RESPIRATORY SYSTEM

June 3, 2024
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RESPIRATORY SYSTEM

1.     Organs and functions

2.     Embryogenesis

3.     Conducting portion:

     а) wall structure;

     b) cells types;

     c) nasal cavity;

     d) larynx;

     e) trachea;

     f) bronchial tree

4. Respiratory portion

5. Air-hematic barrier

6. Pleura

Respiration is a term used to describe two different but interrelated processes: cellular respiration and mechanical respiration. Cellular respiration is the process in which cell derive energy by degradation of organic molecules. Mechanical respiration is the process by which oxygen required for cellular respiration is absorbed from the atmosphere into the blood vascular system and the process by which carbon dioxide is excreted into the atmosphere. Mechanical respiration occurs within the respiratory system.

The respiratory system consists basically of the lungs and airways (i.e., pharynx, larynx, trachea, bronchi). Specialized for gaseous exchange between blood and air, including the uptake of oxygen and release of carbon dioxide, it is functionally divisible into 2 major parts: the conducting and respiratory portions.

The respiratory system is divided anatomically into two pails, the upper and lower respiratory tracts, which are separated by the pharynx. The pharynx is the best considered functionally and histologically as part of the gastrointestinal tract despite its important role as an airway.

Ventilating mechanism. This mechanism, which creates pressure differences that move air into (inspiration) and out of (expiration) the lungs, includes the diaphragm, rib cage, intercostal muscles, abdominal muscles, and the elastic connective tissue in the lungs. Inspiration (inhalation) is active, involving muscle contraction To inhale, the intercostal muscles lift the ribs while the diaphragm and abdominal muscles lower the floor of the thoracic cavity. This enlarges the cavity, creating a vacuum that draws air through the airways. The incoming air expands the airways, inflates the lungs, and stretches the elastic connective tissue. Expiration (exhalation) is more passive: Relaxing the muscles allows the elastic fibers to retract, contracting the lungs and forcing air out.

Conducting portion. The walls of this system of tubes are specialized to carry air to and from the site of gas exchange without collapsing under the pressures created by the ventilating mechanism. This portion also conditions the air, warming, moistening, and cleaning it to enhance gas exchange. It includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.

Respiratory portion. This portion is distinguished by alveoli, small, saccular structures whose thin walls enable the gas exchange between air and blood. Alveoli occur in clusters at the end of the bronchial tree. These clusters extend (like rooms from a hallway) from the walls of respiratory bronchioles, alveolar ducts, and atria and alveolar sacs.

Wall Structure: Like the digestive tract, the tubelike respiratory tract has layered walls whose lining epithelium derives from endoderm. The wall layers include an epithelium, a lamina propria that contains mucous glands as well as cartilages that prevent the tract from collapsing under pressure, smooth muscle that regulates the luminal diameter, and an adventitia that contains collagen and elastic fibers. Each of these layers undergoes gradual changes as the wall’s overall thickness decreases from nasal cavity to alveoli.

Respiratory epithelium. The epithelium lining most of the tract is ciliated pseudostratified columnar with goblet cells; it is generally referred to as respiratory epithelium. As the respiratory tract undergoes successive branching and its lummal diameter decreases, the epithelium gradually drops in height, and loses first goblet cells and then cilia as it approaches the alveoli.

Epithelial cell types

(1)        Ciliated columnar cells are predominate in the tract, has about 300 motile cilia on its apical surface; there are associated basal bodies in the apical cytoplasm. Their main function is protection.

(2)        Mucous goblet cells are the second most numerous type. They secrete the mucus that covers the epithelium and traps and removes bacteria and other particles from inspired air. Cilia projecting from columnar cells sweep the contaminated mucus toward the mouth for disposal.

(3). Endocrine cells produce hormones, which have influence on conducting portion, they regulate muscle contraction.

(4) Basal cells small round cells lie on the basal lamina but do not reach the lumen. They appear to be stem cells that can replace the other cell types.

(5)   Small   granule  cells  resemble  basal  cells,  but  they  contain   many   small cytoplasmic granules and exhibit DNES activity (Clara cells). Their main function is to destroy surfactant.

