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.