12.1 Control of blood flow Flashcards
What drives blood flow?
- Blood flow is driven by PRESSURE GRADIENTS
- Thanks to the pumping of the heart
- Arterial Pressure usually held constant by baroreflex
- Thus if we want to alter the rate of flow of blood to downstream tissues we need to alter the resistance to blood flow
What is the definition of perfusion pressure?
The arterial minus the venous pressure in that organ
What is Darcy’s Law of flow?
Δ Blood Pressure = Blood Flow x Resistance to blood flow
What is Poiseuille’s Law?
- Small changes in the radius of the lumen of blood vessels can have significant effects on the resistance of the vessels
What are the mechanisms of controlling blood flow?
-
LOCAL
- Metabolic, myogenic
- Used to autoregulate blood flow in the face of changing perfusion pressure
- Or to increase blood flow in response to increase demand (eg exercise); active hyperaemia
-
ENDOTHELIAL
- Nitric oxide, prostaglandins etc.
-
HORMONAL (endocrine)
- ADH, adrenaline, Ang II etc.
-
CENTRAL (neural)
- Sympathetic nerves
Briefly explain the myogenic theory
Myogenic theory
Increase BP will increase flow to the tissue and stretches smooth muscle around arteriole, this activates stretch-activated Ca2+ channels in the arteriolar smooth muscle and leads to increased contraction/constriction, increase resistance and decrease flow back down
Briefly explain the metabolic theory
Metabolic theory
Increased pressure* will increase *flow and washes away vasodilatory metabolites faster than they are produced – arteriole constricts and reduces flow back to where it was before the increase in BP
What does increases in sympathetic outflow to the arterioles do?
Increases in the sympathetic outflow to the arterioles causes vasoconstriction
- Post-ganglionic sympathetic neurones release noradrenaline onto arteriolar smooth muscle cells
- Stimulation of α-adrenoreceptors causes a rapid rise in [Ca2+]cyt in the arteriolar smooth muscle cells
- This stimulates the contraction of the arteriolar smooth muscle cells, and therefore vasoconstriction of these arterioles
What happens in skeletal muscles when there is a low concentration of adrenaline?
Low concentrations of adrenaline = VASODILATION (act on Beta adrenoreceptors)
What is the difference between noradrenaline & adrenaline?
NORADRENALINE
- Noradrenaline is the main neurotransmitter of the sympathetic nerves in the cardiovascular system
- ALWAYS causes vasoconstriction
- Acts on on alpha adrenergic receptors
ADRENALINE
- Adrenaline is the main hormone secreted by the adrenal medulla
- Can cause vasoconstriction & vasodilation (in low concentrations)
- Acts on on beta adrenoreceptors at LOW concentrations for vasodilation
- Acts on alpha adrenoreceptors at HIGH concentrations for vasoconstriction
What does intrinsic factors of control of blood flow mean?
INTRINSIC FACTORS
- Regulation of blood flow to an organ by factors originating from within the organ
- e.g. PARACRINE
- Autoregulation by (pathways)
-
Metabolic
- e.g. CO2, H+, K+, adenosine, O2
- Myogenic (vasoconstricting)
-
Endothelial
- NO (vasodilating)
- Endothelin (vasoconstricting)
What does the extrinsic factors of control of blood flow mean & what are the types & give examples?
- Regulation of blood flow to an organ by factors originating OUTSIDE the organ
-
NEURAL
- Sympathetic vasoconstrictor fibres
- Parasympathetic vasodilator fibres (penis, salivary glands, pancreas)
- Sympathetic vasodilator Fibres (sweat glands, cutaneous)
- Nociceptive C-fibres
- Noradrenaline (vasoconstriction)
- Adrenaline (vasoconstriction/vasodilating)
-
ENDOCRINE
- Catecholamines
- Anti-diuretic hormone (vasoconstrictor)
- Angiotensin II (vasoconstrictor)
- Insulin (vasodilator)
- Oestrogen (vasodilator & hypotensive agent)
- Relaxin (vasodilator)
Equation to work out arterial blood pressure
ABP = CO x TPR
Arterial blood pressure = Cardiac output x Total peripheral resistance
What happens in the cardiovascular system in response to exercise?
- ↑Blood Flow to active muscles
- ↑Blood Flow through pulmonary* *circulation
- ↑heat loss via blood flow to skin
-
Maintain Arterial Blood Pressure
- Maintains O2 delivery to active tissues
Explain what the central command does to control blood flow
- A feedforward response that triggers an increase in heart rate prior to exercise onset
- The motor cortex and other motor areas of the brain responsible for triggering skeletal muscle activation also trigger activation of the medullary cardiovascular control centres – leading to an increase in heart rate prior to exercise
-
Cause:
- ↑ Heart rate
- ↑ Cardiac output
- ↑ Contractility
- ↑ Arterial blood pressure
- Central Command also triggers a central resetting of the arterial baroreflex – allowing for GREATER HYPERTENSION during exercise
- Increases in sympathetic nerve activity at the same arterial blood pressure – indicating a modulation of the normal arterial baroreflex.
