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Human Medical Physiology A blog that contains lecture notes in human medical physiology for students of medicine and health sciences . Sample exams and key answers are also available . The aim of this blog is to provide the students with a quick , brief , and precise review , instead of wasting time and efforts reviewing the textbooks . This blog is not an alternative of textbooks in Human Physiology . Cooperation with other instructors and lecturers in Human Medical Physiology is quite welcome. Home
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Introduction Respiratory system is one of the eleven systems of the human organism , that plays important role in maintaining homeostasis . Other than its major function , which is supplying the cells with needed oxygen to produce energy and getting rid of carbon dioxide , it also has other functions , such as : * Vocalization , or sound production. * Participation in acid base balance . * Participation in fluid balance by insensible water elimination (vapors ). * Facilitating venous return . * Participation in blood pressure regulation : Lungs produce Angiotensin converting enzyme ( ACE ) . * Immune function : Lungs produce mucous that trap foreign particles , and have ciliae that move foreign particles away from the lung. They also produce alpha 1 antitrepsin that protect the lungs themselves from the effect of elastase and other proteolytic enzymes Respiration has two forms : 1. External respiration , which is the function of respiratory system , through which the organism gains the oxygen and get rid of carbon dioxide . 2. Internal respiration , which occurs inside the cell , where the oxygen burns glucose to produce energy , CO2 , and synthetic water. In this lecture we are going to study the external respiration . Respiration occurs in three steps : 1- Mechanical ventilation : inhaling and exhaling of air between lungs and atmosphere. 2- Gas exchange : between pulmonary alveoli and pulmonary capillaries. 3- Transport of gases from the lung to the peripheral tissues , and from the peripheral tissues back to blood . These steps are well regulated by neural and chemical regulation.
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Physiologic anatomy of the respiratory system Respiratory tract is subdivided into upper and lower respiratory tract. The upper respiratory tract involves , nose , oropharynx and nasopharynx , while the lower respiratory tract involves larynx , trachea , bronchi ,and lungs . Nose fulfills three important functions which are : 1. warming of inhaled air . b. filtration of air . c. humidification of air . Pharynx is a muscular tube , which forms a passageway for air and food .During swallowing the epiglottis closes the larynx and the bolus of food falls in the esophagus . Larynx is a respiratory organ that connects pharynx with trachea . It is composed of many cartilages and muscles and vocal cords . Its role in respiration is limited to being a conductive passageway for air . Trachea is a tube composed of C shaped cartilage rings from anterior side, and of muscle (trachealis muscle ) from its posterior side.The rings prevent trachea from collapsing during the inspiration. From the trachea the bronchi are branched into right and left bronchus ( primary bronchi) , which enter the lung .Then they repeatedly branch into secondary and tertiary bronchi and then into terminal and respiratory broncholes.There are about 23 branching levels from the right and left bronchi to the respiratory bronchioles , the first upper 17 branching are considered as a part of the conductive zones , while the lower 6 are considered to be respiratory zone. The cartilaginous component decreases gradually from the trachea to the bronchioles . Bronchioles are totally composed of smooth muscles ( no cartilage) . With each branching the diameter of bronchi get smaller , the smallest diameter of respiratory passageways is that of respiratory bronchiole. Lungs are evolved by pleura . Pleura is composed of two layers : visceral and parietal . Between the two layers of pleura , there is a pleural cavity , filled with a fluid that decrease the friction between the visceral and parietal pleura. Respiratory muscles : There are two group of respiratory muscles: 1. Inspiratory muscles : diaphragm and external intercostal muscle ( contract during quiet breathing ) , and accessory inspiratory muscles : scaleni , sternocleidomastoid , internal pectoral muscle , and others( contract during forceful inspiration). 2. Expiratory muscles : internal intercostal muscles , and abdominal muscles ( contract during forceful expiration)
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Pulmonary ventilation Ventilation simply means inhaling and exhaling of air from the atmospheric air into lungs and then exhaling it from the lung into the atmospheric air. Air pressure gradient has to exist between two atmospheres to enable a gas to move from one atmosphere to an other. During inspiration: the intrathoracic pressure has to be less than that of atmospheric pressure. This could be achieved by decreasing the intrathoracic pressure as follows: Depending on Boyle`s law , the pressure of gas is inversely proportional to the volume of its container. So increasing the intrathoracic volume will decrease the intrathoracic pressure which will allow the atmospheric air to be inhaled (inspiration) . As decreasing the intrathoracic volume will increase the intrathoracic pressure and causes exhaling of air ( expiration)
