Heart Structure and Blood Flow

Heart Structure and Blood Flow

• The heart contains four chambers, including the right atrium, right ventricle, left atrium, and left ventricle
• Blood flows in a specific pathway through these chambers
• Deoxygenated blood enters the right atrium via the superior and inferior vena cava
• From the atrium, blood passes through the tricuspid valve into the right ventricle
• Next it goes through the pulmonary valve to the lungs via the pulmonary arteries to get oxygenated
• Oxygenated blood then returns via the pulmonary veins to the left atrium
• From the atrium, it moves through the bicuspid (mitral) valve into the left ventricle
• The left ventricle pumps the now oxygenated blood through the aortic valve into the aorta, which supplies systemic circulation
• The heart is supplied with blood by the coronary arteries, branching off the aorta
• During exercise, the heart increases stroke volume (SV) and heart rate (HR) to meet increased oxygen demand
• Sympathetic nervous system activation and increased venous return allows the heart to meet the body's demands during exercise

Heart Contraction and Rate Control

• The heart contracts due to electrical impulses generated by the sinoatrial (SA) node, or the heart's pacemaker
• The electrical signal spreads through the atrioventricular (AV) node, Bundle of His, and Purkinje fibers to ensure coordinated contraction
• Heart rate control involves intrinsic regulation via the SA node's automaticity
• Extrinsic control is mediated by the autonomic nervous system (ANS)
• Sympathetic stimulation (norepinephrine) increases heart rate
• Parasympathetic stimulation (vagus nerve, acetylcholine) decreases heart rate

Aerobic Exercise and Resting Heart Rate

• Regular aerobic training enhances vagal tone, which reduces resting heart rate
• The pacemaker adapts to a lower default firing rate with regular aerobic exercise

Torsional Contraction

• Torsional contraction refers to the heart's twisting motion during systole and untwisting during diastole
• This type of contraction enhances ventricular filling (diastolic suction), improving stroke volume during exercise

Systole and Diastole

• Systole represents the contraction phase of the heart, when blood is ejected, corresponding to systolic blood pressure (SBP), the peak arterial pressure
• Diastole is the relaxation phase, allowing ventricular filling and corresponds to diastolic blood pressure (DBP), the lowest arterial pressure

Pressure, Flow, and Resistance

• Blood flow (Q) equals pressure (P) divided by resistance (R)
• Higher pressure increases flow, while higher resistance reduces it
• Vasodilation decreases resistance, increasing flow
• Vasoconstriction does the opposite

Blood Flow Control

• Blood flow is regulated by intrinsic (local) and extrinsic (neural/hormonal) factors
• Local factors (autoregulation) include oxygen demand, CO2 levels, and nitric oxide release
• Neural control through sympathetic nerves can vasoconstrict non-essential areas, like the gut and kidneys and dilate active muscles
• Hormones like epinephrine also modulate blood flow

Muscle Blood Flow and Exercise

• Functional sympatholysis occurs during exercise when metabolic byproducts, H+, CO2, adenosine, and NO, override sympathetic vasoconstriction, vasodilating active muscles

Mechanisms for Venous Return

• Contracting muscles squeeze veins, pushing blood toward the heart, called the muscle pump
• Deep breathing creates pressure changes that assist venous return, called the respiratory pump
• Sympathetic stimulation narrows veins, pushing blood toward the heart, called venoconstriction

Functions of Blood

• Transports oxygen, nutrients, hormones, and waste
• Regulates pH balance and temperature
• Protects through immune defense and clotting

External vs Internal Respiration

• External respiration is gas exchange between the lungs and blood, occurring at the alveoli
• In the alveoli, oxygen enters the blood while carbon dioxide exits
• Internal respiration is gas exchange between blood and tissues, where oxygen is delivered to tissues, and carbon dioxide is picked up

Inspiration and Expiration

• Inspiration is an active process that requires contraction of the diaphragm and external intercostals
• This increases lung volume and reduces pressure, causing air to flow in
• Expiration is passive at rest, as the diaphragm relaxes and lung volume decreases, pushing air out
• During exercise, expiration becomes active, involving internal intercostals and abdominal muscles for forced exhalation

Spirometry and Lung Volumes

• A spirometer is a device that measures lung function and volumes and records the following
• Tidal volume (TV) is the amount of air per breath, approximately 500 mL at rest
• Inspiratory reserve volume (IRV) is the extra air inhaled beyond TV
• Expiratory reserve volume (ERV) is the extra air exhaled beyond TV
• Residual volume (RV) is the air remaining after maximal exhalation
• Vital capacity (VC) is the maximum air that can be exhaled after a full inhalation (TV + IRV + ERV)
• Total lung capacity (TLC) is the total volume of lungs (VC + RV)

Partial Pressures of Respiratory Gases

• Partial pressure is the pressure exerted by an individual gas in a mixture
• Total pressure is the sum of individual gas pressures according to Dalton's Law
• Oxygen (O2), carbon dioxide (CO2), and nitrogen (N2) each have different partial pressures that influence diffusion
• Atmospheric oxygen partial pressure is about 159 mmHg at sea level

Gas Partial Pressures and Pulmonary Diffusion

• Gas moves from high to low partial pressure according to Fick's Law
• Oxygen diffuses from alveoli (~100 mmHg) into blood (~40 mmHg)
• Carbon dioxide diffuses from blood (~45 mmHg) into alveoli (~40 mmHg)
• This ensures efficient gas exchange in both lungs and tissues

Gas Exchange and the Respiratory Membrane

• Gas exchange happens in the alveoli across the respiratory membrane
• The respiratory membrane consists of the alveolar wall, capillary wall, and basement membranes
• This thin structure allows for the rapid diffusion of O2 and CO2

Oxygen Cascade

• Ambient air PO₂ is ~159 mmHg.
• Alveoli PO₂ is ~100 mmHg (due to humidification and CO₂ presence).
• Arterial blood PO₂ is ~100 mmHg.
• Tissues (at rest) PO₂ is ~40 mmHg.
• Tissues (during exercise) PO₂ can drop to ~15 mmHg.
• Venous blood PO₂ is ~40 mmHg (at rest) and lower during exercise.

