RespirationRespiration

Mechanics of breathing

Pressure relationships in the thoracic cavity

Atmospheric pressure is the pressure exerted by the gases surrounding the body.

Intrapulmonary pressure is the pressure within the alveoli of the lungs.

Intrapleural pressure is the pressure within the pleural cavity.

Transpulmonary pressure is the difference between the intrapulmonary and intrapleural pressures, which keeps the lungs from collapsing.

A pneumothorax is the presence of air in the intrapleural space.

Pulmonary ventilation

Boyle’s law is the relationship between pressure and volume, such that P1V1=P2V2, which states that there is an inverse proportionality between pressure and volume.

Inspiration is the period when air is flowing into the lungs.

The inspiratory muscles are the diaphragm and the external intercostal muscles.

Expiration is the period when gases are leaving the lungs.

Physical factors influencing pulmonary ventilation

Surface tension is the preferential attraction of water molecules toward each other, rather than to the gas molecules, that produces a state of tension at the liquid surface.

Surfactant is a detergent-like complex of lipids and proteins that interferes with the cohesiveness of water molecules.

Respiratory volumes and pulmonary function tests

Tidal volume (500 ml) is the amount of air inhaled or exhaled with each breath under resting conditions.

Inspiratory reserve volume (3100 ml) is the amount of air that can be forcefully inhaled after a normal tidal volume inhalation.

Expiratory reserve volume (1200 ml) is the amount of air that can be forcefully exhaled after a normal tidal volume exhalation.

Residual volume (1200 ml) is the amount of air remaining in the lungs after a forced exhalation.

Inspiratory capacity (3600 ml) is the maximum amount of air that can be inspired after a normal expiration.

Functional residual capacity (2400 ml) is the volume of air remaining in the lungs after a normal tidal volume expiration.

Vital capacity (4800 ml) is the maximum amount of air that can be expired after a maximum inspiratory effort.

Total lung capacity (6000 ml) is the maximum amount of air contained in the lungs after a maximum inspiratory effort.

Anatomical dead space is the volume of air that fills the conducting respiratory passageways, and never contributes to gas exchange.

Alveolar dead space is the volume of air in the lungs when the alveoli cease to act in gas exchange.

Total dead space is the sum of the anatomical dead space and the alveolar dead space.

A spirometer is an instrument that makes a graphic recording of respiratory volumes.

Total ventilation is the total amount of gas that flows into or out of the respiratory tract in 1 minute.

Forced vital capacity measures the amount of gas expelled when a subject takes a deep breath and then forcefully exhales as rapidly as possible.

Forced expiratory volume determines the amount of air expelled during specific time intervals of the FVC test.

Alveolar ventilation rate measures the flow of fresh gases in and out of the alveoli during a particular time, taking into account the volume of air wasted in the dead spaces.

Nonrespiratory air movements

Nonrespiratory air movements are processes other than breathing that move air into or out of the lungs, such as coughing, sneezing, laughing, and crying.

Gas exchanges in the body

Dalton’s law of partial pressures

Dalton’s law of partial pressure states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture.

The pressure exerted by each gas is its partial pressure, and is directly proportional to its percentage in the total gas mixture.

Henry’s law states that when a mixture of gasses is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure.

Oxygen toxicity develops when PO2 is greater than 2.5-3 atmospheres, due to the large numbers of free radicals.

Transport of respiratory gases by blood

Oxygen transport

Oxyhemoglobin is the hemoglobin-oxygen combination formed when 4 iron atoms in the hemoglobin bind with 4 molecules of oxygen.

Deoxyhemoglobin is the hemoglobin complex after oxygen has been released.

The Bohr effect occurs as a result of declining pH due to increased concentrations of CO2, which weakens the oxygen-hemoglobin bond and thus accelerates oxygen unloading.

Hypoxia is inadequate oxygen delivery to body tissues.

Carbon monoxide poisoning is a type of hypoxia that results when CO, which has an affinity for hemoglobin 200 times greater than that of O2, is introduced into the blood.

Carbon dioxide transport

Between 20% and 30% of transported CO2 is carried within RBCs as carbaminohemoglobin.

Most CO2, 60%-70%, is transported in the plasma as bicarbonate ions.

Carbonic anhydrase is an enzyme found in erythrocytes that catalyzes the following reaction:

CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

The chloride shift exchanges Cl- for HCO3-,

by sending chloride ions into erythrocytes, to compensate for the rapid accumulation of bicarbonate ions in the plasma.

Haldane effect reflects the greater ability of reduced hemoglobin to form carbaminohemoglobin and to buffer H+ by combining with it.

The carbonic acid-bicarbonate buffer system is important in resisting shifts in blood pH.

Control of respiration

Neural mechanisms and generation of breathing rhythm

The dorsal respiratory group and the ventral respiratory group are a cluster of neurons in the medulla oblongata that regulate respiration.

The dorsal respiratory group appears to be the pacesetting respiratory center, and is known as the inspiratory center.

Impulses travel along the phrenic intercostal nerves to excite the diaphragm and external intercostal muscles.

The pneumotaxic center is located in the pons, and seems to inhibit the medulla.

Factors influencing the rate and depth of breathing

The inflation reflex prevents over inflation of the lungs, and is initiated when baroreceptors in the visceral pleurae and conducting passages are stimulated due to over inflation of the lungs.

Chemoreceptors respond to changing levels of carbon dioxide, oxygen, and hydrogen ions in the arterial blood.

Hypercapnia refers to the lowering of the pH of cerebrospinal fluid due to an increase in the PCO2, which excites chemoreceptors and increases the rate and depth of breathing.

Hyperventilation is an increase in the rate and depth of breathing, which enhances alveolar ventilation.

Hypocapnia occurs when low carbon dioxide levels result from hyperventilation, which leads to dizziness due to the constriction of cerebral blood vessels.

Hypoventilation occurs when PCO2 is abnormally low due to inhibited respiration.

Apnea is the complete cessation of breathing, and may result from hypoventilation.

Cells sensitive to arterial oxygen levels are found in the aortic bodies of the aortic arch, and in the carotid bodies in the carotid arteries.

When PO2 becomes the primary stimulus for breathing due to insensitivity of CO2 chemoreceptors following pulmonary disease, it is called the hypoxic drive.

Respiratory adjustments during exercise and at high altitudes

Effects of exercise

Hyperpnea occurs as breathing becomes deeper and more vigorous due to exercise, but metabolic demands match respiratory changes, as opposed to hyperventilation.

Effects of high altitude

Acclimatization occurs when decreased PO2, which is evident at high altitudes, increases ventilation, thus lowering arterial CO2 levels but decreasing hemoglobin affinity for oxygen.

Erythropoitin secretion by the kidneys may increase, thus increasing the concentration of RBCs and the PO2. Go Broncos!