(6) Brush cells also columnar, these cells lack cilia; they often have abundant apical microvilli. Two types are present: One resembles an immature cell and apparently serves to replace dead ciliated or goblet cells. The other has nerve endings on its basal surface and appears to be a sensory receptor.

Small granule cells and brush cells appear only in lower pans of conducting portion and in bronchioles.

Lamina propria consists of loose connective tissue and contains mucous glands in the upper tract (from the nasal cavity to the bronchi). Its elastic fiber content increases toward the alveoli. Skeletal connective tissue support begins as cartilage and bone in the nasal cavity and becomes cartilage only in the larynx. It gradually decreases, disappearing at the level of the bronchioles.

Smooth muscle begins in the trachea, where it joins the open ends of the C-shaped tracheal cartilages. In the bronchi, many layers of smooth muscle cells encircle the walls in a spiral. From this point, the thickness of the muscle layer gradually decreases until it disappears at the level of the alveolar ducts.

NASAL CAVITY

 is divided by the nasal septum into 2 bilaterally symmetric cavities that open to the exterior through the nares (nostrils). Each cavity consists of 2 chambers-a vestibules and a nasal fosseswhich differ in position, size, and wall structure.

Vestibule: The smaller, wider, anterior chamber of each side, it lies just behind the nares. The medial septum and lateral walls are supported by cartilage, and the epithelial lining is a continuation of the epidermis that covers the nose. Just inside, the epithelium is keratinized, containing many sebaceous and sweat glands as well as thick short hairs called vibrissae, which filter large particles from inspired air. Deeper in the vestibule, the epithelium changes from keratinized to nonkeratniized stratified squamous and then to respiratory epithelium just before entering the nasal fossa.

Nasal Fossa: This is the larger, narrower, and more posterior chamber on each side. Here the septum and lateral walls are lined by respiratory epithelium. They are supported by the bone of the skull and contain mucous glands and venous sinuses in the lamina propria.

Three curved bony shelves, termed conches, or turbinate bones, project into each fossa from its lateral wall. These help warm and moisten the air by increasing the mucosal surface area and forming a system of baffles that cause turbulence and slow the airflow through the cavity. Alternating from side to side every 20-30 minutes, venous plexuses (swell bodies) in the conchal mucosa engorge with blood, causing it to swell. This action restricts airflow, directing it through the other side of the nose, and thus helps prevent overdrying of the mucosal surface. Arterial vessels in the fossa walls create a countercur– rent system that warms air by directing blood flow from posterior to anterior (opposite to the flow of inspired air) in a series of small arches. Specialized olfactory epithelium is present in the roof of each fossa.

PARANASAL SINUSES. These are dilated cavities in the frontal, maxillary, ethmoidal, and sphenoidal bones around the nose and eyes. Their thin respiratory epithelial lining has few goblet cells and is bound tightly to the periosteum of the surrounding bones by a lamina propria that contains a few small mucous glands. Mucus produced here drains into the nasal fossa through small openings that are protected by the conches.

NASOPHARYNX

The upper part of the pharynx, the nasopharynx is a broad single cavity overlying the soft palate. It is continuous anteriorly with the nasal fosses and inferior!)’ with the oral part of the pharynx (oropharynx). The walls, lined by respiratory epithelium, are supported by bone and skeletal muscle.

LARYNX

 A bilaterally symmetric tube, the larynx iies in the neck between the base of the oropharynx and the trachea. During swallowing, its opening is protected by the epiglottis. Its walls, supported by several laryngeal cartilages in the lamina propria, contain skeletal muscle and house the vocal apparatus.

Epiglottis: This flap of tissue extends toward the oropharynx from the anterior border of the larynx. It is covered on its superior surface by nonkeratinized stratified squamous epithelium and on its inferior surface by respiratory epithelium. The lamina propria contains a few mucous glands and a small plate of elastic cartilage. During swallowing, the backward motion of the tongue forces the epiglottis over the laryngeal opening, directing food away from the airway and into the esophagus. After swallowing, the elastic cartilage helps to reopen and maintain the airway.

Laryngeal Cartilages: Several cartilages frame the laryngeal lumen and serve as attachments for the skeletal muscles that control the vocal apparatus. The larger thyroid, cricoid, and most of the paired arytenoid cartilages are hyaline, while the smaller onesthe paired cuneiform and corniculate, the epiglottis, and the tips of the arytenoidsare elastic.

