The Cardiovascular System II: The Blood Vessels – Structure, Function, and Physiology

The Cardiovascular System II: The Blood Vessels

Overview of Vascular Systems

The vascular system is essential for transporting blood throughout the body, regulating blood flow to tissues, controlling blood pressure, and secreting various chemicals. It consists of two main circulatory circuits: the pulmonary and systemic vessels.

• Pulmonary vessels: Transport blood from the right ventricle, through the lungs, and back to the left atrium.

• Systemic vessels: Transport blood from the left ventricle throughout the body and return it to the right atrium.

Types of Blood Vessels

There are three primary types of blood vessels, each with distinct structures and functions:

• Arteries: Carry blood away from the heart. Subtypes include elastic arteries, muscular arteries, and arterioles.

• Capillaries: Serve as sites of exchange between blood and tissues, forming capillary beds.

• Veins: Drain blood from capillary beds and return it to the heart. Subtypes include venules, small veins, and large veins.

Structure of Blood Vessel Walls

Arteries and veins share a similar three-layered structure, but with differences in thickness and composition:

• Tunica interna (intima): Innermost layer, composed of simple squamous epithelium (endothelium) and connective tissue; in direct contact with blood.

• Tunica media: Middle layer, primarily smooth muscle and elastic fibers; responsible for vasoconstriction and vasodilation. Thicker in arteries.

• Tunica externa (adventitia): Outermost layer, composed of areolar connective tissue, collagen, elastin fibers, nerves, and lymphatic vessels.

Types of Arteries

• Elastic (Conducting) Arteries: Thick-walled, large diameter, located near the heart. High elastic tissue content allows them to withstand and smooth out pressure fluctuations.

• Muscular (Distributing) Arteries: Medium-sized, regulate blood flow to organs. Thick smooth muscle layer for vasoconstriction and vasodilation.

• Arterioles: Smallest arteries, control blood flow into capillary beds and regulate resistance.

Aging and Pathology of Arteries

• Atherosclerosis: Plaque deposition on vessel walls, commonly in the aorta and coronary arteries.

• Arteriosclerosis: Hardening and loss of elasticity in arteries, often resulting from atherosclerosis.

Types of Capillaries

Capillaries are classified based on their permeability and structure:

• Continuous capillaries: Endothelial cells joined by tight junctions; found in most tissues.

• Fenestrated capillaries: Contain pores (fenestrations); found in kidneys, endocrine glands, and small intestine.

• Sinusoidal capillaries: Discontinuous endothelium with large gaps; found in liver, bone marrow, and spleen.

Capillary Networks

Capillaries form interconnected networks (capillary beds) that regulate blood flow to tissues via precapillary sphincters and metarterioles.

Hemodynamics: Blood Flow and Resistance

Hemodynamics refers to the principles governing blood flow in the circulatory system:

• Laminar flow: Smooth, silent flow in the center of the vessel.

• Turbulent flow: Disrupted flow, often due to vessel constriction or irregularities, producing sounds.

• Blood flow: Volume of blood passing through a vessel per minute; matches cardiac output (about 5 L/min at rest).

• Blood flow is directly proportional to blood pressure and inversely proportional to resistance:

$\text{Blood Flow} \propto \frac{\text{Blood Pressure}}{\text{Resistance}}$

Velocity and Cross-Sectional Area

As blood moves from larger to smaller vessels, total cross-sectional area increases and velocity decreases, allowing efficient exchange in capillaries.

Factors Affecting Blood Flow

• Viscosity: Resistance to flow; increased by higher hematocrit or dehydration.

• Vessel length: Longer vessels increase resistance.

• Peripheral resistance: Determined by vessel radius, viscosity, length, and obstructions.

Blood Pressure

Blood pressure is the force exerted by blood on vessel walls, measured in mmHg. It is highest in the arteries and decreases through the vascular system.

• Factors determining blood pressure: Peripheral resistance, cardiac output, and blood volume.

Cardiac Output and Blood Pressure

Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV):

$\text{CO} = \text{HR} \times \text{SV}$

Blood pressure is determined by the equation:

$\Delta P = \text{CO} \times \text{PR}$

Where $\Delta P$ is the pressure gradient and PR is peripheral resistance.

Vessel Compliance

Compliance is the ability of blood vessels to stretch. Veins are highly compliant, acting as blood reservoirs. Decreased compliance increases blood pressure.

