RESPIRATORY PHYSIOLOGY ease up the respiratory system

respiratory system physiology

1. RESPIRATORY PHYSIOLOGY PRESENTED BY: DR INDRAJEET SINGH

2. FUNCTIONS OF RESPIRATORY SYSTEM The main functions of respiration are to provide oxygen to the tissues and remove carbon dioxide. The four major components of respiration are PULMONARY VENTILATION : inflow and outflow of air between the atmosphere and the lung alveoli DIFFUSION of oxygen (O2) and carbon dioxide (CO2) between the alveoli and the blood TRANSPORT of oxygen and carbon dioxide in the blood and body fluids to and from the body’s tissue cells REGULATION of ventilation and other facets of respiration.

3. INSPIRATION The external intercostals contract and the internal intercostals relax. The ribs are pulled upward and outward. The muscle of diaphragm contract which lowers the diaphragm. Size of thoracic cavity increases, lungs expand Air pressure inside the lungs decreases. In order to balance the air pressure, air rushes from atmosphere into the lungs through air passage. The process of inspiration is active, as it needs energy for muscle contraction.

4. EXPIRATION The internal intercostal muscle contract. Due to contraction of internal intercostal muscles, ribs are pulled back to their normal position. The muscle of diaphragm relaxes and comes to its original dome-shaped. As a result, the size of thoracic cavity decreases and the size of lungs also decreases. Due to which the air pressure increases in lungs. In order to equalize the air pressure, air is expelled out from the lungs to the atmosphere through respiratory tract. Expiration is passive process as it does not require the expenditure of energy

5. MUSCLES INVOLVED IN BREATHING

6. PRESSURES INVOLVED IN BREATHING

7. LUNG COMPLIANCE The extent to which the lungs will expand for each unit increase in transpulmonary pressure (if enough time is allowed to reach equilibrium) is called the lung compliance . The total compliance of both lungs together in the normal adult human averages about 200 milliliters of air per centimeter of water transpulmonary pressure. That is, every time the transpulmonary pressure increases 1 centimeter of water, the lung volume, after 10 to 20 seconds, will expand 200 milliliters .

8. COMPLIANCE DIAGRAM OF LUNGS Diagram relating lung volume changes to changes in pleural pressure, which, in turn, alters transpulmonary pressure. The characteristics of the compliance diagram are determined by the elastic forces of the lungs. These forces can be divided into two parts: elastic forces of the lung tissue (2) elastic forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung air spaces.

9. ELASTIC FORCES OF LUNG TISSUE The elastic forces of the lung tissue are determined mainly by elastin and collagen fibers interwoven among the lung parenchyma. In deflated lungs, these fibers are in an elastically contracted and kinked state; then, when the lungs expand, the fibers become stretched and unkinked, thereby elongating and exerting even more elastic force.

10. ELASTIC FORCES DUE TO SURFACE TENSION When the lungs are filled with air, there is an interface between the alveolar fluid and the air in the alveoli. In lungs filled with saline solution, there is no air-fluid interface, and therefore, the surface tension effect is not present; only tissue elastic forces are operative in the lung filled with saline solution.

11. SURFACE TENSION When water forms a surface with air, the water molecules on the surface of the water have an especially strong attraction for one another. As a result, the water surface is always attempting to contract. This is what holds raindrops together—a tight contractile membrane of water molecules around the entire surface of the raindrop. On the inner surfaces of the alveoli, the water surface is also attempting to contract. This tends to force air out of the alveoli through the bronchi and, in doing so, causes the alveoli to try to collapse. The net effect is to cause an elastic contractile force of the entire lungs, which is called the surface tension elastic force .

12. SURFACTANT Surfactant, secreted by type II alveolar epithelial cells reduces the surface tension. Components : dipalmitoyl phosphatidylcholine, surfactant apoproteins , and calcium ions . In quantitative terms, the surface tension of different water fluids is approximately the following: Pure water - 72 dynes/cm; Alveolar fluids without surfactant – 50 dynes/cm; Alveolar fluids with surfactant - 5 and 30 dynes/cm.

