Flashcards in *physiology 6 (lecture 6) Deck (32):
What are the major resistance vessels in the circulation?
What vessels contain most of the blood volume during rest?
The veins (the capacitance vessels)
What is the main regulator of the HR?
Autonomic nervous system
What are the main regulators of the stroke volume? (3)
Where is the main site of TPR?
How is resistance to blood flow related to blood viscosity and length of blood vessel?
Directly proportional to blood viscosity and length of blood vessel
How is resistance to blood flow related to the radius of the blood vessel?
Inversely proportional to the radius of the blood vessel to the power of 4
Equation for what resistance to blood flow is related to?
blood viscosity X length of blood vessel divided by radius of blood vessel to the power of 4
How is the resistance to blood flow mainly controlled?
By vascular smooth muscle that changes the radius of arterioles
What is involved in the extrinsic control of vascular smooth muscle?
Hormones and nerves
What branch of the autonomic nervous system supplies the vascular smooth muscle?
What receptors on the smooth muscle does the released neurotransmitter act on?
Alpha 1 (noradrenaline)
There is no significant parasympathetic innervation of arterial smooth muscle except in the penis and clitoris
what is the state of the vascular smooth muscle at rest?
It is partially constricted due to the vasomotor tone - tonic discharge of sympathetic nerves resulting in continuous release of noradrenaline
What effect does increased sympathetic stimulation have on vascular smooth muscle?
Increased vasomotor tone = increase vasoconstriction and vice versa
What hormones are involved in the control of vascular smooth muscle? (3)
All cause vasoconstriction
What effect dies adrenaline have on vascular smooth muscle?
Adrenal is released from the adrenal medulla
Adrenal acting on B2 adrenoceptors causes vasodilation (cardiac and skeletal muscle arterioles)
Adrenaline acting on alpha 1 adrenoceptors causes vasoconstriction (predominant in skin, gut and kidney arterioles)
What is the purpose of intrinsic controls of vascular smooth muscle?
To match the blood flow of different tissues to their metabolic needs (they can over red the extrinsic control mechanisms)
They include local chemical and physical factors
What local metabolites (chemicals) cause vasodilation? (6)
Decreased local pO2
Increased local pCO2
Increased local [H+] (decreased pH)
Increased extra-cellular [K+]
increased osmolarity of ECF
adenosine release (from ATP)
What are local humoral agents released in response to?
Tissue injury or inflammation
What are examples of local humoral agents which cause relaxation of arteriolar smooth muscle resulting in vasodilation? (3)
Where is NO released from?
What causes its release?
Continuously produced by the vascular endothelium from the amino acid L-arginine through enzymatic action of nitric oxide synthase (NOS)
Shear stress on vascular endothelium as a result of increased flow causes release of of calcium in vascular endothelial cells and the subsequent activation of NOS
Chemical stimuli can also induce NO formation
How does NO causes vasodilation?
NO diffuses from the vascular endothelium into the adjacent smooth muscles where it activates the formation of cGMP that serves as a second messenger for signalling smooth muscle relaxation
Examples of humoral factors that cause vasocontraction? (4)
In terms for thrombotic, inflammatory and oxidant effect, what are endothelial produced vasodilators?
Anti-thrombotic, anti-inflammatory, anti-oxidants (and vice versa for endothelial produced vasoconstrictors
Apart from chemicals, what else can intrinsically control vascular smooth muscle?
Myogenic response to stretch
What effect does temperature have on vascular smooth muscle?
Cold = vasoconstriction
Warmth = vasodilation
What is the myogenic response to stretch?
If MAP rises, resistance vessels automatically constrict to limit flow
If MAP falls resistance vessels automatically dilate dilate to increase flow
(especially important in tissues lie the brain and kidneys)
When is the maximal force generated in myocytes?
At optimal fibre length
What 5 factors increase venous return? (5)
Increased venomotor tone
Increased skeletal muscle pump
Increased respiratory pump
Increased stroke volume = increased arterial pressure
Increased blood volume
What branch of the autonomic nervous system supplies your blood vessels?
What does increased venomotor tone cause?
Due to increased sympathetic stimulation - increased venous construction
Blood driven to the right atrium increases (venous return), SV and MAP also increase
What does increased muscle activity cause in terms of venous return?
Increased venous return to the heart due to skeletal muscle pump