Proerythroblast is a large basophilic cell containing a large spherical euchromatic nucleus with prominent nucleoli.
Basophilic erythroblast is a strongly basophilic cell with nucleus that comprises approximately 75 % of its mass. Numerous cytoplasmic polyribosomes, condensed chro-matin, no visible nucleoli, and continued hemoglobin synthesis characteristics of this cell.
Polychromatophilic erythroblast is the last cell in this line undergoes mitotic divisions. Its nucleus comprises approximately 50 % of its mass and contains condensed chroma-tin which appears in a «checkerboard» pattern. The po-lychnsia of the cytoplasm is due to the increased quantity of acidophilic hemoglobin combined with the basophilia of cytoplasmic polyribosomes.
Normoblast (orthochromatophilic erythroblast) is a cell with a small heterochromatic nucleus that comprises approximately 25 % of its mass. It contains acidophilic cytoplasm because the large amount of hemoglobin and degenerating organelles. The pyknotic nucleus, which is no longer capable of division, is extruded from the cell.
Reticulocyte (polychromatophilic erythrocyte) is an immature acidophilic denucleated RBC, which still contains some ribosomes and mitochondria involved in the synthesis of a small quantity of hemoglobin. Approximately 1 % of the circulating RBCs are reticulocytes.
Erythrocyte is the mature acidophilic and denucleated RBC. Erythrocytes remain in the circulation approximately 120 days and are then recycled by the spleen, liver, and bone marrow.
New words
reticular – ñåò÷àòûé
sinusoids – ñèíóñîèäû
granulocytes – ãðàíóëîöèòû
agranulocytes – àãðàíóëîöèòû
active – àêòèâíûé
yellow – æåëòûé
glycoprotein – ãëèêîïðîòåèí
erythropoietin – ýðèòðîïîýòèí
amount – êîëè÷åñòâî
hemoglobin – ãåìîãëîáèí
degenerating – äåãåíåðèðóþùèå
condensed – ñæàòûé
22. Hematopoietic tissue. Granulopoiesis, thrombopoiesis
Granulopoiesis is the process of granulocyte formation. Bone marrow stem cells differentiate into all three types of granulocytes.
Myeloblast is a cell that has a large spherical nucleus containing delicate euchromatin and several nucleoli. It has a basophilic cytoplasm and no granules. Myeloblasts divide differentiate to form smaller promyelocytes.
Promyelocyte is a cell that contains a large spherical indented nucleus with coarse condensed chromatin. The cytoplasm is basophilic and contains peripheral azurophi-lic granules.
Myelocyte is the last cell in this series capable of division. The nucleus becomes increasingly heterochromatic with subsequent divisions. Specific granules arise from the Golgi apparatus, resulting in neutrophilic, eosinophilic, and basophilic myelocytes.
Metamyelocyte is a cell whose indented nucleus exhibits lobe formation that is characteristic of the neutrophil, eos-inophil, or basophil. The cytoplasm contains azurophilic granules and increasing numbers of specific granules. This cell does not divide. Granulocytes are the definitive cells that enter the blood. Neutrophilic granulocytes exhibit an intermediate stage called the band neutrophil. This is the first cell of this series to appear in the peripheral blood.
It has a nucleus shaped like a curved rod or band.
Bands normally constitute 0,5–2 % of peripheral WBCs; they subsequently mature into definitive neutrophils.
Agranulopoiesis is the process of lymphocyte and mono-cyte for mation. Lymphocytes develop from bone marrow stem cells (lymphoblasts). Cells develop in bone marrow and seed the secondary lymphoid organs (e. g., tonsils, lymph nodes, spleen). Stem cells for T cells come from bone marrow, develop in the thymus and, subsequently, seed the secondary lym phoid organs.
Promonocytes differentiate from bone marrow stem cells (monoblasts) and multiply to give rise to monocytes.
Monocytes spend only a short period of time in the marrow before being released into the bloodstream.
