Exercise Physiology and the Lungs

• During quiet breathing (rest), humans utilize only tidal volume ($TV$), which represents the normal breathing you do when calm. This is the amount of air that enters or leaves your lungs with each breath. Inspiration is characterized by the pleural space pressure dropping from $-5$ to $-7.5 \ cm \ H_2O$ and then returning to $-5 \ cm \ H_2O$ upon expiration. The pleural pressure is the pressure within the pleural cavity surrounding the lungs, and this change helps the lungs expand and contract.

• Exercise stresses the pulmonary system to move beyond simple tidal volume breathing. In simple terms, as you start exercising, your body requires more oxygen, prompting you to breathe deeper. This is achieved by utilizing the entire vital capacity ($VC$), which is the maximum amount of air you can expel from your lungs after a maximum inhalation. Essentially, this involves breathing all the way up to total lung capacity ($TLC$), which is the maximum amount of air in your lungs after inhaling completely, and breathing all the way down to residual volume ($RV$), which is the air remaining in your lungs after exhaling completely.

• The pressure changes required for exercise are significantly more dynamic compared to rest:

  • To reach $TLC$, pleural pressures must reach approximately $-35 \ cm \ H_2O$. This means your body has to create a strong negative pressure in the pleural cavity to fully expand the lungs.

  • To reach $RV$ (blowing out lungs until only $1$ to $1.5 \ liters$ of air remains), pleural pressures reach approximately $+35 \ cm \ H_2O$. This positive pressure helps to force air out of the lungs more effectively during vigorous exercise.

Functional Anatomy and Muscle Recruitment

• At rest, the primary muscle used for breathing is the diaphragm. This dome-shaped muscle is located just below the lungs and plays a major role in helping our lungs fill with air.

• During exercise, the body recruits accessory muscles to facilitate ventilation:

  • Inspiration: In addition to the diaphragm, external intercostals (muscles between the ribs) and sternocleidomastoids (muscles in the neck) are recruited to assist with inhalation. These muscles help enlarge the pleural space, which makes the pressure inside the lungs more negative and allows them to expand further.

  • Expiration: While resting expiration is mostly passive—meaning it happens without much effort because of the natural recoil of the lungs—exercise requires active recruitment of other muscles. Abdominal muscles and internal intercostals contract to compress the pleural space, creating significantly positive pressure, which aids in pushing air out of the lungs.

• These muscles are amenable to training and strengthening, which means that with regular exercise, they can become stronger and more efficient, leading to an increase in vital capacity ($VC$) over time. This is beneficial for athletes or anyone looking to enhance their respiratory efficiency.

Lung Compliance and Regional Ventilation

• Lung compliance is defined as the change in lung volume for a given change in pressure: $\text{Compliance} = \frac{\Delta V}{\Delta P}$. Greater compliance means your lungs can expand easier for a given pressure change.

• In normal tidal volume breathing, the lungs operate on the steep, compliant part of the pressure-volume curve, where a small pressure change ($2.5 \ cm \ H_2O$) generates a substantial volume change ($0.5 \ liters$). This is like blowing up a balloon when it is still mostly deflated.

• During exercise, the lungs move to the upper part of the compliance curve (the flattening part), where compliance generally decreases because it is harder to add more air to an already inflated lung. Think of a balloon that's nearly full; it's much tougher to squeeze in more air.

• Regional Distribution of Pleural Pressure:

  • Pleural pressure is not uniform across the lungs. At rest, the average pressure is $-5 \ cm \ H_2O$, but it varies:

    • Base of the lung: $-2.5 \ cm \ H_2O$ (this is the lowest pressure).

    • Middle of the lung: $-5 \ cm \ H_2O$ (this is the average pressure).

    • Apex of the lung (top): $-10 \ cm \ H_2O$ (this is the highest negative pressure).

  • Because the apex is at $-10 \ cm \ H_2O$, tidal volume fluctuations (typically between $-5$ to $-7.5 \ cm \ H_2O$) do not effectively distend the apex. Therefore, rest/tidal breathing primarily occurs at the base of the lung, where the pressures are more favorable for air movement.

  • During exercise, the body recruits the apex of the lung by driving pleural pressure more negative than $-10 \ cm \ H_2O$, reaching pressures like $-15$ or $-20 \ cm \ H_2O$, which helps the lungs expand further.

Diffusion and Perfusion Limitations

• At rest, a red blood cell spends approximately $0.75 \ seconds$ in the capillary network. This is the tiny blood vessel system surrounding the alveoli, where gas exchange occurs. Oxygen reaching equilibrium with alveolar pressure ($P\_AO\_2$) takes about one-third of this time ($0.25 \ seconds$). In typical situations, oxygen transfer is mostly perfusion limited.

• During exercise, cardiac output ($CO$)—the amount of blood the heart pumps—can increase about threefold. This means blood moves faster through the capillaries, and a red blood cell might only spend $0.25 \ seconds$ in the capillary during intense activity. This increases the pressure on the diffusion of oxygen.

• As a result, the perfusion/diffusion curve shifts to the right, indicating that while most recreational athletes still reach the oxygen balance, they do so towards the very end of the capillary bed (near the junction with the pulmonary vein). These individuals operate close to the edge of perfusion limitation, meaning they might not be able to get enough oxygen.

• Extreme Athletes: In exceptional athletes (like Tour de France competitors), the cardiac output is so high that red cells spend significantly less than $0.25 \ seconds$ (potentially as low as $0.1 \ seconds$) in the capillary. This leads to a situation where they become diffusion limited, meaning the time they have for oxygen transfer is insufficient for it to reach the necessary equilibrium with the alveolar gas ($100 \ mmHg$). Consequently, their arterial oxygen ($PaO\_2$) may drop from $100$ to $80 \ mmHg$, which results in decreased hemoglobin saturation and oxygen content.