13. WORK OF BREATHING During normal quiet breathing, all respiratory muscle contraction occurs during inspiration, expiration is almost entirely a passive process caused by elastic recoil of the lungs and chest cage. Thus, under resting conditions, the respiratory muscles normally perform “work” to cause inspiration but not to cause expiration. The work of inspiration can be divided into three fractions: compliance work or elastic work : required to expand the lungs against the lung and chest elastic forces tissue resistance work : that required to overcome the viscosity of the lung and chest wall structures airway resistance work : that required to overcome airway resistance to movement of air into the lungs Energy Required for Respiration. During normal quiet respiration, only 3 to 5 percent of the total energy expended by the body is required for pulmonary ventilation. During heavy exercise, the amount of energy required can increase as much as 50-fold, especially if the person has any degree of increased airway resistance or decreased pulmonary compliance. Therefore, one of the major limitations on the intensity of exercise that can be performed is the person’s ability to provide enough muscle energy for the respiratory process alone.

14. PULMONARY VOLUMES & CAPACITIES

15. RESPIRATORY MEMBRANE A layer of fluid containing surfactant that lines the alveolus and reduces the surface tension of the alveolar fluid The alveolar epithelium, which is composed of thin epithelial cells An epithelial basement membrane A thin interstitial space between the alveolar epithelium and the capillary membrane A capillary basement membrane that in many places fuses with the alveolar epithelial basement membrane The capillary endothelial membrane Despite the large number of layers, the overall thickness of the respiratory membrane in some areas is as little as 0.2 micrometer and averages about 0.6 micrometer

16. FACTORS THAT AFFECT THE RATE OF GAS DIFFUSION THROUGH THE RESPIRATORY MEMBRANE (1) the thickness of the membrane (2) the surface area of the membrane (3) the diffusion coefficient of the gas in the substance of the membrane (4) the partial pressure difference of the gas between the two sides of the membrane

17. OXYGEN CONTENT OF BLOOD

18. Distribution of Ventilation Regardless of body position, alveolar ventilation is unevenly distributed in the lungs. The right lung receives more ventilation than the left lung and the upper (dependent) areas of both lungs tend to be better ventilated than the lower areas because of gravitationally induced gradient in intrapleural pressure (Transpulmonary pressure). Pleural pressure decreases about 1cm H2o per 3cm decrease in lung height . Because of higher transpulmonary pressure alveoli in upper lung areas are maximally inflated and relatively non compliant and they undergo little expansion during inspiration. In contrast, the smaller alveoli in dependent areas have a lower Transpulmonary pressure, are more compliant, and undergo greater expansion during inspiration. Perfusion Perfusion of approx. 5l/min of blood flowing through the lungs, and only about 70 – 100ml at any one time are within the pulmonary capillaries undergoing gas exchange.

19. Ventilation / Perfusion Ratio:- Because alveolar ventilation (V A ) is normally about 4L/min. Pulmonary capillary perfusion (Q) is 5L/min. Overall V/Q is about 0.8. It can range from “0” (no ventilation ) to infinite (no perfusion). V/Q normally Ranges from 0.3 to 3.0, majority of the areas however are close to 1.0 Because perfusion increases at a greater rate than ventilation, non dependent (apical) areas tend to have higher V/Q ratios than dependent ( basal) areas. Hypoxic pulmonary vasoconstriction (HPV) It is a unique reflex to try and minimize the perturbations in VA/Q matching (pulmonary arterioles respond to regional hypoxemia by constricting). The arterioles in essentially all other tissues in the body vasodilate in response to hypoxemia. This reflex will tend to redirect blood flow from poorly or nonventilated lung regions to better ventilated regions. The Primary stimulus for HPV is Alveolar hypoxia . Usually seen during one lung ventilation .