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Follow by Email Email address... So. Inspiration could be actively achieved by the contraction of inspiratory muscles : diaphragm and intercostal muscles. While relaxation of the mentioned muscles will passively cause expiration. Contraction of diaphragm will pull the diaphragm down the abdominal cavity ( will move inferiorly) , and then increase the intrathoracic volume ( vertically) . Contraction of external intercostal muscle will pull the ribs upward and forward which will additionally increase the intrathoracic volume ( transversely , the net result will be increasing the intrathoracic volume and decreasing the intrathoracic pressure. Relaxation of diaphragm will move it superiorly during expiration, the relaxation of external intercostal muscles will pull the ribs downward and backward , and the elastic lungs and chest wall will recoil. The net result is decreasing the intrathoracic volume and increasing intrathoracic pressure. All of this occurs during quiet breathing. During forceful inspiration an accessory inspiratory muscle will be involved ( scaleni , sternocleidomastoid , and others) to increase negativity in the intrathoracic pressure more and more. During forceful expiration the accessory expiratory muscles ( internal intercostal muscles and abdominal muscles ) will be involved to decrease the intrathoracic volume more and more and then to increase intrathoracic pressure more and more. The pressure within the alveoli is called intralveolar pressure . Between the two phases of respiration it is equal to the atmospheric pressure. It is decreased during inspiration ( about 1 cm H2O ) and increased during expiration ( about +1 cm H2O ) . This difference allow entering of 0.5 L of air into the lungs. Intrapleural pressure is the pressure of thin fluid between the two pleural layers . It is a slight negative pressure. At the beginning of inspiration it is about -5 cm H2O and reachs -7.5 cm H2O at the end or inspiration. At the beginning of expiration the intrapleural pressure is -7.5 cm H2O and reaches -5 cmH2O at the end of expiration. The difference between intralveolar pressure and intrapleural pressure is called transpulmonary pressure.
Other factors , affecting ventilation : Resistance : Gradual decreasing of the diameter of respiratory airway increase the resistance to air flow. Compliance : means the ease , which the lungs expand.It depends on both the elastic forces of the lungs and the elastic forces , caused by the the surface tension of the fluid, lining the alveoli. Surface tension: Molecules of water have tendency to attract each other on the surface of water adjacent to air. In alveoli the surface tension caused by the fluid in the inner surface of the alveoli may cause collapse of alveoli . The surface tension is decreased by the surfactant . Surfactant is a mixture of phospholipids , proteins and ion m produced by type II pneumocytes. Immature newborns may suffer from respiratory distress syndrome , due to lack of surfactant which is produced during the last trimester of pregnancy. The elastic fibers of the thoracic wall also participate in lung compliance. Lung volumes and capacities: I. Lung`s volumes 1. Tidal volume (TV) : is the volume of air m which is inspired and expired during one quiet breathing . It equals to 500 ml. 2. Inspiratory reserve volume (IRV) : The volume of air that could be inspired over and beyond the tidal volume. It equals to 3000 ml of air. 3. Expiratory reserve volume (ERV) : A volume of air that could be forcefully expired after the end of quiet tidal volume. It is about 1100 ml of air. 4. Residual volume (RV) : the extra volume of air that may remain in the lung after the forceful expiration . It is about 1200 ml of air. 5. Minute volume : the volume of air that is inspired or expired within one minute. It is equal to multiplying of respiratory rate by tidal volume = 12X500= 6000 ml. It is in female lesser than that in male. II. Lung`s capacities : 1. Inspiratory capacity: TV + IRV 2. Vital capacity : TV+IRV+ERV 3. Total lung capacity : TV+IRV+ERV+RV
Alveolar Ventilation: is the volume of air of new air , entering the alveoli and adjacent gas exchange areas each minute . It equals to multiplying of respiratory rate by ( tidal volume - dead space). Va = R rate X (TV- DsV) = 12 X ( 500-150) = 4200 ml of air. Respiratory patterns : The physiologic pattern of breathing expresses the rate , depth and frequency of breathing .In normal breathing pattern the duration of inspiration is 1.5-2 seconds , the same is for expiration and there is a a pause of no breathing that continues for 2 seconds too. The depth is about 500 ml ( tidal volume ) , and the rate is about 12 per minutes. It looks as follows :
Some pathological patterns of breathing : 1. Chein-Stocks breathing : depth starts shallow then progressively increases , then becomes shallow again , after which a period of no breathing ( apnea) follows. It is usually caused by brain injuries and stroke.
