Cardiovascular Physiology and Blood Flow Dynamics

Pathway of Blood Flow Through the Heart and Body

• Deoxygenated Blood Tracking:

  • The cycle begins as deoxygenated blood from the body empties into the Superior and Inferior Vena Cava.
  • Blood enters the Right Atrium.
  • It travels through the Tricuspid Valve (named for its three little leaflets) which prevents the backflow of blood.
  • Blood enters the Right Ventricle.
  • Pressure builds in the ventricle, overcoming the pressure in the atrium, which causes the tricuspid valve to shut.
  • Pressure forces the Pulmonary (Pulmonic) Valve open.
  • Blood is pumped into the Pulmonary Artery (the only artery in the body carrying deoxygenated blood) which branches off to both lungs.

• Oxygenated Blood Tracking:

  • The blood becomes oxygenated in the lungs.
  • Blood returns to the heart via the Pulmonary Veins (there are four pulmonary veins, whereas there are only two pulmonary arteries).
  • Blood enters the Left Atrium.
  • It passes through the Mitral Valve (also known as the Bicuspid Valve), which then shuts.
  • The Left Ventricle contracts, pushing blood through the Aortic Valve.
  • Blood enters the Aorta and is distributed throughout the systemic circulation to the entire body.
  • At the tissue site, oxygen unlatches to oxygenate tissues and picks up $CO_2$ (carbon dioxide).
  • The now deoxygenated blood returns to the Superior and Inferior Vena Cava to restart the process.

The Cardiac Cycle: Key Terminology and Concepts

• Systole: Refers to the contraction or the "squeeze" of the heart muscles, specifically the ventricles.
• Diastole: Refers to the relaxation of the heart muscles. This is when the ventricles fill with blood and the coronary arteries are perfused.
• Blood Pressure Measurement: Expressed as Systole over Diastole (e.g., $120/80 mmHg$).
  • A high systolic pressure (e.g., $200 mmHg$) indicates the heart is working extremely hard, which can lead to a stroke.
• Cardiac Output ($CO$ or $Q$): The total volume of blood ejected from the ventricles each minute.
  • Average resting $CO$ is approximately $5 L/min$, which equals the total blood volume of the human body.
• Stroke Volume ($SV$): The volume of blood ejected from the ventricles during a single contraction.

Hemodynamic Volumes and Ejection Fraction

• End Diastolic Volume ($EDV$): Often equated with Preload, this is the amount of blood in the ventricles at the end of the relaxation period (diastole).
• End Systolic Volume ($ESV$): The amount of blood remaining in the ventricle after a contraction (systole).
  • The heart never pumps out $100\%$ of its volume.
  • A high $ESV$ suggests the heart is weak and failing to pump effectively.
• Ejection Fraction ($EF$): The percentage of blood pumped out of the left ventricle per contraction.
  • Normal range: $55\text{--}70\%$.
  • Heart Failure threshold: Below $50\%$.
  • Critical Failure/Transplant threshold: Below $30\%$.
  • Measurement tool: Echocardiogram (ultrasound of the heart).

Determinants of Stroke Volume: Preload, Afterload, and Contractility

• Preload: The stretching of the cardiac myocytes (heart muscle cells) prior to contraction.
  • It is directly related to the venous return and the $EDV$.
  • Frank-Starling Mechanism: States that the greater the stretch (preload), the greater the force of contraction (up to a physiological limit). This is compared to a rubber band.
• Afterload: The resistance or pressure that the ventricles must pump against to eject blood.
  • It is influenced by blood volume, blood viscosity (thickness), and vascular resistance.
  • The Milkshake Metaphor: Pumping thick blood (thick milkshake) requires more pressure/effort than thin blood (coke).
  • High afterload increases the workload on the heart and can lead to high blood pressure.
• Contractility: The force generated by the myocardium during the shortening of muscle fibers.
  • Inotropes: Drugs that influence contractility.
  • Positive Inotropes: Increase contractility (e.g., Norepinephrine, Dopamine, Dobutamine).
  • Negative Inotropes: Decrease contractility (e.g., Beta blockers, Calcium channel blockers).

