Blood Flow Control Mechanisms

• This lecture discusses the mechanisms for blood flow control, how skeletal and cardiac muscle blood flow matches metabolic demand, and why brain blood flow must remain constant.
• Sympathetic control of skin blood flow and its role in temperature regulation, how lung blood flow matches cardiac output, and the redistribution of systemic blood flow during exercise are covered.

Basic Principles of Blood Flow

• Blood flow to tissues adjusts based on circumstances like eating, inflammation, diving, or exercise.
• Blood flow to individual organs/tissues is regulated independently of cardiac output via local resistance changes.
• Flow equals Pressure divided by Resistance.
  • Vasoconstriction increases resistance and decreases blood flow.
  • Vasodilation decreases resistance and increases blood flow.
• Organs use different mechanisms to control blood flow, including sympathetic/neuronal, endocrine/hormonal, local metabolic/paracrine, and local myogenic vs. metabolic control.
  • Local metabolic/paracrine control is also known as active hyperaemia.
  • Local myogenic vs. metabolic control is also known as flow autoregulation.

Sympathetic and Hormonal Control

• Sympathetic nerve fibers from the medulla innervate arterioles, releasing noradrenaline.
• This is important in muscle, skin, kidney, and gastrointestinal tract TPR and MAP regulation and blood flow redistribution during exercise.
• More noradrenaline released constricts arterioles, decreases blood flow and vice versa.
• The sympathetic nervous system does not influence blood flow in the heart and brain, which use flow autoregulation.
• Vasoactive hormones secreted into the blood control blood flow.
  • Renin/Angiotensin causes vasoconstriction.
  • Atrial Natriuretic Peptide (ANP) causes vasodilation.
• Angiotensin II and ANP are mainly concerned with TPR regulation and therefore MAP in response to blood volume changes.
• Adrenaline can increase or decrease blood flow depending on the receptor subtype expressed on the arterioles.

Active Hyperaemia and Flow Autoregulation

• Active hyperaemia involves increased blood flow because an organ or tissue requires more O2 and fuel to be more metabolically active.
• Active hyperaemia is important in tissues where metabolic activity can change rapidly, and blood flow is controlled by local metabolites.
• Flow autoregulation means an organ or tissue can regulate its activity; a tissue can regulate its blood flow to maintain blood flow despite changing cardiac output or MAP.
• Flow autoregulation has two opposing mechanisms: Metabolic control and myogenic control.
  • Metabolic control increases flow in response to metabolite accumulation.
  • Myogenic control reduces flow in response to increased pressure.
• Auto regulation is especially important in the brain

Control of Skeletal and Cardiac Muscle Blood Flow

• Blood flow must be adjusted to meet changing metabolic demand of the muscle, increasing 6-fold during moderate exercise.
• At rest: muscle arterioles are partially constricted by noradrenaline from sympathetic nerves, acting on α-adrenergic receptors.
• When metabolic demand increases, sympathetic control is overridden by local metabolites and adrenaline, causing vasodilation = Active Hyperaemia.
• Active hyperaemia in skeletal muscle is as follows:
  • Increased work is done by the muscle.
  • Increased consumption of glucose & O2 and the accumulation of CO2 and lactic acid occurs in the muscle.
  • This causes arteriolar vasodilation and increased blood flow, which delivers more O2 and glucose while removing lactate and CO2.
  • This process is also aided by activation of the sympathetic nervous system (SNS), which releases more adrenaline into circulation.

Coronary Circulation

• Coronary blood flow must adjust to meet cardiac muscle's changing metabolic demand.
• During moderate exercise, cardiac output and muscle work double, so coronary blood flow must also double..
• Coronary blood cannot flow when the cardiac muscle is contracted, so blood flow peaks during early diastole, where the myocardium is relaxed.
• Active hyperaemia is achieved through:
  • Increased metabolic demand and increased cardiac output which increases cardiac muscle work.
  • This causes the increased consumption of O2 and increases the production of metabolites.
  • There will also be adenosine from ATP metabolism.
  • All this leads to coronary arteriolar dilation during diastole, which causes increased blood flow in cardiac muscle.

Blood Flow Control in the Brain

• Brain metabolic demand remains relatively constant, even during intense activity.
• Cerebral blood flow must remain relatively constant despite other body changes; therefore, flow autoregulation is achieved in the brain.
• Cerebral arterioles respond to perfusion pressure changes, and flow is tightly controlled:
  • Haemorrhage causes decreased perfusion pressure, and arterioles dilate to prevent low flow
  • Increased CO or MAP increases perfusion pressure, and arterioles constrict in response to prevent high flow.
• If cardiac output increases:
  • Cerebral perfusion pressure increases, and cerebral blood flow increases.
  • CO2 is washed away faster than produced, so the arterioles constrict, decreasing cerebral blood flow back to normal levels.

Skin Blood Flow Control

• Skin blood flow is essential for thermoregulation.
• Unlike other tissues, the aim is not to match skin blood flow to its metabolic demand.
• The main control is neuronal via the sympathetic nervous system to balance heat gain and heat loss.
• A drop in core temperature can be combatted by increased sympathetic output to the skin.
• Increased sympathetic output leads to constriction of arterioles, decreasing skin blood flow and heat lost, which bring core temperature back up.
• A rise in core temperature causes reduced sympathetic output to the skin, leading to dilation of arterioles and increased skin blood flow and heat loss.

Systemic Blood Flow Redistribution During Exercise

• Overall O2 and fuel demand increases 2 to 4-fold during aerobic exercise.
• Heart rate and stroke volume both increase to provide a 2 to 4-fold cardiac output increase.
• Blood is redirected to the parts that need it the most during exercise.
• During moderate exercise when cardiac output doubles from 5 L/min to 10 L/min, blood flow distribution is redirected.
• Skeletal muscle blood flow increases.
• Skin blood flow increases.
• Gastrointestinal tract/kidneys blood flow is reduced by a third.

Blood Flow 'Control' in the Lung

• Blood circulates to the lungs to be oxygenated.
• Pulmonary circulation is in series with the systemic circulation; therefore, cardiac output must be the same on both sides.
• During exercise, the lungs need to absorb more O2 and get rid of more CO2; therefore, lung blood flow increases relative to metabolic demand.
• Pulmonary vascular resistance is low at rest, therefore RV does not need to push as hard as LV to force the same 5L/min through the lungs
• When cardiac output increases, Blood vessels passively dilate and pulmonary vascular resistance falls even lower.
• Blood flow can be increased several-fold without a several-fold increase in pressure.

Summary of Blood Flow Control

• During exercise, the sympathetic nervous system is activated, and cardiac output increases to match blood flow to metabolic demand.
• Muscle blood flow increases several-fold due to sympathetic oversight.
• Coronary blood flow increases with cardiac output under metabolic control.
• Skin blood flow increases to dissipate excess heat.
• Brain blood flow does not change due to metabolic and myogenic control = flow auto-regulation.
• Lung blood flow increases with CO.
• Blood flow is reduced in other organs to redirect to muscles and the heart under sympathetic control.