Chapter 19: The Cardiovascular System – Blood Vessels (Anatomy & Physiology Study Notes)

Blood Vessel Structure and Function

Overview of Blood Vessels

Blood vessels form a closed delivery system that begins and ends at the heart. They are dynamic structures that can constrict, relax, and even multiply, working in conjunction with the lymphatic system to circulate blood throughout the body.

• Arteries carry blood away from the heart.

• Veins carry blood toward the heart.

• Capillaries are exchange vessels, directly serving cellular needs by allowing substances to move across their walls between tissue cells and blood.

Systemic arteries usually carry oxygen-rich blood, while systemic veins carry oxygen-poor blood. The pulmonary circuit is the exception, where pulmonary arteries carry oxygen-poor blood to the lungs and pulmonary veins return oxygen-rich blood to the heart.

Structure of Blood Vessel Walls

General Structure

Except for capillaries, blood vessel walls consist of three layers, or tunics, surrounding a central blood-containing space called the lumen:

• Tunica intima: Innermost layer, in direct contact with blood. Composed of endothelium (simple squamous epithelium) and a subendothelial layer (in larger vessels).

• Tunica media: Middle layer, mostly smooth muscle and sheets of elastin. Responsible for vasoconstriction and vasodilation, thus regulating blood flow and pressure.

• Tunica externa (adventitia): Outermost layer, composed mainly of collagen fibers that protect, reinforce, and anchor the vessel to surrounding structures. Contains nerves, lymphatic vessels, and in larger vessels, its own blood supply (vasa vasorum).

Capillary walls consist only of endothelium with a sparse basal lamina, allowing for efficient exchange of materials.

Types of Blood Vessels

Arteries

• Elastic arteries: Thick-walled, near the heart (e.g., aorta). Act as pressure reservoirs, expanding and recoiling as blood is ejected from the heart. Contain more elastin and are also called conducting arteries.

• Muscular arteries: Distribute blood to specific organs. Have the thickest tunica media relative to their size, more smooth muscle, and are more active in vasoconstriction.

• Arterioles: Smallest arteries, leading into capillary beds. Control blood flow into capillaries via vasodilation and vasoconstriction. Major determinants of total peripheral resistance (TPR).

Capillaries

Capillaries are the smallest blood vessels and are the primary sites for exchange between blood and tissues. Their thin walls consist of only the tunica intima.

• Continuous capillaries: Least permeable, most common. Found in skin, muscles, lungs, and CNS. Endothelial cells joined by tight junctions with intercellular clefts for limited passage of fluids and small solutes.

• Fenestrated capillaries: Have pores (fenestrations) that increase permeability. Found in areas of active filtration (kidneys), absorption (intestines), and hormone secretion (endocrine glands).

• Sinusoidal capillaries: Most permeable, least common. Found in liver, bone marrow, spleen, and adrenal medulla. Large intercellular clefts and fenestrations allow passage of large molecules and cells.

Veins

Veins carry blood toward the heart and act as blood reservoirs, containing up to 65% of the blood supply at any time. They have thinner walls and larger lumens compared to arteries.

• Venules: Formed when capillaries unite. Smallest venules consist only of endothelium and pericytes; larger ones have thin tunica media and externa.

• Veins: Formed from converging venules. Have all three tunics, but walls are thinner and lumens larger than arteries. Adaptations such as valves (especially in limbs) prevent backflow of blood.

• Venous sinuses: Specialized, flattened veins with extremely thin walls, supported by surrounding tissues (e.g., coronary sinus, dural sinuses).

Blood Flow, Pressure, and Resistance

Definitions

• Blood flow (F): Volume of blood flowing through a vessel, organ, or the entire circulation in a given period (ml/min). Equivalent to cardiac output (CO) for the entire vascular system.

• Blood pressure (BP): Force per unit area exerted on a vessel wall by the blood, measured in mm Hg.

• Resistance (R): Opposition to flow, mainly due to friction between blood and vessel walls. Also called total peripheral resistance (TPR).

Three main sources of resistance:

• Blood viscosity (thickness)

• Blood vessel length (longer = more resistance)

• Blood vessel diameter (smaller diameter = more resistance; most important factor)

Relationship:

• Flow (F) is directly proportional to the pressure gradient (ΔP) and inversely proportional to resistance (R):

$F = \frac{\Delta P}{R}$

Blood Pressure Throughout the Circulation

• Blood pressure is highest in the aorta and declines throughout the pathway, reaching 0 mm Hg in the right atrium.

• The steepest drop occurs in the arterioles, where resistance is greatest.

Arterial Blood Pressure

• Systolic pressure: Pressure during ventricular contraction (average 120 mm Hg).