Systemic Blood Pressure and Mean Arterial Pressure (MAP)

Systemic blood pressure declines as blood moves from arteries to veins. MAP is the average pressure in the arteries and is calculated as:

$\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3}(\text{Systolic} - \text{Diastolic})$

Pulse Pressure

Pulse pressure is the difference between systolic and diastolic pressures.

Venous Blood Pressure and Adaptations

Venous pressure is low and steady. Adaptations such as skeletal muscle pumps, valves, and respiratory pumps help return blood to the heart.

Short-Term Regulation of Blood Pressure

Short-term regulation involves the nervous and endocrine systems:

• Hormones: Epinephrine, norepinephrine, thyroid hormone, angiotensin-II, and ANP affect cardiac output and resistance.

• Baroreceptor reflexes: Respond to changes in blood pressure by adjusting heart rate and vessel diameter.

• Chemoreceptor reflexes: Respond to changes in blood oxygen, carbon dioxide, and pH.

Long-Term Regulation of Blood Pressure

Long-term regulation is managed by the urinary and endocrine systems, primarily through control of blood volume:

• Renin-angiotensin-aldosterone system (RAAS): Increases blood volume and pressure.

• Vasopressin (ADH): Promotes water retention.

• Atrial natriuretic peptide (ANP): Promotes water and sodium loss, lowering blood volume.

Disorders of Blood Pressure

• Hypertension: Blood pressure above 120/80 mmHg; can be essential (unknown cause) or secondary (identifiable cause).

• Hypotension: Blood pressure below 90/60 mmHg; can lead to organ failure and is potentially fatal.

Tissue Perfusion and Autoregulation

Tissue perfusion is the delivery of blood to tissues via capillary beds, regulated by local (autoregulation) mechanisms:

• Myogenic mechanism: Alters arteriolar resistance in response to pressure changes.

• Metabolic controls: Mediated by local chemical signals from tissue metabolism.

Capillary Exchange Mechanisms

Capillary exchange occurs via diffusion, transcytosis, and bulk flow:

• Diffusion: Lipid-soluble substances cross membranes; water-soluble substances pass through intercellular spaces or fenestrations.

• Transcytosis: Transport of large molecules via vesicles.

Water Movement Across Capillaries

Water movement is governed by hydrostatic and osmotic pressures:

• Hydrostatic pressure (HP): Pushes water out of capillaries.

• Osmotic pressure (OP): Pulls water into capillaries.

Net Filtration Pressure (NFP)

NFP determines the direction and amount of fluid movement:

• At the arterial end: HP > OP, so fluid moves out (filtration).

• At the venular end: OP > HP, so fluid moves in (absorption).

$\text{NFP} = \text{Hydrostatic Pressure} - \text{Colloid Osmotic Pressure}$

Example (arterial end): $35\ \text{mmHg} - 22\ \text{mmHg} = 13\ \text{mmHg}$ (filtration)

Example (venular end): $15\ \text{mmHg} - 22\ \text{mmHg} = -7\ \text{mmHg}$ (absorption)

Edema and Capillary Exchange

Edema is the accumulation of excess fluid in tissues, caused by increased capillary permeability, decreased plasma proteins, or lymphatic obstruction.

Major Arteries and Veins of the Body

The systemic arteries and veins are organized into regions supplying/draining the head, neck, upper limbs, thorax, abdomen, and lower limbs. The Circle of Willis provides collateral circulation to the brain.

Venous System and Major Veins

Veins return blood to the heart, have thinner walls, larger lumens, and valves to prevent backflow. Major veins are organized similarly to arteries, with additional venous sinuses in the brain and the hepatic portal system in the abdomen.

Summary Table: Pressures Involved in Capillary Exchange

Term	Definition
BP	Blood pressure (hydrostatic pressure in capillary)
IFP	Interstitial fluid pressure
BCOP	Blood colloid osmotic pressure
ICOP	Interstitial fluid colloid osmotic pressure
NFP	Net filtration pressure = (BP - IFP) - (BCOP - ICOP)

References: Amerman, E. C., 2019; Marieb, E. N., et al., 2010; McKinley, M. and O’Loughlin, V. D., 2008; Seeley, R. R., et al., 2010; Tortora and Derrickson, 2014.