Chapter 20
Chapter 20: Anatomy and Physiology - An Integrative Approach
20.1a General Structure of Vessels
The walls of blood vessels consist of three distinct layers known as tunics:
Tunica Intima:
Innermost layer of the vessel wall.
Made up of the endothelium, which is formed from simple squamous epithelium.
Contains a subendothelial layer composed of areolar connective tissue.
Tunica Media:
The middle layer composed of circularly arranged layers of smooth muscle cells intermixed with elastic fibers.
The contraction of this layer results in vasoconstriction, which narrows the lumen of the vessel, while relaxation causes vasodilation, which widens the lumen.
Tunica Externa (Adventitia):
The outermost layer consisting of areolar connective tissue that contains elastic and collagen fibers.
This layer functions to anchor the vessel to other structures and may contain smaller blood vessels known as the vasa vasorum, which supply larger vessels that are too large to receive nutrients by diffusion alone.
The interior of the vessel is termed its lumen.
20.1a General Structure of Vessels (continued)
Comparison of Different Vessel Types
Companion Vessels:
Arteries and veins that lie next to each other serving the same body region.
Arteries:
Have a thicker tunica media compared to veins and a narrower lumen.
Contain a higher concentration of elastic and collagen fibers, allowing them to rebound to their original shape after expansion, contributing to resilience against fluctuations in blood pressure.
Veins:
Possess a thicker tunica externa and larger lumen than arteries.
Contain fewer elastic and collagen fibers, and as a result, their walls may collapse in the absence of blood.
20.1a General Structure of Vessels (comparative details)
Capillaries
Composed solely of the tunica intima.
The walls consist of an endothelium and a basement membrane, allowing for rapid diffusion of gases and nutrients due to the thin structure.
20.1b Arteries
Basic Types of Arteries
Elastic Arteries (Conducting Arteries):
The largest arteries with diameters ranging from 2.5 cm to 1 cm.
These arteries conduct blood from the heart to muscular arteries and contain a large proportion of elastic fibers, facilitating stretch and recoil during the cardiac cycle, thus propelling blood during diastole.
Examples include the aorta, pulmonary trunk, common carotid, and common iliac arteries.
Muscular Arteries (Distributing Arteries):
Medium-sized arteries ranging from 1 cm to 0.3 mm in diameter.
Responsible for distributing blood to specific body regions and have more smooth muscle than elastic arteries, allowing for both vasoconstriction and dilation.
Contain elastic tissue in two layers: the internal elastic lamina between the tunica intima and media, and the external elastic lamina between the tunica media and externa.
Examples comprise brachial artery and coronary arteries.
Arterioles:
The smallest arteries, with diameters measuring between 0.3 mm and 10 micrometers.
Larger arterioles possess three tunics, while smaller arterioles only consist of a thin endothelium and a single layer of smooth muscle, generally in a state of slight constriction known as vasomotor tone, controlled by the vasomotor center in the brainstem.
They play a critical role in regulating systemic blood pressure and blood flow.
20.1c Capillaries
Capillary Characteristics
Small vessels connecting arterioles to venules, averaging 1 mm in length and 8 to 10 micrometers in diameter.
Erythrocytes often travel in a single file, referred to as rouleau formation.
The wall structure includes an endothelial layer supported by a basement membrane, allowing for efficient exchange of gases, nutrients, and waste materials.
Types of Capillaries
Continuous Capillaries:
Endothelial cells form a continuous lining that includes tight junctions connecting adjacent cells but does not create a complete seal.
Intercellular clefts exist, which allow passage of smaller molecules (e.g., glucose) while blocking the movement of larger particles (e.g., cells, proteins).
Commonly found in muscle tissue, skin, lungs, and the central nervous system.
Fenestrated Capillaries:
Similar to continuous capillaries but contain pores (fenestrations) that facilitate the movement of small plasma proteins.
These capillaries are located in areas where significant fluid transport occurs, such as intestinal capillaries (absorbing nutrients) and kidney capillaries (filtering blood for urine formation).
Sinusoids (Discontinuous Capillaries):
Characterized by an incomplete endothelial lining with wide gaps, and sometimes lacking a complete basement membrane.
Allow the transport of large substances, such as formed elements and large proteins.
Commonly located in the bone marrow, spleen, and some endocrine glands.
Capillary Beds
A network of capillaries functioning together, typically fed by a metarteriole, which branches from an arteriole.
The proximal part of the metarteriole is encircled by smooth muscle cells, while the distal part transitions into a thoroughfare channel that connects to a postcapillary venule, draining the capillary bed.
Precapillary sphincters are smooth muscle rings at the origin of true capillaries, regulating blood flow into the capillary bed.
20.1d Veins
Structure of Venules
Smallest veins, measuring 8 to 100 micrometers in diameter, often companion vessels to arterioles, with the smallest being postcapillary venules.
The larger venules contain all three tunics and merge to form larger veins.
Classification of Veins
Small and Medium-Sized Veins:
Companion vessels of muscular arteries, play a role in returning blood to the heart.
