Marieb Human Anatomy & Physiology Chapter 19 - Blood Vessels Flashcards

Overview of the Cardiovascular System: Blood Vessels

The Closed Delivery System: Blood vessels form a system that begins and ends at the heart.
- Dynamic Nature: They are not static tubes; they pulsate, constrict, relax, and can even multiply (angiogenesis).
- Integration: Vessels work in conjunction with the lymphatic system to circulate fluids throughout the body.
General Classification of Vessels:
- Arteries: Carry blood away from the heart toward capillaries.
  - Systemic Arteries: Transport oxygenated blood.
  - Pulmonary Arteries: Transport oxygen-poor blood.
- Capillaries: Known as exchange vessels; they directly serve cellular needs by allowing substances to move across their walls between blood and tissue cells.
- Veins: Carry blood away from capillaries toward the heart.
  - Systemic Veins: Transport oxygen-poor blood.
  - Pulmonary Veins: Transport oxygenated blood.

Blood Vessel Wall Structure

Lumen: The central blood-containing space surrounded by the vessel wall.
Tunics: Most vessel walls (except capillaries) consist of three distinct layers:
Tunica Intima (Innermost Layer):
- Endothelium: Simple squamous epithelium that lines the lumen of all vessels. It is continuous with the endocardium for a slick surface that reduces friction.
- Subendothelial Layer: Consists of a basement membrane and loose connective tissue (present in vessels larger than $10\,mm$ ).
Tunica Media (Middle Layer):
- Composition: Mostly circularly arranged smooth muscle cells and sheets of elastin (in elastic arteries). It is typically the bulkiest layer in arteries.
- Regulation: Controlled by sympathetic vasomotor nerve fibers and various chemicals.
- Functions:
  - Vasoconstriction: Lumen diameter decreases as smooth muscle contracts.
  - Vasodilation: Lumen diameter increases as smooth muscle relaxes.
  - Influence: Critical for maintaining blood flow and blood pressure.
Tunica Externa (Outermost Layer):
- Also Known As: Tunica adventitia.
- Composition: Mostly collagen fibers that protect, reinforce, and anchor the vessel to surrounding structures.
- Contents: Contains nerve fibers, lymphatic vessels, and in larger vessels, the Vasa Vasorum ("vessels of the vessels"), which nourish the external tissues of the vessel wall.

Arterial System: Types and Functions

Elastic Arteries (Conducting Arteries):
- Location: Thick-walled arteries near the heart, such as the aorta and pulmonary trunk.
- Characteristics: Large lumens offer low resistance; contain more elastin than any other vessel type (found in all three tunics).
- Function: Act as pressure reservoirs; they expand and recoil as the heart ejects blood, ensuring continuous flow even between heartbeats.
- Clinical Note: Atherosclerosis stiffens these walls. A weakened wall may balloon out (aneurysm) or burst.
Muscular Arteries (Distributing Arteries):
- Origin: Arise from elastic arteries; deliver blood to specific organs.
- Size: Range from the diameter of a pinky finger to a pencil lead.
- Characteristics: Feature the thickest tunica media proportional to their size. They contain more smooth muscle and less elastic tissue, making them more active in vasoconstriction but less stretchy.
Arterioles (Resistance Vessels):
- Size: Smallest arteries; lead directly into capillary beds.
- Structure: Larger arterioles have all three tunics; smaller ones may be a single layer of smooth muscle over endothelium.
- Function: Control blood flow into capillary beds via vasodilation and vasoconstriction. Changes in their diameter significantly affect resistance.

