Respiratory Physiology PDF

1 Respiratory physiology Airway patency Definition - the ability of the airway to remain open and allow for adequate airflow. A patent airway is essential for proper oxygenation and ventilation of the lungs. Way that it can be compromised: ​ Edmea ​ Crushing injuries to bone and cartilage ​...

1 Respiratory physiology Airway patency Definition - the ability of the airway to remain open and allow for adequate airflow. A patent airway is essential for proper oxygenation and ventilation of the lungs. Way that it can be compromised: ​ Edmea ​ Crushing injuries to bone and cartilage ​ inhalation/swallowing of a foreign object ​ Deviated nasal septum ​ Nasal polyps ​ inflammation of the mucous membranes ​ Airway allergic reactions ​ Vocal chord changes ​ Spasms of smooth muscle ​ Deficiency of surfactant ​ Tumors Observations: ​ Stridor with breathing (noisy) ​ Secretions ​ Snoring ​ Difficulty inhaling/exhaling ​ Coughing ​ Changes in respiratory status ​ i.e. ↓ oxygen saturation Lung blood supply Pulmonary arteries - deoxygenated blood from the right side of the heart for oxygenation Bronchial arteries - deliver oxygenated blood to the lungs primarily perfusing the muscular walls of the bronchi and bronchioles. Ventilation - perfusion coupling Blood flow to each area of the lungs matches with the extent of airflow to the alveoli in that area. Vasoconstriction due to hypoxia - moves pulmonary blood from poorly ventilated areas of the lungs to well-ventilated areas of the lungs. In body tissues hypoxia cases dilation of the blood vessels to increase blood flow  2 Hypoxia - low levels of oxygen Pulmonary ventilation + volume pressure relationship Movement of air between the atmosphere and the alveoli, and consists of inhalation and exhalation and is driven by alternating pressure differences Boyle’s law - the pressure of a gas in a closed container is inversely proportional to the volume of the container. Factors affecting pulmonary ventilation: ​ Surface tension ○​ Causes alveoli to assume the smallest possible diameter ○​ Accounts for ⅔ of elastic recoil which decreases size of alveoli during exhalation ○​ Surfactant reduces surface tension below that of water which prevents alveoli collapse. ○​ Premature infants lack surfactant which makes them prone to alveoli collapse. ​ Respiratory distress syndrome ​ Severe cases may require CPAP ​ Compliance ○​ How much effort is required to stretch the lungs and the chest wall. ​ High compliance = easy ​ Low compliance = hard ○​ It depends of the elasticity and surface tension ​ Scar tissue formation would reduce compliance by reducing elasticity ○​ Analogy - thin balloons that are easy to inflate have high compliance. A heavy and stiff balloon that takes a lot of effort to inflate has low compliance ​ Airway resistance ○​ Airflow is determined by the pressure difference between the alveoli and the atmosphere, divided by airway resistance. ​ Larger diameter airways have decreased resistance ○​ Bronchioles stretch during inhalation, which decreases airway resistance. ○​ Airway diameter is also controlled by smooth muscle in the airway walls. ​ Sympathetic input causes muscle relaxation, leading to bronchodilation (wider airways) and reduced airway resistance. ○​ Conditions that narrow or obstruct airways increase resistance, such as in Chronic Obstructive Pulmonary Disease (COPD).  3 Lung volumes - spirometry At rest ~ 12 breaths per minute ~500 ml of air per breath (tidal volume) Minute ventilation (MV) = 12 breaths per minute x 500 ml = 6 L/min Only ~350 ml (70%) reaches the respiratory zone ~ 150 ml (30%) remains in the conducting zone or anatomic (respiratory dead space) Alveolar ventilation rate = volume of air per min that actually reaches the respiratory zone (~4.2 L/min) Factors affecting volumes and capacities: ​ Gender ​ Height ​ Age ​ Disease Inspiratory reserve volume (IRV) = the maximal volume that can be inspired in addition to a tidal inspiration. Expiratory reserve volume (ERV) = the maximal volume that can be expired in addition to tidal expiration Residual reserve volume = the volume remaining in the lungs at the end of a maximal expiration After ERV is exhaled