Physiology of Respiration PDF

Physiology of Respiration Dr. Hiwa S. Namiq 25-3-2024  Mechanics of Pulmonary Ventilation The lungs can be expanded and contracted in two ways: 1-by downward and upward movement of the diaphragm to lengthen or shorten the chest cavity. 2- by elevation and depression of the r...

Physiology of Respiration Dr. Hiwa S. Namiq 25-3-2024  Mechanics of Pulmonary Ventilation The lungs can be expanded and contracted in two ways: 1-by downward and upward movement of the diaphragm to lengthen or shorten the chest cavity. 2- by elevation and depression of the ribs to increase and decrease the antero-posterior diameter of the chest cavity Muscles that raise the rib cage during inspiration: 1. External intercostals 2. Sternocleidomastoid muscles (lifts sternum) 3. Anterior serrati (lifts many ribs) 4. Scaleni (lifts first two ribs). Muscles that pull the rib cage downward during expiration: 1. Internal intercostals. 2. Abdominal recti (pull downward on the lower ribs and compress the abdominal contents upward against the diaphragm) Contraction and expansion of the thoracic cage during expiration and inspiration, demonstrating diaphragmatic contraction, function of the intercostal muscles, and elevation and depression of the rib cage  Pleural versus Alveolar pressure:* Pleural pressure is the pressure of the fluid in the thin space between the lung pleura and the chest wall pleura. The normal pleural pressure at the beginning of inspiration is about –5 centimeters of water. Alveolar pressure is the pressure of the air inside the lung alveoli. When the glottis is open and no air is flowing into or out of the lungs, the pressures in all parts of the respiratory tree, all the way to the alveoli, are equal to atmospheric pressure, which is considered to be zero reference pressure in the airways—that is, 0 centimeters of water pressure. It fluctuates between (-1 and +1) during inspiration and expiration.  Surfactant and its function The fluid-air surface tension that lines the alveoli tends to collapse the alveoli during expiration. 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, thus trying to collapse the alveoli (called surface tension elastic force).  This collapse is normally prevented by the presence of a substance called surfactant which is secreted by the type II alveolar cells. It is composed of a mixture of phospholipids, proteins and ions. The most important component responsible for reducing the surface tension is a phospholipid called dipalmitoylphosphatidylcholine.  Functions of the Respiratory Passageways A. Trachea, Bronchi, and Bronchioles Multiple cartilage rings keep trachea from collapsing. Less extensive cartilage plates in bronchi also maintain rigidity yet allow sufficient motion for the lungs to expand Intermittent between the cartilaginous rings of trachea and bronchi are smooth muscles Bronchioles (diameters less than 1.5 millimeters) lack such cartilage but still kept expanded mainly by the same transpulmonary pressures that expand the alveoli Wall of bronchioles is entirely smooth m. (except the respiratory bronchiole, which is mainly pulmonary epithelium and underlying fibrous tissue) Many obstructive diseases of the lung result from narrowing of the smaller bronchi and bronchioles due to excessive smooth muscle contraction of their wall. Respiratory passages  Control of bronchiolar musculature Sympathetic Dilation of the Bronchioles 1.Direct control is less effective 2.Sympathetic stimulation from the adrenal medullae (norepinephrine and epinephrine on beta Rc-dilation) Parasympathetic Constriction of the Bronchioles 1.Parasympathetic fibers secret acetylcholine and cause mild to moderate constriction of the bronchioles. 2.Parasympathetic worsens the condition of Asthma Effects of local substances secreted by the lungs: 1.Histamine and slow reactive substance of anaphylaxis (released in the lung tissues by mast cells during allergic reactions-pollen in the air- cause airway obstruction during allergic asthma 2.Smoke, dust, sulfur dioxide and acidic elements in smog initiate local parasympathetic reflex and cause airway obstruction. Pulmonary Capillary Dynamics mmHg  Physical Principles of Gas Exchange After the alveoli are ventilated with fresh air, diffusion of oxygen occurs from the alveoli into the pulmonary blood and carbon dioxide in the opposite direction. All respiratory gases are simple molecules that are free to move among one another- a process called Diffusion (kinetic motion) The rate of diffusion of each gas is directly proportional to the pressure caused by that gas alone (partial pressure of the gas) The air has an approximate composition of 79 per cent nitrogen and 21 per cent oxygen (with small amount of CO2, Helium and water vapor)  Pressure difference causes net diffusion of gases through fluids When the partial pressure of a gas is greater in one area than in another area, there will be net diffusion from the high-pressure area toward the low-pressure area Diffusion of oxygen from one end of a chamber (A) to the other (B). The difference between the lengths of the arrows represents net diffusion. Factors Affecting the Rate of Gas Diffusion Through the Respiratory Membrane 1.Thickness of the membrane. 