Capillaries: The Exchange Vessels

Structure: Composed only of a thin tunica intima. In some cases, a single endothelial cell forms the entire circumference. The lumen is so small that Red Blood Cells (RBCs) pass through in single file.
Vascularization Exceptions: Tendons and ligaments are poorly vascularized; cartilage, epithelia, the cornea, and the lens are avascular. Avascular tissues receive nutrients from adjacent connective tissues or aqueous humor.
Function: Exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid.
Types of Capillaries:
- Continuous Capillaries: Least permeable and most common. Found in skin, muscles, lungs, and the Central Nervous System (CNS). Cells are joined by tight junctions, but have intercellular clefts for limited fluid passage.
- Fenestrated Capillaries: Found where active filtration (kidneys), absorption (intestines), or endocrine secretion occurs. Endothelial cells contain "Swiss cheese-like" pores called fenestrations for increased permeability.
- Sinusoid Capillaries: Most permeable and least common. Found in the liver, bone marrow, spleen, and adrenal medulla. Feature large intercellular clefts, fenestrations, and incomplete basement membranes. Blood flow is sluggish, allowing time for the modification of large molecules and cells.
Capillary Beds:
- Microcirculation: The flow of blood from a terminal arteriole through the capillary bed to a postcapillary venule.
- Composition: A terminal arteriole divides into $10$ to $20$ capillaries.
- Regulation: Controlled by the diameter of the terminal arteriole and upstream arterioles, influenced by local chemical conditions and vasomotor nerve fibers.

Venous System: Blood Reservoirs

Formation: Capillaries drain into venules, which join to form veins. As they converge, diameters increase and walls thicken.
Vein Characteristics:
- Structure: Thinner walls and larger lumens than corresponding arteries. In histology, they often appear collapsed with slit-like lumens.
- Tunics: Thin tunica media; thick tunica externa (collagen and elastic networks).
- Capacitance Vessels: Also called blood reservoirs; they can hold up to $65\%$ of the body's total blood supply at any time.
Adaptations for Venous Return:
- Large Lumens: Offer little resistance to flow.
- Venous Valves: Prevent backflow; most abundant in limbs; resemble semilunar heart valves.

Clinical Homeostatic Imbalance: Varicose Veins

Definition: Tortuous and dilated veins resulting from incompetent (leaky) valves.
Statistics: Affects over $15\%$ of adults, primarily in the lower limbs.
Risk Factors: Heredity, obesity, pregnancy, and prolonged standing. These conditions cause blood to pool, weakening valves and stretching walls.
Hemorrhoids: Varicosities in the anal veins caused by elevated intra-abdominal pressure (e.g., from straining during childbirth or bowel movements).

Physiology of Circulation: Definitions

Blood Flow: The volume of blood flowing through a vessel, organ, or the entire system in a given period ( $ml/min$ ). For the whole system, flow equals Cardiac Output ( $CO$ ).
Blood Pressure (BP): Force per unit area exerted on a vessel wall by blood ( $mm\,Hg$ ). The pressure gradient is the driving force that moves blood from high to low pressure.
Resistance (TPR): Opposition to flow; a measure of friction. Three sources:
- Blood Viscosity: The "stickiness" of blood. Higher viscosity (e.g., in polycythemia) increases resistance; lower viscosity (e.g., in anemia) decreases it.
- Vessel Length: Longer vessels encounter more resistance. Length is relatively constant but increases with tissue growth.
- Vessel Diameter: Smallest vessels provide the most resistance. Resistance varies inversely with the fourth power of the radius: $R \propto \frac{1}{r^4}$ .
  - If the radius is reduced by half, resistance increases by $16\times$ .
  - If the radius is doubled, resistance drops to $\frac{1}{16}$ .

Systemic Blood Pressure

Pressure Gradient: Highest in the aorta ( $120\,mm\,Hg$ ) and declines to $0\,mm\,Hg$ at the right atrium.
Arterial BP:
- Systolic Pressure: Pressure in aorta during ventricular contraction (avg $120\,mm\,Hg$ ).
- Diastolic Pressure: Lowest aortic pressure during heart rest (avg $70$ to $80\,mm\,Hg$ ).
- Pulse Pressure: Difference between systolic and diastolic pressure ( $PP = \text{Systolic} - \text{Diastolic}$ ). Throb felt in arteries.
- Mean Arterial Pressure (MAP): The pressure that propels blood to tissues.
  - Formula: $MAP = \text{Diastolic pressure} + \frac{1}{3} (\text{Pulse pressure})$ .
Capillary BP: Ranges from $35\,mm\,Hg$ (start) to $17\,mm\,Hg$ (end). Low pressure prevents rupture of fragile walls and allows for filtration.
Venous BP: Steady and non-pulsatile. Gradient is small (approx. $15\,mm\,Hg$ from venules to heart).
- Return Mechanisms:
  1. Muscular Pump: Skeletal muscle contraction "milks" blood toward the heart.
  2. Respiratory Pump: Pressure changes during breathing squeeze abdominal veins.
  3. Sympathetic Venoconstriction: SNS reduces venous volume to push blood heartward.

