Microcirculation Flashcards
What are the parts of the microcirculation?
1st order arteriole
terminal arteriole
pericytic venule
venule
capillaries
precapillary sphincter
smooth muscle
What is the overall aim of the cardiovascular system?
adequate blood flow through the capillaries
What is blood flow rate?
volume of blood passing through a vessel per unit time
How do you calculate flow rate?
Q= deltaP/ resistance
flow rate= pressure gradient/ resistance
What is the pressure gradient?
What is resistance?
Hindrance to blood flow due to friction between moving fluid and stationary vascular walls
How do you calculate resistance?
R= 8L/ r^4
l= vessel length
r= vessel radius
it can also be R= (8 x n x L)/ r^4
n= blood viscosity
What happens when you halve the radius?
flow decreases x16
What happens to pressure, resistance and flow when blood pressure increases?
pressure increases
flow increases
What happens to pressure, resistance and flow when there is arteriolar vasoconstriction?
resistance increases
flow decreases
Where does the biggest pressure drop occur?
between arterioles
What regulates the pressure within the system?
arterioles
How do you calculate flow in an organ?
What is special about the pressure gradient in tissues?
They are all the same
- only resistance varies
- it determines flow
What happens if there is no pressure difference?
Blood would not reach tissue capillary beds
Why is vascular tone important?
It allows the arteriole to constrict further or dilate
What happens during relaxation/ vasodilation to radius, resistance and flow?
radius increases
resistance decreases
flow increases
What happens during contraction/ vasoconstriction to radius, resistance and flow?
radius decreases
resistance increases
flow decreases
What is vascular tone?
arteriolar smooth muscle normally displays a state of partial constriction- aka the vascular tone
Radii of arterioles are adjusted independently to accomplish two functions, what are these 2 functions?
Function 1: Match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)
Function 2: Help regulate systemic arterial blood pressure
How is the “Match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)” regulated?
Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation
How is the “Help regulate systemic arterial blood pressure” regulated?
Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated
Explain in more detail the function “Match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)”.
This function may be chemically driven.
Increases metabolites and O2 usage.
Vasodilation of arterioles.
- skeletal muscles dilate to allow more nutrients etc. to a particular tissue
This is called active hyperaemia (chemically driven change)
Explain in more detail the function “Help regulate systemic arterial blood pressure”.
This function may be physically driven.
Decrease blood temperature.
Increase stretch (distension) due to increased BP.
Vasoconstriction of arterioles
- divert blood away
This is called myogenic autoregulation (physically driven change e.g., using an ice pack)
What happens to resistance, flow, perfusion with autoregulation and no autoregulation?
How do you calculate cardiac output?
blood pressure (MAP)/ Total peripheral resistance (TPR)
Q= MAP/ TPR
How does the neural aspect of the arteriole function “help regulate arterial blood pressure” work?
Neural
Cardiovascular control centre in the medulla
Leads to vasoconstriction
Leading to decreased blood flow in specific organs (you want this acutely as this can become dangerous)
How does the hormonal aspect of the arteriole function “help regulate arterial blood pressure” work?
vasopressin/ ADH
Angiotensin II
Adrenaline/ noradrenaline
they all lead to vasoconstriction
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]
What is the diameter of a capillary?
7 micrometers lumen diameter
What is the width of capillary wall?
1 micrometer width
How are capillaries designed to promote?
Ideally suited to enhance Fick’s law
Minimise the diffusion distance
Maximise the surface area and time for diffusion
Why is capillary density important?
highly metabolically active tissues have denser capillary networks
skeletal muscle= 100cm2/g
myocardium/ brain= 500cm2/g
lung= 3500cm2/g
What is a special feature of skeletal muscle?
skeletal muscle has a huge capacity, but limited flow at rest
- at rest most arterioles are closed off, doesn’t function normally
How does cardiac output from rest and exercise differ considering the different organs involved?
What do continuous capillaries have?
H2O filled gap junction
Where can you find fenestrated capillaries?
e.g., in the kidney
What are fenestrated capillaries?
larger gaps between endothelial cells
Where can you find discontinuous capillaries?
bone marrow
- exit route for WBC
liver
- large metabolic capacity
spleen, lymph nodes, and endocrine glands
What are discontinuous capillaries?
even larger gaps and pores
What are the 3 types of capillaries?
What type of capillary is the blood brain barrier?
Continuous
Really small junctions
tight control of what leaves the BBB and enters the brain
What is bulk flow?
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
What are the 2 forces acting on capillaries?
Hydrostatic “pushing” force
Oncotic “pulling” force
What is starling’s hypothesis about fluid movement?
“…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.”
What is the fluid movement for ultrafiltration?
pressure inside the capillary> pressure in the IF
What is the fluid movement during reabsorption?
inward driving pressures> outward pressures across the capillary
Does hydrostatic pressure change across the capillary?
Yes, it decreases
what happens to oncotic pressure across the capillary?
It stays the same
What is oncotic pressure?
form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel’s plasma that causes a pull on fluid back into the capillary.
What is the significance of the fact that ultrafiltration is more effective than reabsorption?
There is a net loss
THE ROLE OF THE LYMPHATIC SYSTEM
What are the parts of the lymphatic system?
Blood capillary
Arteriole
Lymphatic endothelium
Anchoring filament
Interstitial fluid
Openings
Tissue cells
Lymphatic capillary
What is the lymphatic system specialised to do relating to direction of flow?
designed to ensure fluid can only enter the lymphatic system and not leave
What are the different lymph nodes and ducts you need to know?
Right lymphatic duct
Entrance of thoracic duct into subclavian vein
Thoracic duct
Describe the lymphatic system.
There is no pump to induce flow
Drainage-right lymphatic duct- thoracic duct
Right and left subclavian veins
How many litres of lymph a day?
3L
How does lymph move around the body?
Negative pressure of lungs and contraction of skeletal muscle
How does an oedema form with lymph?
rate of production> rate of drainage
(too much fluid leaving capillaries staying in the space)
What causes elephantiasis?
parasitic blockage of lymph nodes
or sometimes rate of production> rate of drainage
What is the pressure entering and leaving arterioles?
93 mmHg entering
37 mmHg leaving
Why are capillaries so important?
critical exchange vessels for nutrients, signals and waste
How would you describe capillaries?
narrow, thin, highly branched
What is density in capillaries proportional to?
metabolic activity
How would you describe lymph vessels?
blind ended, single layered, contains large permeable water-filled one way channels
When is excess interstitial fluid drained?
constantly
What directs lymph flow?
skeletal and respiratory pumps (vessels coalesce to lower neck
What are lymph nodes for?
defence mechanism
What does lymphatic failure lead to?
oedema
