Blood Vessels

Structure of Blood Vessels

Blood vessels are dynamic structures that transport blood throughout the body. They are composed of three main layers, or tunics, each with distinct structural and functional properties.

• Tunica Intima (Inner Layer):

  • Composed of a simple squamous endothelial lining and areolar connective tissue.

  • In arteries, includes an internal elastic membrane (layer of elastic fibers); absent in veins.

• Tunica Media (Middle Layer):

  • Contains concentric sheets of smooth muscle.

  • In arteries, separated from the tunica externa by an external elastic membrane; absent in veins.

• Tunica Externa (Outer Layer):

  • Anchors vessels to surrounding tissues.

  • Arteries: contains collagen and elastic fibers.

  • Veins: contains collagen, elastic fibers, and smooth muscle cells.

  • Vasa vasorum: small vessels in the walls of large arteries and veins, supplying the outer layers.

Comparison of Arteries and Veins:

• Arteries have thicker walls and retain a round lumen; veins have thinner walls and often collapse.

• Arteries are more elastic; veins have larger, flatter lumens and often contain valves (especially in lower limbs).

Structure and Function of Arteries

Introduction to Arteries

Arteries carry blood away from the heart. Their elasticity allows them to absorb pressure waves from each heartbeat, and their contractility enables them to change diameter, regulated by the sympathetic nervous system.

• Vasoconstriction: Contraction of arterial smooth muscle, decreasing diameter.

• Vasodilation: Relaxation of arterial smooth muscle, increasing diameter.

Types of Arteries

• Elastic Arteries (Conducting Arteries):

  • Large vessels (e.g., aorta, pulmonary trunk) near the heart.

  • Tunica media rich in elastic fibers; evens out pulse force.

• Muscular Arteries (Distribution Arteries):

  • Medium-sized; most arteries fall into this category.

  • Tunica media contains many muscle cells.

• Arterioles:

  • Smallest arteries; little or no tunica externa, thin or incomplete tunica media.

Structure and Function of Capillaries

Introduction to Capillaries

Capillaries are the smallest blood vessels, forming networks that permeate all active tissues. They are the principal sites of exchange between blood and interstitial fluid.

• Structure: Endothelial tube with a thin basement membrane; no tunica media or externa; diameter similar to a red blood cell.

Types of Capillaries

• Continuous Capillaries:

  • Complete endothelial lining; found in muscle, skin, CNS.

  • Permit diffusion of water, small solutes, and lipid-soluble materials; restrict most plasma proteins.

• Fenestrated Capillaries:

  • Contain pores (fenestrations) for rapid exchange of water, nutrients, hormones, and small proteins.

  • Locations: choroid plexus, endocrine organs, kidneys, intestinal tract.

• Sinusoids (Discontinuous Capillaries):

  • Largest, most permeable; openings between endothelial cells allow passage of large substances, plasma proteins, and blood cells.

  • Found in liver, spleen, bone marrow; monitored by phagocytic cells.

Capillary Beds (Capillary Plexus)

• Connect one arteriole and one venule.

• Precapillary sphincters guard entrance to each capillary, regulating blood flow in pulses.

Structure and Function of Veins

Introduction to Veins

Veins collect blood from capillaries and return it to the heart. They have larger diameters, thinner walls, and lower blood pressure compared to arteries.

Types of Veins

• Venules: Smallest veins; collect blood from capillaries.

• Medium-sized veins: Most common; thin tunica media, thick tunica externa, often contain valves.

• Large veins: All three tunica layers present; thick tunica externa, thin tunica media.

Venous Valves: Folds of tunica intima that prevent backflow of blood.

Pressure and Resistance

Introduction to Blood Vessel Physiology

Blood vessel physiology involves the movement of blood through the circulatory system, regulated by blood flow, blood pressure, vascular resistance, and tissue perfusion. Blood flows from areas of higher to lower pressure.

• Blood flow: Volume of blood moving through a vessel or organ per minute (mL/min).

Relationship Between Flow, Pressure, and Resistance

• Total capillary blood flow equals cardiac output and is determined by pressure (P) and resistance (R).

• Pressure (P): Generated by the heart to overcome resistance; measured in mmHg.

• Blood flow is directly proportional to pressure and inversely proportional to resistance.

• Pressure Gradient (\( \Delta P \)): Difference in pressure between two ends of a vessel; drives blood flow.

Three Pressures Affecting Blood Flow

• Blood Pressure (BP): Arterial pressure; ranges from 100 mmHg (aorta) to 35 mmHg (capillary bed entrance).

• Capillary Hydrostatic Pressure (CHP): Pressure within capillary beds; declines from ~35 mmHg to 18 mmHg.

• Venous Pressure: Pressure in veins; decreases from venules to right atrium (approaching 0–5 mmHg).

Resistance (R)

• Opposition to blood flow; increased resistance decreases blood flow.

• Most resistance occurs in arterioles (resistance vessels).

