Blood Vessels and Circulation Flashcards

Introduction to Blood Vessels and Circulation

• Core Concepts: The cardiovascular system relies on a complex network of vessels to transport blood, facilitate exchange, and maintain homeostasis. The study of this system involves understanding vessel structure, fluid dynamics, and the regulatory mechanisms controlling flow and pressure.
• Primary Objectives:
  • Describe the structure and function of the various types of blood vessels.
  • Identify where and how fluid and dissolved materials enter and leave the cardiovascular system.
  • Explain regulatory mechanisms for blood flow and factors influencing blood pressure.
  • Discuss the movement of fluids between capillaries and interstitial spaces.
  • Explain the coordination of cardiac, vasomotor, and respiratory centers in tissue blood flow control.

Classes of Blood Vessels

• Arteries: Responsible for carrying blood away from the heart.
• Arterioles: The smallest branches of arteries; they lead into capillary networks.
• Capillaries: The smallest blood vessels and the primary site of exchange between the blood and the surrounding interstitial fluid.
• Venules: Small veins that collect blood from the capillaries.
• Veins: Responsible for returning blood to the heart.

The Structure of Vessel Walls

• Triple-Layered Wall Structure: Both arteries and veins (with the exception of capillaries) possess walls composed of three distinct layers:
  1. Tunica Intima (Inner Layer): Includes the inner endothelial lining composed of simple squamous cells (endothelial cells) and a surrounding connective tissue layer. In arteries, this layer also contains an outer layer of elastic fibers known as the internal elastic membrane.
  2. Tunica Media (Middle Layer): Composed of concentric sheets of smooth muscle.
     • Contraction: Leads to vasoconstriction (vessel diameter decreases).
     • Relaxation: Leads to vasodilation (vessel diameter increases).
     • This layer binds to the inner and outer layers of the vessel wall and is notably thicker in arteries than in veins.
     • In arteries, an external elastic membrane separates the tunica media from the tunica externa.
  3. Tunica Externa (Outer Layer): A connective tissue sheath that stabilizes and anchors the vessel to adjacent tissues.
     • In arteries, it contains collagen and elastic fibers.
     • In veins, it is generally thicker than the tunica media and contains collagen, elastic fibers, and smooth muscle cells.
• Vasa Vasorum: Meaning "vessels of vessels," these are small arteries and veins located within the walls of large arteries and veins. They supply blood to the cells of the tunica media and tunica externa which are too far from the lumen to receive nutrients via diffusion.

Structure and Function of Arteries

• Arterial Characteristics:
  • Elasticity: Allows arteries to expand and constrict passively in response to blood pressure changes.
  • Contractility: The ability to actively change diameter, primarily controlled by the sympathetic division of the Autonomic Nervous System (ANS).
• Types of Arteries:
  • Elastic Arteries (Conducting Arteries): Largest diameter vessels (e.g., pulmonary trunk and aorta). The tunica media contains many elastic fibers and fewer muscle cells. This elasticity allows them to withstand high pressure changes and recoil to even out the "pulse force."
  • Muscular Arteries (Distribution Arteries): Medium-sized arteries that branch off elastic arteries. Their tunica media contains more muscle cells and fewer elastic fibers, allowing for active regulation of blood flow.
  • Arterioles: Small vessels with little or no tunica externa and a thin or incomplete tunica media. Their diameter changes based on local conditions (e.g., dilating when oxygen is low) or sympathetic/endocrine stimulation. They are known as "resistance vessels" because they create resistance ($R$) to blood flow.
• Pathophysiology: Aneurysm:
  • A bulge in an arterial wall caused by a weak spot in the elastic fibers.
  • Internal pressure may rupture the vessel, leading to hemorrhage.
  • The most dangerous locations for aneurysms are the brain or the aorta.

