Chapter 19: Blood Vessels

Blood Vessel Structure and Function

• Definition of Blood Vessels: A delivery system of dynamic structures that begins and ends at the heart.     * They work in conjunction with the lymphatic system to circulate fluids throughout the body.

• Three Main Types of Blood Vessels:     * Arteries: These vessels carry blood away from the heart. They are typically oxygenated, with the exceptions of pulmonary circulation and the umbilical vessels of a fetus.     * Capillaries: These vessels make direct contact with tissue cells to directly serve cellular needs via exchange.     * Veins: These vessels carry blood toward the heart. They are typically deoxygenated, with the exceptions of pulmonary circulation and the umbilical vessels of a fetus.

Structure of the Blood Vessel Wall

• General Architecture: All vessels (except capillaries) consist of a central blood-containing space called a lumen, surrounded by a wall composed of three layers or tunics.

• Capillaries Structure: Consist only of endothelium with a sparse basal lamina.

• Tunica Intima:     * The innermost layer in "intimate" contact with blood.     * Endothelium: A simple squamous epithelium that lines the lumen of all vessels.     * It is continuous with the endocardium of the heart.     * Provides a slick surface to reduce friction.

• Tunica Media:     * The middle layer composed mostly of smooth muscle and sheets of elastin.     * Regulated by sympathetic vasomotor nerve fibers from the autonomic nervous system.     * Vasoconstriction: Decreased lumen diameter caused by smooth muscle contraction.     * Vasodilation: Increased lumen diameter caused by smooth muscle relaxation.     * This is the bulkiest layer and is responsible for maintaining blood flow and blood pressure.

• Tunica Externa:     * The outermost layer, also known as the tunica adventitia.     * Composed mostly of loose collagen fibers that protect and reinforce the wall while anchoring it to surrounding structures.     * Infiltrated with nerve fibers and lymphatic vessels.     * Large veins also contain elastic fibers in this layer.     * Vasa Vasorum: A system of tiny blood vessels found in the external layers of larger vessels that function to nourish the outermost tissue of the vessel wall.

Arterial System: Elastic, Muscular, and Arterioles

• Elastic Arteries:     * Thick-walled vessels with large, low-resistance lumens.     * Includes the aorta and its major branches; also called conducting arteries because they conduct blood from the heart to medium-sized vessels.     * Elastin is found in all three tunics, primarily the tunica media.     * They act as pressure reservoirs that expand and recoil as blood is ejected from the heart, allowing for continuous blood flow downstream between heartbeats.

• Muscular Arteries:     * Derived from elastic arteries; also called distributing arteries because they deliver blood to body organs.     * Diameters range from the size of a pinky finger to pencil-lead size.     * Account for most named arteries in the body.     * Possess the thickest tunica media with more smooth muscle and less elastic tissue (tunica media is sandwiched between elastic membranes).     * Very active in vasoconstriction.

• Arterioles:     * The smallest of all arteries.     * Larger arterioles contain all three tunics; smaller arterioles are mostly a single layer of smooth muscle surrounding endothelial cells.     * Control flow into capillary beds via vasodilation and vasoconstriction.     * Called resistance arteries because changes in their diameter significantly change resistance to blood flow.

Capillaries

• General Characteristics:     * Microscopic vessels with diameters so small only a single Red Blood Cell (RBC) can pass through at a time.     * Walls consist only of a thin tunica intima.     * Supply almost every cell in the body except for cartilage, epithelia, the cornea, and the lens of the eye.     * Primary Function: Exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid.

• Capillary Junctions: All endothelial cells are joined by tight junctions, but they usually contain gaps called intercellular clefts that allow the passage of fluids and small solutes.

