Microcirculation Flashcards
What is the aim of the cardiovascular system?
Overall aim is adequate blood flow through capillaries
What is flow rate?
The volume of blood passing through a vessel per unit time
How do you calculate flow rate?
Flow rate= pressure gradient/ resistance
This is Darcy’s law
How do you calculate the pressure gradient?
Pressure A - Pressure B
Difference between pressures between start an end of arteriole
What effects blood flow?
Blood flow is affected by vessel length and radius and shown by Poiseuille’s law
In vasoconstriction: radius deceases, resistance increases, flow rate decreases
In vasodilation: radius increase, resistance decreases, flow rate increases
Why do radii of arterioles adjust?
- Match blood flow to metabolic needs of specific tissues
2. Help regulate systemic arterial blood pressure
How is blood flow matched to metabolic needs?
This is regulated by local (intrinsic) controls and is independent of nerves or endocrine stimulation
Increased metabolic activity leads to increased O2 usage and metabolite production so leads to vasodilation and active hyperaemia (increase in blood flow)
Its also may be driven by reduction in blood temperature and increased stretch due to increased BP (leads to vasoconstriction)- this is myogenic autoregulation
How is systemic arteriole BP regulated?
Regulated by extrinsic controls which travel via nerve or blood and are usually centrally coordinated
Change in BP is detected by cardiovascular control centre in medulla. This leads to vasoconstriction or vasodilation through neural control
A drop in BP can lead to vasopressin, angiotensin II, noradrenalin and adrenalin release which cause vasoconstriction through hormonal control
What is the purpose of capillary exchange?
To deliver metabolic substances to cells of organisms
Highly metabolic active tissue will have denser capillary networks
Denser capillary network will result in smaller diffusion distance and greater surface area so more efficient gas exchange
Lungs have greatest capillary density because gas exchange is crucial for sustainability of cardiovascular system
How are capillaries structured?
Junction between 2 vascular endothelial cells is a gap junction- not impermeable
Most capillaries are continuous (e.g. blood brain barrier)
Some vessels are fenestrated (have fenestra) which allows things smaller than ~80nm to move through
There is a continuous basement membrane underneath
Some capillaries have a discontinuous membrane- has breaks in membrane and is very leaky
What forces are found in capillaries?
A volume of protein- free plasma filters out of the capillary, mixes with surrounding interstitial fluid and is reabsorbed- bulk flow
There is hydrostatic ‘pushing’ force and an oncotic ‘pulling’ force (sucking blood back into capillary)
The balance between these forces determine fluid accumulation in these tissues
What is starling’s Hypothesis?
There must be a balance between the hydrostatic pressure of blood in capillaries and the osmotic attraction of the blood for the surrounding fluids
What effects fluid movement?
If pressure in the capillary is greater than pressure in interstitial fluid we get ultrafiltration
If inward driving pressure is greater than outward pressure across capillary we get reabsorption
Since ultrafiltration is more effective than reabsorption we get an accumulation of fluid in the interstitial space- the lymphatic system drains this
What’s the importance if lymphatic vessels?
Lymphatic vessels operate under a negative pressure
There is no pump to induce flow
If rate of production of lymph is greater than rate of drainage you get oedema- can lead to elephantiasis