Monocytes are transported in the blood but are also found in connective tissues, body cavities and organs.
Outside the blood vessel wall, they are transformed into macrophages of the mononuclear phagocyte system.
Thrombopoiesis, or the formation of platelets, occurs in the red bone marrow.
Megakaryoblast is a large basophilic cell that contains a U-shaped or ovoid nucleus with prominent nucleoli. It is the last cell that undergoes mitosis.
Megakaryocytes are the largest of bone marrow cells, with diameters of 50 mm or greater. They undergo 4–5 nuclear divi sions without concomitant cytopla-smic division. As a result, the megakaryocyte is a cell with polylobulated, polyploid nucleus and abundant granules in its cytoplasm. As megakaryocyte maturation proceeds, «curtains» of platelet demarcation vesicles form in the cytoplasm. These vesicles coalesce, become tubular, and eventually form platelet demarcation membranes. These membranes fuse to give rise to the membranes of the platelets.
A single megakaryocyte can shed (i. e., produce) up to 3,500 platelets.
New words
capable – ñïîñîáíûé
spherical – ñôåðè÷åñêèé
indented – çàçóáðåííûé
chromatin – õðîìàòèí
23. Arteries
Arteries are classified according to their size, the appearance of their tunica media, or their major function.
Large elastic conducting arteries include the aorta and its large branches. Unstained, they appear yellow due to their high con tent of elastin.
The tunica intima is composed of endothelium and a thin sub jacent connective tissue layer. An internal elastic membrane marks the boundary between the intima and media.
The tunica media is extremely thick in large arteries and con sists of circularly organized, fenestrated sheets of elastic tissue with interspersed smooth muscle cells. These cells are responsi ble for producing elastin and other extracellular matrix com ponents. The outermost elastin sheet is considered as the external elastic membrane, which marks the boundary between the media and the tunica adventitia.
The tunica adventitia is a longitudinally oriented collection of collagenous bundles and delicate elastic fibers with associated fibroblasts. Large blood vessels have their own blood supply (vasa vasorum), which consists of small vessels that branch profusely in the walls of larger arteries and veins. Muscular distributing arteries are medium-sized vessels that are characterized by their predominance of circularly arranged smooth muscle cells in the media interspersed with a few elastin compo nents. Up to 40 layers of smooth muscle may occur. Both internal and external elastic limiting membranes are clearly demonstrated. The intima is thinner than that of the large arteries.
Arterioles are the smallest components of the arterial tree. Generally, any artery less than 0,5 mm in diameter is considered to be a small artery or arteriole. A suben-dothelial layer and the inter nal elastic membrane may be present in the largest of these vessels but are absent in the smaller ones. The media is composed of several smooth muscle cell layers, and the adventitia is poorly devel oped. An external elastic membrane is absent.
New words
endothelium – ýíäîòåëèé
media – ñðåäíÿÿ
arteries – àðòåðèè
to be classified – êëàññèôèöèðîâàííûé
according – ñîîòâåòñòâåííî
their – èõ
size – ðàçìåð
appearance – âèä
tunica – îáîëî÷êà
major – ãëàâíûé
elastic – ýëàñòè÷íûé
conducting – ïðîâåäåíèå
arteries – àðòåðèè
to include – âêëþ÷àòü
aorta – àîðòà
branches – âåòâè
up to – äî
layers – ñëîè
smooth – ãëàäêèé
may – ìîæåò
infima – âíóòðåííÿÿ ïîëîñòü àðòåðèè
24. Capillaries
Capillaries are thin-walled, narrow-diameter, low-pressure vessels that generally permit easy diffusion across their walls. Most capillar ies have a cross-sectional diameter of 7 – 12 mm. They are composed of a simple layer of endothelium, which is the lining of the entire vas cular system, and an underlying basal lamina. They are attached to the surrounding tissues by a delicate reticulum of collagen. Associated with these vessels at various points along their length are specialized cells called pericytes. These cells, enclosed within their own basal lamina, which is continuous with that of the endothelium, contain contractile proteins and thus may be involved in the control of capillary dynamics. They may also serve as stem cells at times of vascular repair. Capillaries are generally divided into three types, according to the structure of their endothelial cell walls
Continuous (muscular, somatic) capillaries are formed by a single uninterrupted layer of endothelial cells rolled up into the shape of a tube and can be found in locations such as connective tissue, muscle, and nerve
Fenestrated (visceral) capillaries are characterized by the presence of pores in the endothelial cell wall. The pores are covered by a thin diaphragm (except in the glomeruli of the kidney) and are usually encountered in tissues where rapid substance interchange occurs (e. g., kidney, intestine, endocrine glands)
Sinusoidal capillaries can be found in the liver, hematopoietic and lymphopoietic organs, and in certain endocrine glands. These tubes with discontinuous endothelial walls have a larger diame ter than other capillaries (up to 40 mm), exhibit irregular cross-sec tional profiles, have more tortuous paths, and often lack a con tinuous basal lamina. Cells with phagocytic activity (macrophages) are present within, or just subjacent to, the en-dothelium.
