Blood Flow and Vessel Anatomy

Blood Flow

• Fluids and nutrients are circulated throughout the body.
• Arteriolar resistance is regulated.
• Silverthorn 7th edition references: 440-443, 475-483, 488-493.
• Silverthorn 8th edition references: 436-440, 476-482, 486-492.

Blood Vessel Anatomy

• Blood vessels have three layers (tunics).
• Tunic Intima: Endothelium, loose connective tissue, internal elastic lamina
• Tunic Media: Smooth muscle, controlled by sympathetic nervous system
• Tunic Externa: Mostly fibrous connective tissue
• Arteries have a thicker tunic media than veins.
• Veins have valves to prevent backflow.
• Capillaries are thin-walled, single-cell layers facilitating nutrient exchange.

Blood Vessel Structure

• Arteries: High pressure, 4.0mm diameter, 1.0mm wall thickness

• Arterioles: 30.0µm diameter, 6.0µm wall thickness

• Capillaries: 8.0µm diameter, 0.5µm wall thickness

• Venules: 20.0µm diameter, 1.0µm wall thickness

• Veins: Low pressure, 5.0mm diameter, 0.5mm wall thickness

• Blood pressure is greatest in the aorta and decreases in the vena cava.

Large Arteries and Veins

• Large arteries (e.g., aorta) comprise 15% of blood volume and have low compliance and capacitance.
• Large veins (e.g., vena cava) comprise 65-80% of blood volume and have high compliance and high capacitance.
• Arteries act as pressure reservoirs.
• Veins act as blood reservoirs during hemorrhage.

Elastic Recoil of Arteries

• Elastic recoil maintains blood flow during ventricular relaxation.

Veins and Venous Return

• Venules drain blood from capillaries to larger veins.
• Veins have less smooth muscle and connective tissue than arteries.
• Veins have valves to prevent backflow.
• Veins carry approximately 70% of the body's blood, acting as a reservoir during hemorrhage.

Capillary Beds

• Capillary beds contain arteriovenous shunts (arteriole to venule) and true capillaries for nutrient exchange.
• Oxygen and nutrients diffuse from capillaries to cells, while carbon dioxide and metabolic waste diffuse from cells into capillaries.
• Capillaries are only one cell layer thick, aiding diffusion.

Fluid Flow

• Fluid flow is directly proportional to the pressure gradient (ΔP).
• Fluid flow is inversely proportional to the resistance of the system (R).
• Flow ∝ ΔP/R

Resistance

• Resistance of a vessel or system inversely affects flow (Flow ∝ 1/R).
• Vessel radius has a significant effect on resistance.
• Resistance increases with length and viscosity.
• Resistance decreases with expanding radius.
• Poiseuille's Law: R = 8Lη/πr² (R is resistance, L is length, η is viscosity, r is radius).

Factors that Alter Arteriolar Resistance

• Myogenic autoregulation,
• Paracrines (local): Active hyperemia, reactive hyperemia,
• Sympathetic control: SNS (norepinephrine), adrenal medulla (epinephrine).

Active Hyperemia

• Increased tissue metabolism releases vasodilators (e.g., adenosine).
• Arterioles dilate increasing blood flow. This increases O2 and nutrient supply.

Reactive Hyperemia

• Reducing blood flow causes metabolic vasodilators to accumulate.
• Arterioles dilate upon resuming blood flow (temporary hyperemia).

Sympathetic Regulation

• Autonomic control of arteriolar diameter with tonic release of norepinephrine.
• Norepinephrine binding to alpha-adrenergic receptors affects blood vessel diameter via calcium-mediated smooth muscle contraction.

Calcium Oscillations

• Calcium oscillations control vascular smooth muscle contraction.