CVS 4 - factors affecting blood flow Flashcards
What does whole blood contain?
erythrocytes (RBCs), leukocytes (WBCs), platelets, plasma proteins
What is haematocrit?
proportion of erythrocytes to total volume
How is haematocrit assessed?
by measuring the Packed Cell Volume (RBC component of whole blood)
What may an increased haematocrit indicate?
dehydration (impacts viscosity and velocity of blood)
What may a decreased haematocrit indicate?
anaemia
What are the layers of centrifuged blood?
plasma (contains plasma proteins), buffy coat, red blood cells
What is the buffy coat?
thin pale layer between plasma and RBCs containing leukocytes and platelets
Appearance of solution containing intact RBCs
cloudy, red appearance (intact erythrocytes disperse light)
Appearance of solution containing haemolysed (ruptured) erythrocytes
transparent, red solution (haemoglobin released - red pigment absorbs but does not disperse light)
What happens when blood is added to 0.9% NaCl (saline) solution?
cloudy mix - no haemolysis as saline is isotonic to RBCs
What happens when blood is added to distilled water?
solution goes red and transparent - haemolysis occurred. Distilled water is hypotonic to erythrocytes (gain water, swell, rupture)
What happens when blood is added to 0.9% NaCl + detergent?
solution turns red and transparent - haemolysis. Detergent dissolves phospholipid membrane
What happens when blood is added to 0.5ml isosmotic sucrose solution (300mosmol/L)?
cloudy mix - no haemolysis. Similar osmolarity to saline and sucrose too large to cross erythrocyte plasma membrane
What happens when blood is added to 0.5ml isosmotic urea solution (300mosmol/L)?
red and transparent - haemolysis. Urea undergoes facilitated diffusion into RBC and water follows. solution is hypotonic to erythrocytes
What happens when blood is added to 0.9% NaCl + urea crystals?
solution goes cloudy - no haemolysis. solution isotonic to erythrocytes
Difference in haemolysis between fresh and stored blood
stored blood undergoes more haemolysis
Why does blood flow vary between organs?
each organ has different metabolic demands
Which organ system is the only exception to blood flow not depending on the metabolic demand?
pulmonary system - blood flow is entire CO as all blood must undergo gas exchange
Examples of blood flow to organs increasing according to a greater metabolic requirement
during exercise, blood flow to skeletal muscle increases due to greater oxygen demand. Following ingestion, blood flow to GI system increases due to greater oxygen demand.
What is the cause of interorgan blood flow differences?
alterations in vascular resistance
How can mechanisms that regulate blood flow be categorised?
local (intrinsic) control and neural/hormonal (extrinsic) control
What is the function of local control of blood flow?
to match blood flow to metabolic requirement of tissue system
How is blood flow controlled locally?
direct action of metabolites on arteriolar resistance
How does neural control of blood flow act?
via action of sympathetic NS on vascular smooth muscle
How does hormonal control of blood flow act?
via action of vasoactive substances (histamine, bradykinin, prostaglandins)
What needs of the tissues are supplied by blood flow?
transport oxygen and nutrients (glucose, amino acids, fatty acids), removal of carbon dioxide and hydrogen ions, maintain ion concentrations, transport hormones
Which three mechanisms can local/intrinsic control of blood flow occur by?
autoregulation, active hyperemia, reactive hyperemia
What is autoregulation?
maintenance of constant blood flow while arterial pressure changes e.g. if coronary arterial pressure decreases, compensatory vasodilation occurs to decrease resistance which maintains constant blood flow
What is active hyperemia?
blood flow to tissues is proportional to its metabolic activity
What happens as a result of active hyperemia when metabolic activity increases?
increased arteriolar dilation which increases blood flow
What is reactive hyperemia?
an increase in blood flow in response to prior period of decreased blood flow
What is the purpose of reactive hyperemia?
to reverse the oxygen debt that has accumulated during the arterial occlusion
What factors determine resistance to blood flow?
vessel diameter (vasoconstriction/vasodilation), vessel length, viscosity of blood
Viscosity of blood usually remains within a narrow range except in which circumstance?
when haemocrit changes (RBC count can impact viscosity)
What can cause an increase in blood viscosity?
dehydration and immobility (so blood flow is reduced)
Which medical condition does an increase in blood viscosity increase the risk of developing?
Deep Vein Thrombosis (DVT)
How can the risk of deep vein thrombosis (DVT) be reduced?
hydration, movement, compression socks
What is vessel resistance (R) directly proportional to?
vessel length (L) and blood viscosity (n). R proportional to nL
What is vessel resistance (R) inversely proportional to?
radius to the power of 4 (R proportional to 1/r4)
What is the equation to calculate blood flow (Q)?
Q= change in pressure (deltaP) / resistance (R)
Which equation describes how flow is related to perfusion pressure, radius, length and viscosity?
Poiseuille’s equation
What is Poiseuille’s equation?
