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Circulatory%20System-%20Blood%20Vessels

Circulatory System Overview

  • The circulatory system is essential for transporting blood throughout the body.

Blood Vessels

  • Three principal categories of blood vessels:

    • Arteries: Carry blood away from the heart.

    • Veins: Carry blood back to the heart.

    • Capillaries: Connect smallest arteries to smallest veins, forming a circuit.

Blood Vessel Wall Structure

  • Artery and vein walls have three tunics (layers):

    1. Tunica Interna (Tunica Intima):

      • Lines the blood vessel.

      • Composed of simple squamous epithelium.

      • Acts as a selectively permeable barrier.

      • Secretes chemicals for vasodilation or constriction.

    2. Tunica Media:

      • Middle layer.

      • Consists of smooth muscle, collagen, and elastic tissue.

      • Strengthens vessels and prevents rupture under blood pressure.

    3. Tunica Externa (Tunica Adventitia):

      • Outermost layer.

      • Made of loose connective tissue, merging with neighboring blood vessels or organs.

      • Contains vasa vasorum: small vessels that supply blood to the outer layer of larger vessels.

Types of Blood Vessels

Arterial System

  • Three types of arteries:

    • Elastic Arteries:

      • Largest; e.g., aorta, carotid, subclavian arteries.

      • Act as pressure reservoirs to maintain blood flow.

    • Muscular Arteries:

      • Distribute blood to specific organs; e.g., brachial, femoral arteries.

    • Arterioles:

      • Smooth muscle regulates blood flow to capillaries.

Aneurysms

  • Weak points in arteries or heart walls.

  • Can form a bulging sac that may rupture, causing hemorrhage.

  • Common sites include the abdominal aorta, renal arteries, and cerebral arteries.

Arterial Sensory Structures

  • Carotid Sinuses:

    • Baroreceptors in the walls of the internal carotid artery that monitor blood pressure.

  • Carotid Bodies:

    • Chemoreceptors that monitor blood chemistry (pH, CO2, O2).

  • Aortic Bodies:

    • Chemoreceptors located near the aortic arch.

Capillaries

  • Function: Exchange vessels for gases, nutrients, wastes, and hormones.

  • Comprised of endothelium and basal lamina.

  • Three types based on permeability:

    • Continuous Capillaries: Most tissues, tight junctions between endothelial cells.

    • Fenestrated Capillaries: Found in organs with rapid absorption/filtration, such as kidneys.

    • Sinusoids: Found in liver, bone marrow, spleen; allow passage of larger molecules and cells.

Capillary Beds

  • Networks of capillaries supplied by a single arteriole or metarteriole.

  • 75% of capillaries can be shut down at any given time, regulated by precapillary sphincters.

Veins

  • Characteristics:

    • Thin-walled, collapse when empty, larger capacity than arteries, act as blood reservoirs.

  • Types of Veins:

    • Postcapillary Venules: Smallest, very porous, no muscle in the wall.

    • Muscular Venules: Slightly larger, thin tunica media with muscle layers.

    • Medium Veins: Venous valves present, may have muscle and collagen in walls.

    • Large Veins: Include vena cava, thickest tunica externa, smooth muscle present.

Circulatory Routes

  • Simple route: heart -> arteries -> arterioles -> capillaries -> veins -> venules.

  • Portal Circulation: Specific routes for circulation between certain organs such as kidneys and liver.

Blood Pressure (BP)

  • Defined as the force exerted by blood against vessel walls.

  • Measured at the brachial artery with a sphygmomanometer.

  • Two key measurements:

    1. Systolic Pressure: Peak pressure during ventricular contraction.

    2. Diastolic Pressure: Minimum pressure during ventricular relaxation.

  • Pulse Pressure: Difference between systolic and diastolic pressure.

  • Mean Arterial Pressure (MAP): Diastolic pressure + one-third of the pulse pressure.

Hypertension and Hypotension

  • Hypertension:

    • Causes serious health issues, can lead to weakened arteries and atherosclerosis.

  • Hypotension:

    • Chronic low blood pressure, can be caused by blood loss or dehydration.

Factors Affecting Blood Pressure

  • Cardiac Output, Blood Volume, and Resistance to Flow all play critical roles.

  • Peripheral Resistance: Affected by viscosity, vessel length, and radius.

Local and Neural Control of Blood Pressure

  • Local Control (Autoregulation): Tissues adjust their blood supply based on needs.

  • Neural Control: The medulla oblongata regulates blood pressure in response to baroreceptor and chemoreceptor input.

Hormonal Control

  • Key hormones affecting blood pressure include:

    • Angiotensin II: Increases BP by causing vasoconstriction.

    • Aldosterone: Promotes Na+ retention, leading to increased blood volume.

    • Antidiuretic Hormone (ADH): Increases blood volume through water retention.

    • Atrial Natriuretic Peptide: Promotes vasodilation and lowers BP.

Capillary Exchange

  • The movement across capillaries occurs via:

    • Diffusion, Transcytosis, Filtration, and Reabsorption.

Edema**

  • Refers to excess fluid accumulation due to

    • Increased capillary filtration, reduced reabsorption, or obstructed lymphatic drainage.

Venous Return

  • Relies on several factors:

    • Pressure gradient, gravity, skeletal muscle pump, respiratory (thoracic) pump, and cardiac suction.

Circulatory Shock**

  • A condition where cardiac output doesn't meet metabolic needs:

    • Types include cardiogenic shock and low venous return shock.

Stroke

  • Sudden loss of brain function due to a blockage or rupture of a blood vessel in the brain. Types include ischemic stroke and hemorrhagic stroke.