Blood Vessels: Structure, Function, and Disorders (Anatomy & Physiology Study Notes)

Blood Vessels

Types of Blood Vessels

The cardiovascular system contains three main types of blood vessels, each with distinct functions and structural features.

• Arteries: Carry blood away from the heart and toward capillaries (efferent vessels). They branch into smaller vessels called arterioles, which further branch into capillaries.

• Veins: Carry blood toward the heart (atria) and away from capillaries (afferent vessels). The smallest veins are called venules.

• Capillaries: Carry blood from arterioles (small arteries) to venules (small veins). Chemical and gas exchange occurs here.

Structure of Arteries

Arterial Wall Layers

Arteries have three structural layers that provide strength and flexibility to withstand high pressure.

• Tunica intima: The innermost layer, consisting of a single layer of squamous epithelial cells (endothelial), surrounded by connective tissue with elastic fibers.

• Tunica media: The middle layer, composed of smooth muscle. It is thick in arteries and is important for blood pressure regulation.

• Tunica externa (adventitia): The outer layer, made of elastic connective tissue. It reinforces the wall and may anchor the vessel to other tissues.

Arterioles

• Small arteries that distribute nutrients, gases, and assist in maintaining arterial blood pressure.

• Carry blood away from the heart.

Artery Structure Diagram

Arteries are classified as elastic or muscular based on the proportion of elastic fibers and smooth muscle in their walls.

• Elastic arteries: Large arteries with a prominent internal elastic membrane.

• Muscular arteries: Medium-sized arteries with a thicker tunica media.

• Arterioles: Smallest arteries, mainly composed of smooth muscle cells and endothelium.

Alternate Routes of Blood Flow

Anastomosis

Anastomosis refers to the joining of blood vessels, forming alternate routes for blood flow.

• Arteriovenous anastomosis: Bypasses the capillary bed, connecting an arteriole directly to a venule.

• Arterial anastomosis: Occurs where arteries fuse before branching into arterioles, ensuring delivery of blood to capillary beds in key areas such as the brain and heart.

Disorders of Arteries

Common Arterial Disorders

Arteries must withstand high pressure and remain free of blockage. Several disorders can compromise arterial function:

• Arteriosclerosis: Thickening and hardening of arterial walls due to calcium deposits, reducing blood flow to tissues.

• Atherosclerosis: A specific type of arteriosclerosis involving the buildup of fatty plaques inside arterial walls.

• Ischemia: Decreased blood supply to a tissue, leading to gradual cell death and potentially complete tissue death (necrosis).

• Gangrene: Occurs when necrosis progresses and tissues die.

• Aneurysm: Abnormal widening of the arterial wall, which can promote thrombus (clot) formation and may burst, causing life-threatening hemorrhage.

• Cerebrovascular Accident (CVA or Stroke): Ischemia of brain tissue caused by embolism (blockage) or hemorrhage.

Structure of Veins

Venous Wall Layers

Veins also have three structural layers, but with differences compared to arteries.

• Tunica intima: Endothelium inner layer; contains valves to prevent backflow of blood.

• Tunica media: Smooth muscle; very thin layer compared to arteries.

• Tunica externa: The thickest layer in veins, providing structural support.

Venules

• Small veins that collect blood for return to the heart.

Functions of Veins

• Collect blood for return to the heart; veins are low-pressure vessels.

• Referred to as Blood Reservoirs: Can expand to hold a larger volume of blood or constrict to hold a smaller volume.

Structure of Capillaries

Capillary Wall

Capillaries are microscopic vessels with only one layer—the tunica intima (endothelial cells), one cell thick.

• Entrance to capillaries is regulated by smooth muscle cells called precapillary sphincters, which control blood flow into each capillary bed.

• A network of capillaries is referred to as a capillary bed.

Functions of Capillaries

• Serve as exchange vessels for nutrients, gases, waste, and fluids between blood and tissues.

Blood Pressure

Definition and Measurement

Blood pressure is the force exerted by blood against the walls of blood vessels. It changes as blood moves through the systemic circuit.

• Highest in the aorta (~120 mm Hg), lowest in the venae cavae (~2 mm Hg).

• Arterial pressure is much higher than venous pressure to overcome peripheral resistance.

• Blood pressure fluctuates between systolic pressure (peak during ventricular contraction) and diastolic pressure (minimum at end of ventricular relaxation).

• Pressure gradient causes blood to circulate: liquids flow from areas of higher to lower pressure.

• Blood pressure is recorded as systolic over diastolic (e.g., 120/80 mm Hg).

Factors Affecting Blood Pressure

• Blood volume: Directly affects blood pressure; low volume lowers BP.

• Heart rate: Direct relationship, provided stroke volume remains constant.

• Strength of heart contractions: Stronger contractions increase BP.

• Blood viscosity: Resistance to flow due to interactions among molecules and suspended materials. Blood is about 5x as viscous as water.

• Peripheral resistance: Mainly determined by vessel diameter; smaller diameter increases resistance and BP.

Venous Return Mechanisms

Maintaining Blood Flow to the Heart

• Strongly beating heart

• Adequate arterial blood pressure

• Valves in veins (one-way flow)

• Muscular compression (skeletal muscle pump)

• Respiratory pump (pressure changes in chest cavity during breathing)

Capillary Pressures and Exchange

Filtration and Reabsorption

• Capillary pressure drops from 35 to 18 mm Hg along the capillary length.

• Capillaries are permeable to ions, nutrients, wastes, gases, and water.

• Filtration: Water and solutes move out of the bloodstream into tissues.

• Most materials are reabsorbed into capillaries; remainder picked up by lymphatic vessels.

Regulation of Blood Flow and Pressure

Homeostatic Regulation

Tissue perfusion (blood flow) is regulated to ensure proper delivery at the right time and location without drastic changes in pressure.