Q is directly proportional to r^4 delta P / nL
What are the limitations of Poiseuille’s equation?
assumes
1. flow is through a uniform straight pipe (no branching, bending)
2. flow is non-pulsatile
3. flow is laminar (smooth)
Where in the blood vessel is velocity of flow greatest?
at the centre (furthest from resistance of wall)
If flow in a tube (vessel) had negligible resistance, how would this affect velocity?
velocity would be the same across the tube
How do pressure differences within the venous system compare with the arterial system?
pressure differences within the venous system are small (graph tapers)
What is the orientation of valves in veins?
venous valves are oriented towards the heart
How is venous return aided?
contraction of skeletal muscles compressing veins, more negative intrathoracic pressure during exercise, sympathetic activation
How is venous return to the heart aided during exercise?
intrathoracic pressure becomes more negative due to frequent respirations which increases the pressure gradient between abdominal and thoracic veins
How does sympathetic activation increase venous return?
sympathetic activation releases noradrenaline which constricts veins
What is venous return to the right ventricle termed?
preload
Define afterload
pressure heart is contracting against to expel blood into the arterial system
According to Starling’s Law, what happens if preload increases?
an increased preload equates to an increased EDV which greater stretches the myocardium leading to increased contraction force (up to a point)
Which diseases when paired with an increase in preload can result in a problem?
coronary artery disease (angina) or heart failure
Function of coronary circulation
perfusion of myocardium to maintain high basal rate of oxygen supply
Function of skeletal muscle circulation
meet metabolic demand of skeletal muscle during exercise
Function of cerebral circulation
maintain cerebral perfusion
What percentage of the resting cardiac output does the heart receive?
5%
What percentage of the body weight does the heart represent?
<0.5%
Which arteries is the blood supply to the entire myocardium derived from?
left and right coronary arteries
Where do the coronary arteries originate?
At the root of the aorta behind cusps of the aortic valve
What portion of the heart is generally supplied by the left coronary artery?
left ventricle and atrium
What portion of the heart is generally supplied by the right coronary artery?
right ventricle and atrium
Which arteries does the left coronary artery divide to form?
left circumflex artery and left anterior descending artery
Where does the left circumflex artery branch to supply?
LA and LV
Where does the left anterior descending artery branch to supply?
descends to the apex and branches to supply the interventricular septum and portion of the right and left ventricles
How is blood returned once it passes through coronary capillaries?
blood collects in venules that drain to form epicardial veins which transport blood to the coronary sinus
Name the largest epicardial vein
great cardiac vein
Where is blood emptied from the coronary sinus?
into the right atrium
Name an alternative pathway for blood drainage from the coronary circulation, other than the coronary sinus
thesbian veins (very small) drain deoxygenated blood directly into cardiac chambers
What is the rate of coronary blood flow at rest?
70-80 ml/min/100g
When does the perfusion of the myocardium from the coronary arteries occur?
during early diastole
Why can perfusion of the myocardium not occur during systole?
contraction of myocardium constricts the vessels
What percentage of left coronary blood flow occurs during diastole?
80%
What is the coronary blood flow during exercise?
300-400 ml/min/100g
What physiological adaptations occur in an athletic heart?
increased number of arterioles and capillaries, exercise induced hypertrophy (different from pathological hypertrophy)
Which circulation is a major influence of total peripheral resistance (TPR)?
skeletal muscle vascular resistance
How is the blood flow in skeletal muscle circulation regulated at rest?
sympathetic innervation
How is blood flow in skeletal muscle circulation regulated during exercise?
by local (intrinsic) control mechanisms
Name local vasodilators in skeletal muscle
lactate, adenosine and potassium ions (K+)
Activation of which receptors causes vasoconstriction in skeletal muscle circulation?
alpha-1 adrenoreceptor
Activation of which receptors causes vasodilation in skeletal muscle circulation?
beta-2 adrenoreceptors
Effect of adrenaline on skeletal muscle circulation
cause vasodilation by binding to beta-2 adrenoreceptors
How is MAP maintained when skeletal muscle has an increased metabolic demand (e.g. during exercise)?
sympathetic vasoconstriction continues in feed arteries and proximal resistance vessels which prevents an excessive decrease in TPR
Which circulation accounts for ~50% of vascular resistance?
cerebral arteries
Name the anastomotic arterial ring at the base of the brain
circle of Willis
Which arteries anastomose to form the circle of Willis?
basilar and internal carotid arteries
What is the function of the circle of Willis?
preserves cerebral perfusion if carotid artery obstruction occurs
What is the effect of cerebral autoregulation when arterial blood pressure falls?
cerebral resistance vessels dilate to maintain perfusion
Below what arterial blood pressure does cerebral blood flow steeply decline (cerebral autoregulation stops)?
~60mmHg
What are the consequences of severe hypotension?
mental confusion and syncope (loss of consciousness)
What is a high carbon dioxide level in the blood termed?
hypercapnia
What is a low carbon dioxide level in the blood termed?
hypocapnia
What is the effect of hypercapnia on cerebral circulation?
hypercapnia causes cerebral vasodilation
What substance mediates cerebral vasodilation?
endothelial NO
What is the effect of hypocapnia on cerebral circulation?
hypocapnia causes cerebral vasoconstriction
How can hyperventilation affect cerebral perfusion?
hyperventilation can reduce cerebral perfusion (dizziness) as hypocapnia occurs which triggers cerebral vasoconstriction
What is the effect of local hypoxia on cerebral circulation?
local hypoxia leads to cerebral vasodilation
Why can systemic hypoxia lead to cerebral vasoconstriction?
systemic hypoxia triggers increased ventilation which results in hypocapnia. This causes cerebral vasoconstriction
Aside from coronary, skeletal muscle and cerebral circulations, name other circulations with specialised local control
pulmonary circulation, skin circulation (for thermoregulation), renal circulation
Which practical investigates cardiovascular reflexes?
tilt table test
What is the initial effect on blood pressure when returning to an upright position after laying down?
decrease in arterial BP
Describe the cardiovascular reflex that occurs when the subject returns to an upright position
- decrease in arterial BP
- detected by baroreceptors in aortic arch and carotid sinus
- decreased baroreceptor firing
- detected by medullary CV centre
- increased sympathetic discharge
- decreased parasympathetic discharge
- increased venous tone and pressure
- increased venous return
- EDV preserved
- CO and MAP maintained
