Respiratory system and gas exchange

1. The respiratory system plays a critical role in enabling the exchange of gases (primarily oxygen and carbon dioxide) between the external environment and the body.
2. This exchange facilitates cellular respiration, a process that provides the energy required for cellular functions.

Structure of the Respiratory system

The respiratory system is made up of the following structures:

1. Nose and Mouth: Air enters the body through the nose or mouth, where it is warmed, moistened, and filtered.
2. Pharynx: A shared pathway for air and food.
3. Larynx: Contains the vocal cords and connects the pharynx to the trachea.
4. Trachea: A tube that carries air to the lungs, supported by cartilage rings.
5. Bronchi: The trachea divides into two bronchi, one for each lung.
6. Bronchioles: Smaller branches of the bronchi that lead to the alveoli.
7. Alveoli: Tiny air sacs where gas exchange occurs.
8. Diaphragm: A muscle that helps control breathing by changing the volume of the thoracic cavity.

Gas Exchange: O₂ and CO₂ Transport

Gas exchange refers to the movement of oxygen and carbon dioxide across the alveolar membrane into and out of the blood.

Process of Gas Exchange

1. Inhalation: Air rich in oxygen enters the body through the nose/mouth and travels through the pharynx, larynx, trachea, bronchi, and bronchioles.
2. Alveoli: The bronchioles lead to tiny air sacs called alveoli. Here, oxygen from the inhaled air diffuses across the thin walls of the alveoli into the capillaries that surround them.
3. Diffusion: Oxygen moves from the high-concentration area (air in the alveoli) to the low-concentration area (blood in the capillaries). Conversely, carbon dioxide, which is in higher concentration in the blood, diffuses from the blood into the alveoli to be exhaled.
4. Exhalation: The body expels carbon dioxide from the lungs back into the atmosphere through the same pathway.

Functions of the Respiratory System

The respiratory system has several important functions:

1. Gas Exchange: Oxygen is taken in from the air and carbon dioxide is expelled.
2. Regulation of Blood pH: By controlling the levels of carbon dioxide, the respiratory system helps maintain the acid-base balance in the blood.
3. Protection: The respiratory system filters out dust, pathogens, and other particles from the air.
4. Sound Production: The larynx contains the vocal cords, which produce sound when air passes through them.

Mechanics of Breathing

Inhalation

1. At rest: The diaphragm contracts and moves downward, expanding the chest cavity. Pressure inside the lungs drops below atmospheric pressure and air flows in.
2. During exercise: The diaphragm is helped by the external intercostal, abdominal, and accessory neck muscles. The chest cavity expands faster and further, drawing in more air per breath.

Exhalation

1. At rest (passive): The diaphragm and external intercostals relax. Their elastic recoil shrinks the chest cavity, pressure inside the lungs rises above atmospheric, and air flows out. No muscle actively pushes air out at rest.
2. During exercise (active): The internal intercostal and abdominal muscles contract to compress the lungs faster and harder than elastic recoil alone could manage. Active exhalation costs energy.

Key Parameters in Respiratory Function

The body adapts to different levels of activity and physical demands by adjusting several key parameters of the respiratory system, including:

1. Minute Ventilation (VE)
2. Tidal Volume (TV)
3. Respiration Rate (RR)

Minute Ventilation

Minute ventilation is an essential measure of lung function because it determines how effectively oxygen is delivered to the blood and how efficiently carbon dioxide is removed.

Minute ventilation is calculated using the formula:

$$\text{Minute Ventilation (VE)} = \text{Tidal Volume (VT)} \times \text{Respiratory Rate (RR)}$$

Where:

1. Tidal Volume (VT) is the volume of air inhaled or exhaled in a single breath.
2. Respiratory Rate (RR) is the number of breaths taken per minute.

Influence of Activity

1. During Exercise: Minute ventilation increases significantly as the body requires more oxygen for energy production and needs to expel more carbon dioxide as a waste product.
2. During Intense Activity: Tidal volume increases (deeper breaths) and respiration rate increases (faster breathing), leading to a higher minute ventilation. This ensures that the muscles are sufficiently supplied with oxygen to sustain energy production and remove CO₂.

Factors Influencing Minute Ventilation

1. Age: As individuals age, their lung capacity and ability to increase minute ventilation during exercise may decrease.
2. Fitness Level: Athletes tend to have a higher minute ventilation during exercise due to better pulmonary efficiency and the ability to breathe more deeply (higher tidal volume).
3. Activity Type and Intensity: Aerobic exercises (such as running or cycling) increase minute ventilation more significantly than activities like weightlifting, which rely more on anaerobic energy pathways.

Tidal Volume (TV)

At rest, tidal volume is usually around 0.5 litres, but this can increase significantly during exercise.

Influence of Activity

1. During Exercise: Tidal volume increases to allow more air (and thus more oxygen) to enter the lungs with each breath. This is especially important during aerobic exercise where oxygen demand is high.
2. Maximal Exercise: Tidal volume may reach its maximum capacity when the body can no longer meet oxygen demands just by increasing breathing rate. This is particularly true for trained athletes who may experience a greater increase in tidal volume during intense activity.

