Unit IV (14-24) - The circulation Flashcards
What is the effect of vasodilation on peripheral vascular resistance?
Decreased peripheral vascular resistance
Increase in which of the following most likely stimulates growth of vessels in a solid tumor? A. Plasma glucose concentration B. Growth hormone C. Vascular endothelial growth factor D. Tissue oxygen concentration
C. VEGF
Ch. 17 p.209-210
How is the velocity of blood flow calculated?
v=F/A
What is the average functional pressure in most vascular beds?
17mmHg
What is Ohm’s law?
F = pressure change / resistance
How is the tendency for turbulent blood flow measured? How is it calculated? At what level does turbulent flow occur?
Reynolds’ number
Re = v x diameter x density / viscosity
200-400 (branches)
2000 (everywhere)
What is conductance?
How is it calculated?
Measure of blood flow through a vessel for a given pressure difference
C = 1/R
What is Poiseuille’s law?
F = (pi x pressure change x radius^4)/(8 x viscosity x length)
Does arterial pressure affect blood flow? Why?
No. Increase in AP initiates compensatory increase in vascular resistance, reduction initiates decreased VR - blood flow autoregulation
How is vascular wall tension calculated?
Laplace law
T = pressure change x (r/wall thickness)
What physical force is most important for development and adaption of the vascular system?
Shear stress
How is vascular distensibility calculated?
VS = increase in volume / (increase in pressure x original volume)
How does the distensibility of veins and arteries differ?
Veins 8x more distensible than arteries
How is vascular compliance calculated?
Vascular compliance = increase in volume / increase in pressure
How can pulse pressure be calculated?
PP = stroke volume / arterial compliance
What is normal RAP?
0mmHg
What could cause a RAP of 20-30?
CHF
Massive transfusion
What are the constituents of the intestitium?
Collagen fibre bundles
Proteoglycan filaments
What is the normal % of free fluid in the interstitium?
<1%
What are the theories that explain acute control of tissue blood flow?
Vasodilator theory - adenosine, CO2, ADP, histamine, K+, H+ - cause dilation of met arterioles and pre-capillary sphincters
Oxygen demand theory - oxygen required for muscle contraction, O2 deficiency = muscle relaxation
What are the two theories that explain auto regulation of blood flow during changes in arterial pressure?
Metabolic theory - vasodilators washed out => vasoconstriction
Myogenic theory - stretch induced vascular depolarisation and contraction
How is acute blood flow regulation different in the brain?
CO2 concentration and H+ play prominent roles in addition to O2
What is the most important endothelial derived vasodilator?
Nitric oxide (NO)
How is NO produced?
By endothelial-derived nitric oxide synthase, from arginine and O2
What is the half life of NO?
6 seconds
How does NO cause vasodilation?
Activates guanylate cyclases in vascular smooth muscle - converts cGTP => cGMP - activates cGMP-dependent protein synthase
What stimulates NO production?
Shear stress
AngII
What is the mechanism of action of sildenafil?
PDE-5 inhibitor
PDE degrades cGMP, inhibition prolongs action of NO
What is the main locally produced vasoconstrictor? What stimulates its production?
Endothelin
Released from damaged endothelium
What are the 4 vascular growth factors?
VEGF
FGF
PDGF
Angiogenin
What determines the vascularity of a tissue?
Its maximum blood flow need
What are the humoral vasoconstrictors? Where are they secreted?
Norepinephrine
Epinephrine
SNS activation - from nerve endings and adrenal medullae
Angiotensin II - RAAS activation
Vasopressin (ADH) - posterior pituitary
What are the humoral vasodilators? Where are they secreted?
Bradykinin - produced from alpha2-globulins in plasma by kalikrein - activated by maceration of blood and tissue inflammation
Histamine - mast cells in damaged tissue
What are the effects of increased concentrations of the following on vascular tone?
a - Ca
b - K
c - Mg
d - H+
e - Acetate/citrate
f - CO2
a - vasoconstriction
b - vasodilation
c - vasodilation (+++)
d - vasodilation
e - vasodilation
f - vasodilation (marked in brain)
Describe the anatomy of the SNS
Sympathetic vasomotor nerves leave the spinal cord through all thoracic and first 2 lumbar spinal nerves, pass into sympathetic chains.