Oxygen Unloading and Carbon Dioxide Removal

• Oxygen unloading is driven by lower PO2 in tissues and enhanced by the Bohr effect
• Increased CO2, temperature, and acidity reduce hemoglobin's affinity for O2 during oxygen unloading
• Carbon dioxide is transported in three ways
  • Dissolved in plasma (~10%)
  • As bicarbonate (HCO3−) (~70%)
  • Bound to hemoglobin (~20%)

Arterial-Venous Oxygen Difference

• The (a-v)O2 difference is the difference in O2 content between arterial and venous blood, reflecting how much oxygen is extracted by tissues
• At rest the difference is ~4-5 mL O2 per 100 mL of blood
• During exercise the difference can increases to 15-18 mL O2 per 100 mL of blood due to greater O2 demand

Pulmonary Ventilation Regulation

• Regulated by central chemoreceptors (detect CO2 and H⁺ in cerebrospinal fluid)
• Also regulated by peripheral chemoreceptors (monitor PO2, PCO2, and pH) and mechanoreceptors (monitor stretch in lungs and muscles)
• During exercise, increased CO2 and H⁺ stimulate ventilation
• Neural input (from motor cortex and muscle afferents) increases breathing rate

Aerobic Exercise and Respiratory Function

• Young individuals experience increased lung efficiency, better oxygen extraction, and enhanced ventilatory capacity with regular aerobic exercise
• Older adults maintain lung function and reduce age-related decline with regular aerobic exercise
• Asthmatics experience improved airway control, reduced symptoms, and better overall respiratory efficiency with regular aerobic exercise

Exercise Training and Respiratory Disease

• Regular exercise lowers mortality from respiratory diseases by
  • Improving lung function and endurance
  • Reducing systemic inflammation
  • Enhancing immune function and reducing risk of chronic respiratory issues

Cardiovascular and Respiratory Responses to Exercise

• Heart rate (HR) increases linearly with exercise intensity until maximum HR is reached
• Stroke volume (SV) increases up to ~50% of VO2max, then plateaus
• Cardiac output (Q = HR × SV) increases proportionally to meet oxygen demands

Determining HRmax

• HRmax = 220 – age (simple but imprecise)
• Alternative: 208 – (0.7 × age) (more accurate)
• Limitations: Individual variability and doesn’t account for fitness levels

Heart Rate Variability

• Indicates autonomic nervous system balance
• Higher HRV suggests better cardiovascular fitness and parasympathetic dominance
• Lower HRV may indicate stress or poor health

Mechanisms for Venous Return During Exercise

• Muscle pump: Rhythmic contractions push blood back to the heart
• Respiratory pump: Changes in thoracic pressure aid venous return

Importance of Stroke Volume

• Higher stroke volume gives greater oxygen delivery to muscles, enhancing VO2max
• Stroke volume is a key determinant of aerobic capacity

The Fick Principle

• Expressed as: VO_2 = Q \times (a-v)O_2 \text{ difference}
• Shows how oxygen consumption depends on cardiac output and oxygen extraction

Frank-Starling Mechanism

• Increased venous return that results in greater ventricular stretch leading to stronger contraction
• Helps maintain stroke volume during exercise

Blood Pressure Response to Exercise

• Systolic BP increases linearly with intensity
• Diastolic BP remains stable or decreases slightly

Cardiovascular Drift

• Heart rate increases while stroke volume decreases during prolonged exercise
• Theories to explain includes increased core temperature that leads to dehydration and reduced plasma volume
• Sympathetic activation that means higher HR reduces ventricular filling

Plasma Volume and Red Blood Cell Changes

• Plasma volume decreases due to sweating
• RBC concentration increases which is called hemoconcentration
• Heat exacerbates plasma loss

Pulmonary Ventilation Response to Exercise

• Initial increase that is neural
• Proportional rise in ventilation with intensity
• At high intensity, respiratory rate increases drastically

Respiratory Terms

• Dyspnea: Shortness of breath
• Hyperventilation: Excessively rapid breathing
• Valsalva maneuver: Holding breath and bearing down (increases BP)
• Ventilatory threshold: Point where ventilation increases disproportionately to VO₂

Exercise-Induced Asthma

• Cold, dry air, or high-intensity exercise can trigger airway constriction
• Common in swimmers, runners, and winter sports athletes

Acid-Base Balance

• The respiratory system removes CO₂, reducing acidity
• It increases ventilation to buffer pH changes

Blood pH and Sprinting

• Arterial blood pH is typically ~7.4
• Muscle pH is typically ~7.1
• After sprinting, pH drops due to lactic acid accumulation

Buffers in Blood and Muscle

• Blood: Bicarbonate (HCO₃⁻) and hemoglobin
• Muscle: Phosphates and proteins

Postexercise Hypotension

• Vasodilation remains high while cardiac output drops, lowering BP

Posture and Ventilation

• Upright posture aids venous return, preventing dizziness