13 - Regional Circulation Flashcards
Three types of factors that influence bloodflow?
- Physical factors
- Neural and neurohumoral factors
- Metabolic Factors
_____________ takes into account the various factors that influence the flow of fluid through a tube
Poiseuille’s Law takes into account the various factors that influence the flow of fluid through a tube
What is Poiseuille’s Law?
Q= (Pi - Po)(π)(r4)/ 8ηl
Q = Flow
Pi and Po = inflow and outflow pressures
r= radius
l = length
η = viscosity of the fluid
What is the principle determinant of vascular resistance to blood flow?
Caliber (r4)
ie diameter
π/8 =
The constant proportionality
Resistance to blood flow is proportional to:
1/radius4
What is the relationship between flow and radius?
As radius decreases, flow decreases
(because as radius decreases, resistance increases to the 4th power)
What is the most important physiological regulator of blood flow?
Change in radius
Radius decreases = flow decreases
How does flow change if there is a pressure difference?
Flow increases with pressure difference (important during exercise)
How does flow change with length of vessel?
Flow decreases (resistance increases) with length of tube (not a very significant physiological regulator)
How does viscosity affect bloodflow?
Flow decreases (resistance increases) with increased viscosity
What are the “special tasks” of the coronary circulation?
- Delivery of oxygen at a high rate to keep pace of cardiac demand
- Virtually all oxygen is being extracted
What is the primary means of increasing oxygen supply to heart muscle?
Increase in coronary blood flow secondary to arteriolar vasodilation
- virtually all the O2 is being extracted from blood passing through the coronary circulation => increasing blood flow = increase O2 delivery
What are three physical factors influencing coronary blood flow?
- changes in aortic pressure
- Compression of coronary arteries by the contracting myocardium
- Autoregulation
Why is coronary blood flow lower during phases of isovolumetric contraction and ejection than during diastole
Coronary arteries are compressed when the myocardium contracts
What is Tachycardia?
- Abnormal increase in heart rate
- Increase HR = Increase time in systole => restrict flow
- BUT the mechanical reduction in flow is compensated by dilation due to increased metabolic demand in the rapidly beating heart
- Increase HR = Increase time in systole => restrict flow
What are the implications of tachycardia on the compression of the coronary arteries by the contracting myocardium?
Bulk of coronary blood flow occurs during diastole
- When HR increases (tachycardia) the duration of diastole decreases much more than the duration in systole
- Therefore, time available for coronary arterial inflow decreases and, as a result, tachycardia may be a problem for someone with coronary artery narrowing
What is autoregulation?
Myogenic tone = shifts diameter depending on HR
Increase HR = Increase diameter
*At any given metabolic rate (heartrate) blood flow increases relatively little with pressure (autoregulation)
Autoregulation:
At any given metabolic rate (heartrate) how does blood flow change?
*At any given metabolic rate (heartrate) blood flow increases relatively little with pressure (autoregulation)
Autoregulation:
At any given perfusion pressure, how does flow change?
At any given perfusion pressure, flow increases with metabolic rate (heart rate); this is Functional or metabolic hyperaemia
What is Functional or metabolic hyperaemia
At any given perfusion pressure, flow increases with metabolic rate
What is required for autoregulation?
Healthy endothelium
Metabolic factors allow vasodilation on endothelium to maintain and increased diameter
Adenosine → NO → vasodilation
Autoregulation is well developed in the coronary circulation but it is reset by ________ to operate at a ______ flow rate
Autoregulation is well developed in the coronary circulation but it is reset by metabolic vasodilation to operate at a higher flow rate
What are neural and neurohumoral factors influencing coronary blood flow?
- Both alpha (constrictors) and beta (dilators) receptors are present on the coronary vessels
- Sympathetic nerves = vasoconstriction
- increase HR → increase sympathetic output → increase vasoconstriction
How do metabolic factors influence coronary blood flow?
- Close relationship between cardiac metabolic activity and coronary flow (vasodilation)
- Supply and demand
What happens when there is a decrease in the ratio of oxygen supply to oxygen demand in the heart?
Vasodilators are released to increase blood flow
(metabolic factors influencing coronary blood flow)
What are five metabolic factors that result in vasodilation of coronary arteries?
- increased lactate
- decreased pH
- decrease o2 tension
- increase potassium
- increased adenosine (increases NO from endothelium)
What three factors function as negative feedback system of coronary blood flow?
Flow, tissue PO2, and adenosine maintain a dynamic balance = neg feedback
Increased blood flow due to adenosine (causes endothelium to release NO = vasodilation) washes out adenosine and increases tissue PO2
What is angina?
Angina is chest pain or discomfort caused when your heart muscle doesn’t get enough oxygen-rich blood
What happens in coronary blood flow when you start to exercise?
- exercise → ↑ Sympathetic activation; ↑ HR (restrict flow); ↑ myocardial contractility increases metabolic demand which evokes vasodilation and ↑flow
What happens if there is a slow obstruction in coronary circulation?