Regulation of Blood Pressure

Main Factors: Blood pressure varies directly with Cardiac Output ( $CO$ ), Total Peripheral Resistance ( $TPR$ ), and Blood Volume.
Biological Relationships:
- $\Delta P = CO \times TPR$
- $MAP = SV \times HR \times TPR$
Short-Term Regulation (Neural and Hormonal):
- Neural Controls: Alter vessel diameter and blood distribution. Operate via baroreceptor reflexes (monitoring stretch) and chemoreceptor reflexes (monitoring $CO_2$ and $pH$ ) in the medulla's cardiovascular center.
- Hormonal Controls:
  - Epinephrine/Norepinephrine: Increase $CO$ and cause vasoconstriction.
  - Angiotensin II: Potent vasoconstriction; stimulates aldosterone and ADH release.
  - ADH (Vasopressin): Water conservation by kidneys; causes vasoconstriction at high levels.
  - Atrial Natriuretic Peptide (ANP): Decreases blood volume and causes vasodilation (lowers MAP).
Long-Term Regulation (Renal):
- Direct Renal Mechanism: Kidneys eliminate more water when BP is high and conserve it when BP is low, independent of hormones.
- Indirect (Renin-Angiotensin-Aldosterone) System:
  1. Drop in MAP causes release of Renin from kidneys.
  2. Renin converts Angiotensinogen to Angiotensin I.
  3. ACE (Angiotensin-converting enzyme) in lung capillaries converts I to Angiotensin II.
  4. Angiotensin II triggers aldosterone (conserves $Na^+$ ), ADH (conserves water), activates the thirst center, and causes vasoconstriction.

Homeostatic Imbalances: Hypertension and Hypotension

Hypertension: Chronic BP above $130/80\,mm\,Hg$ . Known as the "silent killer."
- Primary (Essential): $90\%$ of cases; no known cause; linked to health/genetics. Controlled by diet, exercise, and drugs (diuretics, Beta-blockers, ACE inhibitors).
- Secondary: $10\%$ of cases; due to identifiable disorders (e.g., kidney disease, Cushing's syndrome).
Hypotension: BP below $90/60\,mm\,Hg$ .
- Orthostatic: Temporary drop when standing.
- Chronic: May indicate Addison's disease or malnutrition.
- Acute: Sign of circulatory shock.
Circulatory Shock:
- Hypovolemic Shock: Most common; from large-scale blood/fluid loss.
- Vascular Shock: Extreme vasodilation (e.g., Anaphylactic, Neurogenic, or Septic shock).
- Cardiogenic Shock: Heart failure to circulate blood.

Tissue Perfusion and Autoregulation

Tissue Perfusion: Involved in delivery of $O_2$ /nutrients, waste removal, gas exchange (lungs), absorption (gut), and urine formation (kidneys).
Intrinsic Controls (Autoregulation): Organs regulate their own flow by varying resistance.
- Metabolic Controls: Low $O_2$ or high metabolic products (e.g., $H^+$ , adenosine) trigger vasodilation. Nitric Oxide (NO) is a powerful vasodilator; Endothelins are vasoconstrictors.
- Myogenic Controls: Smooth muscle contracts when stretched by high pressure and relaxes when stretch decreases to maintain constant flow.
- Long-Term: Angiogenesis (forming new vessels).
Extrinsic Controls: Sympathetic nerves and hormones maintain overall MAP, sometimes shunting blood away from less critical areas (e.g., skin, digestive organs) during exercise.