Factors Influencing Tidal Volume

1. Fitness Level: Trained individuals generally have a higher tidal volume at rest and during exercise. They can take in more air per breath due to stronger respiratory muscles and increased lung elasticity.
2. Body Size: Larger individuals generally have larger lungs and a higher tidal volume.
3. Age: As individuals age, the efficiency of the respiratory muscles declines, leading to a decrease in tidal volume and overall lung capacity.

Respiratory Rate (RR)

At rest, the typical rate is about 12-16 breaths per minute, but this rate increases during exercise to facilitate the body's increased need for oxygen and removal of carbon dioxide.

Increased During Exercise

1. Initial Response: When physical activity starts, the respiratory rate increases rapidly to bring in more oxygen and expel CO₂.
2. Intensity Influence: During aerobic exercises (like running), respiration rate increases substantially to provide more oxygen to the blood.
3. Anaerobic Activity: In anaerobic exercise (e.g., sprinting, heavy lifting), the respiration rate spikes quickly due to the demand for oxygen and increased CO₂ production.

Factors Influencing Respiratory Rate

1. Age: Older adults tend to have a slightly higher resting respiratory rate due to decreased lung elasticity and respiratory muscle strength.
2. Fitness Level: Highly trained athletes usually have a lower resting respiratory rate because their lungs are more efficient at oxygenating the blood.
3. Sex Differences: Males generally have a slightly lower respiratory rate than females, but this difference is usually small during exercise.

Control of Ventilation

1. Brainstem control: The brainstem regulates ventilation by responding to several chemical signals at once.
2. Main drivers: Blood carbon dioxide (PCO₂), blood acidity (pH), blood oxygen (PO₂), body temperature, and hormones such as adrenaline.
3. Dominant signal at sea level: Rising CO₂. Chemoreceptors detect the rise and the brainstem orders faster, deeper breathing to clear the excess.
4. At altitude: The lower atmospheric oxygen means oxygen-sensitive receptors take the lead, driving hyperventilation as an adaptation.

Lung Volumes and Capacities

1. Various lung volumes and capacities are used to assess lung function.
2. These volumes are generally determined by factors such as age, sex, body size, and physical conditioning.
   1. Tidal Volume (TV): Approximately 500 ml at rest (see Tidal Volume section above for full detail).
   2. Inspiratory Reserve Volume (IRV): The amount of air that can be inhaled after a normal inhalation (about 3,000 ml).
   3. Expiratory Reserve Volume (ERV): The amount of air that can be exhaled after a normal exhalation (about 1,100 ml).
   4. Residual Volume (RV): The air remaining in the lungs after a forced exhalation (about 1,200 ml).
   5. Vital Capacity (VC): The total amount of air that can be exhaled after a maximum inhalation (TV + IRV + ERV).
   6. Total Lung Capacity (TLC): The maximum amount of air the lungs can hold (VC + RV).

Factors Influencing Respiratory Parameters

Adaptations of the Respiratory System to Exercise

Regular physical training induces several long-term adaptations:

Functional Capacity of the Cardiorespiratory Systems

The respiratory system delivers oxygen into the alveoli, the cardiovascular system carries it to the muscle, and the muscle extracts what it needs. The single best measure of how well the two systems work together during exercise is the rate at which the body actually takes in and uses oxygen.

Maximal Oxygen Uptake (VO₂ max)

1. Definition: The maximum rate at which the body can take in and use oxygen during exercise, also called aerobic capacity.
2. Measurement: Measured in a lab during a progressive exercise test. Workload is raised step by step until the athlete cannot continue. When VO₂ stops climbing, that ceiling is VO₂ max.
3. Variability: A patient with cardiopulmonary disease can have a very low VO₂ max. An elite endurance athlete can have two to three times the population average.

The Fick Equation

1. Formula: VO₂ max = maximum cardiac output × maximum arterio-venous oxygen difference (a-vO₂ diff). The a-vO₂ diff is how much oxygen the working muscle extracts from each litre of blood.
2. Cardiovascular side: Improved by endurance training through cardiac hypertrophy and a larger stroke volume, which raises maximum cardiac output.
3. Muscle extraction side: Improved by skeletal muscle capillarisation. More capillaries per muscle fibre shorten the diffusion distance and widen the a-vO₂ difference.
4. Respiratory contribution: Ventilation must keep the alveoli supplied with fresh air. In healthy people at sea level, the lungs rarely limit VO₂ max; the heart and muscle determine it.

Cardiovascular and Respiratory System Interaction

During exercise, the cardiovascular and respiratory systems work together to ensure sufficient oxygen delivery and carbon dioxide removal.

1. Increased Heart Rate (HR): Delivers oxygenated blood faster.
2. Increased Stroke Volume (SV): More blood per beat = more O₂ delivery.
3. Increased Minute Ventilation (VE): Ensures continuous O₂ supply.
4. CO₂ Clearance: Prevents acidosis and maintains blood pH.