New words
capillaries – êàïèëëÿðû
to thin-walled – îêðóæåííûé òîíêîé ñòåíîé
narrow-diameter – óçêèé äèàìåòð
low-pressure – íèçêîå äàâëåíèå
that – òîò
generally – ãëàâíûì îáðàçîì
permit – ðàçðåøåíèå
easy – ëåãêèé
diffusion – ðàñïðîñòðàíåíèå
cross-sectional – ïîïåðå÷íûé
to be composed – áûòü ñëîæíûì
simple – ïðîñòîé
endothelium – ýíäîòåëèé
lining – âûðàâíèâàíèå
entire – âåñü
vas cular – ñîñóäèñòûé
underlying – ëåæàùèé â îñíîâå
basal – îñíîâíîé
lamina – òîíêàÿ ïëàñòèíêà
25. Veins
Veins are low-pressure vessels that have larger lumina and thinner walls than arteries. In general, veins have more collagenous connec tive tissue and less muscle and elastic tissue than their arterial coun terparts. Although the walls of veins usually exhibit the three layers, they are much less distinct than those of the arter ies. Unlike arteries, veins contain one-way valves composed of exten sions of the intima that prevent reflux of blood away from the heart. Veins can be divided into small veins or venules, medium veins, and large veins.
Venules are the smallest veins, ranging in diameter from approxi mately 15–20 mm (post-capillary venules) up to 1–2 mm (small veins). The walls of the smaller of these are structurally and func tionally like those of the capillaries; they consist of an endothelium surrounded by delicate collagen fibers and some pericytes. In those vessels of increased diameter, circularly arranged smooth muscle cells occur surrounding the intima layer, but unlike in the small arteries, these cells are loosely woven and widely spaced. Venules are important in inflammation because their endothelial cells are sensitive to hista-mine released by local mast cells. This causes endotheli-al cells to contract and separate from each other, exposing a naked basement membrane. Neutrophils stick to the exposed collagen and extravasate (i. e., move out into the connective tissue). Histamine also causes local arterioles to relax, affect ing a rise in venous pressure and increased leaking of fluid. This produces the classic signs of inflammation: redness, heat, and swelling.
Medium veins in the range of 1–9 mm in diameter have a well – developed intima, a media consisting of connective tissue and loosely organized smooth muscle, and an adventitia (usually the thickest layer) composed of collagen bundles, elastic fibers, and smooth muscle cells oriented along the longitudinal axis of the vessel. Venous valves are sheet-like outfoldings of endothelium and underlying connective tissue that form flaps to permit uni-di rectional flow of blood.
