Flashcards in Lab 6: Respiratory System Deck (51)
What is meant by pulmonary ventilation?
Pulmonary ventilation, or breathing, is the inhalation (inflow) and exhalation (outflow) of air and involves the exchange of air between the atmosphere and the alveoli of the lungs.
What does pulmonary ventilation depend upon?
In pulmonary ventilation, air flows between the atmosphere and the alveoli of the lungs because of alternating pressure differences created by contraction and relaxation of respiratory muscles. The rate of airflow and the amount of effort needed for breathing are also influenced by alveolar surface tension, compliance of the lungs, and airway resistance.
What is the most important muscle of inhalation? What is it innervated by?
The most important muscle of inhalation is the diaphragm, the dome-shaped skeletal muscle that forms the floor of the thoracic cavity. It is innervated by fibers of the phrenic nerves, which emerge from the spinal cord at cervical levels 3, 4, and 5. Contraction of the diaphragm causes it to flatten, lowering its dome. This increases the vertical diameter of the thoracic cavity.
State the changes in diaphragm level, pressure differences and change in volume of air during normal inhalation and strenuous breathing.
During normal quiet inhalation, the diaphragm descends about 1 cm (0.4 in.), producing a pressure difference of 1–3 mmHg and the inhalation of about 500 mL of air. In strenuous breathing, the diaphragm may descend 10 cm (4 in.), which produces a pressure difference of 100 mmHg and the inhalation of 2–3 liters of air.
What percentage of air entering is the contraction of the diaphragm responsible for? What can prevent descent of the diaphragm?
Contraction of the diaphragm is re- sponsible for about 75% of the air that enters the lungs during quiet breathing. Advanced pregnancy, excessive obesity, or confining ab- dominal clothing can prevent complete descent of the diaphragm.
What are the second most important muscles of inhalation? Describe them
The next most important muscles of inhalation are the external intercostals. When these muscles contract, they elevate the ribs. As a result, there is an increase in the anteroposterior and lateral diameters of the chest cavity. Contraction of the external intercostals is responsible for about 25% of the air that enters the lungs during normal quiet breathing.
What is the intrapleural pressure during quiet inhalations? What about before inhalation?
During quiet inhalations, the pressure between the two pleural layers in the pleural cavity, called intrapleural (intrathoracic) pressure, is always subatmospheric (lower than atmospheric pressure). Just before inhalation, it is about 4 mmHg less than the atmospheric pressure, or about 756 mmHg at an atmospheric pressure of 760 mmHg.
What happens to the intrapleural pressure when the diaphragm and external intercostals contract? What happens to the parietal and visceral pleurae?
As the diaphragm and external intercostals contract and the overall size of the thoracic cavity increases, the volume of the pleural cavity also increases, which causes intrapleural pressure to decrease to about 754 mmHg. During expansion of the thorax, the parietal and visceral pleurae normally adhere tightly because of the subatmospheric pressure between them and because of the surface tension created by their moist adjoining surfaces. As the thoracic cavity expands, the parietal pleura lining the cavity is pulled outward in all directions, and the visceral pleura and lungs are pulled along with it.
What happens to the alveolar pressure when the lungs increase in volume?
The pressure inside the lungs, called the alveolar (intrapulmonic) pressure, drops from 760 to 758 mmHg. A pressure difference is thus established between the atmosphere and the alveoli. Because air always flows from a region of higher pressure to a region of lower pressure, inhalation takes place. Air continues to flow into the lungs as long as a pressure difference exists.
What extra muscles are active during forceful inhalations?
During deep, forceful inhalations, accessory muscles of inspiration also participate in increasing the size of the thoracic cavity. The muscles are so named because they make little, if any, contribution during normal quiet inhalation, but during exercise or forced ventilation they may contract vigorously. The accessory muscles of inhalation include the sternocleidomastoid muscles, which elevate the sternum; the scalene muscles, which elevate the first two ribs; and the pectoralis minor muscles, which elevate the third through fifth ribs.
Why are normal inhalation and forceful inhalation both active?
Because both normal quiet inhalation and inhalation during exercise or forced ventilation involve muscular contraction, the process of inhalation is said to be active.
Why is normal exhalation considered passive?
Normal exhalation during quiet breathing, unlike inhalation, is a passive process because no muscular contractions are involved. Instead, exhalation results from elastic recoil of the chest wall and lungs, both of which have a natural tendency to spring back after they have been stretched. Two inwardly directed forces contribute to elastic recoil: (1) the recoil of elastic fibers that were stretched during inhalation and (2) the inward pull of surface tension due to the film of alveolar fluid.
