6.6: Microcirculation Flashcards
Calculation for pressure gradient to work out blood flow to capillary
(P) = pressure A - pressure B
(Pressure A - arteriolar pressure)
(Pressure B - capillary network pressure)
Overall aim of cardiovascular system is
Adequate blood flow through capillaries
Blood flow rate is
Volume of blood passing through a vessel per unit time
Darcys law calculation and calculating flow rate
( P = Q x R )
Q = P/R (flow rate = pressure / resistance)
What does an Increase in pressure gradient do to the flow rate
Increases flow rate
Resistance (R) is
Hindrance to blood flow due to friction between moving fluid and stationary vascular walls
3 factors affecting resistance
Vessel length
Vessel radius
Blood viscosity
Impact of vessel radius on resistance
Vessel radius can change rapidly, therefore has the greatest impact on resistance
Halving radius decreases flow x16
What happens to pressure gradient as blood pressure increases
Pressure increases
What happens to pressure during arteriolar vasoconstriction
Remains the same
What happens to resistance during increased blood pressure
Stays the same
What happens to resistance during arteriolar vasoconstriction
Resistance increases
What happens to flow as blood pressure increases
Flow increases
What happens to flow during arteriolar vasoconstriction
Flow decreases
Calculation of blood flow or any organ
Blood flow = change in pressure/ resistance against entire organ
What would happen without a pressure difference to the blood
Blood would not reach tissue capillary beds
How does vasoconstriction affect radius, resistance and flow
Radius decreases
Resistance increases
Flow decreases
How does vasodilation affect radius, resistance and flow
Radius increases
Resistance decreases
Flow increases
Why is it important for smooth muscle to display a state of partial constriction (vascular tone)?
Vascular tone allows for dilation and constriction to regulate blood flow
Why are the radii of arterioles adjusted independently
To accomplish two functions :
Match blood flow to metabolic needs of specific tissues
Help regulate systemic arterial blood pressure
What is matching blood flow to metabolic needs regulated by
Local (intrinsic) controls, and independent of nervous or endocrine stimulation
What is regulating systemic arterial blood pressure controlled by
Extrinsic controls which travel via nerves or blood and are usually centrally coordinated
What is active hyperaemia
Tissue starts working harder:
Increased metabolites and O2 usage
Vasodilation of arterioles
-chemical
What is myogenic autoregulation
E.g twisting ankle
Aim is to induce vasoconstriction
Decrease blood temperature
Increase stretch (distension) due to increased blood pressure
Decrease blood flow
What is stretch distension due to increased blood pressure
Pressure gradient across every micro circulation increases
In tissues which are not being used- autoregulation occurs as increase in arteriole pressure is detected: arterioles constrict to regulate blood flow
Allowing blood flow to heart, brain, lungs
Skeletal muscle arterioles during exercise undergo
Active hyperaemia
Small intestines arterioles during excersise undergo
Myogenic vasoconstriction
Equation for flow across whole circulation
Cardiac output (flow)= blood pressure/total peripheral resistance
How is blood pressure regulated in many tissue beds at once
Cardiovascular centre in medulla sends signal via sympathetic nervous system which innervates arterioles and causes them to constriction
Why does blood flow to specific organs decrease as arterial blood pressure is regulated within the body
Baroreceptors detect decrease in blood pressure, decreasing flow to blood vessels as vasoconstriction occurs only in specific tissues which preserve blood pressure
3 Hormones that control and regulate arterial blood pressure
Vasopressin
Angiotensin
Adrenaline
Leading to vasoconstriction
Why is capillary density important
Ideally suited to enhance diffusion
Specially designed to minimise diffusion distance and maximise surface area and time for diffusion
What is the purpose of capillary exchange
Delivery of metabolic substrates to the cells of the organism - ultimate function of CVS
Capillary networks in highly metabolically active tissues
Denser capillary network
(e.g lung has the highest capillary network to allow most gas diffusion, followed by myocardium/brain and skeletal muscle)
Biggest tissue in body, and it’s characteristic of blood flow
Skeletal muscle
Huge capacity but limited flow at rest
Continuous capillary structure
Single file endothelial cells with water filled gap junctions between them
Small molecules able to pass through gaps
Fenestrated capillary structure
Endothelial cells have greater gaps between them allowing larger molecules to pass through
e.g gut
Discontinuous capillary structure
Larger gaps between endothelial cells
E.g bone marrow and liver
What type of capillary is in the blood brain barrier
Continuous with no gap junctions
How does blood flow into and out of capillaries
Blood forced into capillary at high pressure due to hydrostatic force, some fluid is forced out of gaps between capillaries
What force pushes blood into capillaries
Hydrostatic pushing force
What force returns blood to the capillary
Oncotic pulling force (osmotic)
Bulk flow hypothesis
Volume of protein free plasma filters out of capillary, mixes with surrounding interstitial fluid (IF) and is reabsorbed
Starlings hypothesis
Net fluid movement - hydrostatic pressure and oncotic pressure
What occurs when the pressure inside the capillary is greater than in the IF
Ultrafiltration
What occurs when the inward driving pressures exceed the outward pressures across the capillary
Reabsorption
What is the significance of ultrafiltration being more effective than reabsorption
Role of lymphatic system
Role of lympathic system
Venous return - in one direction
Lymphatic capillaries collect tissue fluid and return it to circulation
Where does the lymphatic system connect to the circulation
Right lymphatic duct - thoracic duct
Right and left subclavian vein
Main characteristic of lymphatic system
No pump to induce flow
3L of fluid returned per day
Why do lymph nodes swell during illness
Bacteria and virus in tissue fluid passes through lymph nodes, immune system activated in nodes, swelling occurs and bacteria and viruses targeted
When does oedema ensure
When rate of production is greater than rate of drainage
Elephantiasis is
Parasitic blockage of lymph nodes
Characteristics of lymphatic system
Blind - ended, single-layered and contain large permeable water filled one way channels
What is capillary density proportional to
Metabolic activity
Arterioles are the
Major resistance vessels
Intrinsic factors affecting arterioles
Chemical and physical (metabolic activity and stretch) to match blood flow to needs
Intrinsic factors affecting atterioles
Neural and hormonal (SNS activity and adrenaline) to regulate arterial blood pressure
Smooth muscle tone affects
Radius, which affects flow
