Respiration

Pulmonary Respiration

Introductory Page

This unit is designed to help refresh the student's memory and understanding of the pulmonary system. After completing these two lessons, the student should have a better understanding of pulmonary adaptions to exercise and the chronic illnesses that can impede performance. In this unit, we will discuss how oxygen is extracted from the surrounding environment and transported to the mitochondria of the cell for its use in aerobic energy conversion. There are three lessons in this unit. The first lesson explains the mechanics of moving air from the surrounding environment and how this movement of air sets the initial stage for oxygen transport. The second lesson will discuss the transport system used to move oxygen to the cell mitochondria and how oxygen enters the cell mitochondria for use in aerobic energy conversion. The third and final lesson in this unit will discuss the neural processes and signals that control this system, and determine if this system limits performance during exercise. Upon completion of these lessons, you should be able to: (1) explain the function of ventilation; (2) explain the mechanics of respiration at rest and during exercise; and (3) develop an understanding of the integration of ventilation and respiration during different types of exercise bouts.

Outline

Lesson 1

Pulmonary ventilation

a) The conducting zone

i) Areas of the conducting zone

ii) Ventilation in terms of inspiration and expiration

b) The respiratory zone

i) Areas of the respiratory zone

Oxygen supply and content in ambient air

i. Lung volumes and oxygen capacities

ii. The ventilatory process

(1) Ventilatory frequency

(2) Minute ventilation

(3) Minute ventilation and oxygen capacity

Alveolar ventilation and residual volumes

1. Alveolar ventilation as a function of tidal volume

2. Alveolar ventilation as a function of minute ventilation

(1) Oxygen capacities and alveolar ventilation

iii) Alveolar ventilation and exercise

(2) Changes in minute ventilation and alveolar ventilation during exercise

(a) Steady state submaximal exercise

(b) Incremental exercise

Lesson 1

Ventilatory Function and Mechanics

When describing respiration, most people assume the term is synonymous with breathing. In fact, breathing or pulmonary ventilation is just a small part of the overall picture of the respiratory process. Respiration, as a physiological parameter, is better defined as the function of gas exchange. The lesson at hand will discuss the pulmonary ventilation portion of respiration. As you move through this lesson, keep in mind that we are discussing the act of breathing, a mechanical process that moves air in and out of the lungs. We will thoroughly discuss respiration (gas exchange) in lesson 2.

Pulmonary Ventilation: The Starting Point for Respiration

To begin this lesson, let us first think about what we all have in common as land dwelling mammals. We require oxygen to sustain life and we use oxygen as our main substrate for energy conversion. With this in mind, we can say that pulmonary ventilation may arguably be the most important process of respiration, in that, its function is to continually renew the air content within the lungs. Without continual renewal of the air content in the lungs, the oxygen that is available for respiration and energy conversion would be depleted quickly. For our purposes, we will separate the anatomy of the lungs into two zones. These zones are called the conducting zone and the respiratory zone. The conducting zone is the portion of the lungs that aids in the movement of air in and out of the surrounding environment, and sets the initial availability of air to move into the respiratory zone. In the conducting zone, no gas exchange occurs. The respiratory zone is the portion of the lungs that includes the alveoli and capillary plexus and is the site where initial gas exchange between the lungs and blood occurs. In this lesson we will learn how each of these zones are ventilated.