• Types of Capillaries:     1. Continuous Capillaries:         * Most common and least permeable.         * Abundant in skin, muscles, lungs, and the Central Nervous System (CNS).         * Often associated with pericytes.         * The blood-brain barrier is formed by unique continuous capillaries that are totally enclosed by tight junctions and lack intercellular clefts.         * Contain pinocytotic vesicles to ferry fluid across the cell.     2. Fenestrated Capillaries:         * Found in areas of active filtration (kidneys), absorption (intestines), or endocrine hormone secretion.         * Contain "Swiss cheese-like" pores called fenestrations to increase permeability.         * Fenestrations are usually covered by a thin glycoprotein diaphragm.     3. Sinusoidal Capillaries:         * The most permeable, occurring in limited locations: liver, bone marrow, spleen, and adrenal medulla.         * Feature large intercellular clefts, fenestrations, few tight junctions, and incomplete basement membranes.         * Large, irregularly shaped lumens allow large molecules and even blood cells to pass.         * Blood flow is sluggish to allow for molecule modification and capture of "prey" by macrophages lining the walls.

Capillary Bed Structure and Microcirculation

• Microcirculation: The flow of blood through a capillary bed from an arteriole to a venule.

• Terminal Arteriole: Branches into $10$ to $20$ exchange vessels (true capillaries) that form the bed.

• Postcapillary Venule: Where the capillaries drain after exchange occurs.

• Special Features (Mesenteric/Serous Membranes):     * Vascular Shunt: A channel that directly connects the arteriole with the venule, bypassing true capillaries. It consists of a metarteriole and a thoroughfare channel.     * Precapillary Sphincter: A cuff of smooth muscle surrounding each true capillary branching off the metarteriole. It acts as a valve to regulate flow and is controlled by local chemical conditions rather than nerves.

The Venous System

• Venules: Formed when capillary beds unite.     * Postcapillary venules consist of endothelium and a few pericytes.     * They are extremely porous, allowing white blood cells (WBCs) and fluid into tissues.

• Veins: Formed when venules converge.     * Have all three tunics but thinner walls and larger lumens than corresponding arteries.     * Tunica externa is the thickest layer, containing collagen and elastic networks.     * Capacitance Vessels: Veins are called blood reservoirs because they can hold up to $65\%$ of the body's total blood supply.

• Venous Adaptations for Return:     * Venous Valves: Prevent backflow; most abundant in the limbs.     * Venous Sinuses: Flattened veins with extremely thin walls (e.g., coronary sinus of the heart, dural sinuses of the brain).

Homeostatic Imbalances and Anastomoses

• Varicose Veins: Dilated and painful veins caused by incompetent (leaky) valves. Linked to heredity, obesity, pregnancy, and prolonged standing. Affects more than $15\%$ of adults.

• Hemorrhoids: Varicosities in the anal veins due to elevated intra-abdominal pressure.

• Vascular Anastomoses: Interconnections between blood vessels.     * Arterial Anastomoses: Provide collateral channels (alternate pathways) to ensure flow if an artery is blocked. Common in joints, brain, and heart; absent in the retina and kidneys.     * Arteriovenous Anastomoses: Shunts like the metarteriole-thoroughfare channel.     * Venous Anastomoses: Very abundant; occlusions rarely block blood flow.

Physiology of Circulation: Flow, Pressure, and Resistance

• Blood Flow: Volume of blood flowing through a vessel or organ in a given period ($ml/min$). For the whole system, it equals Cardiac Output (CO).

• Blood Pressure (BP): Force per unit area exerted on a vessel wall by blood ($mm\,Hg$).     * Measured as systemic arterial BP in large arteries near the heart.     * The pressure gradient (${\Delta}P$) is the driving force that keeps blood moving.

• Resistance (Peripheral Resistance): Opposition to flow; the friction blood encounters.     1. Blood Viscosity: The "stickiness" of blood. Increased viscosity (more proteins/cells) increases resistance.     2. Blood Vessel Length: Longer vessels result in greater resistance.     3. Blood Vessel Diameter: The greatest influence on resistance. Resistance ($R$) varies inversely with the fourth power of the vessel radius ($r$). Formula: $R \propto \frac{1}{r^4}$.         * If the radius doubles, resistance drops to $\frac{1}{16}$ of its original value.         * Small-diameter arterioles are the major determinants of peripheral resistance.         * Abrupt changes or fatty plaques create turbulent flow, increasing resistance.

• Relationship Formula: $F = \frac{\Delta P}{R}$ where $F$ is blood flow, $\Delta P$ is the pressure gradient, and $R$ is resistance.

