Physiology 6 + 7 Flashcards
What are veins?
Capacitance vessels (contain most blood volume during rest ~60%)
What regulates HR? SV?
- HR - autonomic NS
* SV - pre-load, myocardial contractility, after-load
What must be regulated to regulate MAP?
HR, SV and SVR
How is SVR regulated?
Vascular smith muscles
What is the main site of SVR?
Arterioles
What increases SVR and MAP? Decreases?
- Vasoconstriction increases
* Vasodilation decreases
What are vascular smooth muscles controlled by?
Extrinsic and intrinsic mechanisms
What is the relationship between resistance to blood flow and blood viscosity? Blood vessel length? Radius of the blood vessel?
- Resistance increases as blood viscosity increases
- Resistance increases as blood vessel length increases
- Resistance decreases as blood vessel radius increases
What is the equation for resistance to blood flow?
R = n x L/r^4
n = blood viscosity L = length of blood vessel r = radius of blood vessel
What is resistance to blood flow mainly controlled by?
Vascular smooth muscles through changes in radius of arterioles
What does extrinsic control of vascular smooth muscle involve?
Hormones and nerves
What are vascular smooth muscles innervated by?
Sympathetic nerve fibres
What neurotransmitter is involved in vascular smooth muscle contraction?
Noradrenaline acting on a (alpha) receptors
What is vasomotor tone?
Vessels partially constricted at rest
What is equation for resistance? Flow?
Resistance = 1/r^4 Flow = r^4
What is vasomotor tone caused by?
Tonic discharge of sympathetic nerves resulting in continuous release of noradernaline
What will increase vasomotor tone?
Increased sympathetic discharge resulting in vasoconstriction
What will decrease vasomotor tone?
Decreased sympathetic discharge resulting in vasodilation
Is there parasympathetic innervation of smooth muscle?
No, except for penis and clitoris
What hormone is involved in extrinsic control of vascular smooth muscle?
Adrenaline from adrenal medulla
What are the effects of adrenaline? (2)
- Adrenaline acting on alpha receptors causes vasoconstriction
- Adrenaline acting on B2 receptors causes vasodilation
Where are alpha receptors found? B2 receptors?
- Alpha - skin, gut and kidney arterioles
* B2 - cardiac and skeletal muscle arterioles
What is the benefit of the effects of adrenaline being largely organ-specific?
Helps with strategic redistribution of blood e.g. during exercise
What are other hormones that affect vascular smooth muscle? (2)
- Angiotensin II causes vasoconstriction
* Antidiuretic hormone causes vasoconstriction
What does intrinsic control of vascular smooth muscle include?
Chemical and physical factors
What is the role of intrinsic control of vascular smooth muscle?
Match blood flow of different tissues to their metabolic needs
What do intrinsic control mechanisms of vascular smooth muscles have the ability to do?
Ove-ride extrinsic control mechanisms
What are examples of intrinsic chemical control of vascular smooth muscle? (2)
- Local metabolites
* Local humoral agents
How do local metabolites cause VASODILATION and metabolic hyperaemia? (6)
- Decreased local PO2
- Increased local PCO2
- Increased local [H+] (decreased pH)
- Increased extra-cellular [K+]
- Increased osmolality of ECF
- Adenosine release from ATP
What are local humeral agents released in response to?
Tissue injury or inflammation
What are examples of local humeral agents that cause vasodilation of arterioles?
- Histamine
- Bradykinin
- Nitric oxide (NO) - continuously released by endothelial cells of arteries and arterioles
What is nitric oxide produced by?
Continuously produced by vascular endothelium from amino acid L-arginine through enzymatic action of nitric oxide synthase (NOS)
What is nitric oxide?
Potent vasodilator with a short half life of a few seconds
What is NO important in? (2)
Regulation of blood flow and maintenance of vascular health
What results in activation of NOS? (2)
- Stress on vascular endothelium as a result of increased flow
- Causes release of calcium in vascular endothelial cells which activates NOS
What is activation of NOS by increased flow called?
Flow dependent NO formation
What is receptor simulated NO formation?
Chemical stimuli inducing NO formation
How does NO cause vasodilation?
- Diffuses from vascular endothelium into smooth muscle cells
- Activates formation of cGMP which is a second messenger for signalling smooth muscle relaxation
What are examples of local humeral agents that cause vasoconstriction of arterioles? (4)
- Serotonin
- Thromboxane A2
- Leukotrienes
- Endothelin
What is endothelin?
Potent vasoconstrictor released from endothelial cells
What can endothelial damage/dysfunction be caused by? (4)
- High blood pressure
- High cholesterol
- Diabetes
- Smoking
What are features of endothelial-produced vasodilators?
- Anti-thrombotic
- Anti-inflammatory
- Anti-oxidants
What are features of endothelial-produced vasoconstrictors? (3)
- Pro-thrombotic
- Pro-inflammatory
- Pro-oxidants
What are examples of intrinsic physical control of vascular smooth muscle?
- Temperature
- Myogenic response to stretch
- Sheer stress
How does temperature control vascular smooth muscle? (2)
- Cold - causes vasoconstriction
* Warmth - causes vasodilation
How does myogenic response to stretch control vascular smooth muscle?
- If MAP rises, resistance vessels constrict to reduce blood flow
- If MAP falls, resistance vessels dilate to increase flow
What tissues is myogenic response to stretch important in? (2)
Brain and kidneys
How does sheer stress control vascular smooth muscle?
Dilatation of arterioles causes sheer stress in the arteries upstream to make them dilate
What is the effect of vasodilation caused by sheer stress?
Increases blood flow to metabolically active tissues
What factors increase venous return? (5)
- Increased venomotor tone
- Increased skeletal muscle pump
- Increased respiratory pump
- Increased stroke volume
- increased blood volume
What are venous smooth muscles supplied with?
Sympathetic nerve fibres
What does increased VENOmotor tone result in?
- Increased venous return
- Increased SV
- Increased MAP
What does increased VASOmotor tone result in? (2)
- Increased SVR
* Increased MAP
How does skeletal muscle pump increase venous return?
- Large veins in limbs lie between skeletal muscles
* Contraction of muscles aids venous return
What is heart rate influenced by?
- Parasympathetic stimulation
- Sympathetic stimulation
- Hormone adrenaline
What are acute CVS responses to exercise? (7)
- Sympathetic nerve activity increases
- HR and SV increase which increases CO
- Sympathetic vasomotor nerves reduce flow to kidneys and gut - vasoconstriction
- In skeletal and cardiac muscle, metabolic hyperaemia overcomes vasomotor drive - vasodilation
- Blood flow to skeletal and cardiac muscles increase
- Increase in CO increases systolic blood pressure
- Metabolic hyperaemia decreases SVR and decreases DBP (pulse pressure increases)
What are the acute CVS responses to exercise known as?
Post-exercise hypotensive response
What are chronic CVS responses to regular exercise? (6)
- Reduction in sympathetic tone and noradrenaline levels
- Increased parasympathetic tone to the heart
- Cardiac remodelling
- Reduction in plasma renin levels
- Improved endothelial function: increased vasodilators, decreased vasoconstrictors
- Arterial stiffening