The Anatomy and Function of the Conducting Zone of the Lungs

The overall process of pulmonary ventilation is to move air in and out of the lungs (breathing) through the contraction and relaxation of the diaphragm and intercostal muscles. As air movement occurs, the air passes through the conducting zone as it moves toward the respiratory zone. The conducting zone consists of those areas of the lung commonly described as the anatomical dead space (V_D) and includes the trachea and bronchioles. The concept of anatomical dead space means that air moves and resides in the area, but gas exchange does not occur. This area of the conducting zone will be important later in this lesson when we discuss ventilation of the respiratory zone. Ventilation begins at the input area of the conducting zone (nose and mouth), where ambient air moves from the environment into the lungs. This movement of air in and out of the lungs is accomplished by changes in pleural pressure. To move air into the lungs, a negative pressure differential occurs when the diaphragm contracts (an active process) downward, allowing the lungs to expand with air. This action is known as inspiration. The movement of air from the lungs to the ambient environment is accomplished when a positive pressure differential occurs, as the diaphragm relaxes (a passive process), increasing the pressure in the lungs and forcing air out of the nose and mouth. This action is known as expiration. As ventilation occurs, ambient air is moved from the atmosphere into the lungs, and then from the lungs back into the ambient environment.

Note: As we discuss ventilation, we are not speaking of gas exchange, only of air movement and air content in and out of the lungs.

The Respiratory Zone

Ventilation moves air in and out of the respiratory zone through the conduit pathways of the conducting zone. The function of the respiratory zone of the lungs is to provide concurrent gas exchange of O₂ and CO₂ between the lungs and blood. The act of inspiration brings the necessary O₂ into the respiratory zone and the act of expiration removes the CO₂ from the respiratory zone. The respiratory zone consists of those areas of the lung that include the alveoli and capillary plexus. Each alveolar sac in the lung has contact with the capillary plexus. It is at the junction of the alveoli and capillary plexus where O₂ is exchanged from the alveoli to the blood and CO₂ is exchanged from the blood to the alveoli. The process of gas exchange in this area as well as throughout the body is completed by simple diffusion. To understand the diffusion of gases between the lung and blood that will be discussed in lesson two, we need to first examine the properties of the air ventilated and the mechanics of ventilation.

Ambient Air

So, what is the composition of the air that we are moving with ventilation? The ambient air in our environment consists of three major gasses in different proportions. These gasses are Nitrogen (N), Oxygen (O₂), and Carbon Dioxide (CO₂), and are approximately proportioned in the ambient air as 79.04% N, 20.93% O₂, and 0.03% CO₂. Remember Oxygen, the element we need for aerobic energy conversion? As the aerobic energy demand increases, the amount of O₂ needed increases also. Constant ventilation ensures an adequate supply of oxygen (during inspiration). Ventilation also acts as the end process to remove carbon dioxide from the body and put it back into the atmosphere (during expiration).

Ventilation and Oxygen Supply

Now that we know the content of the air we breathe, let us think about how ventilation improves oxygen availability in preparation for gas exchange. By increasing the volume of air that is ventilated, we can increase the availability of O₂ in the lungs. Consider a person who inspires one liter of air into the lungs. The O₂ content of that 1liter of air consists of 209.3 ml of O₂:

20.93%*1liter or 0.2093*1000ml = 209.3ml

If the same person were to inspire two liters of air into the lungs, then the O₂ content of the 2 liters of air would consist of 418.6 ml of O₂:

20.93%*2liters or .2093*2000ml = 418.6 ml

It is safe to assume that for every liter of inspired ambient air, the O₂ content of that air will consist of 209.3 ml of O₂. We can also assume that the greater volume of air that we inspire in a single breath will increase the availability of O₂ for respiration. The next question that comes to mind, is how much air can a normal human being inspire in a single breath? For an answer to this question go to the next page.

Tidal Volume and Oxygen Capacity

Now that we know how much O₂ is available per liter of air inspired, we should probably answer the question previously stated and determine how much air is ventilated by the average individual during a single inspiratory /expiratory (ventilatory) cycle. The volume of air that an individual ventilates in one breath is known as the tidal volume (V_T), and maximum V_T is limited to an individual's lung volume. Normal total lung capacities/volumes for the average human are approximately 6 liters for males and 4 liters for females. The lungs however, retain a residual volume (RV) of air necessary to keep the lungs inflated and aid in oxygen transport during the time intervals between breaths. Approximately 20% of the total lung capacity (TLC) consist of the residual volume. This means that the maximal tidal volume (V_T) for a male with a 6 L TLC is 4.8 liters:

