6.4 Blood vessels and hemodynamics

Blood vessels form the structural network that delivers blood to every tissue in the body. Understanding their design and the physics governing blood flow is central to cardiovascular physiology, since even small structural or functional changes in vessels can dramatically alter tissue perfusion.

This section covers vessel anatomy, the physical laws that govern hemodynamics, and the mechanisms of exchange at the capillary level.

Structure and Function of Blood Vessels

Arteries and Arterioles

Arteries carry blood away from the heart toward the body's tissues. (Note: most systemic arteries carry oxygenated blood, but the pulmonary arteries carry deoxygenated blood to the lungs.)

Their walls are built to handle high pressure. They contain three layers (tunica intima, media, and adventitia), with the tunica media being especially thick and rich in smooth muscle and elastic tissue. The elastic tissue in large arteries like the aorta allows them to stretch during systole and recoil during diastole, smoothing out pulsatile flow.

Arterioles branch off from small arteries and lead into capillary beds. They have a high ratio of smooth muscle to lumen diameter, which gives them the ability to constrict or dilate significantly. This vasomotion is what makes arterioles the primary regulators of blood flow distribution to different tissues.

Capillaries

Capillaries are the smallest blood vessels and the actual site of exchange between blood and tissues. Their walls are just a single layer of endothelial cells, which minimizes the diffusion distance for gases, nutrients, and waste products.

• Intercellular clefts (gaps between endothelial cells) allow passage of water and small water-soluble solutes
• Lipid-soluble substances like $O_2$ and $CO_2$ diffuse directly through the endothelial cell membranes
• The basement membrane and glycocalyx (a carbohydrate-rich layer on the endothelial surface) act as filtration barriers, restricting passage of larger molecules like plasma proteins

Different tissues have different capillary types (continuous, fenestrated, sinusoidal), which reflects how much exchange each tissue requires.

Venules and Veins

Venules collect blood from capillary beds and merge into progressively larger veins. Compared to arterioles, venules have thinner walls and less smooth muscle.

Veins return blood to the heart. They operate under much lower pressure than arteries, so they rely on several mechanisms to maintain venous return:

• Valves inside veins prevent backflow of blood, especially in the limbs
• Skeletal muscle pump: contraction of surrounding muscles compresses veins and pushes blood toward the heart
• Respiratory pump: pressure changes in the thoracic and abdominal cavities during breathing help draw blood back to the heart

Veins also serve as capacitance vessels, holding roughly 60–70% of total blood volume at any given time. Venoconstriction can mobilize this blood reserve when needed (e.g., during exercise or hemorrhage).

Factors Influencing Blood Flow

Blood flow is the volume of blood passing through a vessel (or group of vessels) per unit time, typically expressed in mL/min.

Vessel Radius and Length

Vessel radius is by far the most powerful factor affecting resistance to blood flow. Because of the fourth-power relationship in Poiseuille's law, even a small change in radius produces a large change in flow:

• A 50% reduction in radius increases resistance by roughly 16-fold
• Vasoconstriction decreases radius → increases resistance → reduces flow
• Vasodilation increases radius → decreases resistance → increases flow

Vessel length also affects resistance. Longer vessels create more friction between blood and the vessel wall. However, vessel length doesn't change much in a given individual, so it's not a major regulatory variable day-to-day. (It does become relevant in obesity, where additional vasculature must perfuse extra tissue mass.)

Blood Viscosity and Pressure Gradient

Blood viscosity refers to the internal resistance of blood to flow, largely determined by hematocrit (the percentage of blood volume occupied by red blood cells; normal range is roughly 38–50%).

• Higher hematocrit → higher viscosity → more resistance (as seen in polycythemia, where excess RBCs thicken the blood)
• Lower hematocrit → lower viscosity → less resistance (as seen in anemia, where fewer RBCs thin the blood)

The pressure gradient ($\Delta P$) is the difference in blood pressure between two points along a vessel. This is the driving force for flow. Blood always moves from higher pressure to lower pressure, and a larger gradient produces a greater flow rate.

Poiseuille's Law and Hemodynamics

Applying Poiseuille's Law

Poiseuille's law provides the quantitative relationship between flow and the physical properties of the vessel and fluid:

$Q = \frac{\pi \Delta P r^4}{8 \eta l}$

Where:

• $Q$ = volumetric flow rate
• $\Delta P$ = pressure gradient between the two ends of the vessel
• $r$ = vessel radius
• $\eta$ = blood viscosity
• $l$ = vessel length

The key takeaway is the $r^4$ term. Doubling the radius increases flow by $2^4 = 16$ times. This is why the body primarily regulates blood flow by adjusting vessel radius rather than changing viscosity or vessel length.

