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Showing posts with label Organ. Show all posts

Wednesday, May 5, 2021

Circulatory System: Blood Vessels

Circulatory System: Blood Vessels


Circulatory System: Blood Vessels
Time period: day 18 to birth
Vasculogenesis
Vasculogenesis is the formation of new blood vessels from cells that were not blood vessels before. As if by magic, blood cells and vessels appear in the early embryo. In fact, mesodermal cells are induced to differentiate into haemangioblasts, which further differentiate into both haematopoietic stem cells and angioblasts.
Haematopoietic stem cells will form all the blood cell types, and angioblasts will build the blood vessels. Separate sites of vasculogenesis may merge to form a network of blood vessels, or new vessels may grow from existing vessels by angiogenesis. When the liver forms it will be the primary source of new haematopoietic stem cells during development.

Angiogenesis
Angiogenesis is the development of new blood vessels from existing vessels. Endothelial cells detach and proliferate to form new capillaries. This process is under the influence of various chemical and mechanical factors. Although important in growth this also occurs in wound healing and tumour growth, and as such angiogenesis has become a target for anti‐cancer drugs.
Circulatory System: Blood Vessels, Angiogenesis, Primitive circulation, Aortic arches, Ductus arteriosus, Coronary arteries

Primitive circulation
Near the end of the third week blood islands form through vasculogenesis on either side of the cardiogenic field and the notochord (see Chapter 27). They merge, creating two lateral vessels called the dorsal aortae (Figure 29.1). These blood vessels receive blood from three pairs of veins, including the vitelline veins of the yolk sac (a site of blood vessel formation external to the embryo), the cardinal veins and the umbilical veins (Figure 29.1).
Circulation System: Changes At Birth

Circulation System: Changes At Birth


Circulation System: Changes At Birth
Time period: birth (38 weeks)
Foetal blood circulation
Dramatic and clinically significant changes occur to the circulatory and respiratory systems at birth. Here, we look at changes primarily of the circulatory system and how these changes prepare the baby for life outside the uterus.
If we were to follow the flow of oxygenated blood in the foetus from the placenta (Figure 31.1), we would start in the umbilical vein and track the blood moving towards the liver. Here, half the blood enters the liver itself and half is redirected by the ductus venosus directly into the inferior vena cava, bypassing the liver.
The blood remains well oxygenated and continues to the right atrium, from which it may pass into the right ventricle in the expected manner or directly into the left atrium via the foramen ovale (Figure 31.2). Blood within the left atrium passes to the left ventricle and then into the aorta.
Blood entering the right atrium from the superior vena cava and the coronary sinus is relatively poorly oxygenated. The small amount of blood that returns from the lungs to the left atrium is also poorly oxygenated. Mixing of this blood with the well‐oxygenated blood from the ductus venosus reduces the oxygen saturation somewhat.
Blood within the right ventricle will leave the heart within the pulmonary artery, but most of that blood will pass through the ductus arteriosus and into the descending aorta. Almost all of the well‐oxygenated blood that entered the right side of the heart has avoided entering the pulmonary circulation of the lungs, and has instead passed to the developing brain and other parts of the body (Figure 31.3).
Circulation System: Changes At Birth, Foetal blood circulation, Ductus venosus, Ductus arteriosus, Foramen ovale,

Ductus venosus
The umbilical arteries constrict after birth, preventing blood loss from the neonate. The umbilical cord is not cut and clipped immediately after birth, however, allowing blood to pass from the placenta back to the neonatal circulation through the umbilical vein.
Respiratory System

Respiratory System


Respiratory System
Time period: day 28 to childhood
Introduction
The development of the respiratory system is continuous from the fourth week, when the respiratory diverticulum appears, to term. The 24‐week potential viability of a foetus (approximately 50% chance of survival) is partly because at this stage the lungs have developed enough to oxygenate the blood. Limiters to oxygenation include the surface area available to gaseous exchange, the vascularisation of those tissues of gaseous exchange and the action of surfactant in reducing the surface tension of fluids within the lungs.
Development of the respiratory system includes not only the lungs, but also the conducting pathways, including the trachea, bronchi and bronchioles. Lung development can be described in five stages: embryonic, pseudoglandular, canalicular, saccular and alveolar.
Although not in use as gas exchange organs in utero, the lungs have a role in the production of some amniotic fluid.
Respiratory System, Lung bud, Respiratory tree, Alveoli,

Lung bud
The development of the respiratory system begins with the growth of an endodermal bud from the ventral wall of the developing gut tube in the fourth week (Figure 32.1).
Digestive System: Gastrointestinal Tract

Digestive System: Gastrointestinal Tract


Digestive System: Gastrointestinal Tract
Time period: days 21–50
Induction of the tube
The gut tube forms when the yolk sac is pulled into the embryo and pinched off (see Figure 20.2) as the flat germ layers of the early embryo fold laterally and cephalocaudally (head to tail). Consequently, it has an endodermal lining throughout with a minor exception towards the caudal end. Epithelium forms from the endoderm layer and other structures are derived from the mesoderm.
Initially, the tube is closed at both ends, although the middle remains in contact with the yolk sac through the vitelline duct (or stalk) even as the yolk sac shrinks (Figure 33.1).
The cranial end will become the mouth and is sealed by the buccopharyngeal membrane, which will break in the fourth week, opening the gut tube to the amniotic cavity. The caudal end will become the anus and is sealed by the cloacal membrane, which will break during the seventh week.
Buds develop along the length of the tube that will form a variety of gastrointestinal and respiratory structures (see Chapter 34).
Digestive System: Gastrointestinal Tract, Mesenteries, Story of the hindgut and the cloaca, Twists of the midgut

Divisions of the gut tube
The gut is divided into foregut, midgut and hindgut sections by the region of the gut tube that remains linked to the yolk sac and by the anterior branches from the aorta that supply blood to each part (Figure 33.2).
INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE

INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE


INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE
The tracheobronchial tree and lungs are innervated by the autonomic nervous system. Three types of pathways are involved: autonomic afferent, parasympathetic efferent, and sympathetic efferent. Each type of fiber is discussed here; the neurochemical control of respiration is covered later in the section on physiology (see Plates 2-25 and 2-26).
 