• Diastolic pressure: Pressure during ventricular relaxation (average 70–80 mm Hg).

• Pulse pressure: Difference between systolic and diastolic pressure.

• Mean arterial pressure (MAP): Pressure that propels blood to tissues. Calculated as:

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

Venous Blood Pressure

• Venous pressure is steady and much lower than arterial pressure (~15 mm Hg gradient from venules to heart).

• Adaptations to assist venous return include the muscular pump, respiratory pump, and sympathetic venoconstriction.

Regulation of Blood Pressure

Short-Term Regulation: Neural Controls

• Neural controls maintain adequate MAP by altering vessel diameter and blood distribution.

• Baroreceptors (in carotid sinuses, aortic arch) detect changes in pressure and send signals to the cardiovascular center in the medulla.

• Increased MAP stimulates baroreceptors, leading to vasodilation and decreased heart rate (lowering MAP). Decreased MAP has the opposite effect.

• Chemoreceptors respond to changes in CO2, pH, and O2 levels, increasing CO and MAP when needed.

• Higher brain centers (hypothalamus, cortex) can modify arterial pressure during stress or exercise.

Short-Term Regulation: Hormonal Controls

• Epinephrine and norepinephrine increase CO and vasoconstriction.

• Angiotensin II stimulates vasoconstriction and release of aldosterone and ADH, increasing blood volume.

• Atrial natriuretic peptide (ANP) decreases MAP by promoting vasodilation and reducing blood volume.

• Antidiuretic hormone (ADH, vasopressin) conserves water and causes vasoconstriction.

Long-Term Regulation: Renal Mechanisms

• Kidneys regulate blood volume independently of hormones (direct mechanism) or via the renin-angiotensin-aldosterone system (indirect mechanism).

• Increased blood pressure/volume leads to increased urine output, reducing blood volume and pressure.

• Decreased blood pressure/volume triggers renin release, leading to angiotensin II formation, which increases MAP by vasoconstriction, aldosterone/ADH release, and stimulating thirst.

Capillary Exchange and Bulk Flow

Capillary Transport Mechanisms

• Lipid-soluble substances diffuse directly through endothelial cell membranes.

• Water-soluble substances pass through intercellular clefts or fenestrations.

• Larger molecules (e.g., proteins) are transported via vesicles (endocytosis/exocytosis).

Bulk Flow: Fluid Movements

• Bulk flow is the movement of fluid across capillary walls, driven by hydrostatic and osmotic pressures.

• Hydrostatic pressure (HP): Force exerted by fluid against the vessel wall; pushes fluid out of capillaries (filtration).

• Osmotic (oncotic) pressure (OP): Created by plasma proteins; pulls fluid into capillaries (reabsorption).

• Net filtration pressure (NFP) determines the direction of fluid movement:

$NFP = (HP_c + OP_{if}) - (HP_{if} + OP_c)$

• At the arterial end, NFP is positive (filtration out); at the venous end, NFP is negative (reabsorption in).

• Excess interstitial fluid is returned to the blood via the lymphatic system.

Regulation of Water Balance

Water Intake and Output

• Water intake must equal output (~2500 mL/day).

• Intake: beverages, food, metabolic water.

• Output: urine, insensible loss (skin, lungs), sweat, feces.

• Osmolality is maintained around 280–300 mOsm; regulated by thirst and ADH release.

Regulation of Water Intake and Output

• Thirst mechanism is controlled by the hypothalamic thirst center, stimulated by increased plasma osmolality, dry mouth, or decreased blood volume/pressure.

• Obligatory water losses include insensible loss and minimum urine output.

• ADH regulates water reabsorption in the kidneys; increased ADH decreases urine output, conserving water.

Summary Table: Comparison of Arteries, Veins, and Capillaries

Feature	Arteries	Veins	Capillaries
Wall Thickness	Thick (especially tunica media)	Thin (especially tunica media)	Very thin (only endothelium)
Lumen Size	Smaller, round	Larger, often collapsed	Smallest
Valves	Absent	Present (especially in limbs)	Absent
Main Function	Carry blood away from heart; pressure reservoirs	Carry blood toward heart; blood reservoirs	Exchange of gases, nutrients, wastes

Key Equations

• Flow, pressure, and resistance:

$F = \frac{\Delta P}{R}$

• Mean arterial pressure:

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

• Net filtration pressure (capillaries):

$NFP = (HP_c + OP_{if}) - (HP_{if} + OP_c)$

Additional info:

• These notes are based on Marieb Human Anatomy & Physiology, Chapter 19, and are suitable for college-level Anatomy & Physiology students.

• For further study, review the referenced textbook sections and complete any assigned review questions or quizzes.