Biophysics and tissue fluid Flashcards
What is blood flow equal to?
Change in pressure/ resistance
What is the ‘change in pressure’ of blood flow?
Difference between the pressure at one end of the vessel compared to the other (between left ventricle and right atrium)
How does the length of the tube impact on resistance?
Why?
The LONGER the tube, the HIGHER the resistance
Cohesive forces and adhesive forces - blood sticks to the vessel wall
What has the biggest impact on blood flow?
Why?
RADIUS
Resistance = 1/R^4 Flow = change in pressure/ resistance
What vessels have the highest resistance?
Small arteries and aterioles
What is a key place where blood pressure can be treated?
The arterioles - target the receptors
What happens if blood pressure decreases substantially?
What is this called?
Tissues around the vessels will COMPRESS the blood vessel and cause them to COLLAPSE
Extravascular compression
What would occur to flow when increase the pressure, if vessels were RIDGED?
Increase on a LINEAR scale
What actually happens to flow when increase pressure in the vessel?
Why?
- At FIRST, nothing happens - pressure inside the blood vessel pushes against the extravascular compression
- Until get to a CRITICAL OPENING PRESSURE: pressure inside the blood vessels exceeds the pressure exerted by the extracellular tissue
- Forces the vessel open - blood flow through the tube
BECAUSE: vessels are elastic
What is the ‘law of laplace’?
What does it apply to?
Transmural pressure = Tension/radius
Applies to OPEN vessels
What is the transmural pressure?
Pressure ACROSS the blood vessel wall
from the INSIDE to the OUTSIDE
What happens to the Ptm when radius increase?
Ptm decrease and the SLOWER the blood flow
What happens to the Ptm when radius decrease?
Ptm increase and the FASTER the blood flow
How is tension calculated?
Tension = Pr/W
P = pressure r = internal radius W = wall width
How does the wall width impact tension in the blood vessel?
The smaller the wall width, the greater the tension
What is an anuerysm?
What causes this?
Ballooning of an artery where a segment of wall is weakened
Caused by the pressure of the inside of the vessel wall
What does the tension across an aneurysm depend upon?
How?
What happens when the aneurysm grows?
The RADIUS itself:
Smaller the aneurysm - The greater the pressure across the wall
When grows:
- Pressure across the wall becomes less
- Aneurysm more likely to burst
What is compliance a measure of?
How much ‘give’ - how much pressure within the blood vessel causes it to expand
How does compliance vary?
Varies between blood vessels
How is compliance calculated?
Change in volume / change in pressure
What are the capacitance vessels?
Why?
Veins
More COMPLIANT that arteries - stretch at lower pressures
- Can store more blood
What would happen to blood flow if the vessels were ridgid?
- Would eventually stop further away from the heart (pressure decrease)
- Pulsatile, discontinuous flow
What actually happens to blood flow far away from the heart?
Why?
Pulsatile CONTINUOUS flow
Due to ELASTIC RECOIL in the vessels - Windkessel effect (squeezes the blood to make it continue to flow)
Describe the flow rate in the aorta
Why?
Gets VERY high
Decreases RAPIDLY
(Very elastic)
Describe the flow rate in the pulmonary artery
Why?
More CONSTANT flow rate (not as high, doesn’t decrease rapidly)
Lower pressure system
What happens to the structure of the arteries as go lower down in the system?
- LESS elastic
- MORE muscular
Describe the flow rate in the pulmonary veins
Peak is VERY low and broad
Describe the flow rate in the renal artery
Why?
Peak even lower than the pulmonary veins
Very muscular
What determines if laminar or turbulent flow occurs?
1) DIAMETER of the vessel
2) SPEED of flow
What is laminar blood flow?
SLOW
SMOOTH
STRAIGHT
What are the consequences of laminar blood flow?
1) Blood in the centre of flow stay away from vessel walls - lower the ability to pick up oxygen
2) Layer of blood constantly in contact with the vessel wall
- cause blood clots
When does turbulent blood flow occur?
When the SPEED of flow increases:
- Blood hits the vessel wall and becomes turbulent
What are the ADVANTAGES of turbulent blood flow?
1) Cells in the CENTRE of flow come into contact with the vessel wall - pick up oxygen
2) No stasis of blood - prevent blood clots
What are the DISADVANTAGES of turbulent blood flow?
How is this overcome?
DAMAGE to red blood cells
Overcome as RBC are constantly made
Where does gas exchange occur?
In the capillaries
What is the structure of capillaries?
Thin wall (one cell thick) Fenestrations (gaps between cells)
What is the velocity in the capillaries?
Very low
What are the 2 states of the capillary?
What does this depend upon?
1) Active
2) Inactive (collapsed)
Which state depends upon the state of the PRE-CAPILLARY SPHINCTERS
What do the pre-capillary sphincters do?
CONTROL and DIRECT blood flow into or away from the capillary bed
What happens when pre-capillary sphincters are closed?
Blood is directed to OTHER parts of the body
What is the net fluid transfer from the capillaries to tissues a balance of?
What does the net fluid transfer form?
FILTRATION and ABSORPTION
Forms TISSUE FLUID (extracellular or interstitial)
How is fluid filtered through cells?
1) TRANSCELLULAR (Aquaporins)
2) PARACELLULAR (BETWEEN cells)
What 3 things does tissue fluid formation depend upon?
1) PRESSURE difference between the CAPILLARY and the INTERSTITIAL fluid
2) Difference in COLLOID OSMOTIC PRESSURE (COP)
3) CAPILLARY FILTRATION COEFFICIENT (CFC)
How does the pressure difference between the capillary and interstitial fluid influence tissue fluid formation?
- Higher the pressure
- Higher HYDROSTATIC pressure
- MORE fluid pushed out
What is COP formed by?
How does it influence tissue fluid formation?
The PROTEINS in the fluid (eg. Albumin)
- More proteins in the BLOOD - HIGHER the COP, LESS the fluid will leak out into the tissue
- Less proteins in the blood - LOWER the COP, formation of the tissue fluid is HIGHER
When are the proteins low in the blood?
When starving
What is the CFC?
What does it depend upon?
The ease of flow across a vessel wall
Dependant upon the gaps between the cells of the capillary
What determines how much tissue fluid is made?
Balance between the HP (hydrostatic pressure) and COP (colloid osmotic pressure)
Where is tissue fluid drained?
Into the lymphatic system and then into the great veins of the neck
When is tissue fluid formed?
High blood pressure:
- HP high - pushes fluid out through fenestrations
- COP is LOW
- More tissue fluid is formed
