6.4 Blood vessels and hemodynamics

Blood vessels are the highways of our body, carrying life-giving blood to every nook and cranny. From thick-walled arteries to tiny capillaries, each vessel plays a crucial role in keeping us alive and kicking.

Ever wonder why some parts of your body feel warm while others are cool? It's all about blood flow. Factors like vessel size and blood thickness affect how easily blood moves around, impacting everything from nutrient delivery to temperature regulation.

Structure and Function of Blood Vessels

Arteries and Arterioles

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• Arteries carry oxygenated blood away from the heart to the body's tissues
• Thick walls with smooth muscle and elastic tissue withstand high blood pressure
• Arterioles branch off from arteries and lead to capillaries
  • More smooth muscle relative to diameter allows constriction or dilation to regulate blood flow (vasomotion)

Capillaries

• Smallest blood vessels where exchange of nutrients, gases, and waste occurs between blood and tissues
• Thin walls consisting of a single layer of endothelial cells facilitate diffusion
  • Gaps between endothelial cells allow for passage of fluid and small solutes (lipid-soluble gases, glucose)
  • Basement membrane and glycocalyx act as a filtration barrier for larger molecules (plasma proteins)

Venules and Veins

• Venules receive blood from capillaries and merge to form larger veins
  • Thinner walls and less smooth muscle than arterioles
• Veins carry deoxygenated blood from tissues back to the heart
  • Thinner walls than arteries but contain valves to prevent backflow
  • Skeletal muscle contractions and respiratory pump aid in venous return to the heart

Factors Influencing Blood Flow

Vessel Radius and Length

• Blood flow is the volume of blood flowing through a vessel per unit time
  • Directly proportional to pressure gradient and fourth power of vessel radius
  • Inversely proportional to vessel length and blood viscosity (Poiseuille's law)
• Vessel radius is the most important factor affecting resistance to blood flow
  • Small changes in radius greatly influence flow due to fourth power relationship
  • Vasoconstriction decreases radius and increases resistance, reducing flow
  • Vasodilation increases radius and decreases resistance, increasing flow
• Longer vessels have more surface area for friction, increasing resistance to flow

Blood Viscosity and Pressure Gradient

• Blood viscosity is the thickness or resistance to flow, determined by hematocrit (ratio of red blood cells to plasma)
  • Higher viscosity increases resistance and reduces flow (polycythemia)
  • Lower viscosity decreases resistance and increases flow (anemia)
• Pressure gradient is the difference in blood pressure between two points in a vessel
  • Provides the driving force for blood flow
  • Greater pressure gradient results in greater flow rate

Poiseuille's Law and Hemodynamics

Applying Poiseuille's Law

• Poiseuille's law calculates blood flow based on pressure gradient, vessel radius and length, and blood viscosity
  • $Q = \frac{\pi \Delta Pr^4}{8\eta l}$, where $Q$ is flow rate, $\Delta P$ is pressure gradient, $r$ is radius, $\eta$ is viscosity, and $l$ is length
• Doubling vessel radius increases flow by a factor of 16 due to fourth power relationship
• Increasing vessel length or blood viscosity decreases flow rate by increasing resistance

Arterioles and Resistance

• Arterioles are the primary site of resistance in the cardiovascular system
  • Can constrict or dilate to regulate blood flow to specific tissues based on metabolic needs
  • Sympathetic nervous system stimulation causes vasoconstriction, increasing resistance and decreasing flow (fight-or-flight response)
  • Local factors (tissue metabolites, endothelial factors) cause vasodilation, decreasing resistance and increasing flow (active hyperemia)

Capillary Exchange Mechanisms

Diffusion and Bulk Flow

• Diffusion is the primary mechanism of capillary exchange, driven by concentration gradients
  • Oxygen, carbon dioxide, and lipid-soluble substances move by simple diffusion
  • Fick's law states that diffusion rate is proportional to the concentration gradient and surface area, and inversely proportional to the distance
• Bulk flow is the movement of fluid and solutes across the capillary wall due to hydrostatic and osmotic pressure gradients (Starling's law)
  • Hydrostatic pressure is exerted by blood on the capillary wall, forcing fluid out
  • Osmotic pressure is exerted by plasma proteins, pulling fluid into the capillary

Filtration and Reabsorption

• Filtration occurs at the arterial end of the capillary, where hydrostatic pressure exceeds osmotic pressure
  • Fluid is forced out into the interstitial space
  • Plasma proteins are retained, creating an osmotic gradient
• Reabsorption occurs at the venous end of the capillary, where osmotic pressure exceeds hydrostatic pressure
  • Fluid is drawn back into the capillary
  • Maintains fluid balance between blood and interstitial compartments
• Edema occurs when filtration exceeds reabsorption, leading to excess interstitial fluid accumulation

Active Transport

• Active transport moves larger molecules (glucose, amino acids) across the capillary wall against their concentration gradients
  • Requires carrier proteins and energy in the form of ATP
  • Maintains constant supply of nutrients to tissues despite changes in blood concentration
• Transcytosis is the vesicular transport of macromolecules (hormones, lipoproteins) across endothelial cells
  • Allows for selective uptake and delivery of substances to specific tissues