The Cardiovascular System: Blood Vessels
Cardiovascular System: Blood Vessels
Part 1 Blood Vessel Structure and Function
Blood vessels serve as a delivery system that connects the heart to various parts of the body and works alongside the lymphatic system to circulate fluids.
Blood Vessel Categories:
Arteries:
Transport blood away from the heart.
In the systemic circuit, arteries carry oxygenated blood.
In the pulmonary circuit and umbilical vessels of the fetus, they carry deoxygenated blood.
Capillaries:
Facilitate direct contact with tissue cells; serve the cellular needs directly.
Veins:
Carry blood toward the heart.
In the systemic circuit, veins carry deoxygenated blood.
In the pulmonary circulation and umbilical vessels of the fetus, they carry oxygenated blood.
Figure 19.1: Blood Vessel Relationship
Illustration of the interconnection of blood vessels and lymphatic vessels which include:
Venous System: Large veins (capacitance vessels)
Lymphatic System: Lymphatic vessels
Arterial System: Featuring arteriovenous anastomosis, capillaries, and various sizes of blood vessels.
19.1 Structure of Blood Vessel Wall
Overall Composition:
All vessels have a lumen (central blood-containing space) and are surrounded by a wall.
Endothelium:
Simple squamous epithelium lining the lumen of all vessels.
Continuous with the endocardium, while ensuring a slick surface to minimize friction.
Layers of Blood Vessels (Except for Capillaries):
Tunica Intima:
Innermost layer in intimate contact with blood.
Contains a subendothelial layer of connective tissue (present in vessels larger than 1 mm).
Tunica Media:
Bulky middle layer composed mainly of smooth muscle and sheets of elastin.
Nervous control:
Sympathetic vasomotor nerve fibers are responsible for vasoconstriction (decreased lumen diameter) and vasodilation (increased lumen diameter).
Vital for maintaining blood flow and pressure.
Tunica Externa (Tunica Adventitia):
Outermost layer composed mostly of loose collagen fibers, protecting and anchoring the wall to surrounding structures.
Contains nerve fibers and lymphatic vessels and large veins may have elastic fibers in this layer.
Includes vasa vasorum: tiny blood vessels in larger vessels.
19.2 Arteries
Arteries can be categorized into three groups based on size and function:
Elastic arteries
Muscular arteries
Arterioles
Elastic Arteries
Characteristics:
Thick-walled with a large, low-resistance lumen.
Major examples include the aorta and its main branches, referred to as conducting arteries.
Contains a significant amount of elastin in all tunics, especially in the tunica media.
Primarily serve as pressure reservoirs, expanding and recoiling as blood is ejected from the heart, providing continuous blood flow downstream, even between heartbeats.
Muscular Arteries
Characteristics:
Known as distributing arteries.
Diameter ranges from pinky-finger size to pencil-lead size.
Have the thickest tunica media with more smooth muscle but less elastic tissue.
Active in vasoconstriction and give rise to arterioles.
Arterioles
Characteristics:
Smallest of all arteries.
Larger arterioles contain all three tunics, while smaller ones consist predominantly of a single layer of smooth muscle surrounding endothelial cells.
Responsible for controlling flow into capillary beds via vasodilation and vasoconstriction.
Known as resistance arteries, where diameter changes affect resistance to blood flow.
Lead to capillary beds.
19.3 Capillaries
Description:
Microscopic vessels with diameters small enough for single red blood cells to pass through at one time.
Composed of just a thin tunica intima; in the smallest vessels, a single endothelial cell forms the entire circumference.
Pericytes: Stem cells that stabilize capillary walls, control permeability, and assist in vessel repair.
Supply almost every cell in the body except for cartilage, epithelial tissues, cornea, and lens of the eye.
Functions of Capillaries
Key functions include:
Gas exchange
Nutrient and waste transfer
Hormonal transport between blood and interstitial fluid.
Types of Capillaries
Continuous Capillaries:
Most abundant in skin, muscles, lungs, and CNS.
Unique brain continuous capillaries create the blood-brain barrier with tight junctions and no intercellular clefts, making them the least permeable but most common type.
Fenestrated Capillaries:
Located in areas with active filtration (kidneys), absorption (intestines), or endocrine secretion.
