Chapter 21, Lecture 5: Venus Return, Blood Velocity, Syncope, and Control of Blood Pressure

Venus Return

• Definition: Volume of blood flowing back to the heart through the systemic veins due to the left ventricle's contraction.
• Pressure Gradient:
  • Blood moves down its pressure gradient, similar to solutes moving down a concentration gradient.
  • Pressure is ~16 mmHg at venules and ~0 mmHg at the right ventricle, enabling venous return.
• Factors Affecting Venous Return:
  • Increased pressure in the right atrium/ventricle decreases venous return, potentially due to:
    • Leaky tricuspid valve
    • Congestive heart failure
  • Two auxiliary pumps support venous return:
    • Skeletal Muscle Pump:
      • Leg muscle contraction squeezes veins, pushing blood through valves, preventing backflow.
    • Respiratory Pump:
      • Diaphragm movement during breathing changes pressure dynamics, aiding venous return.

Blood Pressure and Factors Influencing It

• Cardiac Output & Systemic Vascular Resistance Increase:
  • Increased Cardiac Output:
    • Caused by increased heart rate and stroke volume.
    • Influences: Sympathetic impulses, respiratory/skeletal muscle pumps, and blood volume.
  • Increased Systemic Vascular Resistance:
    • Influences: Blood viscosity, vessel length, and vasoconstriction.
• Velocity of Blood Flow:
  • Speed inversely related to cross-sectional area.
  • Fastest in areas of highest blood pressure and slows as cross-sectional area increases.
• Circulation Time:
  • Time for a blood drop to travel from the right atrium and back.

Syncope (Fainting)

• Description: Loss of consciousness not due to trauma, with spontaneous recovery due to insufficient brain blood flow.
• Types of Syncope:
  • Vasodepressor Syncope: Emotional/visual triggers causing heart rate drop and vessel dilation.
  • Situational Syncope: Pressure stress from activities like urination or coughing.
  • Drug-induced Syncope: Due to antihypertensive drugs, tranquilizers affecting blood pressure.
  • Orthostatic Hypotension: Blood pressure drop upon standing, requiring careful posture changes.

Control of Blood Pressure & Blood Flow

• Factors: Heart rate, stroke volume, systemic vascular resistance, and blood volume.
  • Rapid adjustments vs. slow, long-term regulation.
• Medulla Oblongata's Role:
  • Located in the brain stem, controlling vital functions.
  • Input from sensory receptors, directing autonomic responses.
• Sensory Receptors:
  • Proprioceptors: Monitor limb position.
  • Chemoreceptors: Monitor blood chemistry.
  • Baroreceptors: Monitor pressure/stretch in vessels.
• Sympathetic vs. Parasympathetic Responses:
  • Sympathetic (fight-or-flight): Increases heart rate and vessel constriction.
  • Parasympathetic (rest-and-digest): Decreases heart rate via vagus nerve.

Cardiovascular Center and Its Outputs

• Input Sources: Cerebral cortex, limbic system, hypothalamus, proprioceptors, baroreceptors, chemoreceptors.
• Output Pathways:
  • Vagus Nerve (Parasympathetic): Decreases heart rate.
  • Cardiac Accelerator Nerves (Sympathetic): Increases heart rate and contractility.
  • Vasomotor Nerves (Sympathetic): Increases vasoconstriction.