Large veins, such as the external iliac, hepatic portal, and vena cavae, are the major conduits of return toward the heart. The intima is similar to that of medium veins. Although a network of elastic fibers may occur at the boundary between the intima andmedia, a typical internal elastic membrane as seen in arteries is not present. A tunica media may or may not be present. If pre sent, smooth muscle cells are most often circularly arranged. The ad-ventitia is the thickest layer of the wall and consists of elastic fibers and longitudinal bundles of collagen. In the vena cava, this layer also contains well-developed bundles of longitudinally oriented smooth muscle.
New words
vein – âåíà
low-pressure – íèçêîå äàâëåíèå
collagenous – êîëëàãåíîâûé
intima – èíòèìà
reflux – ðåôëþêñ
inflammation – âîñïàëåíèå
longitudinal – ïðîäîëüíûé
flaps – ñòâîðêè
iliac – ïîäâçäîøíûé
hepatic – ïå÷åíî÷íûé
26. Heart
Intrapulmonary bronchi: the primary bronchi give rise to three main branches in the right lung and two branches in the left lung, each of which supply a pulmonary lobe. These lobar bronchi divide repeatedly to give rise to bronchioles.
Mucosa consists of the typical respiratory epithelium.
Submucosa consists of elastic tissue with fewer mixed glands than seen in the trachea.
Anastomosing cartilage plates replace the C-shaped rings found in the trachea and extra pulmonary portions of the pri mary bronchi.
Bronchioles do not possess cartilage, glands, or lymphatic nodules; however, they contain the highest proportion of smooth r muscle in the bronchial tree. Bronchioles branch up to 12 times to supply lobules in the lung.
Bronchioles are lined by ciliated, simple, columnar epithelium with nonciliated bronchiolar cells. The musculature of the bronchi and bronchioles con tracts following stimulation by parasympathetic fibers (vagus nerve) and relaxes in response to sympathetic fibers. Terminal bronchioles consist of low-ciliated epithelium with bronchiolar cells.
The costal surface is a large convex area related to the inner surface of the ribs.
The mediastinal surface is a concave medial surface, contains the root, or hilus, of the lung.
The diaphragmatic surface (base) is related to the convex sur face of the diaphragm. The apex (cupola) protrudes into the root of the neck.
The hilus is the point of attachment for the root of the lung. It contains the bronchi, pulmonary and bronchial vessels, lym phatics, and nerves. Lobes and fissures ventricular con traction (systole). Semilunar valves (aortic and pulmonic) prevent reflux of blood back into the ventricles during ventricular relaxation (diastole). Impulse conducting system of the heart consists of specialized cardiac myocytes that are characterized by auto-maticity and rhythmicity (i. e., they are independent of nervous stimulation and possess the ability to initiate heart beats). These specialized cells are located in the sino-atrial (SA) node (pacemaker), intern-odal tracts, atrioven-tricular (AV) node, AV bundle (of His), left and right bundle branches, and numerous smaller branches to the left and right ventricular walls. Impulse conduct ing myocytes are in electrical contact with each other and with normal contractile myocytes via communicating (gap) junctions. Specialized wide-diameter impulse conducting cells (Purkinje myocytes), with greatly reduced myofilament components, are well-adapted to increase conduction velocity. They rapidly deliver the wave of depolarization to ventricular myocytes.
New words
heart – ñåðäöå
muscular – ìûøå÷íûé
cardiac – ñåðäå÷íûé
to pump – êà÷àòü
endocardium – ýíäîêàðäèóì
innermost – ñàìûé âíóòðåííèé
conducting system – ïðîâåäåíèå ñèñòåìû
subendocardial – âíóòðèñåðäå÷íûé
impulse – èìïóëüñ
fibrosi – ôèáðîçíûå êîëüöà
27. Lungs
Intrapulmonary bronchi: the primary bronchi give rise to three main branches in the right lung and two branches in the left lung, each of which supply a pulmonary lobe. These lobar bronchi divide repeatedly to give rise to bronchioles.
Mucosa consists of the typical respiratory epithelium.
Submucosa consists of elastic tissue with fewer mixed glands than seen in the trachea.