Their endothelial cells contain fenestrations (Swiss cheese-like pores) allowing increased permeability, often covered by a glycoprotein diaphragm.
Example: In some digestive tract organs, fenestration frequency increases during nutrient absorption.
Sinusoidal Capillaries:
Characterized by fewer tight junctions, larger intercellular clefts, and incomplete basement membranes.
Larger lumens typically and found in the liver, bone marrow, spleen, and adrenal medulla.
Slow blood flow allows for modification of large molecules and blood cells passing between blood and tissue and may contain macrophages for immune function.
Capillary Beds
Defined as an intricate network of capillaries that connect arterioles and venules.
Microcirculation: Refers to the flow of blood through these capillary beds.
Capillary beds consist of two primary types of vessels:
Vascular Shunt: Connects an arteriole directly with a venule.
Sequence: Terminal arteriole → Metarteriole → Thoroughfare channel → Postcapillary venule.
True Capillaries: Actual vessels engaged in exchanges, usually numbering 10 to 100 per capillary bed, originating from either a metarteriole or a terminal arteriole.
Precapillary Sphincters: Muscles that regulate blood flow into true capillaries based on local chemical conditions and vasomotor nerve activity.
19.4 Veins
Veins transport blood toward the heart, formed by the convergence of venules.
Venules originate from capillary union and consist of endothelium with few pericytes, permitting fluid and WBC entry into tissues.
Characteristics of Veins:
Thin tunica media yet thick tunica externa composed of collagen fibers and elastic networks.
Features large lumens and thin walls, making them effective storage vessels, known as capacitance vessels (can accommodate up to 65% of blood supply).
Adaptations for blood return to the heart include:
Large-diameter lumens featuring minimal resistance.
Venous Valves: Prevent backflow of blood, particularly concentrated in limbs.
Venous Sinuses: Flattened veins with thin walls primarily composed of endothelium, such as the coronary sinus and dural sinuses of the brain.
19.5 Anastomoses
Little connections between blood vessels that provide alternative pathways for blood flow, ensuring continuous circulation even if one vessel is blocked.
Commonly found in joints, abdominal organs, brain, and heart.
Part 2 Physiology of Circulation
Blood Flow
Measured as milliliters per minute (ml/min), it indicates the volume of blood flowing through a vessel or organ over time.
Equivalent to cardiac output (CO).
Overall blood flow is relatively constant at rest but varies based on individual organ needs at any moment.
Blood Pressure
Defined as the force exerted per unit area on blood vessel walls by the blood, typically measured as systemic arterial BP in large arteries near the heart.
Pressure Gradient: Plays a vital role in keeping blood in motion.
Resistance
Refers to the opposition to flow and the friction encountered by blood as it interacts with vessel walls.
Three main sources of resistance:
Blood Viscosity: Increased viscosity raises resistance.
Total Vessel Length: Longer vessels increase resistance.
Blood Vessel Diameter: The most influential factor, as resistance varies inversely with the fourth power of vessel radius (i.e., doubling the radius decreases resistance by a factor of 16).
Small-diameter arterioles are significant determinants of resistance as their radius changes frequently compared to larger arteries, which maintain relatively constant diameters.
Obstructions (e.g., fatty plaques from atherosclerosis) dramatically alter flow from laminar to turbulent conditions, increasing resistance.
Relationship Between Flow, Pressure, & Resistance
Blood flow (F) is directly proportional to the blood pressure gradient (DP).
As DP rises, blood flow accelerates.
Blood flow is inversely proportional to peripheral resistance (R). - Increases in resistance result in decreased flow, expressed as (F = \frac{DP}{R}).
Resistance is crucial in influencing local blood flow, as it can be easily altered through blood vessel diameter.
19.6 Systemic Blood Pressure
Systemic pressure is highest in the aorta and diminishes as the blood moves through the circulatory system; the most significant drop occurs in the arterioles.
Normative blood pressure follows the pattern:
Systolic Pressure: Peak pressure during ventricular contraction, typically averaging 120 mm Hg in a healthy adult.
Diastolic Pressure: Aortic pressure when the heart is at rest.
Pulse Pressure: The difference between systolic and diastolic pressures.
Pulse: The rhythmic throbbing of arteries due to pulse pressure.
Mean Arterial Pressure (MAP)
Calculated to determine average pressure in the arteries that propels blood to tissues.