Vocal Apparatus: The broad part of the larynx, below the epiglottis and surrounded by the thyroid cartilage, contains 2 bilaterally symmetric pairs of mucosal folds.

1.         False vocal cords (vestibular folds) These are the upper pair of folds in the larynx. They are covered by respiratory epithelium and contain serous glands whose ducts open mainly into the cleft that separates them from the lower pair of folds.

2.    True vocal cords This lower pair of folds is covered by stratified squamous epithelium; each contains 2 major structures: a large bundle of elastic fibers that run front to back, called the vocal ligament; and a bundle of skeletal muscle that runs parallel to the ligament, called the vocal muscle. Air forced through the larynx by the ventilating mechanism causes the true cords to vibrate. The vocal muscle regulates the tension of the cords, while other muscles control the shape and position of the laryngeal lumen. In this way, the laryngeal muscles control the pitch (frequency) and other aspects of the sounds produced by the vibrating cords. The cords also assist the epiglottis in preventing foreign objects from reaching the lungs; they close to build up pressure what coughing is required to dislodge materials blocking the airway.

TRACHEA

This 10-cm tube extends from the larynx to the primary bronchi. It is lined by respiratory epithelium, and its lamina propria contains mixed seromucous glands that open onto its lumen. Its most characteristic feature is the presence of 16-20 C-shaped hyaline cartilage rings whose open ends are directed posteriorly. The opening is bridged by a fibroelastic ligament that prevents overdistention as well as by smooth muscle bundles (tracheal muscle) that constrict the lumen and increase the force of airflow during coughing and forced expiration.

Trachea,is  identified by the presence of hyaline cartilage in its wall. To the left of the cartilage is the mucosa, including epithelium and its underlying connective tissue. The cartilage ring is immediately covered, on both surfaces, with bright pink perichondrium.

The wall of trachea consists of 4 tunics: mucosa, submucosa, fibro-cartilages and adventitia. Mucosa has 3 layers – ciliated epithelium, lamina propria and poorly defined muscularis mucosa. Submucosa contains mucous glands. Hyaline cartilage in the trachea is C-shaped. Outermost tunic – adventitia consists of loose connective tissue with numerous blood vessels and nerves.

LUNGS

Their structure is similar to exocrine gland: they contain secretory portion and ducts. Secretory portion are alveoli with air and ducts are bronchial tree (system of branching bronchi).

BRONCHIAL TREE. This begins where the trachea branches to form 2 primary bronchi, one of which penetrates the hilum of each lung. The hilum is also the site at which arteries and nerves enter and veins and lymphatic vessels exit the organ. These structures, together with the dense connective tissue that binds them, form the pulmonary root. The bronchial tree undergoes extensive branching within the lungs. The changes in wall structure that accompany the progress of the bronchial tree toward the alveoli occur gradually and not at sharp boundaries.

All bronchi have similar structure: mucous, submucous, cartilages and adventitia. Mucous is covered by respiratory epithelium, under which is lining lamina propria and in bronchi appear smooth muscles. Respiratory epithelium undergoes progressive transition from a tall, pseudostratified columnar, ciliated form in primary bronchi to a simple, cuboidal, non-ciliated form in the smallest airways.

Goblet cells are numerous in primary bronchi, but decrease in number and are absent in the terminal bronchioles. Lamina propria has a similar structure as in the upper part of conducting portion. A layer of smooth muscles lies deep in mucosa and becomes increasingly prominent as the airway diameter decreases. It reaches its greatest prominence in the terminal bronchioles. Smooth muscle tone controls the diameter of the conducting passages and thus controls resistance to airflow within the respiratory tree. The autonomic nervous system, adrenal medullary hormones and local factors modulate smooth muscle tone.

Submucosal connective tissue contains serous and mucous glands which becomes progressively less numerous in the narrowed airways and are not present beyond the tertiary bronchi. Cartilages provide a supporting skeleton for the bronchi and prevent the collapse of these airways during respiration. The cartilage framework in primary bronchi is arranged into flattened, interconnected plates of hyaline cartilage. The cartilage framework in secondary bronchi is reduced to a few irregular plates of hyaline cartilage. Tertiary bronchi have some elastic islands. In terminal bronchioles and bronchioles cartilages are absences.