Pass to sympathetic nerves and peripheral portions of spinal nerves
Describe the structure of the vasomotor centre?
- Vasoconstrictor area (acts on all levels of the spinal cord)
- Vasodilator area (inhibits VCA)
- Sensory area (input from vagus/GP nerves - controls VCA and VDA)
What is the sympathetic vasoconstrictor neurotransmitter? Where does it act?
NEpi
Alpha-adrenergic-r of vascular SM
How does SNS activation affect the circulation?
1 - arterioles contracted => ^TPR => ^ABP
2 - veins constricted => ^SV
3 - direct adrenergic stimulation of heart
Where are baroreceptors found?
Carotid arteries and aortic arch
How are signals transmitted from the different baroreceptors?
Carotid arteries => Hering’s nerves => GP nerves => nucleus tractus solatarius (brainstem)
Aortic arch => vagus nerve => NTS (medulla)
What is the effect of baroreceptor stimulation?
Inhibits vasoconstrictor centre
Excites vagal parasympathetic centre
Causes vasodilation, decreased HR and force of contraction
Are baroreceptors important in long term regulation of arterial pressure?
No - reset after 1-2 mins
However, may influence sympathetic nerve activity of the kidneys
Where are the chemoreceptors?
Carotid and aortic arch
What activates the chemoreceptors?
Low O2, high CO2/H+
When are the chemoreceptors important?
At lower pressures than baroreceptors
How do the atrial and pulmonary artery reflexes affect arterial pressure?
Low pressure receptors
Minimise arterial blood pressure change after blood loss
What is the volume reflex?
Stretch of atria activates low pressure atrial receptors => reduced renal sympathetic nerve activity => decreased tubular reabsorption and afferent arteriole dilation
Signals also transmitted to hypothalamus to reduce ADH
Results in increased fluid loss
What is the Bainbridge reflex?
Atrial pressure increase = increased HR
What is the CNS ischaemic response?
When blood flow to brain reduced enough to cause ischaemia, vasoconstrictor centre becomes strongly excited
Increase in BP
Vasoconstriction so intense some peripheral vessels totally occluded
How does anaemia impact cardiac output?
Anaemia => peripheral vasodilation + reduced blood viscosity => reduced TPR => increased CO
What factors influence venous return?
RAP
Degree of filling of the systemic circulation
Resistance to blood flow between peripheral and RA
What factors increase mean circulatory filling pressure?
Increased blood volume
SNS stimulation
What is the formula for calculating venous return?
VR = Pdf - PRA / RVR
Which blood vessel receptors are associated with vasodilation and vasoconstriction?
Alpha - constriction
Beta - dilation
Following cardiac infarction, what 4 factors contribute to the risk of fibrillation?
1 - potassium depletion and accumulation in the ECF - increases irritability
2 - ischameia causes injury current
3 - SNS stimulation
4 - ventricular dilation => circus movements
What is the MOA of digitalis?
Increase calcium in muscle fibres by inhibition of Na-K ATPase = ^Intracellular Na = slowing of Na-Ca exchanger
What 4 effects do SNS stimulation have on fluid balance?
1 - Constriction of renal afferent arteriole = water retention
2 - Activation of alpha-adrenergeric receptors on tubular epithelial cells => na/H2O retention
3 - stimulation of renin release
4 - Stimulation of ADH release
Where are a) ANP and b) BNP released from?
What are their effects?
a - atria
b - ventricles
Increase na excretion
What are the 3 stages of circulatory shock?
1 - nonprogressive
2 - progressive
3 - irreversible
What mechanisms engage in nonprogressive shock?
Baroreceptor reflex
CNS ischaemic response
Reverse stress relaxation of the circulatory system
RAAS activation
ADH secretion
Increased epinephrine and norepinephrine secretion
In which kinds of shock is sympathomimetic therapy valuable?
Neurogenic and anaphylactic