- Collateral vessels develop to provide blood to the myocardium
- Under stress or exercise this may not be adequate and chest pain develops = angina
Three ways to relieve angina?
- Vasodilators - glyceryl trinitrate / NO donor
- Beta-adrenergic blockers = reduce oxygen demand
- Angioplasty-distension of vessel by balloon
wouldn’t give adenosine because endothelium is most likely dysfunctional
What happens when there is a sudden obstruction in coronary circulation?
- Few intracoronary channels exist (not enough time for new blood vessels to form)
- Prolonged severe reduction in coronary blood flow leads to myocardial cell necrosis
- Example:
- Occlusion by thrombosis (blood clot) leads to ischemic necrosis causing cardiac pain and impaired contractility = myocardial infarction
There are ______ arteries that deliver blood to the brain:
There are four arteries that deliver blood to the brain:
- 2 internal carotid arteries
- 2 vertebral arteries
The vertebral arteries join to form the _____ which in conjunction with the branches of the internal carotid arteries forms the ______
The vertebral arteries join to form the basilar artery which in conjunction with the branches of the internal carotid arteries forms the Circle of Willis
What is unique to the brain that would be important when considering cerebral blood flow?
The skull = no room for swelling = internal pressure (intracranial pressure)
- Increase in external pressure Pe = vascular occlusion (eg brain tumour)
What are influencers of cerebral blood flow?
- Intracranial pressure
- Autoregulation
- Arteriolar caliber (r4)
- Neural factors (small role)
- Local factors
- Vasodilation due to PCO2
How is autoregulation of cerebral blood flow accomplished?
Autoregulation is accomplished through myogenic tone
Generally, total cerebral blood flow is _____
Generally, total cerebral blood flow is constant
How does PCO2 influence Cerebral blood flow?
Increase of arterial PCO2 (hypercapnia) = decrease in brain tissue pH = relaxation of arteriolar smooth muscle = vasodilation = increase blood flow
What is hypercapnia?
Increase in arterial PCO2
Effect of arterial pH on cerebral blood flow?
Small effect because H+ does not diffuse readily into brain tissue from the capillaries (blood brain barrier)
What is hypoxia?
Decrease in arterial PO2
Hypoxia in the brain causes:
- _______ brain lactic acid
- _______ brain tissue pH
- Release of ___ from brain cells
All of which have ______ effects
Hypoxia (↓ arterial PO2) causes:
- increase brain lactic acid
- decrease brain tissue pH
- Release of K+ from brain cells
All of which have vasodilating effects
Adenosine in brain?
Brain metabolism is relatively constant therefore adenosine is not as important in the brain as it is in coronary blood flow (heart is metabolically active)
What are some anatomical/mechanical considerations of skeletal muscle blood flow?
- Muscle blood vessels are contained within muscle tissue that results in external pressure (Pe) on the vessels
- During muscle contraction = venous diameter decreases = venous pressure increases = blood is squeezed out of the muscle veins = forced back to heart
During muscle contraction = venous diameter _______ = venous pressure ______ = blood is ______ out of the muscle veins = forced back to heart
During muscle contraction = venous diameter decreases = venous pressure increases = blood is squeezed out of the muscle veins = forced back to heart
Increased venous return = increased ______ (increased ______)
Increased venous return = increased cardiac output (increased stroke volume)
Autoregulation of skeletal blood flow?
Autoregulation is present in resting muscles but is overridden with exercise
Local/Metabolic factors that influence Skeletal Muscle blood flow
- with muscle activity = decreased pH due to decreased PO2 and increased PCO2 = vasodilation
- Increased adenosine, K+, Mg2+, acetate, citrate and pyruvate, temperature (allow extraction O2 from Hb w/ ↑temp)
Sympathetic influence on skeletal muscle blood flow
- Important at rest
- Adrenergic
- Alpha Receptors - vasoconstriction
- Resting muscle BF markedly reduced during intense sympathetic stimulation such as shock
-
Beta Receptors - some effect in resting and early in exercise
- involved in vasodilation
What are the most important factors influencing skeletal muscle blood flow during intense exercise?
Local, metabolic factors > sympathetic factors
How does oxygen delivery effect skeletal blood flow?
Due to the metabolic act of mechanically active muscle, interstitial pH drops and temperature rises
At any given level of saturation hemoglobin O2 is more readily released
What happens to the Hb curve when there is a drop in pH and/or an increase in temperature?
Hb curve shifts to the right = greater O2 extraction
Pulmonary vascular resistance is normally ____% of systemic vascular resistance
Pulmonary vascular resistance is normally 20% of systemic vascular resistance
Pulmonary arterioles tend to be ______ and there is a low level of _______ because of the spongy lung tissue and low intrathoracic pressure
Pulmonary arterioles tend to be relaxed and there is a low level of external pressure (Pe) because of the spongy lung tissue and low intrathoracic pressure
Pulmonary arterioles tend to be relaxed and there is a low level of external pressure (Pe) because of the _________ and ___________
Pulmonary arterioles tend to be relaxed and there is a low level of external pressure (Pe) because of the spongy lung tissue and low intrathoracic pressure
What happens to pulmonary blood flow when PO2 is low?
