Circulation Flashcards
What are the roles of arterioles in systemic circulation?
The main site of systemic vascular resistance
What is the role of the capillaries in systemic vascular circulation?
site of exchange of gas, nutrients and water between blood and tissues
What is the role of the veins in systemic vascular circulation?
they are the capacitance vessels - hold most of the blood volume during rest
passageways of blood from tissues to heart
What is the role of the arteries in systemic vascular circulation?
The passageways of blood from the heart to tissues
What causes an increase in SVR and MAP?
vasoconstriction of smooth muscles
Resistance to blood flow is directly proportional to…
n.L/r4 blood viscosity(n) and length of blood vessel(L); and inversely proportional to the radius of blood vessel (r) to the power 4:
What are the extrinsic controls of the vascular smooth muscles?
- Vasomotor tone
- sympatheic fibres and the neurotransmitter noradrenaline acting on A receptors
- adrenaline (acting on A receptors causes vasoconstriction, acting in B2 receptors causes vasodilation)
A receptors supply skin, gut & kidney arterioles and B2 receptors supply cardiac and skeletal muscle arterioles - Angiotension II - vasoconstriction
- ADH vasopressin causes vasoconstriction
What is vasomotor tone?
- it is caused by tonic discharge of sympathetic nerves resulting in continuous release of noradrenaline
- therefore increasing sympathetic discharge increases vasomotor tone resulting in vasoconstriction and hence decreasing the vasomotor tone resulting in vasodilatation
What are the intrinsic controls of the vascular smooth muscles?
- the intrinsic controls can over-ride the extrinsic control mechanisms
- Chemical
local metabolites
local humoral agents
nitric oxide - Physical
temperature
myogenic response
sheer stress
What is the effect of local metabolites on vascular smooth muscle?
vasodilatation and metabolic hyperaemia are caused by decreased local PO2 increased PCO2 increased local H+ (decreased pH) increased extracellular K+ increased osmolarity of ECF adenosine release from ATP
What are the local humoral agents which can affect vasodilatation?
can sometimes be released as a result of tissue injury and inflammation
the following humoral agents cause relaxation of smooth muscle - vasodilatation
histamine
bradykinin
nitric oxide (NO)
What are the properties of nitric oxide?
- it is continuously produced by the vascular endothelium from the amino acid L-arginine through enzymatic action of NO synthase
- it is a potent vasodilator - with a short half-life important in regulating blood flow and maintaining vascular heath
- NO diffuses from the vascular endothelium into the adjacent smooth activating the formation of cGMP and serving as a second messenger for smooth muscle relaxation
What is the effect of sheer stress on the vascular endothelium?
- dilatation of arterioles causes sheer stress in the arteries upstream making them dilate. Increasing blood flow to metabolically active tissue
- increased flow causes sheer stress
- there is release of calcium in vascular endothelial cells which activate NOS and hence vasodilatation
What mechanism do chemical stimuli use to induce NO formation?
receptor stimulated NO formation
- many vasoactive substances work through stimulation of NO formation
What are the local humoral agents which can affect vasoconstriction?
serotonin
Thromboxane A2
leukotrienes
endothelin (a potent vasoconstrictor released by endothelial cells, it is produced in response to various agents which cause vasoconstriction)
What is the role of the endothelium in the regulation of vascular diameter?
- it has a important role in maintaining vascular health
- the endothelial damage and dysfunction can be caused by HBP, high cholesterol, diabetes and smoking
- endothelial produced vasodilators are anti-thrombotic, anti-inflammatory, anti-oxidants
- endothelial produced vasoconstrictors are pro-thrombotic, pro-inflammatory, pro-oxidants
What is the effect of temperature on vascular smooth muscles?
Cold - causes vasoconstriction
Warmth - causes vasodilatation
What is the myogenic response to stretch?
important in tissues like brain and kidneys helps with regulation of local blood flow
- rise in MAP causes resistance vessels to constrict and limit flow
- fall in MAP causes resistance vessels to dilate and increase flow
What is the autoregulation of celebral blood flow?
the myogenic response to MAP which keeps celebral blood flow constant over a wide range of the MAP
What factors increase venous return?
increased respiratory pump
increased skeletal muscle pump
increased blood volume
increased venomotor tone
What is the effect of increased venous return?
increased arterial pressure
increased end diastolic volume
increased stroke volume in accordance to the Frank- Starling curve
What is the venomotor tone?
- venous smooth muscle is supplied by sympathetic nerve fibres
stimulation of these fibres gives venous constriction - increased venomotor tone increases ve ous return SV and MAP
What is the respiratory pump in venous return?
in inspiration intrathoracic pressure decreases and intraabdominal pressure increases
- there is increased pressure gradient for venous return - suction effects moving the blood in the veins to the heart
- increased rate and depth of breathing causes increased venous return to the heart
What is the skeletal muscle pump in venous return?
large veins in limbs lie between skeletal muscles, the contraction of these muscles aids venous return
- One way venous valves allow blood to move forward towards the heart
What is the acute response to exercise?
- increase in sympathetic nerve activity
- HR and SV increase - CO increase
- vasomotor nerves reduce flow to kidneys and gut through vasoconstriction
- cardiac and skeletal muscle have vasodilatation because metabolic hyperaemia overcomes the vasomotor drive, there is increased blood flow in proportion to metabolic activity
- pulse pressure increases because CO increases systolic BP and metabolic hyperaemia decreases SVR and decreases diastolic BP
- there is a post exercise hypotensive response
What is the effect of sympathetic stimulation on the heart?
- increases heart rate by increasing the rate of firing of the SA node and decreases AV nodal delay
- increases force of contraction
- peak ventricular pressure rises, so the contractility of the heart at a given EDV also rises the frank-starling curve is shifted to the left
What is the effect of noradrenaline on pacemaker cells?
a positive chronotropic effect
- the slope of the pacemaker potential increases causing the threshold to be reached quicker
- there is a higher frequency of action potentials
What is the chronic response to exercise?
- regular aerobic exercise helps reduce BP
Chronic cardiovascular responses to regular exercise may include:
Reduction in sympathetic tone and noradrenaline levels
Increased parasympathetic tone to the heart
Cardiac remodeling
Reduction in plasma renin levels
Improved endothelial function: increased vasodilators decreased vasoconstrictors
decreased Arterial stiffening