INNERVATION OF TRACHEOBRONCHIAL TREE: SCHEMA
INNERVATION OF TRACHEOBRONCHIAL TREE: SCHEMA
Autonomic Afferent Fibers
Afferent fibers from stretch receptors in the alveoli and from irritant receptors in the airways travel via the pulmonary plexus (located around the tracheal bifurcation and hila of the lungs) to the vagus nerve. Similarly, fibers from irritant receptors in the trachea and from cough receptors in the larynx reach the central nervous system via the vagus nerve. Chemoreceptors in the carotid and aortic bodies and pressor receptors in the carotid sinus and aortic arch also give rise to afferent autonomic fibers. Whereas the fibers from the carotid sinus and carotid body travel via the glossopharyngeal nerve, those from the aortic body and aortic arch travel via the vagus nerve. Other receptors in the nose and nasal sinuses give rise to afferent fibers that form parts of the trigeminal and glossopharyngeal nerves. In addition, the respiratory centers are controlled to some extent by impulses from the hypothalamus and higher centers as well as from the reticular activating system.
STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI

STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI


STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI
The trachea or windpipe passes from the larynx to the level of the upper border of the fifth thoracic vertebra, where it divides into the two main bronchi that enter the right and left lungs. About 20 C-shaped plates of cartilage support the anterior and lateral walls of the trachea and main bronchi. The posterior wall, or membranous trachea, is free of cartilage but does have interlacing bundles of muscle fibers that insert into the posterior ends of the cartilage plates. The external diameter of the trachea is approximately 2.0 cm in men and 1.5 cm in women. The tracheal length is approxi- mately 10 to 11 cm.

STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI

Mucous glands are particularly numerous in the posterior aspect of the tracheal mucosa. Throughout the trachea and large airways, some of these glands lie between the cartilage plates, and others are external to the muscle layers with ducts that penetrate this layer to open on the mucosal surface. Posteriorly, elastic fibers are grouped in longitudinal bundles immediately beneath the basement membrane of the tracheal epithelium, and these appear to the naked eye as broad, flat bands that give a rigid effect to the inner lining of the trachea; they are not so obvious anteriorly. More distally, the bands of elastic fibers are thinner and surround the entire circumference of the airways.

Monday, May 3, 2021

INTRAPULMONARY AIRWAYS

INTRAPULMONARY AIRWAYS


INTRAPULMONARY AIRWAYS
According to the distribution of cartilage, airways are divided into bronchi and bronchioles. Bronchi have cartilage plates as discussed earlier. Bronchioles are distal to the bronchi beyond the last plate of cartilage and proximal to the alveolar region. Cartilage plates become sparser toward the periphery of the lung, and in the last generations of bronchi, plates are found only at the points of branching. The large bronchi have enough inherent rigidity to sustain patency even during massive lung collapse; the small bronchi collapse along with the bronchioles and alveoli. Small and large bronchi have submucosal mucous glands within their walls.

INTRAPULMONARY AIRWAYS

When any airway is pursued to its distal limit, the terminal bronchiole is reached. Three to five terminal bronchioles make up a lobule. The acinus, or respiratory unit, of the lung is defined as the lung tissue supplied by a terminal bronchiole. Acini vary in size and shape. In adults, the acinus may be up to 1 cm in diameter. Within the acinus, three to eight generations of respiratory bronchioles may be found. Respiratory bronchioles have the structure of bronchioles in part of their walls but have alveoli opening directly to their lumina as well. Beyond these lie the alveolar ducts and alveolar sacs before the alveoli proper are reached.
STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY

STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY


STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY
The airways are the hollow tubes that conduct air to the respiratory regions of the lung. They are lined throughout their length by pseudostratified, ciliated, columnar epithelium (also referred to as respiratory epithelium) supported by a basement membrane (see Plate 1-24 for details of cell types and their arrangement). The remainder of the wall includes a muscle coat and accessory structures such as submucosal glands, together with connective tissue. In the bronchi, cartilage provides additional support.
In adults, the diameter of the main bronchus is similar to that of the trachea (-2 cm), and the diameter of a terminal bronchiole is about 1 mm. These measurements vary with age and the size of the individual and with the functional state of the airway. For reference purposes, it is helpful to designate airways by their order or generation along an axial pathway. The epithelium is thicker in the larger airways and gradually thins toward the periphery of the lung.
Immediately beneath the basement membrane, elastic  fibers are collected into fine  bands that form longitudinal ridges. In cross-section, the fiber bundles are at the apices of the bronchial folds. The rest of the wall is made up of loose connective tissue containing blood vessels, nerves, capillaries, and lymphatics.

STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY

Blood Supply
The bronchial arteries supply the capillary bed in the airway wall, forming one plexus internal and another external to the muscle layer (see also Plate 1-26).

ANATOMY PHYSIOLOGY

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