- allows us to temporarily increase the driving force (the pressure gradient) for blood flow during exercise
- Increases in sympathetic nerve activity at the same arterial blood pressure – indicating a modulation of the normal arterial baroreflex.
Explain how skeletal muscle pump mechanism inncreases venous return
- Muscle contraction compresses veins and increases blood pressure – increase rate of venous return
- Increases venous return in proportion to rate of activity
Explain how the respiratory muscle pump mechanism leads to venous return
- INSPIRATION triggers a drop in the intrapleural pressure in the thoracic cavity. This leads to a reduced venous pressure in the vena cava – creating an enhanced pressure gradient to drive venous return
- EXPIRATION causes the converse changes – causing compression of the vena cava driving the enhanced blood flow back to the heart
- As hyperventilation quickens, the faster blood is pumped back to the heart
Explain the effect of blood flow in the body during exercise (areas of the body affected vs at rest) show as graph
How does the cardiovascular system respond to changes in flow?
- Large increases in pulmonary blood flow (eg during exercise) only give small increases in pulmonary blood pressure
- Thanks to:
- Recruitment
- Distention
P=FxR (R must fall)
Explain the different zones of the lungs and how they function
ZONE 1
Top of lung; arterial pressure (Pa) is below zero at the very apex due to the height of the lung and the hydrostatic pressure effect. Pv (pulmonary venous pressure) is more negative than arterial
Intra-alveolar (intrapulmonary) pressure zero everywhere (and is not influenced by height of the lung as it under the influence of atmospheric pressure)
Thus, pressure is higher in the alveoli (zero) than in the capillaries (negative) and this will collapse the capillaries; no flow will occur (zone 1)
The lung in this area is ventilated but not perfused (called wasted ventilation)
ZONE 2
Middle of the lung; arterial pressure (Pa) is getting higher due the hydrostatic effect of the height of the lung (not as high up so higher pressure in capillaries), but Pv is still below zero
Intra-alveolar (intrapulmonary) pressure still zero everywhere; hence it is lower than arterial but higher than venous; flow will occur in the first half of the capillary whilst arterial pressure (pressure in the vessel keeping it open) is higher than alveolar pressure (pressure outside the vessel trying to collapse it). As Pa (BP) falls as blood is driven through the vessel, the pressure will eventually fall lower than alveolar pressure and the capillary will collapse. So some blood flow will occur but will cease at the venous end
ZONE 3/4
Bottom of the lung: arterial pressure (Pa) is HIGHEST due to the effect of gravity increasing hydrostatic pressure below the heart (this also increase Pv). This time venous pressure Pv is also higher than alveolar pressure and thus flow will occur over the full length of the capillary
What does functional/metabolic hyperaemia do to blood flow and how?
Functional (Metabolic) Hyperaemia INCREASES blood flow to active skeletal muscle during exercise
A decrease in blood supply or an increase in demand of oxygen (eg in exercise) causes the tissue to release vasodilator metabolites such as:
- Potassium ions
- Hydrogen ions (lactic acid)
- Phosphate ions
- Carbon dioxide
- Prostaglandins
- Adenosine
An increase in blood pressure can increase blood flow, which causes a ‘wash-out’ of vasodilator metabolites, leading to a loss of vasodilator influence and hence a vasoconstriction, reducing blood flow back to the original level
What are the types of hyperaemia and explain them
ACTIVE hyperaemia is the same as metabolic hyperaemia
REACTIVE hyperaemia occurs after vessel occlusion such as during an operation to a limb where blood flow is cut off by using a pneumatic cuff inflated above arterial pressure, then releasing it after the operation, or following occlusion by a clot that then breaks down.
Explain how coronary perfusion works?
- Coronary arteries located in subendocardial region of the myocardium
- Flow in diastole
- Effected by:
- Reduced diastolic time (tachycardia)
- Increasing HR means the whole cardiac cycle has shorten, however, diastole shortens more than systole. This means at high heart rates coronary perfusion will be reduced – at a time when oxygen demand is high.
- Elevated end-diastolic pressure
- If ventricular EDP is raised then that is transmitted into the coronary vessels and pressure their will be higher during diastole when it normally receives its blood supply due to the pressure gradient between the aorta and the coronary vessel in diastole. Thus the pressure gradient to drive blood through the coronary circulation will be reduced and hence blood flow to the myocardium will be _reduced_.
- Reduced arterial pressure
- A reduced arterial BP in the aorta will have the same affect as in point 2; it will _reduce_ the pressure gradient driving blood flow through the coronary vessels during diastole.
- Reduced diastolic time (tachycardia)
- Metabolic methods increase flow
- These problems magnified in coronary artery disease where there may be narrowing of the coronary vessels and thus increased resistance