20. Dead Space Any portion of an inspired breath that does not enter gas exchanging lung units is Dead space (VD). Minute ventilation (VE) is the sum of alveolar ventilation (VA) and dead space ventilation (VD). Dead space can be subdivided into Physiologic dead space and Anatomical dead space (breathing circuit). Physiologic dead space is further subdivided into Airway dead space and Alveolar dead space. Airway dead space is relatively constant but does vary directly with lung volume. Bronchodilation increases airway dead space. Airway dead space is decreased by endotracheal intubation . Alveolar dead space , however, becomes clinically important during positive pressure ventilation and in any condition of altered hemodynamics. Decreased cardiac output, pulmonary embolism, and changes in posture will all have clinically important effects on alveolar dead space. Shunt or venous admixture Portion of the venous blood returned to the heart that passes to the arterial circulation without being exposed to normally ventilated lung units. Shunt may be Extrapulmonary (blood does not pass through the lungs , thebesian veins, and bronchial circulation; 1% total pulmonary circulation) or Pulmonary ( venous blood passing through lung regions with decreased or no alveolar ventilation ) Shunt has a large effect on Pao2 but a limited effect on Paco2. Shunt is the commonest cause of hypoxemia during anesthesia.

21. • DISSOLVED O2 IN PLASMA – 1.5% BOUND TO Hb – 98.5% Each 100 mL of oxygenated blood contains the equivalent of 20 mL of gaseous O2. • The heme portion of hemoglobin contains four atoms of iron, each capable of binding to a molecule of O2. • As blood flows through tissue capillaries, the iron–oxygen reaction reverses. Hemoglobin releases oxygen, which diffuses first into the interstitial fluid and then into cells. OXYGEN TRANSPORT Oxygen and hemoglobin bind in an easily reversible reaction to form oxyhemoglobin. O2 +Hgb = 4HgbO2

22. Oxygen cascade :- It describes the process of stepwise decrease in partial pressure of oxygen from atmospheric air to the mitochondria.

23. OXYGEN DISSOCIATION CURVE The oxygen dissociation curve is a graph with oxygen partial pressure along the horizontal axis and oxygen saturation on the vertical axis, which shows an S-shaped relationship

24. A rightward shift in the oxygen hemoglobin dissociation curve lowers o2 affinity, displaces o2 from hemoglobin and makes more o2 available to tissues, a leftward shift increases hemoglobin’s affinity for o2 , reducing it’s avaibility to tissues. Left shifted oxy – Hb curve- Alkalosis ( metabolic and respiratory – the Haldane effect), hypothermia, abnormal fetal Hb, carboxyhemoglobin, methemoglobin, and decreased RBC 2,3-diphosphoglycerate (2,3- DPG) content. Right – shifted oxy-Hb curve- Acidosis (metabolic and respiratory – the Bohr effect), hyperthermia, abnormal Hb, increased RBC 2,3- DPG content.

25. CO2 TRANSPORTATION Normal resting conditions, each 100 mL of deoxygenated blood contains the equivalent of 53 mL of gaseous CO2, which is transported in the blood in three main forms: Dissolved CO2 – 7% Carbamino compounds – 23% Bicarbonate ions – 70%

26. HAMBURGER PHENOMENON AND CHLORIDE SHIFT

27. CO2 DISSOCIATION CURVE The arterial point (a) and the venous point (v)

28. BOHR EFFECT AND HALDANE EFFECT BOHR EFFECT: Increased CO2 causes right shift, Hb easily unloads O2. HALDANE EFFECT: Binding of oxygen with hemoglobin tends to displace carbon dioxide from the blood.

29. BLOOD VOLUME OF THE LUNGS Normally ~ 450 ml. Increase in alveolar pressure can decrease volume. Influence cardiac pathology on blood volume: - Failure of left ventricle - Mitral stenosis - Mitral regurgitation

30. DISTRIBUTION OF BLOOD FLOW TROUGH THE LUNGS Hydrostatic pressure - Influenced by gravity 30 cm lung height Lung apex: 15 mmHg less pressure than heart level Lung base: 8 mmHg greater pressure than heart level

31. LUNG CIRCULATIONS The lung has two circulations, a high-pressure, low-flow circulation - systemic arterial blood to the trachea, the bronchial tree (including the terminal bronchioles), the supporting tissues of the lung, and the outer coats (adventitia) of the pulmonary arteries and veins. a low-pressure, high-flow circulation - supplies venous blood from all parts of the body to the alveolar capillaries where oxygen (O2) is added and carbon dioxide (CO2) is removed.