Anastomosing cartilage plates replace the C-shaped rings found in the trachea and extra pulmonary portions of the pri òàãó bronchi.
Bronchioles do not possess cartilage, glands, or lymphatic nodules; however, they contain the highest proportion of smooth r muscle in the bronchial tree. Bronchioles branch up to 12 times to supply lobules in the lung.
Bronchioles are lined by ciliated, simple, columnar epithelium with nonciliated bronchiolar cells. The musculature of the bronchi and bronchioles con tracts following stimulation by parasympathetic fibers (vagus nerve) and relaxes in response to sympathetic fibers. Terminal bronchioles consist of low-ciliated epithelium with bronchiolar cells.
The costal surface is a large convex area related to the inner surface of the ribs.
The mediastinal surface is a concave medial surface, contains the root, or hilus, of the lung.
The diaphragmatic surface (base) is related to the convex sur face of the diaphragm. The apex (cupola) protrudes into the root of the neck.
The hilus is the point of attachment for the root of the lung. It contains the bronchi, pulmonary and bronchial vessels, lym phatics, and nerves. Lobes and fissures.
The right lung has three lobes: superior, middle and inferior.
The left lung has upper and lower lobes.
Bronchopulmonary segments of the lung are supplied by the segmental (tertiary) bronchus, artery, and vein. There are 10 on the right and 8 on the left.
Arterial supply: Right and left pulmonary arteries arise from the pulmonary trunk. The pulmonary arteries deliver deoxygenated blood to the lungs from the right side of the heart.
Bronchial arteries supply the bronchi and nonrespiratory por tions of the lung. They are usually branches of the thoracic aorta.
Venous drainage. There are four pulmonary veins: superior right and left and inferior right and left. Pulmonary veins carry oxygenated blood to the left atrium of the heart.
The bronchial veins drain to the azygos system.
Bronchomediastinal lymph trunks drain to the right lymphatic duct and the thoracic duct.
Innervation of Lungs: Anterior and posterior pulmonary plexuses are formed by vagal (parasympathetic) and sympathetic fibers. Parasympathetic stimulation has a bronchoconstrictive effect. Sympathetic stimulation has a bronchodilator effect.
New words
lungs – ëåãêèå
intrapulmonary bronchi – âíóòðèëåãî÷íûå áðîíõè
the primary bronchi – ïåðâè÷íûå áðîíõè
lobar bronchi – äîëåâûå áðîíõè
submucosa – ïîäñëèçèñòàÿ îáîëî÷êà
28. Respiratory system
The respiratory system is structurally and functionally adapt ed for the efficient transfer of gases between the ambient air and the bloodstream as well as between the bloodstream and the tissues. The major functional components of the res piratory system are: the airways, alveoli, and bloodvessels of the lungs; the tissues of the chest wall and diaphragm; the systemic blood vessels; red blood cells and plasma; and respi ratory control neurons in the brainstem and their sensory and motor connections. LUNG FUNCTION: provision of 02 for tissue metabolism occurs via four mechanisms. Ventilation – the transport of air from the environment to the gas exchange surface in the alveoli. 02 diffusion from the alveolar air space across the alveolar-capillary membranes to the blood.
Transport of 02 by the blood to the tissues: 02 diffusion from the blood to the tissues.
Removal of C02 produced by tissue metabolism occurs via four mechanisms. C02 diffusion from the tissues to the blood.
Transport by the blood to the pulmonary capillary-alveolar membrane.
C02 diffusion across the capillary-alveolar membrane to the air spaces of the alveoli. Ventilation – the transport of alveolar gas to the air. Functional components: Conducting airways (conducting zone; anatomical dead space).
These airways are concerned only with the transport of gas, not with gas exchange with the blood.
They are thick-walled, branching, cylindrical structures with ciliated epithelial cells, goblet cells, smooth muscle cells. Clara cells, mucous glands, and (sometimes) cartilage.
Alveoli and alveolar septa (respiratory zone; lung parenchyma).