Structure and Function of Capillaries

• General Features: Smallest vessels with very thin walls. They form microscopic networks (capillary beds) that permeate all active tissues. Their primary function is the exchange of materials (diffusion) between blood and interstitial fluid or blood and air.
• Capillary Structure: Consists of endothelium inside a thin basement membrane. They lack a tunica media and tunica externa.
• Types of Capillaries:
  • Continuous Capillaries: Feature a complete endothelial lining. They are the most common type, found in all active tissues except epithelia and cartilage. They permit diffusion of water, small solutes, and lipid-soluble materials, but block blood cells and plasma proteins.
  • Specialized Continuous Capillaries: Found in the CNS and thymus; they have very restricted permeability due to tight junctions (e.g., the blood-brain barrier).
  • Fenestrated Capillaries: Contain pores in the endothelial lining, allowing rapid exchange of water and large solutes. Found in endocrine organs (pituitary, thyroid), kidneys, and the intestinal tract.
  • Sinusoids (Sinusoidal Capillaries): Feature large gaps between endothelial cells and a thin or absent basement membrane. Located in the liver, spleen, bone marrow, and endocrine organs. They permit free exchange of water and large plasma proteins. Macrophages monitor blood at sinusoids.
• Capillary Beds (Capillary Plexus):
  • Groups of capillaries connecting an arteriole to a venule.
  • Precapillary Sphincter: A circular muscle band at the entrance of each capillary. It constricts to stop/slow flow or relaxes to allow flow.
  • Vasomotion: The cyclic contraction and relaxation of these sphincters, causing blood flow to change routes constantly to ensure all tissues receive nutrients/oxygen.
  • Collaterals: Multiple arteries contributing to one capillary bed to ensure circulation if one path is blocked. Fusion of two collaterals is an arterial anastomosis.
  • Arteriovenous Anastomosis: Direct connections between arterioles and venules that bypass the capillary bed entirely when dilated.

Structure and Function of Veins

• General Features: Collect blood from tissues and return it to the heart. Compared to arteries, veins have larger diameters, thinner walls, and lower blood pressure.
• Venous Valves: Folds of the tunica intima that prevent the backflow of blood. Muscular compression (skeletal muscle pump) helps push blood toward the heart.
• Types of Veins:
  • Venules: Smallest veins collecting blood from capillaries.
  • Medium-Sized Veins: Thin tunica media with few muscle cells; thick tunica externa with collagen/elastic fibers.
  • Large Veins: Thin tunica media and a very thick tunica externa (e.g., Venae Cavae).

The Distribution of Blood

• Blood Volume Allocation:
  • Heart, Arteries, Capillaries, and Pulmonary Circuit: $30-35\%$ of blood volume.
  • Venous System: $65-70\%$ of blood volume.
  • Large Venous Networks (liver, bone marrow, skin): Contains approximately $20\%$ of total blood.
• Capacitance: The relationship between blood volume and pressure, or the ability of a vessel to stretch. Veins are "capacitance vessels" because they can accommodate large volume changes with minimal pressure changes, acting as a blood reservoir.
• Venous Response to Blood Loss: Vasomotor centers stimulate sympathetic nerves, causing venoconstriction and the redistribution of the venous reserve (especially from the liver, skin, and bone marrow).

Pressure and Resistance

• Circulatory Flow Forces: Capillary blood flow is determined by the interplay between Pressure ($P$) and Resistance ($R$).
  • Flow ($F$) is proportional to the pressure gradient ($\Delta P$).
  • Flow ($F$) is inversely proportional to resistance ($R$): $F = \frac{\Delta P}{R}$.
• Measurement Categories:
  1. Blood Pressure (BP): Arterial pressure measured in $mmHg$.
  2. Capillary Hydrostatic Pressure (CHP): Pressure within capillary beds.
  3. Venous Pressure: Pressure within the venous system.
• Total Peripheral Resistance (TPR): The resistance of the entire cardiovascular system, which circulatory pressure must overcome. Factors include:
  • Vascular Resistance: Caused by friction between blood and vessel walls. Depends on vessel length (constant in adults) and diameter (variable). Resistance increases exponentially as diameter decreases: $R \propto \frac{1}{r^4}$.
  • Viscosity: Resistance to flow caused by molecules in the liquid. Whole blood is $5 \times$ more viscous than water.
  • Turbulence: Swirling action disturbing smooth flow; occurs in heart chambers and great vessels, or due to atherosclerotic plaques.
• Pressure, Velocity, and Area Relationships:
  • As total cross-sectional area increases (as in capillaries), velocity of flow decreases.
  • Capillaries have the largest total cross-sectional area and the slowest blood flow, providing maximal time for exchange.
  • As blood enters the venous system, total cross-sectional area decreases, and flow velocity increases.