Specialized Circulatory Pathways

Skeletal Muscle: At rest, only $25\%$ of capillaries are open. During activity, active hyperemia (metabolic autoregulation) can increase flow to over $70\%$ of total blood volume.
Brain: Flow kept at constant $750\,ml/min$ . MAP below $60\,mm\,Hg$ leads to syncope (fainting); above $160\,mm\,Hg$ leads to cerebral edema.
Skin: Primary function of blood flow is temperature regulation via venous plexuses ( $50\,ml/min$ to $2500\,ml/min$ ).
Lungs: Short pathway; low pressure ( $24/10\,mm\,Hg$ ). Autoregulation is unique: low $O_2$ causes vasoconstriction to shunt blood toward $O_2$ -rich areas.

Circulatory Pathways

Pulmonary Circulation: Short loop from right ventricle to lungs to left atrium.
Systemic Circulation: Long loop from left ventricle to body tissues back to right atrium.
Unique Drainages:
- Coronary Sinus: Drains myocardium into right atrium.
- Dural Venous Sinuses: Drain brain blood.
- Hepatic Portal System: Drains digestive system blood into the liver before returning to the heart.

Developmental Aspects

Blood Islands: Collections of mesodermal cells that form rudimentary vascular tubes.
Timing: Heart pumps by week $4$ of development.
Fetal Shunts:
- Foramen ovale and ductus arteriosus bypass lungs.
- Ductus venosus bypasses the liver.
- Umbilical vessels circulate to/from the placenta.

Low pressure in capillaries has several important reasons:

Prevention of Rupture: The relatively low pressure, which ranges from $35 \text{ mm Hg}$ at the start to $17 \text{ mm Hg}$ at the end of the capillary bed, helps prevent damage to the fragile capillary walls.
Facilitation of Exchange: Low pressure allows for better exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid. High pressure might force fluids and solutes out too rapidly, hindering the exchange process.
Filtration: The low pressure in capillaries is essential for facilitating filtration. It allows sufficient time for nutrients and gases to diffuse into tissues while ensuring that waste products effectively move from tissues back into the blood for removal.

Precapillary Sphincters

Precapillary Sphincters: These are smooth muscle cuffs that surround the entrance to capillary beds.
Function: They regulate blood flow into the capillary beds by contracting to reduce blood flow or relaxing to allow blood flow. This control is influenced by local tissue oxygen and nutrient needs, thus playing a significant role in tissue perfusion and regulating blood flow according to metabolic demands.

Blood flow to the lungs is distinct from blood flow to skeletal muscle, the brain, and the skin due to the unique physiological roles and requirements of each organ:

Pulmonary Circulation:
- Blood flows from the right ventricle to the lungs at relatively low pressure (around $24/10 \text{ mm Hg}$ ). This low pressure prevents damage to the delicate capillary networks within the lungs and allows for effective gas exchange.
- Benefit: Low pressure in the pulmonary circulation facilitates optimal exchange of oxygen and carbon dioxide, which is critical for respiration without causing rupture of fragile capillary walls.
Skeletal Muscle:
- At rest, only about 25% of capillaries are open, but during activity, blood flow can increase dramatically (active hyperemia) to over 70% of total blood volume.
- Benefit: This ability allows for increased perfusion to meet the heightened metabolic demands during exercise, aiding in energy supply and waste removal.
Brain:
- The brain receives a consistent blood flow of approximately 750 ml/min, with autoregulation mechanisms that maintain this flow relatively constant, regardless of systemic blood pressure within certain limits.
- Benefit: This stability is crucial as the brain is highly sensitive to fluctuations in oxygen and nutrient supply, ensuring optimal function and preventing syncope under low perfusion conditions.
Skin:
- Blood flow in the skin varies significantly, serving primarily for temperature regulation, with flow rates ranging from $50 \text{ ml/min}$ to $2500 \text{ ml/min}$ depending on environmental conditions and body temperature.
- Benefit: By adjusting blood flow, the skin can effectively dissipate heat or conserve warmth, contributing to homeostasis and temperature regulation.

In summary, the distinct flow patterns and pressures in these vascular systems are beneficial as they ensure that each organ receives the appropriate amount of blood flow necessary for its specific functions, enhancing overall physiological efficiency.