2. Kaussmaul breathing : deep and rapid respiratory pattern. Usually caused by severe metabolic acidosis .
3. apneustic breathig : prolonged inspiration , followed by prolonged expiration . It may be caused by damage to respiratory centers in the brain.
Gas exchange After the air enters the alveoli , a gas exchange occurs between the alveoli and the pulmonary capillaries . Here we have to notice that the alveolar wall is lined by many capillaries , so the blood flows in the alveolar wall as a sheet. Gas exchange takes place in the respirator units of lungs. Each respiratory unit is composed of : respiratory bronchiole , alveolar duct , alveolar sac , and pulmonary capillaries.The components of the respiratory units are quite thin and interconnecting with capillaries to form what is known the respiratory membrane. Respiratory units have a surface area of 50-100 quadrant meters. This huge surface area with thin structures allow rapid gas diffusion between the alveoli and capillaries.
Pulmonary capillaries have many differences that makes gas exchange between the capillaries and surrounding interstitium is somehow different from exchange of gases between systemic capillaries and the interstitium. These differences are: 1. Hydrostatic pressure in the pulmonary capillaries is lower than that of systemic capillaries ( 7 mm Hg ) 2. Interstitial hydrostatic pressure is lower ( more negative) 3. The oncotic pressure in the pulmonary capillaries is lower than that in systemic capillaries , because pulmonary capillaries are more leaky to plasma proteins. 4. The pulmonary capillaries walls are thin and weak and could be ruptured easily . Gases are exchanged by diffusion . For example : O2 diffuses from the alveoli into the capillaries via the interstitium , while the CO2 diffuses from the pulmonary capillaries into the alveoli via the interstitium. A gas has to pass 5 layers during its diffusion , which are : 1. pneumocytes type I 2. alveolar basal membrane 3. interstitium between alveoli and capillaries. 4. basal membrane of the capillaries 5. endothelial cells of the capillaries.
The diffusion rate of a given gas depends on many factors: 1. The pressure gradient ( delta pressure ) : The partial pressure of O2 in the alveoli is about 104 mm Hg , while in the arteriolar side of pulmonary capillaries its partial pressure is about 40 mm Hg . This causes diffusion of O2 from a region of high pressure ( alveoli ) to a region of lower pressure ( capillaries) for the O2. While the partial pressure of carbon dioxide is about 46 mm Hg in the capillary blood , and its partial pressure in alveoli is about 40 mm Hg , this tiny pressure gradient causes the diffusion of CO2 from the capillary blood into the alveoli . 2. Solubility of gases (S) : when the solubility is increased the rate is increased. Solubility of CO2 is higher than that of O2. 3. Surface area ( A) : when the surface area increases , the diffusion rate increases. 4. Molecular weight (MW) : diffusion rate is inversely proportional with the molecular weight of a gas. 5. Distance ( D) : the diffusion rate is inversely proportional with the distance that the gas has to pass. Clinical application: * Pulmonary edema increases the distance and so , decreases the diffusion rate of gases and impair gas exchange . * Emphysema decreases the surface area and so , it decreases diffusion rate and impairs gas exchange.