Primary (Principal) Bronchi: There are 2 primary bronchi, one entering each lung. Their histologic appearance is quite similar to that of the trachea, but their cartilage rings and spiral bands of smooth muscle completely encircle their respective lumens. The path of the right primary bronchus is more vertical than that of the left. As a result, foreign objects that reach the bronchi are more likely to lodge in the right side of the bronchial tree.

Secondary Bronchi: These lobar bronchi are branches that arise directly from the rjrimarv bronchi; each supplies one pulmonary lobe Since trie right lung has 3 lobes and the left only 2, the right primary bronchus gives rise to 3 secondary bronchi and the left primary bronchus gives rise to 2. Their histologic structure is similar to that of the primary bronchi except that their supporting cartilages (and those of the smaller bronchi) are arranged as irregular plates, of cartilage, rather than as rings.

Tertiary Bronchi: Arising directly from the secondary bronchi, which they resemble histologically, each of these segmental bronchi supplies one bronchopulmonary segment (pulmonary lobule). Although each lung has 10 such segments, the different number of secondary bronchi causes the tertiary branching pattern to differ between the right and left lungs. Except for a decrease in overall diameter, the histologic appearance of tertiary bronchi is identical to that of secondary bronchi. Tertiary bronchi may branch several times to form successively sraaller branches that are considered bronchi as long as their walls contain cartilage and glands.

Bronchioles: These are branches of the smallest bronchi. The largest bronchioles differ from the smallest bronchi only by the absence of cartilage and glands in their walls. Large bronchioles are lined by typical respiratory epithelium; as they branch further, the epithelial height and complexity decrease to simple ciliated columnar or cuboidal. Each bronchiole gives rise to 5-7 terminal bronchioles.

Terminal Bronchioles: The smallest components of the conducting portion of the respiratory system, these are lined by ciliated cuboidal or columnar epithelium and have no goblet cells. (The elimination of goblet cells before the cilia in the lower parts of the bronchial tree is important in preventing individuals from drowning in their own mucus.) The lining here also includes dome-shaped cilia-free Clara cells, whose cytoplasm contains glycogen granules, lateral and apical Golgi complexes, elongated mitochondria, and a few secretory granules. Although the function of these cells is unclear, they may be serous secretory cells substituting for mucous goblet cells at this level. Each terminal bronchiole branches to form 2 or more respiratory bronchioles, which form a respiratory portion. Morpho-functional unit of respiratory portion is acinus, which consists of respiratory bronchioles, alveolar duets and alveolar sacs.

A branching portion of the respiratory tree. No cartilage is visible here, so we’ll say it’s a bronchiole. Lymphatic nodules may be found randomly along the respiratory tree. Note also the cross-cuts of pulmonary artery branches accompanying the tree. Cuts of alveoli of the lung fill the surrounding field.

Respiratory Bronchioles: These are the first part of the respiratory portion, with a cuboidal epithelial without cilia, lining that resembles that of the terminal bronchioles but which is interrupted by thin-walled saccular evaginations called alveoli. The number of alveoli increases as the respiratory bronchioles proceed distally. As the alveoli increase iumber, the cilia decrease until they disappear. Lamina propria and smooth muscles are bad developed.

Alveolar Ducts: These are simply the distal extensions of the respiratory bronchioles where the alveoli are so dense that the wall consists almost entirely of these sacs, and the lining has been reduced to small knobs of smooth muscle covered by cilia-free simple cuboidal cells. The knobs appear to project inio the elongated lumen of the duct, each resting atop a thin septum that separates adjacent alveoli. The alveolar clue: can thus be likened to a long hallway with so many doorways leading to small rooms (alveoli), that the hallway (the alveolar duct) appears almost to lack walls.