Arterial Blood Pressure Details

• Systolic Pressure: Peak arterial pressure during ventricular systole.
  • Diastolic Pressure: Minimum arterial pressure during ventricular diastole.
  • Pulse Pressure: The difference between systolic and diastolic pressure; represents the force of heart contraction.
  • Mean Arterial Pressure (MAP): A single calculated pressure value: $MAP = \text{diastolic pressure} + \frac{1}{3} \text{pulse pressure}$.
• Clinical Ranges:
  • Normal BP: $120/80\,mmHg$.
  • Hypertension: Abnormally high BP (greater than $130/80\,mmHg$). Increases heart workload and damages myocardium.
  • Hypotension: Abnormally low blood pressure.

Venous Return and Capillary Exchange

• Venous Return Mechanisms: Pressure in the venous system is low (approximately $16\,mmHg$ at venules to $2\,mmHg$ at the Vena Cava).
  • Muscular Compression: Skeletal muscles squeeze veins.
  • Respiratory Pump: Inhalation decreases thoracic pressure and increases abdominal pressure, pulling blood into the thoracic cavity. Exhalation pushes blood into the right atrium.
• Capillary Exchange Processes:
  1. Diffusion: Movement from high to low concentration. Routes include gaps between endothelial cells (water/glucose), fenestrations, or direct diffusion through membranes (lipids/gases).
  2. Filtration: Solutes driven across a membrane by hydrostatic pressure (CHP). Water and small solutes are pushed into interstitial fluid; large solutes (proteins) are left behind.
  3. Reabsorption: Movement back into blood via osmosis, driven by Blood Colloid Osmotic Pressure (BCOP), which is created by suspended blood proteins.
• Net Filtration Pressure (NFP): The difference between Net Hydrostatic Pressure and Net Osmotic Pressure.
  • Formula: $NFP = (CHP - IHP) - (BCOP - ICOP)$.
  • At the Arterial End: CHP is high, NFP is positive; fluid moves out (filtration).
  • At the Venous End: BCOP dominates, NFP is negative; fluid moves in (reabsorption).
  • Fluid Balance: Capillaries filter about $24\,L/day$ and reabsorb $20.4\,L/day$. The remaining $3.6\,L/day$ is collected by lymphatic vessels and returned via the subclavian vein.
• Edema: Abnormal accumulation of interstitial fluid. Causes include tissue damage (leaky walls), starvation (low plasma proteins/BCOP), or high circulatory pressure (high CHP).

Cardiovascular Regulation and Perfusion

• Tissue Perfusion: Blood flow through tissues to deliver $O_2$ and remove wastes. Controlled by cardiac output, peripheral resistance, and BP.
• Regulatory Mechanisms:
  1. Autoregulation: Localized, immediate adjustments via precapillary sphincters.
     • Vasodilators: Low $O_2$, high $CO_2$, low pH (lactic acid), or Nitric Oxide (NO) increase local flow.
     • Vasoconstrictors: Thromboxanes released by platelets decrease local flow.
  2. Neural Mechanisms:
     • Cardiovascular Centers: Cardioacceleratory (increases CO) and Cardioinhibitory (decreases CO).
     • Vasomotor Center: Controls vessel diameter. Sympathetic stimulation (Norepinephrine) causes vasoconstriction.
     • Baroreceptor Reflex: Located in carotid and aortic sinuses. High BP causes inhibition of sympathetic neurons (vasodilation); low BP stimulates them (vasoconstriction).
     • Chemoreceptor Reflex: Monitors arterial blood ($pH, O_2, CO_2$) via carotid/aortic bodies and medulla surface.
  3. Endocrine Mechanisms:
     • Epinephrine/Norepinephrine: Increase CO and cause peripheral vasoconstriction.
     • Angiotensin II: Produced in response to low BP/volume; potent vasoconstrictor and triggers ADH/Aldosterone release.
     • Antidiuretic Hormone (ADH): Promotes water retention and vasoconstriction.
     • Erythropoietin (EPO): Stimulated by low BP to increase Red Blood Cell formation.
     • Natriuretic Peptides (ANP and BNP): Released by the heart due to excessive stretching. They promote water/sodium loss, decrease thirst, and cause vasodilation to lower BP and volume.