adjusted independently to accomplish which two functions?
- Match blood flow to the metabolic needs of the tissue
- Regulate systemic arterial blood pressure
Through what two mechanisms do the aterioles help regulate arterial blood pressure?
Hormonal and Neural
How does the neural control of blood vessel radii occur?
The cardiovascular control center in the medulla sends signals to vasocontrict to increase blood pressure
Which hormones are involved in vasoconstriction?
Vasopressin/ADH
Angiotensin II
Adrenaline/noradrenaline
Where can the greatest pressure drop in the CVS be observed?
From one end of the arteriole to the other
What is the equation linking mean arterial pressure, cardiac output and total peripheral resistance?
MAP = CO x TPR
What is the purpose of capillary exchange?
The delivery of metabolic substrates to the cells of the organisms
What is the lumen diameter of a capillary?
7 micrometers
What is the cell width of a capillary?
1 micrometer
What are capillaries specially designed to do?
Minimise the diffusion distance
Maximise surface area and time for diffusion
Which tissues have highly dense capillary networks?
More metabolically active tissues
Which tissue has a large capacity but limited flow at rest?
Skeletal muscle
Why does the lung have such a dense capillary network?
Because of gas exchange which occurs in the lungs
What percentage of arterioles are active at rest in skeletal muscle?
10%
What happens to the arterioles during exercise in sketeal muscle?
They vasodilate due to active hyperaemia
What is the definition of bulk flow?
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid and is then reabsorbed
What is the difference in CO of skeletal muscle when at rest and during exercise?
It increases by 5x
What are the very small gaps between endothelial cells in continuous capillaries called?
H2O filled gap junctions
What substances can move across the H2O filled gap junctions?
Very small molecules like sodium ions
What are fenestrated capillaries?
Where you have little gaps/holes in the capillaries
Where are fenestrated capillaries found?
In the glomerulus, intestinal villi and endocrine glands
What are some tissues in which continuous capillaries are found?
Fat
Muscle
Nervous System
In which capillary can glucose pass through the wall without the need for glucose transporters?
Fenestrated capillaries
What are discontinuous capillaries?
Where there are large caps in the endothelial cells to allow large molecules to pass through
Where might you find discontinuous capillaries?
In the liver, bone marrow and spleen
What type of capillary structure forms the blood brain barrier?
Continuous, without water filled gap junction
What force pushes plasma out the capillary?
Hydrostatic pushing force
What force causes plasma to be reabsorbed?
Oncotic pulling force
What acts as an oncotic force to pull plasma back into the capillaries?
Plasma proteins in blood vessels
What determines transudation of fluids?
Capillary pressure
What occurs if pressure inside the capillaries was greater than in the interstitial fluid?
Ultrafiltration
What occurs if inward driving pressures are greater than outward pressures across the capillaries?
Reabsorption
What is the significance of the fact that ultra-filtration is more effective than reabsorption?
More fluid is lost into the surrounding tissue than is gained - therefore the excess fluid which is lost is picked up by lymphatic vessels and returned to circulation to maintain blood pressure
What drives the lymphatic system given there is no central pump?
Pressure differences (results in flow)
Where do the lymphatics drain into?
Into the right lymphatic duct and thoracic duct
How much Lymph is drained a day?
3L
What occurs if the rate of lymph production is greater than the rate of lymph drainage?
Oedema – swelling due to interstitial fluid buildup
What is elephantiasis?
- A parasitic disease which arises due to the blockage of lymph nodes
- Thus rate of production exceeds rate of drainage, resulting in oedema
What is systemic arterial blood pressure regulated by?
Extrinsic controls which travel via nerves or blood
How does renin regulate blood pressure?
- Renin from adrenal gland converts angiotensiogen to angiotensin I
- ACE in the vasculature of lungs and kidneys converts angiotensin I to II
- Angiotensin II increases blood volume and vasoconstriction, increasing BP
How do ADH and adrenaline regulate blood pressure?
- Both increase water reabsorption in the kidney
- ADH binds to vascular smooth muscle to stimulate vasoconstriction
