Pressure relationships in thethoracic 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 withinthe pleural cavity.Transpulmonary pressure is the differencebetween the intrapulmonary and intrapleuralpressures, which keeps the lungs fromcollapsing.A pneumothorax is the presence of air in theintrapleural 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 flowinginto the lungs.The inspiratory muscles are the diaphragmand the external intercostal muscles.Expiration is the period when gases areleaving the lungs.
Physical factors influencingpulmonary 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 andpulmonary 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 theamount of air that can be forcefully exhaledafter a normal tidal volume exhalation.Residual volume (1200 ml) is the amount ofair remaining in the lungs after a forcedexhalation.
Inspiratory capacity (3600 ml) is themaximum amount of air that can be inspiredafter a normal expiration.Functional residual capacity (2400 ml) isthe volume of air remaining in the lungsafter a normal tidal volume expiration.
Vital capacity (4800 ml) is the maximumamount of air that can be expired after amaximum inspiratory effort.Total lung capacity (6000 ml) is themaximum amount of air contained in thelungs after a maximum inspiratory effort.
Anatomical dead space is the volume of airthat fills the conducting respiratorypassageways, and never contributes to gasexchange.Alveolar dead space is the volume of air inthe lungs when the alveoli cease to act ingas exchange.
Total dead space is the sum of theanatomical dead space and the alveolar deadspace.A spirometer is an instrument that makes agraphic recording of respiratory volumes.Total ventilation is the total amount of gasthat flows into or out of the respiratory tractin 1 minute.
Forced vital capacity measures the amountof gas expelled when a subject takes a deepbreath and then forcefully exhales asrapidly as possible.Forced expiratory volume determines theamount of air expelled during specific timeintervals of the FVC test.
Alveolar ventilation rate measures the flowof fresh gases in and out of the alveoliduring a particular time, taking into accountthe 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.
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 ofgasses is in contact with a liquid, each gaswill dissolve in the liquid in proportion toits partial pressure.Oxygen toxicity develops when PO2 isgreater than 2.5-3 atmospheres, due to thelarge numbers of free radicals.
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 ofdeclining pH due to increasedconcentrations of CO2, which weakens theoxygen-hemoglobin bond and thusaccelerates oxygen unloading.
Hypoxia is inadequate oxygen delivery tobody tissues.Carbon monoxide poisoning is a type ofhypoxia that results when CO, which has anaffinity for hemoglobin 200 times greaterthan 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 inerythrocytes that catalyzes the followingreaction:CO2 + H2O <-> H2CO3 <-> H+ + HCO3-The chloride shift exchanges Cl- for HCO3-,by sending chloride ions into erythrocytes,to compensate for the rapid accumulation ofbicarbonate ions in the plasma.
Haldane effect reflects the greater ability ofreduced hemoglobin to formcarbaminohemoglobin and to buffer H+ bycombining with it.The carbonic acid-bicarbonate buffersystem is important in resisting shifts inblood pH.
Neural mechanisms andgeneration 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 intercostalnerves to excite the diaphragm and externalintercostal muscles.The pneumotaxic center is located in thepons, and seems to inhibit the medulla.
Factors influencing the rate anddepth 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 thepH of cerebrospinal fluid due to an increasein the PCO2, which excites chemoreceptorsand increases the rate and depth ofbreathing.Hyperventilation is an increase in the rateand depth of breathing, which enhancesalveolar ventilation.
Hypocapnia occurs when low carbondioxide levels result from hyperventilation,which leads to dizziness due to theconstriction of cerebral blood vessels.Hypoventilation occurs when PCO2 isabnormally low due to inhibited respiration.
Apnea is the complete cessation ofbreathing, and may result fromhypoventilation.Cells sensitive to arterial oxygen levels arefound in the aortic bodies of the aortic arch,and in the carotid bodies in the carotidarteries.
When PO2 becomes the primary stimulus forbreathing due to insensitivity of CO 2chemoreceptors following pulmonarydisease, it is called the hypoxic drive.
Respiratory adjustmentsduring exercise and at highaltitudes
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!