2.Surface area of the membrane. 3.Pressure difference of the gas between the two sides of the membrane. 4.Diffusion coefficient of the gas in the membrane. The following figure shows the slow rate of renewal of the alveolar air. In the first alveolus of the figure, excess gas is present in the alveoli but note that even at the end of 16 breaths the excess gas still has not been completely removed from the alveoli. Expiration of a gas with successive breaths The figure below demonstrates graphically the rate at which excess gas in the alveoli is normally removed, showing that with normal alveolar ventilation, about one half the gas is removed in 17 seconds. When a person’s rate of alveolar ventilation is only one-half normal, one half the gas is removed in 34 seconds, and when the rate of ventilation is twice normal, one half is removed in about 8 seconds. Rate at Which Alveolar Air Is Renewed by Atmospheric Air  OXYGEN CONCENTRATION AND PARTIAL PRESSURE IN THE ALVEOLI Normal VR is near 5 L/min Effect of alveolar ventilation on the alveolar PO2 at two rates of oxygen absorption from the alveoli—250 ml/min and 1000 ml/min. Point A is the normal operating point.  CO2 CONCENTRATION AND PARTIAL PRESSURE IN THE ALVEOLI Effect of alveolar ventilation on the alveolar PCO2 at two rates of carbon dioxide excretion from the blood—800 ml/min and 200 ml/min. Point A is the normal operating point.  Diffusion of Gases Through the Respiratory Membrane A respiratory unit is composed of a respiratory bronchiole, alveolar ducts, atria, and alveoli The alveolar walls are extremely thin, and capillaries are arranged as an interconnecting network. Gas exchange between the alveolar air and the pulmonary blood occurs through the respiratory membrane Respiratory Membrane (Pulmonary membrane)  Effect of the Ventilation-Perfusion Ratio on Alveolar Gas Concentration ❖ Normally to some extent, some areas of the lungs are well ventilated but have almost no blood flow, whereas other areas may have excellent blood flow but little or no ventilation ❖ The ratio between alveolar ventilation and alveolar blood flow is called the ventilation-perfusion ratio. ❖ This is expressed as follow: VA/Q Where VA = alveolar ventilation Q = blood flow When both are normal for a given alveolus, the ratio is also normal. When ventilation is zero but perfusion normal, the ratio=zero (shunted blood-physiologic shunt) When ventilation is normal but no perfusion, the ratio is infinity (Physiologic dead space) At both extremity, there is no gas exchange at the alveoli Diffusion of Oxygen from the Alveoli to the Pulmonary Capillary Blood Uptake of oxygen by the pulmonary capillary blood  f blood r e o a d m ixtu s venou O =95 P 2 with a Changes in PO2 in the pulmonary capillary blood, systemic arterial blood, and systemic capillary blood, demonstrating the effect of “venous admixture.”  Oxygen diffuses from the alveoli into the pulmonary capillary blood because the oxygen partial pressure in the alveoli (Po2 = 104 mm Hg ) is greater than the Po2 in the pulmonary capillary blood (40 mm Hg). In the other tissues of the body, a higher Po2 in the capillary blood (95 mm Hg) than in the tissues (40 mm Hg) causes oxygen to diffuse into the surrounding cells. Diffusion of oxygen from a tissue capillary to the cells  Diffusion of Carbon Dioxide from the Peripheral Tissue Cells into the Capillaries and from the Pulmonary Capillaries into the Alveoli Uptake of carbon dioxide by the blood in the tissue capillaries Diffusion of carbon dioxide from the pulmonary blood into the alveolus  Role of Hemoglobin in Oxygen Transport Normally, about 97% of the oxygen is transported from the lungs to the tissues in reversible chemical combination with hemoglobin (when PO2 is high oxygen binds with the hemoglobin, but when PO2 is low, oxygen is released from the hemoglobin). The remaining 3 per cent is transported