Lab 4: Respiratory Systems

Objective

• To understand the microscopic and gross anatomy of the respiratory tract.
• To observe and measure the mechanics of breathing, respiratory volumes, and the control of breathing.
• To observe and understand the role of buffers in maintaining pH balance in the body.
• To name and describe lung volumes and capacities.
• To describe how the structures of the lungs and thoracic cavity control the mechanics of breathing.

Introduction

Breathing is primarily an involuntary process, typically occurring at an average of 15 times per minute during rest for humans. The respiratory system's primary function is to deliver oxygen (O₂) to the tissues and remove carbon dioxide (CO₂), which is a waste product of cellular metabolism. O₂ diffuses into cells for metabolic reactions that produce adenosine triphosphate (ATP), while CO₂ diffuses out to be expelled via the lungs during exhalation. The main structures involved in the human respiratory system include the nasal cavity, trachea, and lungs.

Basic Principles of Gas Exchange

Gas exchange relies on the process of diffusion, where gas molecules move from areas of high concentration to areas of low concentration, driven by a concentration gradient. Blood low in O₂ and high in CO₂ exchanges gases with the air in the lungs, which has higher O₂ concentration and lower CO₂ concentration.

Partial Pressure

Partial pressure is defined as the measure of concentration of individual gas components in a mixture. The total pressure exerted by this mixture is equal to the sum of the partial pressures of its constituents. The rate of gas diffusion aligns with its partial pressure within the gas mixture.

Lung Volumes and Capacities

Lung capacities can differ widely across species; for instance, cheetahs have evolved greater lung capacity to meet their high metabolic demands during rapid running, while elephants have larger lung volumes to accommodate their larger body sizes.

Human Lung Volumes and Capacities

• The average total lung capacity for adult males is approximately 6 liters.
• Tidal Volume (TV): the volume of air inhaled with a normal breath (average: 0.5 liters).
• Inspiratory Capacity (IC): the volume of air inhaled during a deep breath.
• Residual Volume (RV): air remaining in the lungs after forceful exhalation.
• Human lung size is influenced by:
    - Genetics
    - Gender
    - Height

Air volume in the lungs can be quantified using lung volumes and capacities, where volume is a singular measurement for an action (e.g., inhalation/exhalation), and capacity combines multiple volumes (e.g., from a maximal exhalation).

Table 1: Lung Volumes and Capacities (Avg Adult Male)

Lung Volume Characteristics

• Tidal Volume (TV): This is roughly equal to 0.5 liters, analogous to a 20-ounce drink bottle.
• Expiratory Reserve Volume (ERV): Extra air exhaled beyond a normal breath.
• Inspiratory Reserve Volume (IRV): Additional air inhaled after a normal breath.
• Residual Volume (RV): It is crucial as it prevents lung tissues from sticking together and avoids excessive energy expenditure during lung reinflation. This volume cannot be measured directly, only calculated.

Capacities Definitions

• Vital Capacity (VC): Total air capacity measurable during a respiratory cycle (sum of ERV, TV, and IRV).
• Inspiratory Capacity (IC): Total air that can be inhaled after normal expiration (sum of TV and IRV).
• Functional Residual Capacity (FRC): Total air remaining after a normal expiration (sum of ERV and RV).
• Total Lung Capacity (TLC): Total possible lung air volume measured post-maximal inhalation (sum of RV, ERV, TV, and IRV).

Measuring Lung Volumes

Lung volumes are measured via spirometry, which enables evaluation of various respiratory parameters, including FEV1 (how much can be expelled in one second) and FVC (total air pressure expelled). The ratio of these measurements, FEV1/FVC, is instrumental in diagnosing disorders like asthma, emphysema, and fibrosis.

Interpretation of FEV1/FVC Ratio

• A high FEV1/FVC ratio indicates problems with lung stiffness and is associated with conditions like fibrosis, where lungs recoil effectively and air is exhaled quickly.
• Conversely, a low FEV1/FVC ratio indicates resistance issues typical of asthma, where expelling air becomes a prolonged effort.

Mechanics of Human Breathing

Boyle’s Law

Boyle’s Law illustrates that within a closed system, pressure and volume are inversely related (Figure 3). An increase in thoracic cavity volume results in decreased pressure, leading to air influx during inhalation, while decreased volume during exhalation results in increased pressure, propelling air outward from the lungs.

Inhalation Process

During inhalation:

• Diaphragm contraction increases thoracic volume and decreases internal pressure, allowing air to enter.
• Intercostal muscle contraction expands the chest cavity.
• The increase in thoracic cavity volume is primarily due to expanding alveolar space, as the bronchioles and bronchi are rigid structures.

Exhalation Process

During exhalation:

• Elastic recoil of the lung tissues expels air as the diaphragm and intercostal muscles relax.
• The movement during expiration is passive, occurring naturally without muscle contractions.

Pleural Mechanics

Each lung is encased in a double layer of pleura.

• The visceral pleura directly covers each lung, while the parietal pleura lines the thoracic cavity.
• Intrapleural space contains fluid, reducing friction during lung expansion and contraction.
• Pleurisy is an inflammation condition that causes pain due to increased thoracic pressure and lung volume reduction.

Exercises

Exercise 1: Mechanism of Breathing (Balloon-and-Bell Jar Model)

Materials: Balloon-and-bell jar

• The bell jar simulates thoracic pressure dynamics. Pulling down the rubber sheet (diaphragm) demonstrates inhalation mechanics.
• Upon pulling down, volume inside increases, causing a pressure decrease that draws air in through the represented trachea (glass tube), inflating the balloons (lungs).

Observations during Breathing Cycle

The lung model demonstrates:

• Inhalation: When the diaphragm moves down, thoracic volume rises; air inflates the balloons until pressure outside balances internal lung pressure.
• Exhalation: The diaphragm moving up reduces thoracic volume and increases pressure, deflating the balloons as air exits, maintaining equilibrium of pressures.

Real Breathing Factors

• The model lacks ribs and intercostal muscle dynamics. In reality, these muscles contribute substantially to thoracic volume changes, particularly during heavy inhalation and exhalation.
• The shape and movement of the diaphragm differ; it is dome-shaped and flattens in reality, not just pulled flat.

Procedure

• Record your observations during breath pressures and dynamic changes using the model contrasted with human physiology.

Exercise 2: Breathing Measurements

Monitor breathing rate and depth during different activities:

1. Record breath-holding times under various conditions after inspiration and expiration.
2. Explore the effects of rapid breathing and exercise on breath-holding capabilities, illustrating changes in oxygen demand and CO₂ production associated with physical exertion.

Effect of Exercise

Exercise elevates the need for O₂ as muscle cells demand it for energy, simultaneously increasing CO₂ output, thereby necessitating greater respiratory activity.

• The recovery time after increased respiration offers insights into individual fitness levels.
• Average respiratory rates in resting state: 12-15 breaths/minute.
• Alveolar ventilation is crucial for CO₂ removal and maintaining blood pH.

Procedure Steps for Measurements

1. Calculate breath-holding duration after different breathing conditions. Record times and observe the physiological impacts on volume and rate of respiration under various levels of effort.

Caution: Individuals with respiratory or cardiac conditions should not perform these tests without prior consultation. Stop activities immediately if dizziness occurs.

The exercises aim to elucidate the physiological principles governing respiration dynamics and imply how these can be impacted by both internal and external conditions.