Respiratory Physiology


INTRODUCTION• Pulmonary ventilation consists of two phases: inspiration and expiration. It is a mechanical process thatdepends on volume changes in the thoracic cavity.• Volume changes lead to pressure changes and pressure changes lead to flow of gasses to equalize thepressure.• Boyle’s Law gives the relationship between pressure and volume of a gas: At a constant temperature,the pressure of a gas varies inversely with its volume P1V1=P2V2• During inspiration, the external intercostals and the diaphragm contract and the size of the thoraciccavity increases• During expiration, the inspiratory muscles relax and the thoracic volume decreases. As the gas moleculeswithin the lungs are forced closer together, the intrapulmonary pressure rises to a point higher thanatmospheric pressure which causes gases to flow out of the lungs

  1. What are the two phases of pulmonary ventilation?: Inspiration and expiration

  2. How does Boyle's Law describe the relationship between pressure and volume of a gas?: At a constant temperature, the pressure of a gas varies inversely with its volume (P1V1=P2V2)

  3. What happens to the thoracic cavity during inspiration?: Increases as the external intercostals and diaphragm contract.

  4. What occurs during expiration in terms of thoracic volume and gas pressure?: The thoracic volume decreases as the inspiratory muscles relax, causing gas molecules within the lungs to be forced closer together, leading to an increase in intrapulmonary pressure higher than atmospheric pressure, resulting in the flow of gases out of the lungs.


MECHANICS OF RESPIRATION• During inspiration, the inspiratory muscles (external intercostals and the diaphragm)contract and the size of the thoracic cavity increases. As air flows in, the diaphragmflattens out and the height of the thoracic cavity increases. As the external intercostalscontract, they lift the rib cage and pull the sternum superiorly.• Expiration: As the inspiratory muscles relax and resume their resting length, the thoracicand intrapulmonary volumes decrease

  1. What muscles are involved in inspiration during respiration?: External intercostals and the diaphragm

  2. What happens to the thoracic cavity during inspiration?: The size of the thoracic cavity increases

  3. What occurs to the diaphragm during inspiration?: It flattens out

  4. What happens to the rib cage during inspiration?: It is lifted by the external intercostals

  5. What occurs to the thoracic and intrapulmonary volumes during expiration?: They decrease


RESPIRATORY SOUNDS• As air flows in and out of the bronchial tree, two sounds can be auscultated with astethoscope.• Bronchial sounds are produced by air rushing through the trachea and the bronchi• Vesicular sounds are the result of air filling the alveolar sacs

RESPIRATORY VOLUMES ANDCAPACITIES– SPIROMETRY• Normal quiet breathing moves about500 ml of air in and out of the lungswith each breath. As we saw with thefirst activity, a person can usuallyforcibly inhale or exhale much moreair than is exchanged in normal quietbreathing.• Respiratory volumes can be measuredwith an apparatus called aspirometer.

RESPIRATORY VOLUMES• The four respiratory volumes of interest are tidal, inspiratory reserve, expiratory reserve, and residual.• During normal quiet breathing, about 500 ml of air moves into and out of the lungs with each breath.This respiratory volume is the tidal volume.• The amount of air that can be inspired forcibly beyond the tidal volume is the inspiratory reservevolume (IRV).• The expiratory reserve volume (ERV) is the amount of air that can be expelled from the lungs after anormal tidal volume expiration.• Even after the most strenuous expiration, about 1200 ml of air remains in the lungs, this is the residualvolume (RV), which helps to keep the alveoli open and prevent lung collapse

  1. What is the respiratory volume that represents the amount of air moving into and out of the lungs during normal quiet breathing?: Tidal volume: About 500 ml of air.

  2. What is the respiratory volume that indicates the amount of air that can be forcibly inspired beyond the tidal volume?: Inspiratory reserve volume (IRV).

  3. Which respiratory volume refers to the amount of air that can be expelled from the lungs after a normal tidal volume expiration?: Expiratory reserve volume (ERV).

  4. What is the respiratory volume that remains in the lungs even after the most strenuous expiration, helping to keep the alveoli open and prevent lung collapse: Residual volume (RV).


RESPIRATORY CAPACITIES• The respiratory capacities include inspiratory, functional residual, vital, and total lung capacities.The respiratory capacities always consist of two or more lung volumes.• Inspiratory capacity (IC) is the total amount of air that can be expired after a normal tidalvolume expiration, so it is the sum of TV and IRV.• Functional residual capacity (FRC) represents the amount of air remaining in the lungs after anormal tidal volume and the combination of RV and ERV.• Vital capacity (VC) is the total amount of exchangeable air. It is the sum of TV, IRV, and ERV.• Total lung capacity (TLC) is the sum of all lung volumes

ROLE OF THE RESPIRATORY SYSTEM IN ACID-BASEBALANCE OF BLOOD• Blood pH must be relatively constant for the cells of the body to function optimally.• Arterial blood pH at 7.4 ± 0.02• When carbon dioxide diffuses into the blood from the tissue cells, much of it enters the red blood cells, where itcombines with water to form carbonic acid.• Carbonic acid dissociates to form bicarbonate and H+• H+ ions remaining in the RBCs are neutralized by combining with hemoglobin• HCO3-diffuses out of the RBCs into plasma and CL- diffuses into the RBCs• If the H+ concentration in blood rises, H+ combines with HCO3- to form carbonic acid• If the H+ concentration in the blood drops, HCO3- will dissociate to release more HCO3- and H+ ions into the blood