Oxygen Delivery and Consumption Formulas

• Oxygen Delivery Equation: $DO\_2 = Q \times CaO\_2$. This formula tells us how much oxygen our body delivers to tissues.

  • $Q$ = Cardiac Output (how much blood the heart pumps per minute).

  • $CaO\_2$ = Oxygen Content in the blood.

• Oxygen Content ($CaO\_2$) Formula: $(1.34 \times \text{grams of Hb} \times \text{saturation}) + (0.003 \times PaO\_2)$. This tells us how much oxygen is actually available for the body to use.

  • $1.34$ is a constant (amount of milliliters of oxygen carried per gram of hemoglobin ($Hb$)).

  • The $PaO\_2$ term represents the dissolved oxygen in the plasma, which is a very small part of the total oxygen content.

• Resting Statistics:

  • Normal $CO$: $5,000 \ ml/min$. This is an average amount of blood circulated by the heart.

  • Hemoglobin ($Hb$): $15 \ g/dl$. Hemoglobin is the protein in red blood cells that carries oxygen.

  • Saturation: $98\%$. This tells how much of the hemoglobin is saturated with oxygen.

  • Total Delivery: Approximately $1,000 \ ml \ O\_2/min$. This is how much oxygen is delivered to the body per minute.

  • Oxygen Consumption ($VO\_2$): $250 \ ml \ O\_2/min$. This is the amount of oxygen used by the body.

  • Oxygen Returned to Right Heart: $750 \ ml \ O\_2/min$ (approx. $75\%$ saturation). This is how much oxygen is left after the body has used what it needed.

Competitive Advantages and Blood Doping

• To maintain high oxygen delivery despite diffusion limitations (which can decrease saturation), extreme athletes focus on increasing the hemoglobin ($Hb$) levels in their blood. This is crucial since more hemoglobin means more oxygen can be transported.

• Methods to increase hemoglobin include:

  1. Erythropoietin (EPO): A hormone that stimulates the bone marrow to produce more red blood cells.

  2. Blood Transfusions: Athletes can donate their own blood, concentrate the red cells, and re-infuse them before competition. This boosts the number of red blood cells available to carry oxygen.

  3. Altitude Training: Training at high altitudes helps stimulate the production of EPO and leads to an increase in red blood cell production due to lower oxygen levels in the environment. This is why many athletes train in places like Colorado Springs.

• Using these methods allows athletes to "jump" to a higher hemoglobin line on the oxygen content graph. This means they can meet the high metabolic demands of their tissues even when their oxygen saturation is not at optimal levels.

Rightward Shift of the Oxyhemoglobin Dissociation Curve

• Exercise causes a rightward shift in the oxyhemoglobin dissociation curve (characterized by the Bohr effect), which means that hemoglobin binds oxygen less tightly. This is beneficial because it allows hemoglobin to release more oxygen easily to the tissues that need it.

• Four factors that drive this rightward shift, all present during exercise:

  1. Increased $PaCO\_2$ (higher carbon dioxide levels).

  2. Decreased pH (due to lactic acidosis). Lower pH levels indicate more acidity in the blood, often a result of intense exercise.

  3. Increased temperature (the byproduct of metabolism during exercise). Heat promotes oxygen release from hemoglobin.

  4. Increased $2,3\text{-DPG}$ (2,3-diphosphoglycerate), a metabolic byproduct that also encourages oxygen release.

Regulation of Ventilation During Exercise

• Initial Phase: Ventilation (breathing rate, denoted as $V\_E$) increases immediately at the start of exercise, sometimes even before starting the activity. This increase can be attributed to mental (psychic) signals or hormonal factors in anticipation of the effort about to begin.

• Steady State Phase: During steady-state exercise, the primary driver of ventilation is the production of $CO\_2$, which acts via central chemoreceptors located in the brain. Ventilation ($V\_E$) parallels $CO\_2$ production ($VCO\_2$) closely during this phase, meaning as you produce more $CO\_2$, your breathing rate increases to remove it, regardless of your oxygen consumption ($VO\_2$).

• Anaerobic Threshold: During intense exercise, such as sprinting, $CO\_2$ production can exceed the rate at which oxygen can be consumed. This excess $CO\_2$ arises due to the production of lactic acid. This condition increases the Respiratory Quotient ($R = \frac{\text{Production of } CO\_2}{\text{Consumption of } O\_2}$), indicating the body’s reliance on anaerobic metabolism.

• Metabolic Acidosis: In high-intensity work rates (e.g., handling workloads of $180 \ watts$), bicarbonate levels start to drop as it buffers lactic acid. This results in a lower pH in the blood. The body compensates for this by increasing ventilation to blow off excess $CO\_2$ (another acid), maintaining the balance of bicarbonate to $CO\_2$ ratio in the bloodstream.

• Oxygen Debt: After exercise stops, ventilation remains high for about $30$ to $90 \ minutes$. This increased breathing rate serves to repay oxygen debt—restoring normal oxygen levels after intense workouts while also blowing off the accumulated $CO\_2$ and replenishing oxygen stores in the body.

Summary of Integration

• Alveolar $CO\_2$ is defended at approximately $40 \ mmHg$ even at high work rates, which demonstrates how the body tightly regulates carbon dioxide levels.

• Alveolar oxygen levels remain relatively constant, and can potentially increase slightly during extreme work due to hyperventilation.

• Ultimately, $CO\_2$ acting via central chemoreceptors is the main regulatory force for adjusting respiratory rates during and after exercise.