These are the sites of gas exchange.
Cell types include: Type I and II epithelial cells, alveolar macrophages.
The blood-gas barrier (pulmonary capillary-alveolar membrane) is ideal for gas exchange because it is very thin (‹ 0,5 mm) and has a very large surface area (50 – 100 m
2). It consists of alveolar epithelium, basement membrane interstitium, and capillary endothelium.
New words
respiratory – äûõàòåëüíûé
air – âîçäóõ
bloodstream – êðîâîòîê
airways – âîçäóøíûå ïóòè
alveoli – àëüâåîëû
blood vessels – êðîâåíîñíûå ñîñóäû
lungs – ëåãêèå
chest – ãðóäü
diaphragm – äèàôðàãìà
the systemic blood vessels – ñèñòåìíûå êðîâåíîñíûå ñîñóäû
red blood cells – êðàñíûå êðîâÿíûå êëåòêè
plasma – ïëàçìà
respi ratory control neurons – äûõàòåëüíûå íåéðîíû êîíòðîëÿ
brainstem – ñòâîë ìîçãà
sensory – ñåíñîðíûé
motor connections – ìîòîðíûå ñâÿçè
ventilation – âåíòèëÿöèÿ
transport – òðàíñïîðòèðîâêà
environment exchange – îêðóæàþùàÿ ñðåäà
surface – ïîâåðõíîñòü
29. Lung volumes and capacities
Lung volumes – there are four lung volumes, which when added together, equal the maximal volume of the lungs. Tidal volume is the volume of one inspired or expected normal breath (average human = 0,5 L per breath). Inspiratory reserve volume is the volume of air that can be inspired in excess of the tidal volume. Expiratory reserve volume is the extra an that can be expired after a normal tidal expiration.
Residual volume is the volume of gas that re lungs after maximal expiration (average human = 1,2 L).
Total lung capacity is the volume of gas that can be con tained within the maximally inflated lungs (average human = 6 L).
Vital capacity is the maximal volume that can be expelled after maximal inspiration (average human = 4,8 L).
Functional residual capacity is the volume remaining in the lungs at the end of a normal tidal expiration (average luman = 2,2 L).
Inspiratory capacity is the volume that can be taken into the lungs after maximal inspiration following expiration of a normal breath. Helium dilution techniques are used to determine residual volume, FRC and TLC. A forced vital capacity is obtained when a subject inspires maximally and then exhales as forcefully and as completely as possible. The forced expiratory volume (FEV1) is the volume of air exhaled in the first second. Typically, the FEV1 is approximate 80 % of the FVC.
GAS LAWS AS APPLIED TO RESPIRATORY PHYSIOLOGY: Dalton's Law: In a gas mixture, the pressure exerted by each gas is independent of the pressure exerted by the other gases.
A consequence of this is as follows: partial pressure = total pressure x fractional concentration. This equation can be used to determine the partial pressure of oxygen in the atmosphere. Assuming that the total pressure (or barometric pressure, PB) is atmospheric pressure at sea level (760 mmHg) and the fractional concentration of O
2is 21 %, or 0,21: P02 = 760 mmHg × 0,21 = 160 mmHg. As air moves into the airways, the partial pressures of the va-ri ous gases in atmospheric air are reduced because of the addi tion of water vapor (47 mmHg). Henry's Law states that the concentration of a gas dissolved in liquid is proportional to its partial pressure and its solubility coef fi-cient (Ks). Thus, for gas X, [X] = Ks × Px
Fick's Law states that the volume of gas that diffuses across a barrier per unit time is given by:
Vgas = Y x D x (P1 – P2)
where A and T are the area and thickness of the barrier, P1 and P2 are the partial pressures of the gas on either side of the barrier and D is the diffusion constant of the gas. D is directly proportional to the solubility of the gas and inversely proportional to the square root of its molecular weight.