• Factors affecting resistance:

  • Vessel Diameter: Most important factor. Vasoconstriction decreases diameter (increases resistance and BP); vasodilation increases diameter (decreases resistance and BP).

  • Blood Viscosity: Thickness of blood; higher viscosity increases resistance (e.g., dehydration, polycythemia).

  • Vessel Length: Longer vessels increase resistance; obesity increases total vascular length and can contribute to hypertension.

  • Turbulence: Swirling action disturbs smooth flow; atherosclerotic plaques increase turbulence.

• Total Peripheral Resistance: Combined resistance of all systemic blood vessels; arterioles contribute most.

Overview of Cardiovascular Pressures

• Systolic Pressure: Pressure during ventricular contraction.

• Diastolic Pressure: Pressure during ventricular relaxation.

• Example: 120/80 mmHg (systolic/diastolic).

• Mean Arterial Pressure (MAP): Average pressure driving blood into tissues.

  • \( MAP \approx \text{Diastolic Pressure} + \frac{1}{3} (\text{Systolic} - \text{Diastolic}) \)

• Venous Pressure and Venous Return: Amount of blood returning to the right atrium per minute; assisted by skeletal muscle compression and the respiratory pump.

• Blood Flow Velocity: Highest in the aorta, lowest in capillaries; slow capillary flow allows efficient exchange.

• Elastic Rebound: Large arteries stretch during systole and rebound during diastole, maintaining continuous blood flow.

Capillary Exchange

Capillary Pressures and Exchange Mechanisms

Materials move across capillary walls by diffusion, filtration, and reabsorption.

• Diffusion: Movement down concentration gradients; lipid-soluble substances (O2, CO2) cross membranes, water-soluble substances move through clefts or fenestrations.

• Filtration: Pressure-driven movement of water and small solutes out of capillaries; driven by hydrostatic pressure.

• Reabsorption: Osmosis-driven movement of water into capillaries; driven by blood colloid osmotic pressure (BCOP), mainly due to albumin.

Bulk Flow and Net Filtration Pressure (NFP)

• Bulk flow ensures constant exchange between plasma and interstitial fluid, distributing nutrients and removing wastes.

• Lymphatic vessels return excess interstitial fluid and proteins to the blood.

• Net Filtration Pressure (NFP): Determines direction of fluid movement.

  • \( NFP = \text{Hydrostatic Pressure} - \text{Colloid Osmotic Pressure} \)

• Capillaries filter more fluid than they reabsorb; excess is returned by the lymphatic system (~3.6 L/day).

Tissue Perfusion

Overview

Tissue perfusion is the amount of blood flowing through tissues, delivering oxygen and nutrients and removing wastes. It is regulated by cardiac output, peripheral resistance, and blood pressure.

• Blood flow to a region increases when cells become active.

• Cardiovascular regulation ensures appropriate changes in blood flow without compromising vital organs.

Vasomotion and Local Control

• Vasomotion: Cyclical contraction and relaxation of precapillary sphincters, altering capillary blood flow.

• Autoregulation: Immediate, localized adjustments in blood flow via changes in peripheral resistance; does not involve nervous or endocrine systems.

• Local Vasoconstrictors: Reduce blood flow (e.g., endothelins from damaged endothelium).

• Local Vasodilators: Increase blood flow (e.g., low O2, high CO2, lactate, nitric oxide, high K+/H+, histamine, elevated temperature).

Neural Mechanisms

• Cardiovascular Centers (Medulla Oblongata):

  • Cardioacceleratory Center (CAC): Increases heart rate and contractility.

  • Cardioinhibitory Center (CIC): Decreases heart rate and contractility.

  • Vasomotor Center: Controls vasoconstriction (arteriole contraction) and vasodilation (smooth muscle relaxation).

  • Vasomotor Tone: Maintained by constant sympathetic stimulation.

• Reflex Control:

  • Baroreceptor Reflexes: Respond to changes in blood pressure; stretch receptors in carotid sinuses, aortic arch, and right atrium.

  • Atrial (Bainbridge) Reflex: Increased atrial stretch increases heart rate.

  • When BP rises: decreased cardiac output, vasodilation; when BP falls: increased cardiac output, vasoconstriction.

  • Chemoreceptor Reflexes: Respond to changes in pH, O2, and CO2; located in carotid and aortic bodies, and brainstem.

Endocrine Mechanisms

• Epinephrine and Norepinephrine: From adrenal medulla; increase cardiac output and vasoconstriction.

• Antidiuretic Hormone (ADH): From posterior pituitary; elevates BP, reduces water loss at kidneys.

• Angiotensin II: Formed in response to low renal perfusion; causes vasoconstriction, aldosterone and ADH release, and stimulates thirst.

• Erythropoietin (EPO): From kidneys in response to low O2; increases RBC production.

• Natriuretic Peptides (ANP, BNP): From heart in response to excessive stretching; lower blood volume and BP, reduce heart stress.