Systemic and Arterial Blood Pressure

• Pressure Distribution: Highest in the aorta ($120\,mm\,Hg$) and lowest in the right atrium ($0\,mm\,Hg$). The steepest drop occurs in the arterioles.

• Arterial BP Factors: Determined by the elasticity (compliance) of arteries and the volume of blood forced into them.

• Pulsatile Pressure:     * Systolic Pressure: Pressure in use during ventricular contraction (avg $120\,mm\,Hg$).     * Diastolic Pressure: Lowest aortic pressure during heart rest (avg $80\,mm\,Hg$).     * Pulse Pressure: $Systolic - Diastolic$.     * Mean Arterial Pressure (MAP): The pressure that propels blood to tissues.     * MAP Calculation: $\text{MAP} = \text{Diastolic Pressure} + \left(\frac{1}{3} \times \text{Pulse Pressure}\right)$.

Capillary and Venous Pressure

• Capillary BP: Ranges from $35\,mm\,Hg$ at the start to $17\,mm\,Hg$ at the end. Low pressure is necessary to prevent the rupture of thin walls and to allow for filtrate forcing into interstitial spaces.

• Venous BP: Small gradient of about $15\,mm\,Hg$.

• Aids to Venous Return:     1. Muscular Pump: Skeletal muscle contraction "milks" blood toward the heart.     2. Respiratory Pump: Pressure changes during breathing (abdominal squeeze and thoracic expansion) move blood.     3. Sympathetic Venoconstriction: Smooth muscle in veins constricts under sympathetic control.

Regulation of Blood Pressure

• Main Factors: Cardiac Output (CO), Peripheral Resistance (PR), and Blood Volume.

• Formula: $BP \propto CO \times PR$.

• Short-Term Control (Neural):     * Cardiovascular Center: Clusters of sympathetic neurons in the medulla (cardioinhibitory, cardioacceleratory, and vasomotor centers).     * Vasomotor Tone: Continuous moderate constriction of vessels via vasomotor fibers.     * Baroreceptor Reflexes: Located in carotid sinuses and aortic arch. High BP stimulates them to inhibit the vasomotor/cardioacceleratory centers and stimulate the cardioinhibitory center, leading to vasodilation and decreased CO.     * Chemoreceptor Reflexes: Detect high $CO_2$, low $pH$, or low $O_2$; stimulate the cardioacceleratory and vasomotor centers to increase BP.     * Higher Brain Centers: The hypothalamus increases BP during stress and redistributes flow during exercise.

• Short-Term Control (Hormonal):     * Epinephrine/Norepinephrine: Increase CO and vasoconstriction.     * Angiotensin II: Potent vasoconstrictor.     * ADH (Antidiuretic Hormone): High levels cause vasoconstriction.     * Atrial Natriuretic Peptide (ANP): Decreases BP by decreasing blood volume (antagonizes aldosterone).

• Long-Term Control (Renal):     * Direct Renal Mechanism: Kidneys eliminate more urine when BP is high to reduce volume.     * Indirect (Renin-Angiotensin-Aldosterone) Mechanism: Decreased BP causes renin release $\rightarrow$ converts angiotensinogen to Angiotensin I $\rightarrow$ ACE converts it to Angiotensin II.     * Functions of Angiotensin II: Stimulates aldosterone, causes ADH release, triggers thirst, and acts as a vasoconstrictor.

Homeostatic Imbalances

• Hypertension: Sustained arterial pressure of $140/90\,mm\,Hg$ or higher.     * Primary Hypertension: $90\%$ of cases; no single cause (linked to diet, obesity, smoking, age).     * Secondary Hypertension: Due to identifiable disorders like kidney disease or hyperthyroidism.

• Hypotension: BP below $90/60\,mm\,Hg$.     * Orthostatic: Temporary drop when standing up.     * Chronic: May indicate Addison's disease or malnutrition.     * Acute: Sign of circulatory shock.

• Circulatory Shock:     * Hypovolemic Shock: Results from large-scale blood loss.     * Vascular Shock: Results from extreme vasodilation.     * Cardiogenic Shock: Inefficient heart cannot sustain circulation.