Factors Affecting Blood Flow

Factors Affecting Blood Flow

• Blood flow through tissues and organs is influenced by vascular resistance.
• Vasodilation decreases resistance and increases blood flow.
• Vasoconstriction has the opposite effect.

Estimated Distribution of Cardiac Output at Rest

• The gastrointestinal tract and liver receive 24% of cardiac output at rest (1,400 mL/min).
• Kidneys receive 19% (1,100 mL/min).
• The brain receives 13% (750 mL/min).
• The heart itself receives 4% (250 mL/min).
• Skeletal muscles receive 21% (1,200 mL/min).
• Skin receives 9% (500 mL/min).
• Other organs combined comprise 10% (600 mL/min).

Physical Laws Describing Blood Flow

• Blood flow is driven by a pressure difference across the vascular system (ΔP = P1 - P2).
• Flow rate is directly proportional to the pressure difference.
• Flow rate is inversely proportional to resistance (Flow = ΔP/R).
• Resistance is proportional to vessel length (L) and blood viscosity and inversely proportional to the 4th power of the vessel radius.

Poiseuille's Law

• Poiseuille's Law describes factors affecting blood flow.
• Blood flow is directly proportional to the fourth power of the radius (r4)and the pressure difference (ΔP).
• Blood flow is inversely proportional to the length of the vessel (L) and blood viscosity (η).
• Flow = ΔPr^4 / (8η*L)

Extrinsic Regulation of Blood Flow

• Sympathoadrenal activation increases cardiac output (CO) and peripheral/visceral resistance, increasing blood flow to skeletal muscles.
• Skeletal muscle arterioles dilate in response to epinephrine and norepinephrine, which promotes blood flow.
• Blood is diverted from visceral and skin tissues to skeletal muscle.
• Parasympathetic nervous system stimulation generally has vasodilatory effects, primarily on the digestive tract, genitalia, and salivary glands.

Paracrine Regulation of Blood Flow

• Endothelium releases paracrine factors, such as nitric oxide (NO), bradykinin, and prostacyclin that promote vascular relaxation.
• Nitric oxide is crucial in setting the resting tone of blood vessels.
• Vasodilator drugs like nitroglycerin act through NO.
• Endothelin 1 is a vasoconstrictor produced by the endothelium.

Intrinsic Regulation (Autoregulation) of Blood Flow

• Intrinsic mechanisms maintain a relatively constant blood flow despite blood pressure changes.
• Myogenic control: vascular smooth muscle contracts when stretched (high pressure) and relaxes when not stretched (low pressure).
• Metabolic control: blood flow adjusts to match local tissue needs. Low O2, high CO2 or H+ levels stimulate vasodilation to increase blood flow to meet metabolic demands (active hyperemia).

Aerobic Requirements of the Heart

• The heart needs a consistent, adequate blood supply to meet its high aerobic demands.
• Each myocardial cell is close to a capillary.
• Myocardium contains many mitochondria and aerobic enzymes.

Coronary Blood Flow Regulation

• Sympathetic activity affects coronary blood flow; norepinephrine causes vasoconstriction; epinephrine causes vasodilation.
• Exercise-induced dilation is primarily due to intrinsic factors.

Circulatory Changes During Exercise

• Exercise initiates sympathetic activity leading to vasodilation in active skeletal muscles due to epinephrine and norepinephrine release.
• Blood flow is diverted from non-essential tissues (viscera, skin).
• Brain blood flow remains relatively constant.
• With prolonged exercise, intrinsic vasodilation factors in active skeletal muscles are more significant.
• Sympathetic stimulation increases cardiac output (CO) and stroke volume (SV), but also increases vascular resistance due to increased heart rate (HR) and ejection fraction.

Cerebral Circulation

• The brain receives approximately 15% of resting cardiac output (750 mL/min).
• Cerebral blood flow is relatively constant across various conditions to maintain consciousness.
• Cerebral blood flow is primarily regulated intrinsically.
• Cerebral arterioles constrict when blood pressure increases and dilate when it decreases.
• Arteriolar dilation/constriction is responsive to local changes in CO2 levels and neural activity (metabolic regulation).

Cutaneous Blood Flow

• Skin acts as a heat exchanger for thermoregulation.
• Skin blood flow adjusts to maintain a core body temperature of 37°C.
• Changes in blood flow are mediated by arterial dilation/constriction with the participation of arteriovenous anastomoses.
• Sympathetic activity dramatically changes blood flow to skin, directing blood flow to/away from the skin depending on temperature or the "fight or flight" response.