6 L * 0.8 = 4.8 L

Conversely, the maximal tidal volume (V_T) for a female with a 4 liter TLC is 3.2 liters:

4 L * 0.8 = 3.2 L

From the previous page, we can conclude that the greatest amount of O₂ available to a male during a single ventilatory cycle (breath) would be:

4.8 L * 0.2093 = 1.004 L O₂

The greatest amount of O₂ available to a female during a single breath would be:

3.2 L * .2093 = 696.7 ml

Note: Keep in mind, the preceding illustrations are for the maximal tidal volume. At rest, V_T is much lower, approximately 600 ml for males and 500 ml for females.

Ventilatory Frequency

Since the volume of oxygen is limited to the V_T during a single ventilatory cycle, another factor that improves the availability of oxygen for transport and exchange must include the number of ventilatory cycles in a certain time frame. Think back to the energy module. When we discussed energy conversion, there was a time element involved (kcal/minute). If oxygen is necessary for energy conversion in aerobic metabolism, then the same time element must be involved. Therefore, we must think in terms of liters/minute or ml/minute when discussing ventilation and respiration. You have learned, thus far, that the maximal V_T for a single ventilatory cycle is dependent on the TLC and the RV of the individual. To further understand the availability of O₂ for energy conversion, you must include the number of ventilatory cycles during a minute. Ventilatory frequency (f) is the term that describes the number of breaths taken in one minute. As frequency of breaths increase the rate of air movement in and out of the lungs increases. Ventilatory frequency for most healthy individuals is approximately 12 breaths per minute at rest and can range from 15 breaths per minute for light exercise, up to 60 breaths per minute during all out maximal effort.

Minute Ventilation

Now that we have considered V_T and f separately, let us integrate these two factors and develop a formula that will allow us to determine the amount of air cycled through the lungs in any given minute. Luckily for us, others have already developed the formula to determine this. Since we are using a time constraint (minute) to determine the volume or air cycled during ventilation, we will call the product of the formula minute ventilation (V_E). The formula used to determine V_E is:

V_E = V_T * f

V_E = (Volume of air/breath) * (Number of breaths/minute) = Volume of air/minute

Minute ventilation for a healthy individual can range from approximately 6 L/min at rest to above 200 L/min during all out maximal effort depending on the lung volume, breathing patterns, and body size of the individual. At this time I should point out that this description of V_E is describing the ventilation of the lungs as a whole, meaning that V_E incorporates both the conducting zone and the respiratory zone. To determine how V_E is affected by breathing patterns, Click here to determine the V_E for three different scenarios of ventilation.

Remember: V_E is the amount of air not oxygen that is cycled through the lungs in any given minute.

Alveolar Ventilation and Residual Volume

Up to this point, we have examined ventilation of the whole lung and have not tried to delineate the differences between air movement in the conducting zone and respiratory zone. Now, let us begin to describe the proportional amount of ventilated air that actually reaches the respiratory zone. Remember the respiratory zone is where gas exchange occurs, so we need ventilation of the alveoli for gas exchange. Alveolar ventilation (V_dotA) is the movement of air in and out of the alveolar sacs. This does not mean that ALL of the air is expelled from the alveoli during expiration. Remember, we have a Residual Volume of air that keeps the lungs inflated and aids in gas exchange between breaths. A small portion of the RV remains in the alveoli at all times for the purpose of gas exchange between breaths, however, the greater portion of the RV is contained in the anatomical dead space (V_D) of the conducting zone. The air that remains in the V_D acts as a buffer by mixing with newly inspired air and protects the alveolar air from severe fluctuations in mixed gas concentrations during inspiration. By protecting from severe fluctuations in air concentrations in the alveoli, the V_D allows for consistency of blood gas composition during minute ventilation. Go to the next page of this lesson to determine how alveolar volume (V_A) and V_D are integrated in the overall concept of minute ventilation.