Atria and Alveolar Sacs: Atria are the distal terminations of alveolar ducts. The arrangement is comparable to a long hallway (alveolar duct) leading to a rounded foyer (atrium). The foyer has small doorways leading to some small rooms (alveoli), but also has 2 or more larger doorways leading into short, dead-end hallways (alveolar sacs). The short hallways are also lined by small rooms (alveoli). The difference between atria and alveolar sacs is that the atria open into alveolar ducts, alveoli, and alveolar sacs, while the alveolar sacs open only into alveoli and atria. Although these distinctions can be made fairly easily in sections cut longitudinally through the entire system of passageways beginning with the alveolar duct, such perfect cuts are relatively rare in standard slides of lung tissue. More often, the various components are cut in oblique or cross section, and only the openings to the alveoli are seen, making it hard to distinguish between the sacs and the atria. In such cases, the only useful clue is the size of the knobs that project into the passageways. Those projecting into alveolar sacs lack smooth muscle and are thus smaller than those projecting into either the atria or the alveolar ducts.

ALVEOLI. Occurring only in the respiratory portion (which their presence distinguishes from the conducting portion), these small sacs open into a respiratory bronchiole, an alveolar duct, an atrium, or an alveolar sac. They are separated from one another by thin walls termed interalveolar (or alveola;-) septa. Alveoli wall is covered by simple squamous epithelium, which contains three cell types.

Type I cells also called type I alveolar cells, type 1 pneumocytes, and squamous alveolar cells, these are squamous epithelial cells that make up 97% of the alveolar surfaces. They are specialized to serve as very thin (often only 25 nra in width) gas-permeable components of the blood-air barrier. Their organelles (eg, Golgi complex, endoplasmic reticulum, mitochondria) cluster around the nucleus. Much of the cytoplasm is thus unobstructed by organelles, except for the abundant small pinocytotic vesicles that are involved in the turnover of pulmonary surfactant and the removal of small particles from the alveolar surfaces. They attach to neighboring epithelial cells by desmosomes and occluding junctions. The latter reduce pleural effusionleakage of tissue fluid into the alveolar lumen. Type 1 cells can be distinguished from the nearby capillary endothelial cells by their position bordering the alveolar lumen and by their slightly more rounded nuclei. Main function is gas exchange.

Type II cells are also called type II alveolar cells, type II pneumocytes, great alveolar cells cells, cover the remaining 3% of the alveolar surface. They are interspersed among the type I cells, to which they attach by desmosomes and occluding junctions. Type II cells are roughly cuboidal with round nuclei; they occur most often in small groups at the angles where alveolar septal walls converge. In the electron microscope they contain many mitochondria and a well-developed Golgi complex but they are mainly characterized by the presence of large, membrane-limited lamellar (multilamellar) bodies. These structures, which exhibit many closely apposed concentric or parallel membranes (lamellae), contain phospho­lipids, glycosaminoglycans, and proteins. Type II cells are secretory cells. Their secretory product, pulmonary surfactant, is assembled and stored in the lamellar bodies, which also carry it to the apical cytoplasm. There, the bodies fuse with the apical plasma membrane and release surfactant onto the alveolar surface.

The large, vacuolated, rather ragged looking vesicles in the cytoplasm of the Type II cell are lamellar bodies containing the precursor of alveolar surfactant.

Alveolar macrophages known also as dust cells, these large monocytederived representatives of the mononuclear phagocyte system are found both on the surface of alveolar septa and in the interstitium. Macrophages are important in removing any debris that escapes the mucus and cilia in the conducting portion of the system. They also phagocytose blood cells that enter the alveoli as a result of heart failure. These alveolar macrophages, which stain positively for iron pigment (hemosiderin), are thus designated heart failure cells. They came from the blood and phagocyse toxins and nicotine.

Pulmonary Surfactant: Continuously synthesized and secreted by type II alveolar cells onto the alveolar surfaces, pulmonary surfactant is removed from these surfaces by alveolar macrophages and by type I and II alveolar cells Its composition and continuous turnover allow it to serve 2 major functions. Not only does it reduce surface tension in the alveoli, it is also thought to have some bactericidal effects, cleaning the alveolar surface and preventing bacterial invasion of the many capillaries in the septa. The surfactant forms a thin 2-layer film over the entire alveolar surface. The film consists of an aqueous basal layer (hypophase) composed mainly of protein, which is covered by a monomolecular film of phospholipid (mainly dipalmitoyl lecithin) whose fatty acid tails extend into the lumen. By reducing surface tension, the surfactant helps prevent collapse of the alveoli curing expiration. It thus eases breathing by decreasing the force required to reopen the alveoli during the next inspiration. Because surfactant secretion begins in the last weeks of fetal development, premature infants often suffer a condition called hyaline membrane disease, evidenced by respiratory distress (labored breathing) caused by the lack of surfactant. Luckily, surfactant secretion can be induced by administering glucocorticoids, significantly improving the infant’s condition and chances for survival.