Regulation of Reaspiration Respiration is mainly controlled by the nervous system. There are many centers in the brain that regulate the respiratory function. The goal of regulatory centers is to control both the rate and depth of respiration to maintain normal level of O2 and CO2 in the blood. I. Respiratory centers in the brain: 1. Respiratory center in the medulla oblongata : This center plays the major role in regulation. It is composed of two groups of neuron ( remember : The respiratory centers are poorly defined collection of neurons , they are not located in special nuclei and strictly defined groups of neuron) : * Dorsal respiratory neurons : Associated with inspiration , when they are active , their action potential travels through the spinal cord, then through the phrenic and intercostal nerves to stimulate the respiratory muscles. It was proposed that intrinsic , periodic firing of these dorsal neurons is responsible for the basic rhythm of respirationm as a result these neurons exhibit spontaneously a cycle of activity every few seconds. * Ventral groups of neurons : Associated of forceful expiration only . Keeping in mind that expiration is a passive process , they are (silent) during inspiration and quiet expiration. They are active during forceful expiration and inhibit the dorsal group. So: The ventral and dorsal groups of respiratory neurons are connected to each other and act in synchrony to keep the respiratory movement symmetric.
2. Respiratory centers in the Pons : There are two centers in the pons , that are related to respiratory control : * Apneustic center: located in the lower pons . The exact function of this center is not well known , but it is proposed that this center stimulate the inspiratory neurons . Any lesion in this center is followed by pathological pattern of respiration with frequent apnoea , as the breathing become shallow . * Pneumotaxic center : Located in the upper part of the pons . It has an inhibitory effect on the medullary dorsal inspiratory neurons and thus terminate inspiration . It is responsible for what is known as Hering - Breur Reflex , as follows : When the lung inflated during inspiration , stretch mechanoreceptors in the wall of the lung are stimulated and send nerve impulse to the pneumotaxic center in the pons. The pneumotaxic center then sends nerve impulses to the the dorsal neurons and inhibits them to terminate inspiration. When inspiration is terminated the expiration starts spontaneously as a passive process . When expiration starts the activity of the stretch receptors ceases.
Chemoreceptors : They are specialized neurons that are stimulated by changes in the concentration of O2 and CO2 gases as well as changes in pH of the blood. There are two types of chemoreceptors: * Central chemoreceptors : Found in the medulla oblongata and exposed to the CSF as they are exposed to the local blood flow and metabolism in this area . They are connected to the respiratory center in the medulla oblongata. Actually : The central chemoreceptors are stimulated by changes in H+ concentration. H+ can not cross the blood brain barrier . When CO2 concentration in blood increase CO2 cross the blood brain barrier and reacts with the fluid in CSF as follows : CO2 + H2O H2CO3 HCO3- + H+ The increased H+ as a result stimulate the respiratory center and causes hyperventilation to expell CO2 and reduce its level in blood. Central chemoreceptors are more important in respiratory regulation because they are stimulated even with minimal changes in Co2 concentration ( 5 mm Hg change in PaCO2 ) may stimulate them. * Peripheral chemoreceptors : Those chemoreceptors are located at the carotid bifurcation and at the aortic arch . They are small vascular sensory organs , encapsulated with the connective tissue . The peripheral chemoreceptors are connected to the respiratory center by glossopharyngeal and vagal nerve . They get stimulated only in hypoxia when PaO2 is decresed less than 50% of its normal value , they stimulate respiration . 3. Other centers : In some extent , respiration is also controlled by higher brain centers such as cerebral cortex , which is necessary during talking , coughing , and vomiting . Holding breath voluntarily is also controlled by the cerebral cortex .
Gas Transport After diffusion of O2 from the alveoli to the pulmonary capillaries , it will be transported by blood to the peripheral tissue in two forms: 1. Dissolved gas : About 2% of O2 is transported as dissolved gas . In blood the amount of O2 dissolved is proportional to its partial pressure. In arterial blood the PaO2 is about 100 mm Hg , in each 100 ml arterial blood there is only 0.3 ml dissolved O2 , so the transport of O2 only as dissolved gas is not enough at all. 2. Oxyhemoglobin : Hemoglobin is composed of protein structure (Globin) + Nonprotein prosthetic group ( Heme). Globin has 4 protein polypeptide ( 2 alpha- and 2 Beta- ) . Each polypeptide is tightly bound to heme group that contains iron ion ,capable to bind an O2 molecule . Thus one hemoglobin can carry 4 molecules of O2.
