AH

Cardiovascular System Part 2 Notes

Cardiac Output

  • Cardiac Output (CO) is defined as the volume of blood ejected from the left ventricle (or the right ventricle) into the aorta (or pulmonary trunk) each minute.
  • It is calculated as Stroke Volume (SV) multiplied by Heart Rate (HR): CO = SV \times HR
  • In a resting adult male, the typical cardiac output is approximately 5.25 L/min.
  • Factors that increase either Heart Rate or Stroke Volume will lead to an increase in Cardiac Output.
  • Cardiac Output is a critical measure as it indicates the rate at which oxygenated blood is delivered to body cells, which is essential for their function and survival.

Cardiac Output Variation in Trained vs. Untrained Individuals

  • Trained athletes have a significantly greater capacity to increase Cardiac Output above resting levels compared to untrained individuals.
  • Athletes may experience a seven to eight-fold increase in Cardiac Output from their resting level, demonstrating a large reserve capacity.
  • Non-trained individuals typically exhibit a smaller increase, around four to five-fold, showing a smaller reserve capacity.

Distribution of Cardiac Output During Rest and Heavy Exercise

  • At rest, Cardiac Output is approximately 5 L/min, while during heavy exercise, it can increase to 25 L/min.
  • Distribution of Cardiac Output varies:
    • Rest:
      • Brain: 15-20% (~0.75 L/min)
      • Heart: 4-5%
      • Skeletal Muscle: 15-20%
      • Kidneys: 20-25%
      • Abdominal Organs: 20%
      • Skin: 3-5%
      • Other: 4-5%
    • Heavy Exercise:
      • Brain: 3-5%
      • Heart: 4-5%
      • Skeletal Muscle: 70-85% (~20 L/min)
      • Kidneys: 0.5-1%
      • Abdominal Organs: 3-4%
      • Skin: 5-20% (depending on ambient and body temperatures)
      • Other: 2-4%

Factors Influencing Cardiac Output

  • Cardiac Output is determined by the formula: CO = HR \times SV
  • To increase Cardiac Output, either Stroke Volume or Heart Rate (or both) must increase.
  • Maximal exercise capacity can result in a Cardiac Output of roughly 30 L/min.
  • Stroke Volume typically increases until about half of maximal Cardiac Output is reached.
  • Beyond this point, further increases in Cardiac Output primarily depend on increases in Heart Rate.

Regulation of Stroke Volume

  • Stroke Volume (SV) is the volume of blood ejected by a ventricle during each contraction.
  • Stroke Volume is regulated by three primary factors:
    • Preload
    • Myocardial Contractility
    • Afterload

Preload

  • Preload is the degree of stretch of the ventricle before it contracts.
  • According to Starling's Law of the Heart, the greater the stretch (due to more blood in the ventricle during diastole), the greater the force of contraction during systole.
  • Preload is proportional to the End Diastolic Volume (EDV).

Myocardial Contractility

  • Myocardial Contractility is the strength of contraction at a given preload.
  • Factors that increase contractility include sympathetic stimulation and certain drugs like digitalis.

Afterload

  • Afterload is the pressure that must be overcome to open the semilunar valves (aortic pressure of ~80 mmHg or pulmonary trunk pressure of ~20 mmHg).
  • Conditions that increase Afterload, such as hypertension or atherosclerosis, reduce the Stroke Volume.

Regulation of Heart Rate

  • Adjustments to Heart Rate are important for short-term control of Cardiac Output.
  • Heart Rate regulation occurs through two main mechanisms:
    • Autonomic Regulation
    • Chemical Regulation

Autonomic Regulation of Heart Rate

  • Originates in the Cardiovascular Center of the Medulla.
  • Receives input from:
    • Proprioceptors (monitoring limb position)
    • Chemoreceptors (monitoring chemical changes in the blood)
    • Baroreceptors (measuring stretching of major arteries and veins)

Sympathetic Stimulation

  • Occurs when:
    • Proprioceptors detect movement.
    • Chemoreceptors detect increased CO_2 or H^+ concentration in the blood.
    • Baroreceptors detect increased blood pressure.
  • Results in:
    • Increased rate of SA node depolarization, increasing Heart Rate.
    • Increased contractility, allowing a greater volume of blood to be ejected during systole.

Parasympathetic Stimulation

  • Occurs when:
    • Proprioceptors detect reduced movement.
    • Chemoreceptors detect decreased CO_2 or H^+ concentration.
    • Baroreceptors detect decreased blood pressure.
  • Results in:
    • Slower rate of SA node depolarization via the Vagus nerve (CN X), decreasing Heart Rate.
    • No direct effect on contractility.

Chemical Regulation of Heart Rate

  • Various chemicals influence Heart Rate including:
    • Hormones
    • Cations

Hormones

  • Epinephrine and norepinephrine (from the adrenal medulla) and thyroid hormones increase Heart Rate and contractility.

Cations

  • Ionic imbalances interfere with the depolarization of the heart, affecting Heart Rate.
  • Elevated blood levels of K^+ or Na^+ decrease Heart Rate.
  • Elevated interstitial Ca^{2+} levels increase Heart Rate.