Keep in mind that Poiseuille's law assumes steady, laminar flow through a rigid tube. Real blood vessels are elastic and blood flow is pulsatile, so the equation is an idealized model. Still, it captures the core relationships accurately enough to be clinically and physiologically useful.

Arterioles and Resistance

Arterioles are called the resistance vessels because they account for the greatest drop in blood pressure across the cardiovascular system. Their ability to change diameter makes them the main control point for distributing cardiac output.

Regulation of arteriolar tone happens through multiple mechanisms:

• Sympathetic nervous system: norepinephrine binds alpha-1 adrenergic receptors on arteriolar smooth muscle → vasoconstriction → increased resistance (part of the fight-or-flight response and blood pressure regulation)
• Local metabolic factors: active tissues release metabolites ($CO_2$, $H^+$, adenosine, $K^+$) that cause local vasodilation → decreased resistance → increased flow to match metabolic demand. This is called active hyperemia.
• Endothelial factors: endothelial cells release nitric oxide (NO), which relaxes smooth muscle and promotes vasodilation, and endothelin-1, which promotes vasoconstriction
• Myogenic response: arteriolar smooth muscle contracts in response to stretch (increased transmural pressure), helping to autoregulate flow

Capillary Exchange Mechanisms

Diffusion and Bulk Flow

Diffusion is the primary mechanism for gas and nutrient exchange at the capillary level. Substances move down their concentration gradients across the thin capillary wall.

Fick's law of diffusion states:

$J = -D \cdot A \cdot \frac{\Delta C}{\Delta x}$

Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, $A$ is the surface area, $\Delta C$ is the concentration difference, and $\Delta x$ is the diffusion distance. In practical terms: diffusion is faster when the concentration gradient is steeper, the surface area is larger, and the wall is thinner.

• $O_2$, $CO_2$, and other lipid-soluble substances cross by simple diffusion through endothelial membranes
• Water-soluble solutes (ions, glucose) pass through intercellular clefts or specific transport mechanisms

Bulk flow is the movement of fluid (and dissolved solutes) across the capillary wall driven by pressure differences rather than concentration gradients. This is governed by Starling forces:

• Capillary hydrostatic pressure ($P_c$): pushes fluid out of the capillary
• Interstitial hydrostatic pressure ($P_i$): opposes filtration (usually near zero or slightly negative)
• Plasma colloid osmotic pressure ($\pi_c$): generated by plasma proteins (mainly albumin), pulls fluid into the capillary
• Interstitial colloid osmotic pressure ($\pi_i$): pulls fluid out of the capillary (usually small)

Filtration and Reabsorption

The balance of Starling forces shifts along the length of the capillary:

1. At the arteriolar end, capillary hydrostatic pressure is relatively high (~35 mmHg in many tissues) and exceeds the opposing osmotic pressure (~25 mmHg). The net force pushes fluid out into the interstitial space. This is filtration.
2. Along the capillary, hydrostatic pressure drops as blood moves through the narrow vessel.
3. At the venular end, hydrostatic pressure has fallen (~15 mmHg) below the plasma osmotic pressure. The net force pulls fluid back into the capillary. This is reabsorption.

Filtration slightly exceeds reabsorption overall, so there's a small net outward movement of fluid. The lymphatic system picks up this excess interstitial fluid and returns it to the venous circulation, maintaining fluid balance.

Edema occurs when filtration significantly exceeds reabsorption and lymphatic drainage. Common causes include increased capillary hydrostatic pressure (e.g., heart failure), decreased plasma protein concentration (e.g., liver disease, nephrotic syndrome), increased capillary permeability (e.g., inflammation), or lymphatic obstruction.

Active Transport and Transcytosis

Most small molecules cross the capillary wall passively, but some substances require active mechanisms:

• Active transport uses carrier proteins and ATP to move molecules like glucose and amino acids against their concentration gradients. This helps maintain a steady nutrient supply to tissues even when blood concentrations fluctuate.
• Transcytosis is vesicular transport across endothelial cells. The cell engulfs a substance on one side (via endocytosis), shuttles the vesicle across, and releases it on the other side (via exocytosis). This is how large molecules like albumin, certain hormones, and lipoproteins cross the capillary wall in a controlled, selective manner.