Vasoconstriction
- reduces blood flow to poorly ventilated alveoli = shunting of pulmonary blood flow to the better ventilated regions and thereby improve the O2 saturation of the systemic blood
Hypoxia in the lungs causes _______ of pulmonary circulation
Hypoxia in the lungs causes vasoconstriction of pulmonary circulation
In systemic hypoxia causes vasodilation
How does the nervous system affect pulmonary circulation?
Nervous - little influence
Postural influence on pulmonary circulation?
- gravity affects the regional distribution of blood flow in the lungs
- due to low pressure and great distensibility (low resistance bed) of the pulmonary vessels
- In upright posture, blood pressure in the apical vasculature is lower than in the base due to hydrostatic effects
In upright posture, blood pressure in the ___________ (pulmonary) is lower than in the ____ due to hydrostatic effects
In upright posture, blood pressure in the apical vasculature is lower than in the base due to hydrostatic effects
In upright posture, blood pressure in the apical vasculature is lower than in the base due to ______ effects
In upright posture, blood pressure in the apical vasculature is lower than in the base due to hydrostatic effects
Structural features unique to fetus:
- Adult heart = 4 chambers, 2 pumps in a series that perfuses to separate circulatory systems (pulmonary and systemic)
- Fetal Heart =
Structural features unique to fetus:
- Adult heart = 4 chambers, 2 pumps in a series that perfuses to separate circulatory systems (pulmonary and systemic)
- Fetal Heart = four-chambered; arranged as a parallel system where right ventricle and left ventricle outputs mix (O2 from placenta NOT lungs)
Label A B and C of the fetal heart:
A = Ductus Venosus
B = Foramen Ovale
C = Ductus Arteriosus
The fetal heart is a parallel system due to a series of ______
The fetal heart is a parallel system due to a series of four shunts
- role of the Ductus venosus
Ductus venosus = directs blood to inferior vena cava
Role of foramen Ovale in fetal circulation?
Passes blood from right to left side of circulation due to 2 holes in the right and left atrium
High pressure in Right Atrium forces blood to Left atrium and then to left ventricle
Therefore oxygen rich umbilical blood travels through the ivc (inferior vena cava) to the LV that supplies oxygen to the heart and brain
What causes blood to flow through the foramen ovale?
High pressure in Right Atrium forces blood to Left atrium and then to left ventricle
Therefore oxygen rich umbilical blood travels through the ivc (inferior vena cava) to the LV that supplies oxygen to the heart and brain
Role of Ductus Arteriosus in fetal circulation?
Carries output of right ventricle = bypasses pulmonary
Closes within 48 hours of birth under the influence of O2 and Prostaglandin inhibition
What causes the ductus arteriosus to close?
Carries output of right ventricle
Closes within 48 hours of birth under the influence of O2 and Prostaglandin inhibition
In the fetus, gas exchange occurs in the _____, therefore pulmonary blood flow supplies only the _________
In the fetus, gas exchange occurs in the placenta, therefore pulmonary blood flow supplies only the nutritional requirements for lung development
Primary changes in pulmonary and systemic vascular resistance at birth:
- Severing umbilical cord
- loss of blood flow through placenta (low resistance shunt) approximately doubles systemic resistance (left side)
- Increases in aortic, left ventricle and left atrium pressure
- Pulmonary ventilation
- Pulmonary resistance decreases due to lung expansion
- Vessels are not compressed by fluid and O2 causes vasodilation
- ↓pulmonary pressure and right ventricle and right atrium pressure is reduced
- closes foramen ovale (Pright < Pleft)
- ↓pulmonary pressure and right ventricle and right atrium pressure is reduced
How does severing the umbilical cord modify blood flow in baby?
- Severing umbilical cord
- loss of blood flow through placenta (low resistance shunt) approximately doubles systemic resistance (left side)
- Increases in aortic, left ventricle and left atrium pressure
How does Pulmonary ventilation change after birth?
- Pulmonary resistance decreases due to lung expansion
- Vessels are not compressed by fluid and O2 causes vasodilation
- ↓pulmonary pressure and right ventricle and right atrium pressure is reduced
- closes foramen ovale (Pright < Pleft)
- ↓pulmonary pressure and right ventricle and right atrium pressure is reduced
Low right atrial pressure and high left atrial pressure causes what effect in infant circulation?
closes foramen ovale and most fuse with the septum
What causes closure of ductus arteriosus?
O2 is the trigger of vasoconstriction
Prostaglandin E2 = vasoconstriction
What causes closure of the ductus venosus?
Mechanical closure due to loss of blood flow (no more blood going through because the placenta is gone)
