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.