Hematopoietic tissue. Erythropoiesis

Hematopoietic tissue is composed of reticular fibers and cells, blood vessels, and sinusoids (thin–walled blood channels). Myeloid, or blood cell–forming tissue, is found in the bone marrow and provides the stem cells that develop into erythrocytes, granulocytes, agranulocytes, and platelets. Red marrow is characterized by active hemato–poiesis; yellow bone marrow is inactive and contains mostly fat cells. In the human adult, hematopoiesis takes place in the mar row of the flat bones of the skull, ribs and sternum, the vertebral column, the pelvis, and the proximal ends of some long bones. Erythropoiesis is the process of RBC formation. Bone marrow stem cells (colony–forming units, CFUs) differentiate into proerythroblasts under the influence of the glycoprotein erythropoietin, which is produced by the kidney.

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 ap proximately 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 – сетчатый

sinusoids – синусоиды

granulocytes – гранулоциты

agranulocytes – агранулоциты

active – активный

yellow – желтый

glycoprotein – гликопротеин

erythropoietin – эритропоэтин

amount – количество

hemoglobin – гемоглобин

degenerating – дегенерирующие

condensed – сжатый

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 – способный

spherical – сферический

indented – зазубренный

chromatin – хроматин

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 sev eral smooth muscle cell layers, and the adventitia is poorly devel oped. An external elastic membrane is absent.

New words

endothelium – эндотелий

media – средняя

arteries – артерии

to be classified – классифицированный

according – соответственно

their – их

size – размер

appearance – вид

tunica – оболочка

major – главный

elastic – эластичный

conducting – проведение

arteries – артерии

to include – включать

aorta – аорта

branches – ветви

up to – до

layers – слои

smooth – гладкий

may – может

infima – внутренняя полость артерии

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 reticu–lum of collagen. Associated with these vessels at various points along their length are specialized cells called pe–ricytes. 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 glome–ruli 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, hemato–poietic 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 (macropha–ges) are present within, or just subjacent to, the en–dothelium.

New words

capillaries – капилляры

to thin–walled – окруженный тонкой стеной

narrow–diameter – узкий диаметр

low–pressure – низкое давление

that – тот

generally – главным образом

permit – разрешение easy – легкий

diffusion – распространение

cross–sectional – поперечный

to be composed – быть сложным

simple – простой

endothelium – эндотелий

lining – выравнивание

entire – весь

vas cular – сосудистый

underlying – лежащий в основе

basal – основной

lamina – тонкая пластинка

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 – вена

low–pressure – низкое давление

collagenous – коллагеновый

intima – интима

reflux – рефлюкс

inflammation – воспаление

longitudinal – продольный

flaps – створки

iliac – подвздошный

hepatic – печеночный

Heart

The heart is a muscular organ, composed primarily of cardiac muscle tissue, which contracts rhythmically to pump blood throughout the body. Structure of the heart wall: the walls of the heart are constructed in layers that are similar to those of the major blood vessels.

Endocardium is the innermost layer of the heart and is lined with endothelium. Veins, nerves, and components of the impulse conduc–ting system are present in the suben–docardial connective tissue layer.

Myocardium is composed of branching, anastomotic cardiac myocytes attached to one another by intercalated disks. Most of these cells are involved in the pumping function of the heart; others are specialized for the control of rhythmicity (impulse conducting system) or secretion (myocardial endocrine cells).

Epicardium is a serous membrane that forms the visceral lining of the pericardium. Its external mesothelium is supported by a loose connective tissue subepicardial layer.

Cardiac skeleton is composed mainly of dense connective tissue and consists of the annuli fibrosi, the trigonum fibrosum, and the septum membranaceum.

Cardiac valves are composed of dense fibrous tissue covered by endothelium. Unidirectional flow is maintained from the.

Right atrium to the right ventricle (tricuspid valve).

Right ventricle to the pulmonary artery (pulmonic semilunar valve). Left atrium to the left ventricle (mitral/bicuspid valve).

Left ventricle to the aorta (aortic semilunar valve).

Tricuspid and mitral valves are attached to papillary muscles by cords of fibrous connective tissue (chordae tendineae) and prevent reflux of blood into the atria during 26б 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 (Pur–kinje 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 – сердце

muscular – мышечный

cardiac – сердечный

to pump – качать

endocardium – эндокардиум

innermost – самый внутренний

conducting system – проведение системы

subendocardial – внутрисердечный

impulse – импульс

fibrosi – фиброзные кольца

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 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.

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 nonrespirato–ry 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 hasa broncho–constrictive effect. Sympathetic stimulation has a broncho–dilator effect.

New words

lungs – легкие

intrapulmonary bronchi – внутрилегочные бронхи

the primary bronchi – первичные бронхи

lobar bronchi – долевые бронхи

submucosa – подслизистая оболочка

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 blood vessels 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 O 2 for tissue metabolism occurs via four mechanisms. Ventilation – the transport of air from the environment to the gas exchange surface in the alveoli. O ₂ diffusion from the alveolar air space across the alveolar–capillary membranes to the blood.

Transport of O ₂ by the blood to the tissues: O ₂ diffusion from the blood to the tissues.

Removal of CO ₂ produced by tissue metabolism occurs via four mechanisms. CO ₂ diffusion from the tissues to the blood.

Transport by the blood to the pulmonary capillary–alveolar membrane.

CO ₂ 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 in–terstitium, and capillary endothelium.

New words

respiratory – дыхательный

air – воздух

bloodstream – кровоток

airways – воздушные пути

alveoli – альвеолы

blood vessels – кровеносные сосуды

lungs – легкие

chest – грудь

diaphragm – диафрагма

the systemic blood vessels – системные кровеносные сосуды

red blood cells – красные кровяные клетки

plasma – плазма

respi ratory control neurons – дыхательные нейроны контроля

brainstem – ствол мозга

sensory – сенсорный

motor connections – моторные связи

ventilation – вентиляция

transport – транспортировка

environment exchange – окружающая среда

surface – поверхность

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