Summary of Factors Influencing Blood Pressure

  • Several factors impact Mean Arterial Pressure (MAP):
    • Increased blood volume
    • Skeletal muscle pump activity
    • Respiratory pump activity
    • Venoconstriction
    • Decreased parasympathetic impulses
    • Increased sympathetic impulses and adrenal medulla hormones
    • Increased venous return
    • Increased red blood cell count (polycythemia)
    • Increased blood viscosity
    • Increased total blood vessel length
    • Increased body size (obesity)
    • Decreased blood vessel radius (vasoconstriction)

Blood Vessels

  • There are 5 main types of blood vessels:
    • Arteries: Carry blood away from the heart.
    • Arterioles: Smaller arteries branching into tissues.
    • Capillaries: Allow exchange of substances between blood and body tissues.
    • Venules: Formed by merging capillaries.
    • Veins: Carry blood from the tissues back to the heart.

Histology of Blood Vessels

  • Most blood vessels (except capillaries) have three main layers:
    • Tunica Interna (Intima): Innermost layer.
    • Tunica Media: Middle layer.
    • Tunica Externa (Adventitia): Outermost layer.

Tunica Interna (Intima)

  • Innermost layer, closest to the lumen.
  • Composed of endothelium (simple squamous epithelium).

Tunica Media

  • Middle layer consisting of elastic and smooth muscle fibers.

Tunica Externa (Adventitia)

  • Outermost layer consisting of elastic and collagen fibers.

Differences Between Artery and Vein Histology

  • Arteries have a much thicker Tunica Media compared to veins to handle higher pressures.
  • Veins have valves, whereas arteries do not.
  • The lumen is smaller in arteries compared to veins.

Hemodynamics: Factors Affecting Blood Flow

  • Blood flow (cardiac output) to various parts of the body depends on:
    • Blood Pressure: The pressure difference drives blood flow through a particular tissue.
    • Vascular Resistance: The resistance to blood flow in specific blood vessels.

Blood Pressure

  • Blood flows from regions of higher pressure to regions of lower pressure.
  • The greater the pressure difference, the greater the blood flow.
  • Systolic Blood Pressure is the highest pressure attained in the arteries during systole.
  • Diastolic Blood Pressure is the lowest pressure attained in the arteries during diastole.
  • Pressure is highest as blood leaves the ventricles (120 mmHg from the left ventricle, 20 mmHg from the right ventricle).
  • Pressure progressively falls as the distance from the ventricle increases.
  • In capillaries, pressure fluctuations disappear.
  • At the venous end of capillaries, blood pressure is about 16 mmHg.
  • Blood pressure continues to drop as blood enters systemic venules and veins (5-10 mmHg).
  • As blood reaches the right ventricle, blood pressure is 0 mmHg.

Vascular Resistance

  • Vascular Resistance is the opposition to blood flow due to friction between blood and the walls of the blood vessels.
  • It depends on three factors:
    • Diameter of the lumen
    • Blood viscosity
    • Total vessel length

Diameter of the Lumen

  • The smaller the lumen, the greater the resistance to blood flow.
  • Resistance is inversely related to the fourth power of the diameter of the blood vessels lumen: R = \frac{1}{d^4}. If the diameter of a blood vessel decreases by half, the resistance will increase 16 times.
  • As arterioles dilate, resistance decreases, and blood pressure falls.

Blood Viscosity

  • A measure of the "thickness" of the blood.
  • The higher the blood’s viscosity, the higher the resistance.
  • Any condition that increases the viscosity of the blood (dehydration, polycythemia) increases blood pressure.

Total Vessel Length

  • Resistance to blood flow through a vessel is directly proportional to the length of the blood vessel.
  • The longer the vessel, the greater the resistance.
  • Obese people often have hypertension because additional blood vessels are needed for each extra kg of body fat.

Blood Flow Shunting

  • Through pressure and resistance changes, blood flow is shunted to various parts of the body.
  • Elevated hydrostatic pressures force blood to contracting muscle during exercise.
  • Elevated resistance through reduced lumen diameter can reduce blood flow to non-essential parts of the body (the kidney and intestines).
  • Increased lumen diameter due to vasodilation decreases the resistance, increasing the blood flow to essential parts of the body (working muscles).

Effects of Dehydration on Blood Flow and Performance

  • If the viscosity of the blood elevates (e.g., dehydration), that can reduce the flow of blood through all the vasculature.
  • Mild dehydration (1-2% loss of body weight) increases Heart Rate, increases body temperature, increases fatigue, and decreases endurance, severely reducing physical performance.

Venous Return

  • The volume of blood flowing back to the heart through the systemic veins is called the venous return.
  • Normally, the pressure difference from the venules (16mmHg) to the right ventricle (0mmHg) is sufficient to cause blood to return to the heart.
  • Besides the heart, two other mechanisms “pump” blood back up to the heart:
    • Skeletal Muscle Pump
    • Respiratory Pump

Skeletal Muscle Pump

  • Contraction of muscles in the leg compresses the vein, pushing blood to the heart through the proximal valve, and the distal valve closes as some blood is pushed against it.
  • When the muscle relaxes, the previously opened proximal valve closes, and the distal valve opens because blood pressure is higher in the foot than the leg. As a result, the vein fills with blood.

Respiratory Pump

  • During inhalation, the diaphragm moves downward, causing a reduction in pressure in the thoracic cavity and an increase in pressure in the abdominal cavity.
  • As a result, abdominal veins are compressed, and a greater volume of blood moves from the compressed abdominal veins into the decompressed thoracic veins, and then into the right atrium.
  • The valves prevent the backflow of blood during the reversal of pressures during exhalation.
  • Elevated venous return can increase preload, which can increase the stroke volume.