Blood vessels, pressure and flow Flashcards
Structural features of arteries
Thick outer layer of collagen fibres
Thick inner layer of muscle and elastin
Endothelium (made of endothelial cells)
Narrow central lumen
Structural features of veins
Thin outer layer
Thin inner layer of muscle and elastin
Endthelium
Wide central lumen
Valves
Functions of endothelial cells:
Local blood pressure control
Prevent platelet aggregation and blood clot formation
Angiogenisis and vessel remodelling, Permeability barrier for nutrients/ fluids
Can release constrictors and dilators
Can influence proliferatie state of smooth muscle
Can release free radicals that oxidise LDL or be damaging
Express molecules with tether inflammatory cells
Function of smooth muscle:
Present in all vessels excluding smallest capillaries
Determines radius - contracts/ relaxes
Secretes ECM (gives elastic properties)
Multiplies in some disease
What is the importance of compliane/ arterial elasticity?
Allows large arteries to act as pressure reservoirs, prevents pressure falling to 0 as blood leaves arteries
How are the different blood vessels specialised to their function?
Aorta and arteries: small amount of blood at high pressure -> very thick walled and elastic allowing compliance
Arterioles: variable resistance system distributing blood -> dissipate most of pressure, protects capillaries which aren’t designed for high pressure
Capillaries: vast suface area for exchange of substances -> one cell thick
Venules, veins and venae cava: collecting and resevoir system containing most of blood at low pressure -> very distensible
How to calculate mean arterial blood? pressure? What is it?
DP + 1/3 (PP)
Pulse pressure = difference between DP and SP
MAP = average pressure pushing blood around system
How is does fluid move in and out of capillaries?
Arterial end: fluid moves out
- Hydrostatic pressure (blood exerting pressure against capillary walls) greater in capillary than out greater than colloid pressure (plasma proteins in plasma mean inside capillary is greater)
Venous end: fluid in
- Hydrostatic pressure reduced and is lower than colloid pressure
What are the exeptions to normal transport of fluid?
In lung capillaries:
- Pulmonary hydrostatic pressure much lower than systemic pressure so is equal to colloid pressure
- No net fluid transfer (if anything fluid moves from alveoli into blood)
- Prevents pulmonary oedema
How does lymphatic system work?
Prevents oedema from fluid not returned to blood as lymph capillaries and vessesl move fluid back to blood.
Are able to contract to move it back
System also responsible for moving absorbed fat into circulation
Central control of blood pressure:
Able to alter TPR which impacts MAP (MAP = CO x TPR)
- Sympathetic nerves on some vessels release noradrenaline
- NA binds to alpha 1 receptors
- Vascular muscle contracts
- TPR increases
- Blood pressure increases
How to prazosin and phentolamine use the process of central blood pressure control to act as drugs?
Prozosin: inhibits alpha 1 receptors, decreases vasoconstrictor tone
Phentolamine: binds to alpha 1 receptors but also alpha 2 receptors on nerve ending with prevents positive feedback loop
Define active hyperaemia
changes in oxygen/ carbon dioxide/ cellular metabolites causing arterioles to dilate
Define Reactive hyperaemia
Blocking blood flow temporarily
What are examples of local control of arterioles?
- Changes in oxygen/ carbon dioxide/ cellular metabolites can dilate arterioles - ACTIVE HYPERAEMIA
- Blocking blood flow temporarily induces REACTIVE HYPERAEMIA
- Flow can be maintained by FLOW AUTOREGULATION
Factors controlling blood flow within vessels
Radius (larger = less resistance)
Length of tube (shorter = less resistance)
Pressure gradient across length of tube
Viscosity of liquid
Exercise
Clinical significance of blood vessel radius
Pressure gradient changes from high to low throughut arterial tree.
Resistance is proportional to diameter
As branching increases down arterial tree resistance increases as diameter decreases
Mechanisms of encouraging venous return:
- Valves: prevent the back flow
- Wide lumen: decreases resistance
- Venous pressure important in determining tissue fluid balance in capillaries (preventing backlog increasing hydrostatic pressure at venous end)
- Skeletal muscles pump
- Respiratory movement can aid venous return
- Sympathetic nerves: noradrenaline contricting veins leads to increased venous return to heart
How do baroreceptors regulate blood pressure (e.g. decreased arterial pressure)?
- Respond to stretching (sensitive around MAP) with increased firing
- Reflex via medullary cardiovascular centre causes decrease in parasynthetic outflow to heart
- Also causes increase in sympathetic outflow to heart, arterioles and veins
Where are baroreceptors located?
Common carotid arteries
Carotid sinus
Aortic arch
What local controls can effect MABP?
Cardiac output (either stroke volume or heart rate)
Total peripheral resistance
Local controls - act with central controls
Capillary fluid shift
What is response in brain to high blood pressure?
- Baroreceptor inputs information to nucleus tractus solitari (NTS) alpha 2 receptors which:
- Inhibits bulbar circulatory centres –> decrease NA binding to alpha 1 on vessels –> decreased peripheral resistance. Also decrease NA binding to beta 2 on heart –> decreased cardiac output
- Activates vagal nucelus –> increased ACh –> increased binding to M2 receptors –> decreased CO
What is response in brain to high blood pressure?
- Baroceptor input to nucleus tractus solitari beta 2 receptors
- Activates bulbar circulatory centres –> increased NA –> increased binding to alpha 1 receptors on vessels –> increased peripheral resistance. also increased binding of NA to beta 1 receptors on heart –> increased caridac output
- Inhibits vagal nucleus –> less ACh –> less binding to M2 receptors on heart –> increased CO
Centrally acting drugs involved in controlling BP
Clonidine-alpha2 agonist (activates alpha 2) -> lowers BP
Alpha-methyldopa - converted into an apha 2 agonist –> reduction in production of neurotransmitters
Local control of blood pressure:
Independed on nerves/ hormones
Metabolites, blood gases and endothelium involved
Carbond dioxide concentration (lower causes vasoconstriction)
Local control of blood pressure:
Independed on nerves/ hormones
Metabolites, blood gases and endothelium involved
Carbond dioxide concentration (lower causes vasoconstriction)