Alveolar Lining Regeneration: Daily turnover of about 1% of the type II cells, whose mitotic progeny form both type I and type II cells, allows for normal alveolar lining renewal. When these lining cells are destroyed by inhaling toxic gases, replacements for both types of cells are similarly derived from the surviving type II cells.

Interalveolar Septa. The structural features of these septa, which are specialized for gas exchange, are critically important to respiratory function. The septa consist of 2 simple squamous epithelial layers with the interstitium sandwiched between them. The interstitium consists of continuous (nonfenestrated) capillaries embedded in an elastic connective tissue that includes elastic and collagen fibers, ground substance, fibroblasts, mast cells, macrophages, leukocytes, and contractile interstitial cells that contract in response to epinephrine and histamine. This elastic tissue is an important component of the ventilating mechanism. Gas exchange occurs between the air in the alveolar lumen and the blood in the interstitial capillaries.

Alveolar pores. One or more pores may interrupt each septum. These connect adjacent alveoli and may help to equalize pressure and allow collateral air circulation thus maximizing the use of available alveoli when some small airways are blocked.

Blood-air barrier

 This term refers to the structures that oxygen and carbon dioxide must cross to be exchanged. It includes the following layers: a. The layer of pulmonary surfactant on the alveolar surface: b The cytoplasm of the squamous epithelial (type I alveolar) cells; c. The fused basal lamina sandwiched between the type 1 alveolar and capillary endothelial ceils; and d The cytoplasm of the squamous endothelial cells lining the interstitial capillaries.

THE PLEURA

This serous membrane has 2 layers, one covering die lungs (visceral pleura) and the other covering the internal wall of the thoracic cavity (parietal pleura). Like the peritoneum and the pericardium, the pleura consists of a thin squamous mesothelium attached to the organ or wall by a thin layer of connective tissue that contains collagen and elastic fibers. Bordered by the mesothelial cells, the narrow pleural cavity lies between the parietal and visceral pleurae. The cavity normally contains only a thin film of lubricating fluid that (together with the smooth mesothelial surfaces) reduces the jriction between the lung surfaces and thoracic walls that would otherwise accompany the respiratory movements. Certain diseases and wounds allow excess air or fluid to enter the pleural cavity, increasing its size and restricting respiratory movement. White small amounts с fair and fluids can be absorbed, larger amounts may precipitate lung collapse and require medical intervention.

 

References:

A-Basic:

1.                 Practical classes materials

http://intranet.tdmu.edu.ua/data/kafedra/internal/histolog/classes_stud/English/medical/III%20term/20%20Respiratory%20system.%20Urinary%20system.htm

2.                 Lecture presentations

http://intranet.tdmu.edu.ua/ukr/kafedra/index.php?kafid=hist&lengid=eng&fakultid=m&kurs=2&discid=Histology, cytology and embryology

3.                 Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 159-176

4.                 Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 234-251, 302-328

5.                 Inderbir Singh Textbook of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002. – P. 207-216, 258-272

6.                 Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 664-740

 

B – Additional:

1.                                         Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. Lippincott Williams and Wilkins, 2008. – P. 333-381

2.                                         Junqueira L. Basic Histology / L. Junqueira, J. Carneiro, R. Kelley. – [seventh edition]. – Norwalk, Connecticut : Appleton and Lange, 1992. – P. 358-377, 392-409

3.                                         Charts:

http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#inf3

4.                                         Disk:

http://intranet.tdmu.edu.ua/data/teacher/video/hist/  

5.                                         Volkov K. S. Ultrastructure of the main components of body / K. S. Volkov. – Ternopil : Ukrmedknyha, 1999. – P. 78-84, 88-94

http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf

http://en.wikipedia.org/wiki/Circulatory

http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html

http://www.udel.edu/biology/Wags/histopage/histopage.htm

 

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