7. Microcirculation Flashcards
Microcirculation
Every organ in body has its own microcirculation
Why do we regulate blood flow to each microcirculation?
Different amounts of blood go to different tissues dependent on need
What equation connects blood flow, pressure gradient and resistance?
Flow = pressure gradient/ resistance
What factors affect vascular resistance?
Vessel radius
Vessel length
Blood viscosity
What is the relationship between resistance and radius?
Poiseuille’s law: resistance is inversely proportional to r^4
Halving the radius decreases the flow 16x
What are the major resistance vessels?
Arterioles
What effect does increasing blood pressure have on blood flow?
Increases pressure gradient
Increases flow
What effect does arteriolar vasoconstriction have on blood flow?
Decreases radius of arterioles supplying tissue
Increases resistance
Decreases flow
What is usually the pressure at the start of the arteriole?
MAP ~ 93 mmHg
What is usually the pressure at the end of the arteriole?
Changes depending on which tissue bed,
but often ~37 mmHg
Why can change in blood pressure be substituted by MAP in the flow equation for an organ?
Arterial BP always MAP
BP in veins is usually ~0 mm Hg
so the change in BP through a capillary bed is ~ MAP.
What is the normal state of vascular smooth muscle? Why?
Normally in a state of partial vascular constriction (vascular tone)
Allows further constriction or dilation
What are the 2 controls of vessel radius?
Intrinsic controls (independent of nervous/ endocrine stimulation): match blood flow to metabolic needs of specific tissue Extrinsic controls (via nerves/ blood, centrally coordinated): regulate systemic arterial BP
Describe how vessel radius responds to the chemical environment.
When tissues are highly metabolically active they will produce a lot of ATP and use up a lot of O2.
Increased uptake of O2 is detected by the tissues, which sends a message to the arteriolar smooth muscle to dilate.
= active hyperaemia.
Describe how vessel radius responds to the physical environment.
Decrease in blood temperature causes vascular smooth muscle to constrict so that less blood reaches the surface and so less heat is radiated away.
Describe myogenic autoregulation
Start exercising BP increases
Increases pressure gradient across every tissue bed
Can’t happen as not enough blood to perfuse every tissue in the body
(Prioritise muscles, brain, heart. Divert away from gut)
So you get myogenic autoregulation in less prioritised places e.g. gut
Initially causes increased flow to tissue
Sensed by stretch receptors in arterioles
Signal to arterioles to constrict to decrease blood flow back to normal levels
How can the flow rate, pressure gradient and resistance equation be applied to the entire circulation?
Flow rate is CO
Pressure gradient is MAP
Resistance is TPR
CO = MAP / TPR
What are the 2 pathways controlling arterial blood pressure?
Neural
Hormonal
Where is the centre that regulates arterial blood pressure found?
In the medulla:
cardiovascular control centre
What are the main vasoconstrictor hormones?
Vasopressin
Angiotensin II
Adrenaline
Noradrenaline
What is capillary exchange?
Delivery of metabolic substrates to the cells of an organism
What tissues have a high capillary density?
Skeletal muscle
Myocardium
Brain
Lungs
Why is capillary density important?
Need to ensure every cell is close to a capillary
Ideally suited to enhance diffusion:
Minimise diffusion distance
Maximise SA and time for diffusion
What is the function of pre-capillary sphincters and where are they found?
Skeletal muscle
Allow certain capillaries to be ‘shut down’ at rest, otherwise vast amounts of blood would flow to skeletal muscle unneccesarily
What are the 3 main types of capillary and how do they differ?
CONTINUOUS: small water filled gap junctions that allow the passage of electrolytes and small molecules
FENESTRATED: slightly bigger gaps allowing slightly larger molecules to pass through
DISCONTINUOUS: large gaps in the capillary
What is the most common type of capillary?
Continuous
Where are fenestrated capillaries found and why?
Glomerulus
need to filter molecules of a certain size
Where are discontinuous capillaries found and why?
Bone marrow: WBCs need to exit bone marrow tissue into blood
Liver: deals with metabolism, need to allow substances to access liver tissue to be metabolised
How is the blood brain barrier different to other capillaries and why?
Blood brain barrier is a continuous capillary structure with tight junctions instead of gap junctions, giving brain much tighter control over what accesses brain tissue
If very lipid soluble, will access brain anyway
If water soluble, will need a protein transporter
Brain is more protected
What is the difference between hydrostatic and oncotic pressure in capillaries?
Hydrostatic = ‘pushing’ force Oncotic = ‘pulling’ force
Why is oncotic force needed?
BP generated by the heart forces blood through capillary at relatively high speed
Would cause fluid to be squeezed out into tissues and BP would decrease
Starlings forces
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
Describe the pressure changes across a capillary
Pressure drops across capillary
So hydrostatic pressure is much higher at beginning than end
No protein outside capillaries, but lots in, so oncotic pressure remains constant
As blood enters capillaries, net force pushes fluid out
At end, net force pushes fluid back in
Ultrafiltration
If pressure inside the capillary > in the IF
Reabsorption
If inward driving pressures > outward pressures across the capillary
What is the significance of the fact that ultrafiltration is more effective than reabsorption?
Always losing fluid from blood system, if there wasn’t a mechanism to draw fluid back in BP would just keep dropping
necessitates lymphatic system
Describe 5 characteristics of the lymphatic system.
Consists of blind-ended lymphatic capillaries
Next to blood capillaries
Valves - prevent backflow
All but the right upper quadrant (RUQ) of the body drains via the thoracic duct into the left subclavian vein
RUQ drains into the left subclavian vein
What are the key roles of the lymphatic system?
Return excess fluid back into blood
Immune surveillance system: If infection present in fluid in lymphatic capillaries, detected when passes through lymph nodes, activates production of lymphocytes
What ensues if rate of production (amount of fluid lost into tissues) > rate of drainage?
Oedema
Volume of lymph fluid produced per day
3L