in the dissolved state in the water of the plasma and blood cells. Oxygen-Hemoglobin Dissociation Curve Oxygen-hemoglobin dissociation curve shows the percentage of hemoglobin bound with oxygen. The blood that leaves the lungs and enters the systemic arteries has a PO2 of about 95 mm Hg and oxygen saturation of systemic arterial blood averages 97 per cent. While venous blood returning from the peripheral tissues has a PO2 of about 40 mm Hg and the saturation of hemoglobin averages 75 per cent. Oxygen-hemoglobin dissociation curve Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids Once oxygen has diffused from the alveoli into the pulmonary blood, it is transported to the peripheral tissue capillaries almost entirely in combination with hemoglobin. The presence of hemoglobin in the red blood cells allows the blood to transport 30 to 100 times as much oxygen as could be transported in the form of dissolved oxygen in the water of the blood  Transport of Carbon Dioxide in the In Blood the body’s tissue cells, oxygen reacts with various foodstuffs to form large quantities of carbon dioxide. This carbon dioxide enters the tissue capillaries and is transported back to the lungs. Most of the carbon dioxide (about 70 per cent) is transported in the form of (Bicarbonate ions) after CO2 combines with water on the surface of RBC to form carbonic acid, which then dissociate to give HCO3- and H+. This reaction is accelerated by the enzyme carbonic anhydrase on the red cell membrane. Also about 23% of CO2 is transported in form of carbaminohemoglobin (Hb-CO2) Normally, the rest of all the carbon dioxide (7%) is transported in a dissolved state to the lungs.  f t shi ide lor Transport of carbon dioxide in the blood Ch  Regulation of Respiration A. Neural control of respiration: Respiratory Center. The respiratory center is composed of groups of neurons located bilaterally in the medulla oblongata and pons of the brain stem. It is divided into three groups of neurons: 1. Dorsal respiratory group which mainly causes inspiration. 2. Ventral respiratory group which mainly causes expiration. 3. Pneumotaxic center which mainly controls rate and depth of breathing.  Dorsal Respiratory Group of Neurons—Its Control of Inspiration and of Respiratory Rhythm Most of the neurons of DRG are located within the nucleus of the tractus solitaries in the medulla of brain stem. This nucleus is the sensory termination of both the vagal and the glossopharyngeal nerves that transmit sensory signals from: Chemoreceptors Baroreceptors Receptors in the lungs The DRG generates rhythmical inspiratory discharges to the inspiratory muscles (mainly diaphragm) in a ramp manner (2 by 3 seconds).  Ventral Respiratory Group of Neurons—Functions in Both Inspiration and Expiration Features of this group of neurons: 1. Remain almost inactive during normal quiet respiration and not participate in basic rhythm of respiration. 2. Operates as an overdrive mechanism when only high levels of pulmonary ventilation are required (e.g. in heavy exercise) via powerful expiratory signals to abdominal muscles  B. Chemical Control of Respiration Excess carbon dioxide or excess hydrogen ions in the blood mainly act directly to stimulate the respiratory center. But oxygen does not have a significant direct effect on the respiratory center. Instead it acts almost entirely on peripheral chemoreceptors. Chemosensitive Area of the Respiratory Center None of the 3 neuron groups which control respiration is affected directly by changes in blood carbon dioxide concentration or hydrogen ion concentration. A Chemosensitive area is highly sensitive to changes in either blood Pco2 or hydrogen ion concentration, and it in turn excites the other portions of the respiratory center. Stimulation of brain stem inspiratory area by signals from the Chemosensitive area. Note that carbon dioxide in the fluid gives rise to most of the hydrogen ions (Q/ Which one have greater effect? H+ or CO2 or O2) Effects of increased arterial blood PCO2 and decreased arterial pH (increased hydrogen ion concentration) on the rate of alveolar ventilation.  Peripheral Chemoreceptor System for Control of Respiratory Activity Chemoreceptors, are located in several areas outside the brain, They are especially important for detecting changes in oxygen in the blood and to a lesser extent to changes in carbon dioxide and hydrogen ion concentrations Most of the chemoreceptors are in the carotid bodies (few are also in the aortic bodies) Respiratory control by peripheral chemoreceptors (carotid and aortic bodies)