When does exhalation start? What happens during exhalation
When the inspiratory muscles relax. As the diaphragm relaxes, its dome moves superiorly owing to its elasticity. As the external intercostals relax, the ribs are depressed. These movements decrease the vertical, lateral, and anteroposterior diameters of the thoracic cavity, which decreases lung volume. In turn, the alveolar pressure increases to about 762 mmHg. Air then flows from the area of higher pressure in the alveoli to the area of lower pressure in the atmosphere
When does exhalation become active? What happens during
Exhalation becomes active only during forceful breathing, as occurs while playing a wind instrument or during exercise. During these times, muscles of exhalation—the abdominal and internal intercostals—contract, which increases pressure in the abdominal region and thorax. Contraction of the abdominal muscles moves the inferior ribs downward and com- presses the abdominal viscera, thereby forcing the diaphragm superiorly. Contraction of the internal intercostals, which extend inferiorly and posteriorly between adjacent ribs, pulls the ribs inferiorly.
When can intrapleural pressure exceed atmospheric pressure?
During a forceful exhalation, such as during a cough.
Apart from air pressure differences, what are the three other factors that affect the rate of airflow and the ease of pulmonary ventilation?
Surface tension of the alveolar fluid, compliance of the lungs, and airway resistance
What exerts the force of surface tension? Where does ST arise? What is its affect?
As noted earlier, a thin layer of alveolar fluid coats the luminal surface of alveoli and exerts a force known as surface tension. Surface tension arises at all air-water interfaces because the polar water molecules are more strongly attracted to each other than they are to gas molecules in the air. In the lungs, surface tension causes the alveoli to assume the smallest possible diameter. During breathing, surface tension must be overcome to expand the lungs during each inhalation. Surface tension also accounts for two-thirds of lung elastic recoil, which decreases the size of alveoli during exhalation.
What is the purpose of surfactant? What is it?
The surfactant (a mixture of phospholipids and lipoproteins) present in alveolar fluid reduces its surface tension below the surface tension of pure water.
What does a deficiency of surfactant in premature infants cause? What are consequences of this?
A deficiency of surfactant in premature infants causes respiratory distress syndrome, in which the surface tension of alveolar fluid is greatly increased, so that many alveoli collapse at the end of each exhalation. Great effort is then needed at the next inhalation to reopen the collapsed alveoli.
What is meant by compliance? What does it mean when it is high or low?
Compliance refers to how much effort is required to stretch the lungs and chest wall. High compliance means that the lungs and chest wall expand easily; low compliance means that they resist expansion.
In the lungs, what are the two principal factors that compliance is related to?
Elasticity and surface tension
What is the usual compliance of the lungs?
The lungs normally have high compliance and expand easily be- cause elastic fibers in lung tissue are easily stretched and surfactant in alveolar fluid reduces surface tension.
What is decreased compliance a common feature of? What diseases does it occur in?
Pulmonary conditions that (1) scar lung tissue (for example, tuberculosis), (2) cause lung tissue to become filled with fluid (pulmonary edema), (3) produce a deficiency in surfactant, or (4) impede lung expansion in any way (for example, paralysis of the intercostal muscles). Decreased lung compliance occurs in emphysema due to destruction of elastic fibers in alveolar walls.
What is airflow equal to?
Airflow equals the pressure difference between the alveoli and the atmosphere divided by the resistance.
What offers resistance to the normal flow of air into and out of the lungs?
The walls of the airways, especially the bronchioles.
What happens to the bronchioles when the lungs expand during inhalation and during exhalation?
As the lungs expand during inhalation, the bronchioles enlarge because their walls are pulled outward in all directions. Larger-diameter air- ways have decreased resistance. Airway resistance then increases during exhalation as the diameter of bronchioles decreases.
How is airway diameter regulated by the degree of contraction or relaxation of smooth muscle in the walls of the artery?
Signals from the sympathetic division of the autonomic nervous system cause relaxation of this smooth muscle, which results in broncho- dilation and decreased resistance.
How is resistance affected by diseases?
Any condition that narrows or obstructs the airways increases resistance, so that more pressure is required to maintain the same airflow. The hallmark of asthma or chronic obstructive pulmonary disease (COPD)—emphysema or chronic bronchitis— is increased airway resistance due to obstruction or collapse of airways.
What is the term for the normal pattern of quiet breathing? What does this consist of?
Eupnea can consist of shallow, deep, or combined shallow and deep breathing. A pattern of shallow (chest) breathing, called costal breathing, consists of an upward and outward movement of the chest due to contraction of the external intercostal muscles. A pattern of deep (abdominal) breathing, called diaphragmatic breathing, consists of the outward movement of the abdomen due to the contraction and descent of the diaphragm.