Large Veins:
Accompany elastic arteries; many of these veins contain multiple valves which are crucial in preventing blood from pooling in the limbs and ensuring that blood flows toward the heart.
Valves consist of folds of the tunica intima and are reinforced with elastic and collagen fibers, reflecting a structure similar to the heart's semilunar valves.
Systemic Veins as Blood Reservoirs:
At rest, about 70% of the blood volume is in systemic circulation, with systemic veins comprising approximately 55%.
Systemic arteries make up about 10% and systemic capillaries about 5%.
Pulmonary circulation contains about 18% of the blood, and the heart retains about 12%.
Dynamics of Blood Movement in Veins
Blood can transition from venous reservoirs into circulation through vasoconstriction (such as during exertion) and can revert to reservoirs via vasodilation (during rest).
20.2 Total Cross-Sectional Area and Blood Flow Velocity
The cross-sectional area of a single vessel is defined as its lumen diameter.
The collective total cross-sectional area for a specific vessel type (artery, capillary, or vein) is the sum of diameters for all vessels of that type.
Capillaries, with their vast network, have the largest total cross-sectional area.
Blood Flow Velocity: Inversely related to total cross-sectional area; hence, blood flow is relatively slow in capillaries, allowing sufficient time for exchanges between blood and tissue fluids.
20.3 Capillary Exchange
Mechanisms of Exchange
Capillaries facilitate the exchange of various substances (gases, nutrients, wastes, hormones) between blood and surrounding tissues using methods such as diffusion, vesicular transport, and bulk flow.
Bulk Flow Mechanism
Bulk Flow:
A process where fluids flow down the pressure gradient; substantial amounts of fluids and dissolved substances move simultaneously.
The direction of movement depends on the net pressure from opposing forces, primarily hydrostatic pressure and colloid osmotic pressure.
Filtration:
The movement of fluid out of the blood occurs primarily at the arterial end of the capillary where hydrostatic pressure is high.
Fluid and small solutes pass easily through the openings in the capillary walls, while larger solutes are retained in the blood.
Reabsorption:
Counter to filtration, this is the movement of fluid back into the blood that predominantly takes place at the venous end of the capillary.
Hydrostatic Pressure (HP)
Blood Hydrostatic Pressure (HPb): This is the force exerted by the blood per unit area on the vessel wall, promoting filtration from the capillary.
Interstitial Fluid Hydrostatic Pressure (HPif): This is less than zero in most tissues, representing the force exerted by interstitial fluid on the outside of the blood vessel.
Colloid Osmotic Pressure (COP)
Refers to the pull exerted on water due to the presence of proteins (colloid).
Blood Colloid Osmotic Pressure: Draws fluid back into the blood due to the presence of blood proteins (e.g., albumins) and promotes reabsorption, while Interstitial Fluid Colloid Osmotic Pressure draws fluid into interstitial fluid but is relatively low due to a lack of proteins.
Net Filtration Pressure (NFP)
Defined as the difference between net hydrostatic and net colloid osmotic pressures, where:
Net Hydrostatic Pressure = Difference between blood and interstitial fluid hydrostatic pressures.
Net Colloid Osmotic Pressure = Difference between blood and interstitial fluid osmotic pressures.
The NFP changes along the length of the capillary, being higher at the arterial end (favoring filtration) and lower at the venous end (favoring reabsorption).
Lymphatic System Role
Collects excess interstitial fluid that is not reabsorbed at the venous end of capillaries (approximately 15% of fluid) and filters this fluid before returning it to the venous circulation.
20.4 Local Blood Flow
Not all capillaries are filled simultaneously; local blood flow varies and is measured in milliliters per minute—this flow must be sufficient to maintain adequate tissue perfusion.
Factors affecting local blood flow include:
Degree of tissue vascularity
Myogenic response / regulation
Local regulatory factors altering blood flow
Total blood flow to the area.
Local Short-Term Regulation
Local blood flow can vary directly based on immediate metabolic needs or tissue damage.
Vasoactive Chemicals: These modify blood flow by:
Vasodilators: Cause dilation of arterioles and relax precapillary sphincters, resulting in increased blood flow to capillary beds.
Vasoconstrictors: Constrict arterioles and activate precapillary sphincters, reducing blood flow.
Autoregulation: Refers to how tissues self-regulate their blood supply in response to metabolic activity, where signals (low oxygen, high carbon dioxide, lactic acid, etc.) can act as vasodilators when activity increases.
Example: In situations like reactive hyperemia, there is an increase in blood flow following temporary disruption, assisting in the restoration of oxygen and nutrient levels and the elimination of wastes.
20.5 Blood Pressure
Overview of Blood Pressure
Blood Pressure: Represents the force of blood against vessel walls, with the gradient (change from one end of a vessel to the other) propelling blood through the vascular system.
The highest blood pressure occurs in arteries (particularly during systole) and the lowest in veins.
Systolic Pressure: The pressure in the arteries when ventricles contract, typically higher (e.g., 120 mm Hg).