New words
lung – ëåãêîå
tidal – âäûõàåìûé è âûäûõàåìûé
inspired – âäîõíîâëåííûé
breath – äûõàíèå
human – ÷åëîâåê
residual – oñòàòî÷íûé
helium – ãåëèé
dilution – ðàñòâîðåíèå
techniques – ìåòîäû
the conducting – ïðîâåäåíèå
30. Ventilation
Total ventilation (VT, minute ventilation) is the total gas flow into the lungs per minute. It is equal to the tidal volume (VT) x the respiratory rate (n). Total ventilation is the sum of dead space ventilation and alveolar ventilation.
Anatomic dead space is equivalent to the volume of the conducting airways (150 mL in normal individuals), i. e., the trachea and bronchi up to and including the terminal bronchioles. Gas exchange does not occur here. Physiologic dead space is the volume of the respiratory tract that does not participate in gas exchange. It includes the anatomic dead space and partially functional or nonfunctional alveoli (e. g., because of a pulmonan embolus preventing blood supply to a region of alveoli). In normal individuals, anatomic and physiologic dead space are approximately equal. Physiologic dead space can greatly exceed anatomic dead space in individuals with lung disease.
Dead space ventilation is the gas flow into dead space per minute. Alveolar ventilation is the gas flow entering functional alveoli per minute.
Alveolar ventilation: It is the single most important parameter of lung function. It cannot be measured directly. It must be adequate for removal of the CO
2produced by tissue metabolism whereas the partial pressure of inspired O
2is 150 mmHg, the partial pressure of O
2in the alveoli is typically 100 mmHg because of the displacement of O
2with CO
2. PAo2 cannot be measured directly.
New words
total – îáùåå êîëè÷åñòâî
ventilation – âåíòèëÿöèÿ
flow – ïîòîê
per minute – â ìèíóòó
equal – ðàâíûé
airways – âîçäóøíûå ïóòè
exchange – îáìåí
tract – òðàêòàò
to be measured – áûòü èçìåðåííûì
directly – íåïîñðåäñòâåííî
displacement – ñìåùåíèå
31. Air flow
Air moves from areas of higher pressure to areas of lower pres sure just as fluids do. A pressure gradient needs to be established to move air.
Alveolar pressure becomes less than atmospheric pressure when the muscles of inspiration enlarge the chest cavity, thus lowering the intrathoracic pressure. Intrapleural pressure decreases, causing expansion of the alveoli and reduction of intra-alveolar pressure. The pressure gradient between the atmosphere and the alveoli drives air into the airways. The opposite occurs with expiration.
Air travels in the conducting airways via bulk flow (mL/min). Bulk flow may be turbulent or laminar, depending on its velocity. Velocity represents the speed of movement of a single particle in the bulk flow. At high velocities, the flow may be turbulent. At lower velocities transitional flow is likely to occur. At still lower velocities, flow may be laminar (streamlined). Reynold's number predicts the air flow. The higher the number, the more likely the air will be turbulent. The velocity of particle movement slows as air moves deeper into the lungs because of the enormous increase in cross-sectional area due to branching. Diffusion is the primary mechanism by which gas moves between terminal bronchioles and alveoli (the respiratory zone).
Airway resistance: The pressure difference necessary to produce gas flow is directly related to the resistance caused by friction at the airway walls. Medium-sized airways (› 2 mm diameter) are the major site of airway resistance. Small airways have a high individual resistance. However, their total resistance is much less because resistances in parallel add as reciprocals.
Factors affecting airway resistance: Bronchocon-striction (increased resistance) can be caused by parasympathetic stimulation, histamine (immediate hyper-sensitivity reaction), slow-reacting substance of anaphylaxis (SRS-A = leukotrienes C4, D4, E4; mediator of asthma), and irritants. Bronchodilation (decreased resistance) can be caused by sympathetic stimulation (via beta-2 receptors). Lung volume also affects airway resistance. High lung volumes lower airway resistance because the surrounding lung parenchyma pulls airways open by radial traction. 