Block 2 W4 Flashcards
What is the normal blood pressure?
120/80
Due to our environment and increases with age.
Why is high blood pressure needed?
Overcome effects of gravity to pump blood to brain.
High metabolic rate
High rate of toxin production
What are the functions of the heart?
- Pump function
2. Endocrine function - atrial natriuretic hormone
Describes the pressure difference between right and left side of heart.
Right side has lower pressure than left:
pulmonary arterial pressure = 0-5 — 4-12
systemic arterial pressure = 20-25 — 90-130
What is the blood pressure equation?
BP = CO x TPR
What is the blood flow equation?
Q = △T/R
Or
CO = mean arterial pressure - right atrial pressure/TPR
Q - flow (CO) ml/min
△T - pressure gradient mmHg
R - resistance mmHg/ml/min
Pressure gradient drives blood flow, thus blood flows from high to low pressure.
Blood flow is inversely proportional to resistance of vessels.
In which system is the pressure drop greatest?
Greater drop in pressure in systemic (arterioles) than pulmonary system as systemic resistance is greater than pulmonary.
Pressure is highest in aorta and large arteries and lowest in vena cava.
How do metabolising tissues during exercise receive more blood?
Tissues determine how much blood is needed. Task of circulation is to maintain continuous pressure for tissues.
Peripheral resistance within muscles decreases - change in pressure remains the same -> increases flow.
Viscera blood flow decreases during exercise.
Describe the resistance equation.
R = 8nl/πr^4
R - resistance
n - viscosity of blood
l - length of blood vessel
r^4 - radius of blood vessel to 4th power.
Resistance is directly proportional to viscosity of blood. Directly proportional to length of vessel. Inversely proportional to 4th power of vessel radius - flow is very sensitive to radius.
What is the role of pre-capillary sphincters?
Pre-capillary sphincters - arterioles with rings of smooth muscles around them -> control peripheral resistance as it enables vasodilation and vasoconstriction.
Tissues at work releases metabolic products e.g. K+, H+, adenosine, hypoxia, CO2, sheer stress -> causes relaxation of pre-capillary sphincters - vasodilation.
Describe the velocity of blood flow equation.
V = Q/A
V - velocity cm/sec
Q - flow ml/min
A - cross-sectional area cm^2
Velocity is directly proportional to blood flow and inversely proportional to the cross-sectional area at any level of CV system.
e.g. blood velocity is higher in aorta (small cross-sectional area) than capillaries (large cross-sectional area).
Low velocity of blood in capillaries optimises condition for nutrient exchange.
Define laminar and turbulent flow.
Laminar flow is streamlined with fastest velocity in centre of vessel and slowest velocity at wall of vessel.
Turbulent flow has random direction due to blockage of vessel.
Define bruits.
Audible vibrations of turbulent flow of blood in a vessel.
Describe the capacitance equation.
Describes the distensibility of blood vessels. Capacitance is inversely related to elastance (stiffness).
Describes how volume changes in response to change in pressure.
C = V/P
C - capacitance/compliance ml/mmHg
V - volume ml
P - pressure mmHg
Greater in veins than arteries so more blood contained in veins.
Capacitance of arteries decreases with age - arteries become stiffer and less distensible.
How is mean arterial pressure calculated?
Average arterial pressure with respect to time.
Diastolic pressure + 1/3(pulse pressure).
How does tissue demand control cardiac output?
By matching venous return to cardiac output.
As venous return increases, cardiac output increases too.
Blood is pumped by heart (cardiac output) at the rate entering the heart (venous return) regardless of after load (BP).
As BP increases, blood to tissues remain stable.
What is the short term BP control?
Fast, neurological baroreceptor mechanism via ANS.
Sympathetic fibres innervate all vessels except capillaries - release NA -> binds a-receptors = vasoconstriction -> increases peripheral resistance -> increased BP.
What are the effects of SNS stimulation?
- constriction everywhere except muscles -> vasodilation as muscles need blood flow
- venous constriction -> increased venous return -> increases CO
- increased inotropy, chronotropy and lusitropy.
Describe the arterial baroreceptors.
Stimulated by stretch and transmit to vasomotor centre in medulla.
Situated in walls of carotid sinus near the bifurcation of common carotid arteries and aortic arch.
Systole -> firing rate increases. Diastole -> firing rate decreases.
Firing rate increases more dramatically over physiological range.
Describe the barorceptor reflex.
Pressure falls -> decreases stretch on carotid sinus walls -> decreases firing rate to carotid sinus nerve -> medulla -> increases sympathetic tone to heart and vessels and decreases parasympathetic tone to heart -> increases BP.
Pressure rises -> increases stretch on carotid sinus walls -> increases firing rate to carotid sinus nerve -> medulla -> decreased sympathetic tone and increases parasympathetic tone -> decreases BP.
What are the responses of the vasomotor centre?
- increased chronotropy due to decreases parasympathetic tone and increased sympathetic tone to SA node
- increased inotropy and stroke volume
- increased vasoconstriction of arterioles due to increased sympathetic tone -> TPR and pressure increases
- increases vasoconstriction of veins -> decreases unstressed volume and increases venous return to heart.
Describe the blood pressure changes that occur when moving from supine to standing position.
Stand -> venous pooling in lower extremities due to high compliance of veins -> venous return decreases -> SV and CO decreases -> arterial pressure decreases due to reduction in CO -> sensed by carotid sinus baroreceptors -> baroreceptor reflex -> BP increases.
Describe the blood pressure changes that occur during exercise.
The central command originates in motor cortex or from reflexes initiated in muscle proprioceptors when exercise is anticipated and causes changes:
- sympathetic tone increased
- parasympathetic tone decreased
- CO increased
- venous return increased due to venoconstriction
- arteriolar resistance in skin, splanchnic regions, kidneys and inactive muscles is increases -> blood flow to these organs decreases
Increased metabolic activity of skeletal muscle releases vasodilator metabolites (lactate, K+ and adenosine) -> causes arteriolar vasodilation in active skeletal muscles and reduces TPR -> increased blood flow to skeletal muscle.
What are the other neural mechanisms of regulating BP.
- Atrial stretch -> causes renal arteriole dilation and decreases ADH
- Chemoreceptors -> low BP -> reduced partial pressure of O2 -> activate vasomotor centre -> rise in BP.
- CNS ischaemia -> increased PCO2 -> chemoreceptors in vasomotor centre respond by increases sympathetic tone of heart and vessels -> rises BP.
What is the long term control for BP?
Slow, hormonal renin-angiotensin-aldosterone system.
Define pressure diuresis.
As BP increases, urine output increases.
In kidney diseases - to remove same amount of salt water, need higher BP -> renal diseases causes hypertension.
High salt diet - need higher BP to get rid of it.
Describe the renin-angiotensin-aldosterone system.
Decrease in renal perfusion pressure -> juxta-glomerula apparatus secretes renin (enzyme) -> catalyses conversion of angiotensinogen (liver made) into angiotensin I in circulation.
Angiotensin converting enzyme (ACE) catalyses conversion of angiotensin I to angiotensin II -> effector molecule.
What are the effects of angiotensin II?
- stimulates aldosterone secretion by adrenal cortex -> increases Na+ reabsorption by renal distal tube which increases ECF volume, blood volume and pressure.
- increases Na+/H+ exchanger in proximal convoluted tube - directly increases Na+ reabsorption.
- increases thirst
- vasoconstriction of arterioles -> increases TPR and BP.