Recorded as the upper number of blood pressure (e.g., 120/80).
Diastolic Pressure: The pressure when ventricle relaxation occurs, reflected as the lower number (e.g., 80 mm Hg).
Pulse Pressure: The difference between systolic and diastolic pressures (e.g., 40 mm Hg in the case of 120/80), serving as an indicator of arterial health and elasticity.
Mean Arterial Pressure (MAP): The average arterial pressure throughout the cardiac cycle, being calculated as: MAP = Diastolic Pressure + rac{1}{3} Pulse Pressure
E.g., for 120/80, ( MAP = 80 + \frac{40}{3} = 93 ) mm Hg, where MAP < 60 may indicate insufficient perfusion.
Capillary Blood Pressure: Gradually changes, averaging around 40 mm Hg at the arterial end and declining to below 20 mm Hg at the venous end, facilitating both filtration and reabsorption without damaging the capillaries.
Venous Blood Pressure: Ranges low, with pressure of 20 mm Hg in venules and effectively 0 in the vena cava. Venous return relies on the pressure gradient and mechanisms including the skeletal muscle pump and respiratory pump.
Regulation of Venous Pressure
Skeletal Muscle Pump: Assists in blood return from limbs; contracting muscles squeeze veins, propelling blood forward, aided by valves that prevent backflow.
Respiratory Pump: Changes in thoracic and abdominal pressure during inhalation and exhalation supporting venous return to the heart.
Systemic Circulation Blood Pressure Gradient
The systemic blood pressure gradient signifies the difference in pressure between arteries near the heart and the vena cava, averaging about 93 mm Hg, functioning as the driving force for blood movement through the vascular system.
20.5b Resistance
Overview of Resistance
Resistance refers to the friction encountered by blood as it flows through the vessels, opposing blood flow. Factors affecting resistance include:
Viscosity: The thickness of blood; higher viscosity means greater resistance. Blood viscosity is about 5 times more than that of water due to formed elements and proteins. Conditions like anemia (lower viscosity) and dehydration (higher viscosity) can change resistance.
Vessel Length: Longer vessels increase resistance due to increased friction along the vessel. Changes may occur with weight gain (angiogenesis, increasing resistance) and weight loss (vessel regression, reducing resistance).
Vessel Radius: Smaller radii lead to greater resistance. Blood flows with laminar flow, faster at the center and slower at the walls. The relationship between flow and radius is defined as:
F \propto r^4A change in radius from 1 mm to 2 mm results in a change in flow of 16 times greater.
Relationship of Blood Flow
The total blood flow (F) through the system is proportional to the pressure gradient (ΔP) divided by resistance (R):
F \propto \frac{\Delta P}{R}Greater pressure gradients yield increased total blood flow, while elevated resistance decreases that flow.
20.6 Regulation of Blood Pressure and Blood Flow
Maintaining blood pressure within appropriate ranges is essential for tissue perfusion, facilitated by the regulation of cardiac output, resistance, and blood volume through nervous and endocrine systems.
Neural Regulation Overview
Short-term regulation of blood pressure occurs through autonomic reflexes involving nuclei in the medulla oblongata that quickly adjust cardiac output and resistance according to immediate needs (for instance, when standing up).
The Cardiovascular Center in the medulla contains two autonomic nuclei: the cardiac center and the vasomotor center.
The cardiac center influences blood pressure by adjusting cardiac output.
The vasomotor center affects blood pressure by regulating vessel diameter, with sympathetic activation leading to vasoconstriction, increasing resistance and blood pressure.
Cardiac Center Functions
The cardiac center contains two nuclei:
Cardioacceleratory Center: Originates sympathetic pathways extending to the sinoatrial (SA) node and myocardium, increasing heart rate and contractility, thus elevating cardiac output and blood pressure.
Cardioinhibitory Center: Originates parasympathetic pathways to the SA and atrioventricular (AV) nodes, decreasing heart rate and the conduction of electrical signals, decreasing cardiac output and blood pressure.
Vasomotor Center Activity
It leads to sympathetic pathways that release norepinephrine (NE) affecting blood vessel smooth muscle.
Activation of the vasomotor center stimulates the adrenal medulla to release epinephrine (EPI) and norepinephrine (NE), both of which target alpha-1 and beta-2 receptors in blood vessel smooth muscle, which results in either vasoconstriction or vasodilation.
Baroreceptors and Chemoreceptors
Baroreceptors: Nerve endings sensitive to stretches in the vessel wall, located in the tunica externa of aortic arch and carotid sinuses. They modulate blood pressure and are integral in regulating systemic blood pressure.
Chemoreceptor Reflexes: Stimulation of chemoreceptors induces feedback mechanisms that maintain blood chemistry homeostasis, affecting both the respiratory and cardiovascular systems. Main chemoreceptors are located in the aortic and carotid bodies, responding to changes in carbon dioxide, pH, and oxygen levels to effectuate adjustments in blood flow and pressure.