air remains in the lungs because the sub-atmospheric intrapleural pressure keeps alveoli slightly inflated and some air remains in non-collapsible airways. Inspiratory Capacity (IC = TV + IRV) = the maximal volume inspired following a normal expiration Vital Capacity (VC = ERV + TV + IRV) = the maximal volume that can be expired following a maximal inspiration, i.e. the largest possible breath you can make Functional Residual Capacity (FRC = RV + ERV) = the volume in the lungs at the end of normal expiration when all the muscles of breathing are relaxed Total Lung Capacity (TLC = RV + ERV + TV + IRV = FRC + IC) = the volume in the lungs at the end of a maximal inspiration.  4 Gas Laws : Dalton’s Law and Henry’s Law Dalton’s Law - is important for the understanding how gases move down their pressure gradients by diffusion Henry’s Law - helps explain how the solubility of the gas relates to its diffusion. Dalton’s Law Each gas in a mixture of gases excerpts its own pressure as if no other gases were present. ​ Pressure of a specific gas in a mixture is called its partial pressure (Px) ○​ Can be calculated by multiplying the percentage of gas in the mixture by the total pressure ○​ The total pressure of the mixture is calculated by adding all of the partial pressures. ​ Partial pressures determine movement of O2 and CO2 during respiration ​ Gases move from areas of high partial pressure to areas of low partial pressure ​ Alveolar air has less O2 (13.6% vs 20.9%) and more CO2 (5.2% vs 0.04%) Henry’s Law The quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility. ​ The ability of a gas to stay insulation is greater when its partial pressure is higher and when it has a high solubility in water Carbon dioxide is dissolved in blood plasma because the solubility of carbon dioxide is 24 times greater than that of oxygen. Respiration Partial pressure determines the movement of oxygen and carbon dioxide between the atmosphere and lungs, between the lungs and blood, and the blood and body cells. Each gas diffuses across a permeable membrane from an area where its partial pressure is greater to the area where its partial pressure is less. The rate of pulmonary and systemic gas exchange depends of several factors: 1)​ partial pressure difference of the gases - e.g. s;latitude sickness a)​ Increases in altitude = decrease total atmospheric pressure and decrease in peripheral oxygen. b)​ Decrease pulmonary alveolar peripheral oxygen and oxygen diffuses into the blood more slowly 2)​ Surface area available for gas exchange - emphysema- alveolar walls deteriorate 3)​ Diffusion distance - pulmonary edema - build up of fluid 4)​ Molecular weight and solubility of gases a)​ Oxygen decrease molecular weight than carbon dioxide - should diffuse faster b)​ But solubility of carbon dioxide is about 24x greater than that of oxygen c)​ Net outward carbon dioxide diffusion occurs 20x more rapidly than net inward oxygen diffusion. Respiration is the exchange of gases.  5 ​ external respiration (pulmonary) - is the gas exchange between the alveoli and the blood ​ Internal respiration (tissue) - is the gas exchange between the systemic capillaries and the tissues of the body. Exchange refers to the movement of oxygen OR carbon dioxide from a region of high partial pressure to a region of low partial pressure NOT THE EXCHANGE OF O2 FOR CO2. Diffusion occurs until the partial pressure in the blood matches that in the alveoli for O2 and alveoli matches the blood for CO2. O2 transport ​ In the blood, some O2 is dissolved in the plasma as a gas ​ Most O2 (about 98.5%) is carried attached to haemoglobin (Hb) which is called oxyhaemoglobin. The most important factor that determines how much O2 binds to haemoglobin is the PO2; “The higher the PO2 the more O2 combines with Hb” Relationship between haemoglobin and PO2 The percentage saturation of haemoglobin expresses the average saturation of haemoglobin with oxygen. For instance, if each haemoglobin molecule has bound two oxygen molecules, then the haemoglobin is 50% saturated because each haemoglobin molecule can bind to the maximum of four oxygen. Factors affecting affinity of haemoglobin for O2 Most important factor