Combination of O2 with Hemoglobin is a reversible combination. O2+ Hb HbO2 Oxyhemoglobin is different from (Methmoglobin) which contains iron in oxidized form , that lacks the electron , which is necessary to bind O2 ) . Now we have to distinguish between three terms: 1. O2 contents of blood : involves O2 in dissolved form + O2 combined to Hb ( Oxyhemoglobin) . 2- Carrying capacity of Hemoglobin: Each gram of Hb can carry 1.39 ml O2in blood. In each ml blood there is an average amount of 15 , so the O2 carrying capacity is 20.8 ml. These two things depend on the Hemoglobin concentration in blood . 3- Hemoglobin saturation : The percentage of binding of O2 to Hb. It equals to : O2 combined to Hb/ O2 capacity . In arterial blood Hb is 97.5% saturated. If we have three patients: One of them has anemia , the other has normal Hb , while the third one has polycythemia . In the three patients the O2 contents and the O2 capacity may vary , but the O2 saturation may be the same. Hb affinity to O2 depends on the PaO2 . In peripheral tissue The PaO2 decreases and the dissociation occurs (uloading of O2 ) to supply the peripheral tissue by O2 . In a partial pressure from 60mm Hg - 100 mm Hg ) the O2 saturation does not increase that much only 10% . This is very beneficial for organism m because if there is limited diffusion of O2 , due to high altitude for example or due to respiratory disease , the saturation curve would not be affected that much. This is called plateau of the hemoglobin dissociation curve. In peripheral tissue , when the PaO2 decreases less than 60 mm Hg the decreasing in saturation has a steep pattern. at 27 mm Hg (active tissue metabolism) the SaO2 becomes 50%.
O2 dissociation curve is also determined ( next to PaO2) by many other determinants , such as: PaCO2 : When it is increased the dissociation increased and the curve is shifted to the right (Bohr`s effect ) . H+ concentration: when it is increased ( pH decreased) the curve is also shift to the right . Tempertaure : Increased temperature shifts the curve to the right . DPG : Disphosphoglycerate ( DPG ) is an end product or erythrocytes` metabolism. Increased DPG also shift the curve to the right. Shift to right means increase unloading of O2 .
CO intoxication shifts the curve to the left . Transport of CO2 : CO2 is transported from peripheral tissue to the lungs by three means: 1. Dissolved in plasma ( 10% ) . This is due to the higher solubility of CO2 . 2. A bicarbonate (60%) : CO2+H2O H2CO3 HCO3- + H+. 3. Carbamino compound: (30 % ) Binding of CO2 to the amine group of Hb. Note : Please don`t forget to read about : Haldane effect and chloride shift.
5 comments: Mohammad Odeh 6 December 2013 at 08:07 Question How does the respiratory system facilitate the venous return ? Reply Replies Hasan Arafat 9 December 2013 at 06:16 When you breathe, the intrathoracic pressure decreases, this 'sucks' blood from the abdominal veins to the thoracic veins. You can imagine yourself drinking juice using a straw, in this case, the juice can resembles the abdomen, the straw resembles veins and your mouth ( the oral cavity) resembles the thorax, decreasing the pressure of the oral cavity facilitates the movement of fluid from the can to your mouth. This is the best answer I could think of....
mohammed odeh 21 December 2013 at 15:08 Thanks :* Reply
Abdulrahman Aqra
20 December 2013 at 04:34
Well Done Hasan Reply
madhurima roy 24 June 2015 at 23:31 well...it works through what is called as respiratory pump... during inspiration the thoracic cavity expands that makes the intrathoracic pressure to fall and intraabdominal pressure to increase.due to this the diameter of the inferior vena cava increases and the amount of blood flowing to the heart increases thus increasing the venous return. Reply
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