Circluation and Short Term BP (PNB)

UNIT 4: CIRCULATION

• Instructor: Dr. John Redden

• Institution: Physiology and Neurobiology, University of Connecticut

BLOOD VESSELS

• Types of Blood Vessels:

  • Arteries: Carry blood away from the heart (Distribution).

  • Veins: Carry blood toward the heart (Collection).

  • Capillaries: Sites of exchange between blood and tissues.

BLOOD FLOW

• Blood Pressure Measurement:

  • Systolic pressure: Pressure during heart contraction.

  • Diastolic pressure: Pressure during heart relaxation.

  • Mean arterial pressure: Average pressure in the arteries.

• Circulatory Pathway:

  • Aorta → Elastic arteries → Muscular arteries → Arterioles → Capillaries → Venules → Large veins (e.g., venae cavae).

RESISTANCE

• Definition: Opposition to blood flow due to friction in blood vessels.

• Factors Affecting Resistance:

  • Viscosity of Blood (e.g., hematocrit levels).

  • Blood Vessel Radius:

    • Resistance (R) is inversely proportional to the fourth power of radius (R = 1/r^4).

    • Very small changes in radius result in significant changes in resistance.

• Poiseuille’s Law: Flow is proportional to the radius of the blood vessel.

TENSION

• Aneurysms:

  • Law of La Place states that tension within a vessel is proportional to its radius.

  • Areas studied include:

    • Arterioles

    • Arteries

    • Veins

    • Capillaries

BLOOD VESSEL STRUCTURE

• Layers of Blood Vessels:

  1. Tunica Interna (Intima): Endothelium and sub-endothelial layer.

  2. Tunica Media: Contains smooth muscle and elastin.

  3. Tunica Externa: Composed of collagen and elastic fibers.

ARTERIES

• Classification:

  1. Elastic (Conducting): Large diameter, walls stretch to accommodate blood.

  2. Muscular (Distribution): Medium-sized, regulate blood flow by changing diameter.

• Function: Distribution and acting as pressure reservoirs.

• Compliance: Low compliance means less stretch under pressure.

ARTERIOLES

• Function:

  • Small vessels that determine blood flow to tissues and total peripheral resistance (TPR).

VASOCONSTRICTION AND DILATION**

• Mechanism:

  • Vascular smooth muscle regulates blood flow through constriction and dilation, affecting various body regions differently during rest and vigorous exercise.

ARTERIOLE REGULATION

• Local Regulation:

  • Vasodilation due to low oxygen or high levels of CO2 and H+.

  • Vasoconstriction can occur due to vasoconstrictive agents like thromboxane (TXA2).

• Intrinsic Regulation: Blood flow adjusts to meet tissue needs without manipulating mean arterial pressure (MAP).

REGULATION: ANS & ENDOCRINE

• Autonomic Nervous System:

  • Sympathetic Nervous System regulates vasoconstriction (Alpha receptors) and vasodilation (Beta receptors).

• Extrinsic Regulation: Adjusting blood flow based on the body’s overall needs:

  • Vasodilation: Beta2 receptors via nitric oxide and atrial natriuretic peptide.

  • Vasoconstriction: Alpha adrenergic receptors and hormones like angiotensin II.

CAPILLARIES

• Structure:

  • Composed primarily of the tunica interna.

  • Red blood cells pass through in single file without smooth muscle around them.

• Classification of Capillaries:

  1. Continuous: No breaks, exchange without loss of RBC & plasma (most tissues).

  2. Fenestrated: Allow exchange of small peptides and signaling molecules (e.g., hypothalamus).

  3. Sinusoid: Allow exchange of large proteins (e.g., liver).

CAPILLARY EXCHANGE**

• Filtration: Movement of fluid out of capillaries due to hydrostatic pressure.

• Reabsorption: Movement of fluid back into blood caused by osmotic pressure.

• Noteworthy Note: More fluid is filtered than reabsorbed.

THE LYMPHATIC SYSTEM

• Function:

  • Absorbs excess fluid from tissues and returns it to the circulatory system.

  • Lymphatic vessels drain into systemic circulation via subclavian veins to the superior vena cava.

  • Plays a role in moving insoluble molecules, flushing toxins, and improving nutrient distribution.

EDEMA**

• Definition: Fluid accumulation in tissues due to failure of the lymphatic system.

• Causes:

  1. Increased capillary pressure.

  2. Decreased plasma proteins.

  3. Increased capillary permeability.

  4. Lymph blockage.

VEINS**

• Characteristics:

  • Low-pressure conduits returning blood to the right atrium.

  • Highly compliant (can stretch easily) which accommodates large volumes of blood with little pressure increase.

ADAPTATIONS OF VEINS* *

• Str’ucture:

  • Composed of tunica externa, tunica media, and tunica intima (endothelium).

• Description of Types:

  • Large Veins: High compliance with large lumens.

  • Medium-Sized Veins: Transition structures between large veins and venules.

  • Venules: Small diameter vessels leading from capillary beds.

PATHOLOGY: VARICOSE VEINS**

• Description:

  • Veins that have widened and stretched due to valve incompetence, leading to backflow.

VEINS AS BLOOD RESERVOIRS**

• Function:

  • Due to high compliance, veins can store large amounts of blood without significant pressure increases.

VENOUS RETURN**

• Mechanism:

  • Contraction of skeletal muscles compresses veins to raise venous pressure and prevent backflow.

  • Breathing creates negative pressure in thoracic cavity, aiding venous return.

CHANGING VENOUS RETURN USING ANS**

• Effect of Sympathetic Stimulation:

  • Increases venous tone and decreases compliance, thus affecting central venous pressure.

BLOOD PRESSURE REGULATION**

• General Regulation Centers:

  • Located in the medulla, involving the cardioacceleratory and cardioinhibitory centers controlling sympathetic and parasympathetic innervations respectively.

BARORECEPTORS & CHEMORECEPTORS**

• Location:

  • Baroreceptors located in carotid sinuses and aortic arch; sensitive to changes in MAP.

• Mechanism:

  • Increased arterial pressure results in increased firing rate of baroreceptors, influencing cardiovascular responses.

CHEMORECEPTOR REFLEX**

• Function:

  • Responds to low O2, high CO2, and low pH by stimulating cardiovascular centers, causing vasoconstriction and increased cardiac output to maintain MAP.

HEMORRHAGE**

• Effects:

  • In blood loss situations, arterial pressure decreases, resulting in various compensatory physiological changes to stabilize blood volume and pressure.