Blood Vessels: Anatomy and Physiology

  • New lecture topic: Blood vessels.

  • Structure of the PowerPoint presentation: Three lectures planned, different from the heart topic covered in three rather than four.

Overview of Circulatory System

  • Closed circulatory system: - Contrast with open circulatory systems where blood spills out and bathes tissues directly.

    • Blood remains within blood vessels, comprising a continuous loop.

Types and Sizes of Blood Vessels

  • Blood vessels are categorized by their size and function.- Arteries:

    • Largest: Elastic arteries (for large blood flow).

    • Smaller: Muscular arteries (distribute blood to specific regions).

    • Smallest: Arterioles (adjusts based on local tissue conditions).

    • Capillaries:

    • Smallest vessels facilitating material exchange between blood and tissues.

    • Veins:

    • Smallest: Venules (merge into larger veins).

    • Larger veins return blood to the heart.

Vasa Vasorum

  • Definition: Blood vessels that supply the walls of larger blood vessels (muscular layer).

  • Important for providing oxygen and nutrients to the blood vessel's own tissues, similar to coronary arteries for heart muscle.

Structure of Blood Vessel Layers

  • Three main layers of blood vessels:- Tunica Externa (Adventitia):

    • Outermost layer, made mostly of loose connective tissue.

    • Tunica Media:

    • Middle layer, primarily smooth muscle.

    • Thick in arteries; enables vasoconstriction and vasodilation.

    • Contains elastic tissue in larger vessels.

    • Tunica Intima (Interna):

    • Innermost layer made of endothelium (squamous epithelial tissue) with a basement membrane.

    • In larger vessels contains loose areolar connective tissue (lamina propria).

Elastic and Muscular Arteries

  • Elastic (Conducting) Arteries: - Characterized by thick sheets of elastic fibers in the tunica media.

    • Allows arteries to expand during ventricular contraction (systolic pressure) and recoil during relaxation (diastolic pressure).

  • Muscular (Distributing) Arteries: - Contain two laminae of elastic tissue but less elastic fibers in the tunica media.

    • Responsible for distributing blood flow through vasoconstriction and vasodilation in response to body needs.

Control of Blood Flow

  • Sympathetic Nervous System Control:- Controls vasoconstriction and vasodilation of blood vessels.

    • Fight or Flight response:

    • Blood flows to skeletal and cardiac muscles, constricting areas like the kidneys and digestive organs.

    • Sympathetic Innervation:

    • Nerve fibers originating from the spinal cord release neurotransmitters such as norepinephrine that bind to adrenergic receptors on smooth muscle cells in the vessel walls. This leads to vasoconstriction, increasing blood pressure and redirecting blood flow to vital areas during stress or physical activity.

  • Chemical Control in Smaller Blood Vessels:- Local tissue conditions affect blood vessel dilation:

    • Increased carbon dioxide and hydrogen (lower pH) cause vasodilation due to increased tissue activity (aerobic/anaerobic).

    • Temperature increase also causes vasodilation.

    • Potassium concentration rise due to tissue damage leads to dilation.

    • Nitric oxide: another chemical inducing vasodilation.

Arterioles and Capillary Function

  • Arterioles: - Smallest arteries with no elastic fibers, regulating blood flow via muscle contraction.

  • Precapillary Sphincters: - Control the flow into capillaries based on tissue requirements, responding to chemical signals.

  • Vasomotion: - Intermittent contraction and relaxation of sphincters affecting blood delivery to capillaries.

Capillary Structure and Types

  • Capillaries:- Smallest vessels for exchange between blood and tissues.

    • Composed primarily of tunica intima with endothelial cells and basement membrane.

  • Types of Capillaries:- Continuous Capillaries:

    • Least leaky, found in muscle, lungs, and especially brain (blood-brain barrier).

    • Fenestrated Capillaries:

    • Have pores (fenestrations) for larger molecule passage, found in kidneys, endocrine glands, and areas of fluid filtration.

    • Sinusoidal Capillaries:

    • Most leaky, with large intercellular clefts, found in bone marrow, spleen, and liver.

Venous System and Blood Return

  • Venules:- Small veins allowing some exchange such as white blood cells in areas needing immune response.

  • Venous Sinuses:- Specialized veins for holding large volumes (e.g., coronary sinus).

  • Vein Structure:- Lower pressure than arteries, leading to less smooth muscle and extended capacity.

    • Contain valves to prevent backflow due to low pressure.

Varicose Veins

  • Condition arising from damaged valves, leading to blood pooling in veins, causing inflammation and pain.

Circulatory Routes

  • Normal blood flow:- Parallel through capillary beds allowing distinct regions to receive oxygenated blood.

  • Portal Systems:- Series of connected capillary beds delivering nutrients or hormones directly from point A to B without dilution in general circulation.

    • Example: Hepatic portal system delivering absorbed nutrients from intestines to the liver.

Anastomoses

  • Arterial Anastomoses: Connections between arteries allowing collateral circulation during blockages.

  • Venous Anastomoses: Bypass pathways for blood flow during vein blockage, preventing pressure backup.

  • Arteriovenous (AV) Anastomoses:- Normal: Help in heat conservation in superficial areas (fingers, toes).

    • Abnormal: AV malformations, particularly dangerous in the brain, can lead to aneurysms and strokes.

  • AV Fistula:- Created surgically for dialysis patients providing high-pressure access to blood filtration systems.

Blood Distribution:

  • At rest, most blood volume in veins; during activity blood flow to skeletal muscles can increase significantly (20x).

Mechanisms of Exchange at Capillaries

  • Diffusion:- Ultimate endpoint in material exchange; substances move from higher to lower concentration.

  • Transcytosis:- Mechanism for larger molecules crossing endothelial cells using vesicular transport.

  • Bulk Flow:- Movement of water (along with dissolved molecules) through clefts for quicker nutrient delivery to tissues.

Pressure Mechanisms in Bulk Flow

  • Hydrostatic Pressure:- Pressure-driven flow in blood vessels; higher at arterial end due to heart pumping.

  • Colloid Osmotic Pressure:- Draws fluid back into circulation, primarily driven by plasma proteins (e.g., albumin).

Net Filtration Pressure

  • Determines whether liquids enter or leave capillaries:- Arterial end: positive net pressure favors filtration out.

    • Venous end: negative net pressure favors reabsorption back into circulation.

  • Starling's Law of Capillary Dynamics:- Most fluid filtered out is reabsorbed, with excess being collected by lymphatic system for return to blood.