Microcirculation Flashcards
What is the overall aim of the CVS?
Adequate blood flow
through the capillaries
What is the microcirculation?
- Arterioles
- Capillaries
- Venules
What is blood flow rate?
Volume of blood passing through a vessel per unit time
How do you calculate flow rate?
Q (flow rate) = Pressure gradient / Resistance
What is pressure gradient?
Pressure gradient (DP) = Pressure A – Pressure B
What will an increase in pressure gradient do?
Increase flow rate
What is resistance?
Hindrance to blood flow due to friction between moving fluid and stationary vascular walls
What impacts resistance?
- Vessel length
- Vessel radius (changes)
- Blood viscosity
What happens in increased blood pressure?
Pressure gradient: increase
R:
F: increase
What happen in arteriolar vasoconstriction?
Pressure gradient:
R: increase
F: decrease
When is the biggest pressure difference?
One end of arteriole to another
How do you calculate flow rate for an organ?
Forgan = pressure gradient (MAP) / Rorgan
-Without this pressure difference blood would not reach tissue capillary beds
What happens in vaso contraction?
r: decrease
R: increase
F: decrease
What happens in vasodilation?
r: increase
R: derease
F: increase
-
Why is vascular tone important?
- Arteriolar smooth muscle normally displays a state of partial constriction (this is vascular tone)
- If you need capacity to both dilate and contract (can contract further or dilate)
How many ways can radii of arterioles adjust independently?
Radii of arterioles are adjusted independently to accomplish two functions
What is the first function? How is this regulated?
- Function 1: Match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)
- Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation
What is the second function? How is this regulated?
- Function 2: Help regulate systemic arterial blood pressure
2 Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated
How does the arterioles match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs) chemically?
- If more active: function may be chemically driven
- increase metabolites + O2 usage
- Therefore vasodilation of arterioles (tissue dilates)
- Called active hyperaemia
How does the arterioles match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs) physically?
- function may be physically driven
1. Decrease blood temp
2. Increase stretch (distension) due to increase BP - Therefore vasoconstriction of arterioles
- Called myogenic auto regulation
What happens if BP increases?
- Flow to nay tissue bed increases
- As blood flow increases it stretches
- If tissue realises it doesn’t need this responds and contracts
- To ensure that blood doesn’t increase to every tissue
In exercise what happens?
- Skeletal muscle arterials go to active hyperaemia
2. Small intestine arterioles go toward myogenic vasoconstriction
Generate and equation for flow across the whole circulation and rearrange to generate a BP equation?
Cardiac output (Q) = Blood pressure (MAP) / totally peripheral resistance (TPR)
How do arterioles help regulate arterial blood pressure (centrally)?
- Cardiovascular control centre in medulla (e.g. loss of blood) for vasoconstriction (to maintain BP)
- This would decrease blood flow to specific organs as vasoconstriction decrease flow
- Brain is no1 💓
What hormonal controls cause vasoconstriction?
- Vasopressin/ADH
- Angiotensin II
- Adrenalin/noradrenaline
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 structure of capillaries?
- 7 muM lumen diameter
- 1 muM cell width
- Highly branched network
- Thin wall
- No tissue to far away
Why is capillary density important?
- Different in different tissue beds
- Ideally suited to enhance diffusion Fick’s law
- Specially designed to:
1. Minimise the diffusion distance
2. Maximise the surface area and time for diffusion
What do different tissues need capillary density?
- Highly metabolically active tissues have denser capillary networks
- Skeletal muscle = 100cm2/g
- Myocardium/brain = 500cm2/g (metabolically)
- Lung = 3500cm2/g (for efficiency)
- Skeletal muscle has a huge capacity, but limited flow at rest
What are continuous capillaries?
- one cell thick
- very small H2O filled gap junction
- Diffuse across cell or gap
- Most common
What is fenestrated capillaries?
- Lots of fenestrae
- Larger holes around 80nM
- In glomerulus
What is discontinuous capillaries?
- Larger and less common
- Bone marrow and in liver
What type of capillary is in blood brain barrier?
-Continuous for protection and continue with no gap junctions so difficult to leave and enter -not lipid soluble won’t get in
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
- Bulk Flow
What are the different forces with blood flow?
- Hydrostatic ‘pushing’ force (forced out of capillary)
- Oncotic ‘pulling force’ (to return blood, more protein in blood), draws fluid back in
What is Starlings hypothesis?
Starling’s hypothesis (1896)
“…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 happens
If pressure inside the capillary > in the IF?
Ultrafiltration
What happens If inward driving pressures > outward pressures across the capillary?
Reabsorption
What is the significance of the fact that ultrafiltration is more effective than reabsorption?
Overall net loss, role of lymphatic system (return fluid back)
What is the lymphatic system?
- Present in all tissues and are blind-ended, single-layered and contain large permeable water-filled one-way channels
- Drain excess interstitial fluid constantly – skeletal and respiratory pumps direct lymph flow – vessels coalesce to lower neck
- Nodes = defence mechanism, lymphatic failure -> oedema