is PO2 that determines the percent of oxygen saturation but other factors include: ​ Acidity ​ Peripheral carbon dioxide ​ Temperature ​ 2,3-bis-phosphoglycerate (BPG) The Bohr effect first described in 1904 by the Danish physiologist Christian Bohr Hb O2 binding affinity is inversely related both to acidity and to the concentration of CO2 Bohr effect refers to the shift in the O2 dissociation curve caused by changes in the concentration of CO2 or the pH of the environment. The main acids produced by metabolically active tissues are lactic acid and carbonic acid (from increased CO2 production) ↑ H+ results in haemoglobin proteins releasing their load of O2. Thus, lowered pH drives O2 off haemoglobin, making more O2 available for tissue cells  6 Conversely, a decrease in CO2 provokes an increase in pH (↓ H+), which results in haemoglobin picking up more O2 BPG is formed in red blood cells when they break down glucose to produce ATP in glycolysis When BPG combines with haemoglobin, the haemoglobin binds O2 less tightly at the heme group sites. The greater the level of BPG, the more O2 is unloaded from haemoglobin In the case of carbon monoxide, it binds high affinity to Hb , but increases the affinity of other binding sites for O2 (left shift) preventing O2 dissociation. So CO poisoning, patients can have normal pO2 but suffer severe tissue hypoxia. CO2 transport ​ Carbon dioxide is transported in the blood in three different forms: ○​ 7% is dissolved in the plasma as a gas ○​ 70% is converted into carbonic acid by carbonic anhydrase (CA) before dissociating into bicarbonate and protons. ○​ 23% is attached to the Hb forming carbaminohemoglobin (but not at the same binding site a oxygen) Control of respiration ​ The respiratory muscles contract as a result of nerve impulses transmitted from centres in the brain and relax in the absence of nerve impulses ​ The nerve impulses are sent from clusters of neurons located bilaterally in the brain stem ​ This widely dispersed group of neurons collectively called the respiratory centre They are divided into two principal areas based of their location and function: 1)​ Medullary respiratory centre - in the medulla oblongata 2)​ Pontine respiratory group - in the pons Medullary respiratory centre ​ Dorsal respiratory group (DRG) ​ Ventral respiratory group (VRG) Normal quiet breathing  7 DRG neurons generate impulses (2 secs) causing diaphragm and external intercostals to contract - inhalation DRG then inactive, diaphragm and external intercostals relax (3 secs) Allows passive recoil of the lungs and thoracic wall. VRG contains neurons called the pre-Bötzinger complex (BOT-zin-ger) Important in the rhythm of breathing Analogous to the one in the heart, is composed of pacemaker cells that set the basic rhythm of breathing. Other neurons in VRG become activated when forceful breathing is required Regulation of the respiratory centre – cortical control Activity of the respiratory centre can be modified in response to inputs from other brain regions, receptors in the peripheral nervous system, and other factors in order to maintain the homeostasis of breathing Cerebral cortex has connections with the respiratory centre We can voluntarily alter our pattern of breathing including breath holding – protective The ability to not breathe is limited by the buildup of CO2 and H+ in the body. ↑ PCO2 and H+ concentrations stimulate DRG neurons of the medullary respiratory center Nerve impulses are sent along the phrenic and intercostal nerves to inspiratory muscles, and breathing resumes Chemoreceptor regulation of breathing Certain chemical stimuli modulate how quickly and how deeply we breathe Chemoreceptors in two locations of the respiratory system monitor levels of CO2, H+, and O2 and provide input to the respiratory centre Central chemoreceptors located in or near the medulla oblongata and respond to changes in H+ concentration or PCO2 in cerebrospinal fluid Peripheral chemoreceptors located in the aortic bodies, clusters of chemoreceptors located in the wall of the aortic arch, and in the carotid bodies and are part of the peripheral nervous system and are sensitive to changes in PO2, H+, and PCO2 in the blood.