32. Respiratory Control Central Nervous System Dissolved CO2 in plasma diffuses easily across the blood–brain barrier into the cerebrospinal fluid (CSF) where it interacts with H2O to form H+ and HCO3 . The H concentration in the CSF is the primary controller for normal minute ventilation. The brainstem central chemoreceptor is acutely sensitive to changes in pH. The central chemoreceptor is acutely sensitive to CNS depressants. Opioids, sedatives, and most general anesthetics decrease the respiratory response to hypercapnia. B. Peripheral chemoreceptors Located primarily in the carotid bodies at the bifurcation of the carotid arteries and also in aortic bodies above and below the aortic arch. These receptors respond primarily to changes in Pao2. There is some tonic activity from these peripheral chemoreceptors, but they do not normally stimulate ventilation until the Pao2 falls to below a threshold of approximately 70 to 80 mm Hg . This threshold will be lowered in individuals who are adapted to altitude and in some chronic respiratory or congenital hypoxic cardiac diseases.

33. Obesity The increased weight of the abdominal contents and chest wall impose a restrictive ventilatory pattern on the respiratory system with a decrease of all lung volumes but a preservation of the FEV1/FVC ratio (but a fall in FRC leads to increased veno-arterial shunt and a tendency to desaturate during induction and maintenance of anesthesia and in the postoperative period). Th e challenge in respiratory management of the obese patient perioperatively is to minimize the fall in FRC (regional anesthesia/analgesia, avoiding long-acting muscle relaxants, positioning, and use of postoperative CPAP ) Sleep-Disordered Breathing Approximately 20% of the population has disorders of respiration during sleep, ranging from simple snoring to obstructive sleep apnea . Obstructive sleep apnea (OSA ) is defined by more than 5 episodes per hour of apnea, each 10 seconds. b. The disturbance of normal sleep leads to daytime somnolence and the periods of hypoxia may contribute to cardiovascular morbid The obesity hypoventilation syndrome is a combination of obesity, hypoventilation, and severe OSA, which has been called the Pickwickian syndrome

34. Altered Barometric Pressures – a. Altitude The ambient PO2 decreases proportionally as the barometric pressure falls with increases in altitude. There are acute and chronic adaptations to the hypoxia associated with altitude. Rapid adaptation involves hyperventilation, driven by the peripheral chemoreceptors to decrease the alveolar PCO2 and thus increase the alveolar Pao2. Secondary alkalinization of blood and CSF returns to normal aft er several days at altitude as bicarbonate is excreted. Increased pulmonary pressures due to HPV triggered by hypoxia can lead to high altitude pulmonary edema (treated with oxygen, diuretics, and pulmonary vasodilators). Anesthesia at mild elevations is generally uncomplicated as long as oxygen saturation is monitored and adequate supplemental oxygen is provided. Most modern commercial vaporizers deliver reasonably accurate dosages of volatile anesthetics at modest elevations. Pressure in the air-filled cuff of an endotracheal tube or laryngeal mask airway will increase and decrease significantly with changes in ambient pressure, which may be associated with medical air transport.

35. b. Hyperbaric oxygen Hyperbaric oxygen is delivered in a chamber pressurized to two to three times atmospheric pressure (ATM) (1,400 to 2,100 mm Hg). Indications include gas embolism, decompression sickness, necrotizing soft tissue infections, and carbon monoxide poisoning. At high Fio2 levels, above 2 ATM, hyperoxia may cause convulsions. Prolonged exposure to a high Pao2 causes pulmonary oxygen toxicity and a restrictive lung disease

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