Due to longer diastolic duration, MAP is not simply the average of systole and diastole; it is calculated as follows: MAP = Diastolic Pressure + \frac{1}{3} (Pulse Pressure)
Example: If BP = 120/80:
Pulse Pressure = 120 - 80 = 40
Thus, MAP = 80 + \frac{1}{3}(40) = 93 mm Hg
Both pulse pressure and MAP decrease as the distance from the heart increases.
27. Clinical Monitoring of Circulatory Efficiency
Vital Signs: Include pulse and blood pressure, as well as respiratory rate and body temperature.
Taking Pulse: Typically taken at the wrist via the radial artery and is commonly monitored, although pulse points can be found throughout the body.
Measuring Blood Pressure: Typically assessed indirectly using a sphygmomanometer, where a cuff is wrapped around the upper arm, inflated, and then slowly deflated while listening for Korotkoff sounds through a stethoscope:
Systolic Pressure: The first appearance of sound as blood flows through the artery (spurt).
Diastolic Pressure: The cessation of sound (steady flow).
Capillary Blood Pressure
Generally ranges from 35 mm Hg at the arterial end of the capillary bed to 17 mm Hg at the venous end.
Low capillary pressure is desirable to prevent rupture of thin-walled capillaries and to promote filtration into interstitial spaces.
Venous Blood Pressure
Steady in nature with no pulsations; the pressure gradient shows a drop from the aorta (60 mm Hg) to the venae cavae (15 mm Hg).
Owing to the low venous pressure:
Blood flows smoothly out of cut veins.
Blood spurts from arteries when cut due to higher pressure, necessitating adaptations for venous return.
Muscular Pump: Skeletal muscle contractions propel blood back toward the heart while preventing backflow through valves.
Respiratory Pump: Pressure changes during breathing compress abdominal veins and facilitate venous blood flow toward the heart.
Sympathetic Venoconstriction: Responds to smooth muscle contractions that enhance blood return to the heart.
19.8 Regulation of Blood Pressure
Key Contributors
The heart, blood vessels, kidneys, and brain work collectively to maintain blood pressure.
Blood pressure is directly tied to cardiac output, resistance, and blood volume.
Relationships expressed as:
DP = CO \times R
Adjustments in any one of these metrics can be compensated by alterations in the others.
19.8.1 Nervous System Short-Term Regulation
Higher brain centers (particularly the hypothalamus) control blood pressure, especially during stress and changes in body temperature.
Hormonal Influences:
Hormonal regulation affects blood pressure chiefly by altering resistance through changes in blood vessel diameter.
19.8.2 Long-Term Mechanisms: Renal Regulation
Long-term mechanisms focus primarily on altering blood volume, which is a key determinant of cardiac output.
An increase in blood volume leads to a rise in MAP, while a decrease results in lower MAP.
Excessive salt intake, vigorous exercise, and blood loss can significantly influence overall blood volume.
Renal Mechanisms for Blood Pressure Control
Direct Renal Mechanisms:
Fast filtering speeds lead to increases in the volume of fluid excreted by the kidneys.
Conversely, low volume or pressure allows kidneys to filter fluid more gradually.
Indirect Renal Mechanism:
Known as the renin-angiotensin-aldosterone system (RAAS).
When blood pressure is low, the kidneys release renin, which catalyzes the conversion of angiotensinogen to angiotensin I; angiotensin I is then converted to angiotensin II.
Angiotensin II Effects
Stabilizes mean arterial pressure and balances extracellular fluid volume via four main mechanisms:
Stimulates adrenal cortex for aldosterone secretion.
Promotes posterior pituitary to release ADH (antidiuretic hormone).
Activates the hypothalamic thirst center.
Acts as a potent vasoconstrictor to increase blood pressure.
19.8.3 Summary: Hormonal Mechanisms
A comprehensive overview of the correlation between various hormonal activities and blood pressure regulation through both direct and indirect renal processes.
This integration of renal function and hormonal influences showcases the complexity and intricacy of ensuring optimal blood pressure and fluid dynamics in the body.
19.8.4 Overall Regulation Mechanisms
The equilibrium between cardiac output, blood vessel resistance, and overall blood volume leads to stable blood pressure homeostasis.