RESPIRATORYACIDOSIS/ALKALOSIS• With hypoventilation or hyperventilation,the amount of carbonic acid in the bloodcan greatly be altered• Increased with hypoventilation• Decreased with hyperventilation• Respiratory Acidosis: elevated CO2,decreased bicarbonate, lowered pH• Respiratory Alkalosis: decreased CO2,increased bicarbonate, raised pH

A spirometer consists of a weighted drum, containing air, inverted over a chamber of water. The air-filled chamber is connected to the subject's mouth by a tube. In some types of spirometers (as the one depicted in

L: Fig. 37.4(b), p.558) when the subject inspires, air is removed from the chamber, causing the drum to sink and producing an upward deflection. The deflection is recorded by the stylus on the graph paper on the rotating drum (kymograph). When the subject exhales, air is added, causing the drum to rise, producing a downward deflection.

(See Fig. below or L Fig. 37.2 (a), p.556) These spirometric studies measure lung capacities, rates and depths of ventilation and are used to diagnose respiratory disorders such as bronchial asthma, emphysema and other respiratory dysfunctions. The spirometer used in this lab differs from that described above, but generally operates in a similar fashion. The teaching assistants will instruct you on its use and operation.

Each inspiration of normal, quiet breathing pulls about 500 ml of air into the respiratory passageways.

The same amount moves out with each expiration, and this volume of air inspired (or expired) is called the tidal volume (TV). Only about 350 ml of this tidal volume reaches the alveoli. The remaining 150 ml amount is called dead air because it remains in the "dead spaces" of the respiratory tract (nose, pharynx, trachea and bronchi) where there is no gas exchange with the respiratory capillaries in those areas.

  1. What is the volume of air inspired or expired with each normal, quiet breath?: 500 ml

  2. What is the term for the volume of air inspired or expired with each breath that reaches the alveoli?: Tidal volume (TV)

  3. What is the term for the 150 ml of air that remains in the "dead spaces" of the respiratory tract where there is no gas exchange?: Dead air

If we take a very deep breath, we can inspire much more than 500 ml. The additional inhaled air, called the inspiratory reserve volume (IRV), averages 3100 ml above the 500 ml of the TV. Thus, our respiratory system can pull in as much as 3,600 ml of air (the total of TV+ IRV) (See IC below). If we inspire normally and then expire as forcibly as possible beyond our normal TV, we can push out 1,200 ml of air in addition to the 500 ml TV. This extra 1200 ml of forcibly expired air is called the expiratory reserve volume (ERV). Even after the ERV is expelled, a considerable amount of air still remains in the lungs because the lower intrathoracic pressure surrounding the lungs keeps the alveoli slightly inflated. This air, the residual volume (RV), amounts to about 1200 ml. When the thoracic or chest cavity is opened, the intrathoracic pressure equilibrates with and equals the atmospheric pressure, which forces out the RV. The lungs still contain a small amount of air called minimal volumes MV) (which can be demonstrated by placing a piece of lung in water and watching it float).

  1. What is the additional inhaled air called when we take a very deep breath: Inspiratory Reserve Volume (IRV)

  2. How much additional air can the respiratory system pull in when taking a deep breath?: Up to 3,600 ml (TV + IRV)

  3. What is the term for the extra air that can be forcibly expired beyond the normal Tidal Volume (TV)?: Expiratory Reserve Volume (ERV)

  4. How much air remains in the lungs after the ERV is expelled?: About 1200 ml, known as the Residual Volume (RV)

  5. What is the term for the small amount of air that remains in the lungs after the RV is expelled?: Minimal Volume (MV)

Lung capacities can be calculated by combining various volumes. Inspiratory capacity (IC), the total

inspiratory ability of the lungs, is the sum of the, I plus the IR = 3,000 ml. Functional residual capacity

(FRC) is the sum of RV plus ERV = 2,400 ml. Vital capacity (VC) is the sum of the IRV, TV and the ERV

(4800ml). Finally, total lung capacity is the sum of all volumes (6,000 ml).

Also: IRV = VC - (TV + ERV), since VC = TV+IRV+ERV; VC is the total exchangeable air of the lungs.

Residual volume (RV is the non-exchageable air. Also: IC = VC - ERV.

  1. What is Inspiratory Capacity (IC)?: The total inspiratory ability of the lungs, calculated as the sum of TV and IRV, which equals 3,000 ml.

  2. What is Functional Residual Capacity (FRC)?: The sum of Residual Volume (RV) and Expiratory Reserve Volume (ERV), totaling 2,400 ml.

  3. What is Vital Capacity (VC)?: The sum of Inspiratory Reserve Volume (IRV), Tidal Volume (TV), and Expiratory Reserve Volume (ERV), amounting to 4,800 ml.

  4. What is Total Lung Capacity?: The sum of all lung volumes, totaling 6,000 ml.

  5. How can Inspiratory Reserve Volume (IRV) be calculated?: IRV = VC - (TV + ERV), as VC = TV + IRV + ERV.

  6. What is Residual Volume (RV)?: The non-exchangeable air in the lungs.

  7. How can Inspiratory Capacity (IC) be calculated?: IC = VC - ERV.