The Respiratory System
• Composed of structures that allow:
o Passage of air from outside the body to the lungs
o Gas exchange to occur
• Three main functions:
o Supply O2nto the blood
o Remove CO2 from the blood
o Regulate blood pH (acid-base balance)
• Divided into two zones:
o Conductive zone
o Respiratory zone
The Conductive Zone
• The conductive zone is composed of structures that transport air to the lungs:
o Mouth and nose
o Larynx
o Trachea
o Primary and secondary bronchi
o Tertiary and terminal bronchioles
• Filters, warms and humidifies air taken in with each breath
The Respiratory Zone
• The Respiratory zone is composed of structures involved with the exchange of gases
o Respiratory bronchioles
o Alveolar ducts
o Alveolar sacs
• The structure of the lungs provides a large surface area and a minimal distance for diffusion of gases to occur, maximizing the rate of gas exchange.
Mechanisms of Breathing
• Inspiration:
o Contraction of diaphragm
o Thoracic cavity expands
 Air pressure in thoracic cavity is lower than air pressure outside the body
o Air rushes in to lungs (from high to low) to restore balance
• Expiration:
o Relaxation of diaphragm
o Thoracic cavity reduces
 Air pressure in thoracic cavity is higher than air pressure outside fo the body
o Air rushes out of the lungs (from high to low) to restore balance.
Ventilation
• Ventilation (VE) is the volume of air moved by the lungs in 1 minute
• Influenced by two factors:
o Tidal Volume (TV)
 Volume of air in each breath
 A typical TV is 0.5 L/breath at rest
 During exercise, it can increase to about 4 L/breath
o Respiratory frequency (f)
 Number of breaths taken per minute
 At rest, f is about 12 breaths/min
 During exercise, it can increase to about 40 breaths/min
o VE (L/min) = TV (L/breath) x f (breaths/min)
Respiratory Control Centres
• Respiratory control centres are found within brain stem and are regulated by the autonomic nervous system.
o The area of the brain stem that is most important in regulating ventilation is the medulla oblongata. There are 2 main parts.
 Inspiratory centre
• Signals 15-20 breaths per minute at rest
• Increases during exercise
 Expiratory centre
• Two main functions:
o Ensure the inspiratory muscles never completely relax
o Stimulate forceful expiration when required (during exercise)
Lung Volumes
• Lung Volumes are divided into two categories:
o Static lung volumes
 Determined by the actual structure of the lung
• Total lung capacity (TLC)
o Maximum volume of air that lungs can hold
• Vital capacity (VC)
o Maximum amount of air that can be exhaled following a maximal inhalation
o Includes tidal volume (TV), inspiratory reserve volume (IRV) and expiratory reserve volume (ERV)
o VC = IRV + TV + ERV
• Residual volume (RV)
o Air that remains in lungs following a maximal exhalation
 TLC = VC + RV
o Dynamic lung volumes
 Dependent on volumes as well as movement/flow of air
Gas Exchange
• Diffusion mediates gas exchange
o Diffusion is the movement of a gas, liquid, or solid from a region of high concentration to low concentration
 Can only occur if a difference in concentration exists(called a concentration gradient
o The concentrations of specific gases involved in respiration are measured using a system called partial pressures
• Diffusion pathway
o Area through which gases move from the lungs into the blood; from the blood into the tissue, and back
• Rates of diffusion depend on :
o Size of concentration of gradient
• The greater the concentration gradient, the faster the rate of diffusion
o Thickness of barrier btw two areas
• Thinner barriers increase the rate of diffusion
o Surface area between two areas
• The larger the surface area, the greater the rate of diffusion
• The respiratory system takes advantage of all of these factors to maximize the rate of diffusion
Oxygen Transport
• Oxygen(O2) transport within the blood achieved in two ways:
1. A certain amount of O2 can dissolve in the plasma
 Represents 2% of O2 found in the blood
2. The remaining 98% is transported by binding the hemoglobin
 Oxyhemoglobin dissociation curve- “S” shaped curve that illustrates the relationship between the PO2 and the saturation of hemoglobin
 When the PO2 is low, saturation is low. When the PO2 increases, the saturation of hemoglobin increases until it levels off.
 This ensures that the O2 will be delivered to where it is needed (low PO2 regions)
Carbon Dioxide Transport
• Carbon dioxide(CO2) transport is achieved in three ways:
1. About 5-10 % is dissolved within the plasma
2. About 20% binds to hemoglobin forming carbaminohemoglobin
 When O2 is removed at the tissue (low PO2), CO2 binds to the hemoglobin. When the PO2 increases (near lungs) CO2 is removed and exhaled.
3. Bicarbonate system
 The remaining 70-75% diffuses into the red blood cells and reacts with water to form carbonic acid. The carbonic acid dissociates into a hydrogen ion and a bicarbonate ion and both are transported to the lungs
 At the lungs, the reactions are reversed and the CO2 is exhaled.
Ventilation
• Pulmonary ventilation (VE) is closely matched to the rate and/or intensity to the work being done.
• These increases can be divided into three phases.
1. Rapid On Phase – VE increases at a very rapid rate.
2. A slower exponential increase.
3. A new steady state level determined by the iintensity of the exercise and the fitness level of the individual.
• The increases in VE are due initially to a rise in TV and as exercise continues, a rise in f.
• VE plays an important role in the regulation of blood pH.
• As exercise progresses, blood pH decreases as lactic acid is deposited into the blood.
• VE increases, causing more H+ ions to combine with HCO3- ions in the bicarbonate system.
• This lowers the H+ concentration which helps to increase blood pH to normal levels.
External and Internal Respiration
• External respiration (occurs in the lung) increases as a result of two main factors:
o Increase in pulmonary ventilation (VE)
 Maintains necessary gradients in the partial pressures of both O2 and CO2 to maintain gas exchange
o Increase in blood flow to the lungs
 Caused by an increase in cardiac output
• Internal respiration involves exchange of gases at tissue level. During exercise, the extraction O2 at tissues is increased.
• Occurs as a result of four main factors:
1. Increase in partial pressure of oxygen (PO2) gradient
o The skeletal muscle increases cellular respiration and O2 is used to generate ATP.
o This increased use of O2 results in a decrease in the PO2 within the skeletal muscle which increases the concentration gradient between the blood in the capillaries and the working muscle.
2. Increase in partial pressure of carbon dioxide (PCO2)
3. Decrease in pH
4. Increase in temperature
o These three factors influence the binding of O2 with hemoglobin and result in the increase in the unloading of O2
o These factors lead to a shift of the oxyhemoglobin dissociation curve to the left. This is called the Bohr shift.
o This means that for a given PO2, more O2 will be unloaded which increases the rate of internal respiration.
• One way to determine how much O2 has been delivered to skeletal muscle is to measure the amount of O2 in the arterial blood before it arrives at the muscle, and then measure the venous blood that leaves the same muscle.
• This is called the a-VO2 difference.
a-VO2 Difference (diagram)

Share on Facebook