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The Circulatory System - Blood Vessels and Circulation

The Circulatory System - Blood Vessels and Circulation

General Anatomy of 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 and smallest veins to create a circuit.

Structure of Blood Vessel Walls

  • Three Layers of Blood Vessels (Tunics):

    • Tunica Interna (Tunica Intima):

      • Lines the blood vessel and is exposed to blood.

      • Composed of simple squamous epithelium (endothelium), which acts as a selectively permeable barrier.

      • Secretes chemicals to stimulate dilation or constriction and usually repels blood cells and platelets to prevent clotting.

      • In inflammation, produces cell-adhesion molecules to congregate leukocytes.

    • Tunica Media:

      • Middle layer made of smooth muscle, collagen, and elastic tissue.

      • Strengthens vessels, prevents rupture due to blood pressure.

      • Contraction controls blood vessel diameter.

    • Tunica Externa (Tunica Adventitia):

      • Outermost layer consists of loose connective tissue.

      • Merges with nearby structures, anchors the vessel, provides passage for nerves and lymphatic vessels.

      • Contains vasa vasorum—small vessels that supply blood to the outer half of the wall in larger vessels.

Types of Arteries

  • Classification by Size:

    • Conducting (Elastic) Arteries:

      • Largest, e.g., aorta and pulmonary trunk.

      • Have internal and external elastic laminae for expansion and recoil during heartbeats.

    • Distributing (Muscular) Arteries:

      • Medium-sized arteries that distribute blood to specific organs.

      • E.g., brachial and femoral arteries.

    • Resistance (Small) Arteries:

      • Thicker tunica media in proportion to lumen; controls blood flow to organs.

      • Arterioles are the smallest, control blood supply and resistance.

Aneurysms

  • Definition:

    • Weak point in an artery or heart wall forming a bulging sac, can lead to rupture.

    • Most common in abdominal aorta and can cause pain or hemorrhage.

    • Causes include congenital weakness, trauma, infections, especially due to atherosclerosis and hypertension.

Arterial Sense Organs

  • Carotid Sinuses:

    • Baroreceptors that monitor blood pressure.

  • Carotid Bodies and Aortic Bodies:

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

    • Vagus Nerve

Capillaries

  • Function:

    • Exchange vessels for gases, nutrients, hormones, and waste between blood and tissue.

    • Nearly eveey cell in body is close to capillary

      • Tendons, ligaments

  • Types of Capillaries:

    • Continuous Capillaries: Found in most tissues, small gaps allow small solutes to pass.

    • Fenestrated Capillaries: Have pores for rapid absorption in kidneys and intestines.

    • Sinusoids: Large gaps allow proteins and blood cells to pass; found in liver and spleen.

Veins

  • Capacitance Vessels:

    • Thin-walled and flaccid; contain 64% of blood at rest.

    • Steady blood flow

    • Expands easily and subject to low blood pressure (averages 10 mm Hg).

Types of Veins

  • Postcapillary Venules:

    • Smallest veins, very porous for fluid exchange.

  • Muscular Venules:

    • Slightly larger, with some smooth muscle.

  • Medium and Large Veins:

    • Larger diameter, valves prevent backflow; includes examples like vena cava.

Circulatory Routes

  • Simplest Route:

    • Heart → Arteries → Arterioles → Capillaries → Venules → Veins.

  • Portal System:

    • Blood flows through two capillary networks before returning to the heart (e.g., intestines to liver).

Blood Pressure, Resistance, and Flow

  • Blood Pressure (BP):

    • Force against vessel walls measured at brachial artery; recorded as systolic and diastolic pressure.

  • Peripheral Resistance:

    • Opposition to blood flow influenced by blood viscosity, vessel length, and radius.

    • Arterioles are primary regulators of resistance, affecting blood flow significantly.

Regulation of Blood Pressure and Flow

  • Vasomotion:

    • Alteration of blood vessel diameter through local, neural, and hormonal controls.

    • Local control adjusts perfusion based on tissue needs; neural control involves reflex adjustments; hormonal influences can affect vessel contraction or dilation.

Mechanisms of Venous Return

  • Includes pressure gradient, skeletal muscle pump, thoracic pump, and cardiac suction.

  • Exercise increases venous return through all mechanisms; inactivity leads to pooling and possible complications.

Circulatory Shock

  • Types:

    • Cardiogenic shock (heart failure) and Low Venous Return (LVR) shock (various causes such as bleeding, obstruction).

    • Response varies from compensated recovery to decompensated shock leading to positive feedback loops affecting tissue health.

Special Cases: The Brain and Other Organs

  • Blood flow monitoring through autoregulation, ensuring optimal perfusion based on activity (e.g., dilation in low brain perfusion).

  • Transient Ischemic Attacks (TIAs): Brief episodes signaling possible strokes; the importance of immediate medical evaluation.