• Regulated by autoregulation, neural processes, and endocrine processes.

Autoregulation

• Local adjustments in blood flow made by precapillary sphincters in response to tissue conditions.

• Vasodilators: Promote dilation and increased blood flow (e.g., low O2, high CO2, histamine, nitric oxide).

• Vasoconstrictors: Stimulate constriction and decreased blood flow.

Neural Control

• Cardiovascular center (medulla oblongata): Maintains adequate blood flow by responding to changes in arterial pressure or blood gas levels.

• Contains cardioacceleratory center (increases cardiac output via sympathetic innervation) and cardioinhibitory center (reduces cardiac output via parasympathetic innervation).

• Vasomotor center: Controls diameter of arterioles and peripheral resistance; sympathetic stimulation causes vasoconstriction.

Reflexes

• Baroreceptor reflexes: Respond to changes in blood pressure; found in carotid sinuses, aortic arch, and right atrium.

• Chemoreceptor reflexes: Respond to changes in CO2, O2, and pH; located in carotid and aortic bodies, and medulla oblongata.

Hormonal Regulation of Blood Pressure

Short-Term and Long-Term Hormonal Effects

• Epinephrine (E) and norepinephrine (NE): Stimulate cardiac output and peripheral vasoconstriction (short-term).

• Antidiuretic hormone (ADH), angiotensin II, erythropoietin (EPO): Raise BP when too low (long-term).

• Atrial natriuretic peptide (ANP): Lowers BP when too high.

Key Hormonal Mechanisms

• ADH: Released from posterior pituitary in response to decreased blood volume or increased plasma osmotic concentration; causes vasoconstriction and water conservation by kidneys.

• Angiotensin II: Formed when BP decreases; stimulates cardiac output, arteriolar constriction, release of ADH and aldosterone, and thirst center.

• EPO: Released by kidneys when BP or plasma oxygen drops; stimulates RBC production, increasing blood volume and oxygen-carrying capacity.

• ANP: Released by atrial walls when BP increases; increases sodium and water loss by kidneys, reduces thirst, blocks release of ADH, aldosterone, E, NE, and stimulates arteriolar dilation.

Cardiovascular Responses to Exercise and Hemorrhage

Exercise

• Cardiac output and blood distribution change; output can increase significantly.

• Extensive vasodilation and increased venous return occur.

• Blood flow is shifted away from nonessential organs during high exertion.

Hemorrhage

• Short-term: Carotid and aortic reflexes increase cardiac output and peripheral resistance; venous reservoir mobilized; sympathetic activation triggers vasoconstriction and hormone release.

• Long-term: Fluid retention, increased thirst, and RBC production restore blood volume and pressure.

Clinical Concepts

Hypertension

• Blood pressure consistently exceeds 140/90 mm Hg.

• Risk factors: Genetics, stress, age, obesity, and more.

• If untreated, can lead to heart disease, kidney failure, and stroke.

Pulse

• Definition: Alternate expansion and recoil of the blood vessel wall.

• Pulse points: Locations where the pulse can be easily felt.

Shock

• Failure of the circulatory system to deliver oxygen to tissues adequately, resulting in cell impairment.

• Types: Cardiogenic (heart failure), hypovolemic (blood volume drop), neurogenic (nerve-induced vessel dilation), anaphylactic (severe allergic reaction), septic (infection-related toxins).

Circuits of the Cardiovascular System

Systemic and Pulmonary Circulation

• Systemic circulation: Carries oxygenated blood throughout the body; returns deoxygenated blood to the heart.

• Pulmonary circulation: Carries deoxygenated blood from the right ventricle to the lungs for gas exchange, then returns oxygenated blood to the left atrium.

Major Vessels

• Arteries: Coronary (heart), axillary (armpit), brachial (arm), radial (lateral hand), femoral (thigh), popliteal (leg and foot).

• Veins: Superior vena cava (upper body), inferior vena cava (lower body), coronary (heart), femoral (thigh), popliteal (leg), hepatic portal vein (liver and gallbladder).

Hepatic Portal Circulation

• Unique route through the liver; veins from digestive organs drain into the hepatic portal vein, passing through the liver before reaching the heart.

• Assists with homeostasis of blood glucose and removal of toxins.

Fetal and Newborn Circulation

Fetal Circulation

• Before birth, lungs are collapsed and nutrients/gases are supplied via the placenta.

• Key structures: Placenta, umbilical arteries and vein, ductus venosus, ductus arteriosus, foramen ovale.

Circulatory Changes at Birth

• Lungs expand; pulmonary vessels open.

• Ductus arteriosus contracts and becomes ligamentum arteriosum.

• Foramen ovale closes, becoming fossa ovalis.

Age-Related Changes in Blood Vessels

• Linked to arteriosclerosis (thickening and toughening of arterial walls).

• Elastic walls of arteries become less tolerant of pressure increases, leading to aneurysms.

• Calcium deposits and atherosclerotic plaques increase risk of stroke and myocardial infarction.

Dental Implications

• Be aware of allergies and risk of anaphylactic shock, especially in children.

• Blood pressure considerations: Positioning of dental chair, risk of postural hypotension.

• Drug-related changes: Dry mouth, taste alterations, oral lesions.

Summary Table: Comparison of Arteries, Veins, and Capillaries

Feature	Arteries	Veins	Capillaries
Direction of Blood Flow	Away from heart	Toward heart	Between arterioles and venules
Wall Structure	Thick, elastic, muscular	Thin, less muscular, valves present	Single layer (endothelium)
Pressure	High	Low	Variable
Function	Transport blood under pressure	Return blood, reservoir	Exchange of substances

Additional info: Some details and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.