Hemodynamic Monitoring and Resistance Units

• PA Catheter (Swan-Ganz): A specialized catheter used to measure preload and pressures within the heart.
  • It is inserted into a large vein and "floated" until it reaches the pulmonary artery.
  • Pulmonary Capillary Wedge Pressure ($PCWP$): The measurement obtained when the balloon on the catheter is inflated, providing the best approximation of left-sided preload.
• Systemic Vascular Resistance ($SVR$): Reflects the afterload on the Left Ventricle.
  • Increased by vasoconstriction, plaque, or high blood viscosity.
• Pulmonary Vascular Resistance ($PVR$): Reflects the afterload on the Right Ventricle.
  • The pulmonary system is normally a low-pressure, low-resistance system.

The Vascular System Structure

• Arteries: High-pressure vessels that carry blood away from the heart. They are muscular, elastic, and found deeper in the tissue. The pulse is felt here due to the expansion and relaxation of the walls.
• Arterioles: Known as the Resistance Vessels. They contain smooth muscle that can constrict or dilate to regulate systemic blood pressure and distribute blood flow (shunting blood from the gut to the brain/muscles during fight-or-flight).
• Capillaries: The site of gas exchange.
  • Internal Respiration: Gas exchange between blood and systemic tissues.
  • External Respiration: Gas exchange between lungs and blood.
• Veins: Known as Capacitance Vessels. Characterized by low pressure and high distensibility, they hold approximately $60\%$ of the total blood volume.

Neurological Control of the Heart and Vasculature

• Vasomotor Center: Located in the Medulla Oblongata. It controls vessel diameter through sympathetic nervous impulses.
  • Vasomotor Tone: A state of continuous sympathetic impulses that keeps vessels in a slightly constricted state, allowing for either further constriction or dilation as needed.
• Baroreceptors (Stretch Receptors): Located in the walls of the carotid arteries and the aortic arch. They respond instantly to changes in blood pressure.
  • Carotid Receptors: Innervated by the Glossopharyngeal Nerve (9th Cranial Nerve).
  • Aortic Arch Receptors: Innervated by the Vagus Nerve (10th Cranial Nerve).
  • If pressure is too high, sensors signal the brain to dilate vessels and decrease heart rate. If pressure is low (e.g., standing up too fast), they signal for constriction.
  • Carotid Massage: A clinical technique to stimulate baroreceptors and slow down tachycardia.

Calculations and Mathematical Models

• Cardiac Output Formula:$CO = SV \times HR$

  • Example: If $SV = 70\u2009mL$ and $HR = 72\u2009bpm$, then $CO = 5,040\u2009mL/min$ or approximately $5.0\u2009L/min$.

• Mean Arterial Pressure ($MAP$): The average pressure in the arterial system through one cardiac cycle. It is the best indicator of tissue perfusion.     $MAP = \frac{SBP + (2 \times DBP)}{3}$

  • Normal Range: $80\text{--}100\u2009mmHg$.
  • Critical Point: $MAP < 60\u2009mmHg$ leads to organ failure.

Pulmonary Distribution and Lung Zones

• Gravity Dependence: Blood flow in the lungs is heavily affected by gravity. In an upright person, most blood flow is at the base (bottom) of the lungs.
• Zones of the Lung:
  • Zone 1 (Apex/Top): High ventilation, low perfusion (Potential Dead Space).
  • Zone 2 (Middle): Equal ventilation and perfusion ($V/Q$ match).
  • Zone 3 (Base/Bottom): High perfusion, low ventilation (Potential Shunting).
• Hypoxic Pulmonary Vasoconstriction ($HPV$): A unique lung mechanism where pulmonary vessels constrict in response to low oxygen levels to reroute blood to better-ventilated areas of the lung.

Pathological Conditions and Clinical Observations

• Heart Failure Backups:
  • Left-Sided Failure: Blood backs up into the lungs, causing Pulmonary Edema, shortness of breath, and white-out on X-rays.
  • Right-Sided Failure: Blood backs up into the systemic system, causing Jugular Venous Distension ($JVD$), swollen legs (pedal edema), and ascites (distended belly).
• Pulse Points for Palpation: Temporal, Carotid, Apical, Brachial, Radial (thumb side), Femoral (groin), Popliteal (back of knee), Posterior Tibial (ankle), and Dorsalis Pedis (top of foot).