General Properties of the BBB 8 страница

Sinuses

Paranasal sinuses are air-filled spaces in the bones of the walls of the nasal cavity.

Pharynx

Figure 75. Diagram of the relationship of the pharynx to the respiratory and digestive system

The pharynx con­nects the nasal and oral cavities to the larynx and esopha­gus. The pharynx is located posterior to the nasal and oral cavities and is divided re­gionally into the nasopharynx and oropharynx, respec­tively. The auditory (Eustachian) tubes connect the nasopharynx to each middle ear. Diffuse lym­phatic tissue and lymphatic nodules are found in the wall of the nasopharynx. The concentration of such nodules in the posterior wall is called the pharyngeal tonsil.

Larynx

Figure 76. The area of the vocal folds

The passageway for air between the oropharynx and tra­chea is the larynx. It is a complex tubular segment of the respiratory system that is formed by irreg­ularly shaped plates of hyaline and elastic cartilage.

Vocal Folds Control the Flow of Air Through the Larynx and Vibrate to Produce Sound

The vocal folds, also referred to as vocal cords, are two folds of mucosa that project into the lumen of the larynx.A supporting ligament and skeletal mus­cle, the vocalis muscle, is contained within each vocal fold-ligament. The ventricular folds located above the vocal folds are the "false vocal cords".

Stratified Squamous and Ciliated, Pseudostratified Columnar Epithelium Line the

The luminal surface of the vocal cords is covered with stratified squamous epithelium. The rest of the larynx is lined with the ciliated, pseudostratified columnar epithelium that characterizes the respiratory tract. The connective tissue of the larynx contains mixed mucoserous glands that secrete through ducts onto the laryngeal surface.

Trachea

Figure 77. Photomicrograph showing the relationship between the trachea and the esophagus at the base of the neck

The tracheais a short tube. It serves as a conduit for air. The trachea extends from the larynx to about the middle of the thorax, where it divides into the two primary bronchi. The wall of the trachea consists of four layers: Mucosa, composed of ciliated, pseudostratified epithe­lium and an elastic fiber-rich lamina propria Submucosa composed of a slightly more dense con­nective tissue than the lamina propria. Cartilaginous layercomposed of C-shaped hyaline car­tilages. Adventitia, which binds the trachea to adjacent struc­tures

1. Mucosa

Ciliated columnar cells, mucous (goblet) cells, and basal cells are the principal cell types in the tracheal epithelium. Brush cells are also present but in small numbers, as are the small granule cells.

Ciliated cells, the most numerous of the cell types. Cilia appear as short hair-like projec­tions from the apical surface. The cilia provide a coordinated sweeping mo­tion of the mucous coat.

Mucous cells are similar in appearance to the intestinal goblet cells and, thus, are often referred to by the same name. They accumulate mucinogen granules in their cytoplasm.

Brush cells are columnar cells that bear microvilli. The brush cell is regarded as a receptor cell.

Small granule cells are a representation in respiratory epithelium of the general class of enteroendocrine cells of the gut and gut derivatives. First type of small granule cell produces a catecholamine. A second cell type produces a polypeptide hormone.

Figure 79. Diagram of a brush cell(a) and small granule cell(b)

Basal cells serve as a reserve population by maintaining individual cell replacement in the epithelium.

Basement Membrane and Lamina Propria A Thick "Basement Membrane" Is Characteristic of Tracheal Epithelium Basementmembrane appears as a glassy or homogeneous layer.

The lamina propria appears as a typical loose connec­tive tissue. It is very cellular, containing numerous lym­phocytes, many of which infiltrate the epithelium.

The boundary between mucosa and submucosa is defined by an elastic membrane.

2. Submucosa

In the trachea the submucosa is a relatively loose connective tis­sue similar in appearance to the lamina propria. Submucosal glands composed of mucus-secreting acini with serous demilunes are also present in the sub­mucosa. The submucosal layer ends where its connective tissue fibers blend with the perichondrium of the cartilage layer.

3. Cartilaginous layer

The tracheal cartilages and trachealis muscle separate submucosa from adventitia.

A unique feature of the trachea is the presence of the C-shaped hyaline cartilages that are stacked on one another to form a supporting structure. Fibroelastic tissue and smooth muscle, the trachealis muscle, bridge the gap between the free ends of the C-shaped cartilages at the posterior border of the trachea, adjacent to the esophagus.

4. The adventitia

The outer layer, lies peripheral to the car­tilage rings and trachealis muscle. It consists of loose connective tissue.

Bronchi

The trachea divides into two branches forming the pri­mary or extrapulmonary bronchi (right and left bronchi). On entering the lungs the bronchi become the intrapulmonary bronchi, which branch immediately to give rise to the lobar bronchi (sec­ondary bronchi).

The left and right lung is further divided into eight and ten bronchopulmonary segments.

Thus the lobar bronchi divide to give rise to segmental bronchi (tertiary bron­chi); a segmental bronchus and the lung pa­renchyma that it supplies constitute a bronchopulmonary segment.

Bronchial Structure

The bronchi initially have the same general histologic structure as the trachea. At the point where the bronchi en­ter the lungs to become intrapulmonary bronchi, the struc­ture of the bronchial wall changes. The cartilage rings are replaced by cartilage plates of irregular shape. Bronchi can be identified by their cartilage plates and a circular layer of smooth muscle. The second change observed in the wall of the intrapulmonary bronchus is the addition of smooth muscle to form a complete circumferential layer.

The wall of the bronchus consists of five layers.

Mucosa is composed of a pseudostratified epithelium having the same cellular composition as the trachea. The lamina propria is similar to that of the trachea but is reduced in amount in proportion to the diameter of the bronchi.

Muscularis is a continuous layer of smooth muscle in the larger bronchi. It may appear discon­tinuous in smaller bronchi. Contraction of the muscle maintains the appropriate diameter of the air­way.

Submucosa remains as a relatively loose connective tis­sue. Glands are present as well as adipose tissue in the larger bronchi.

Cartilage layer consists of discontinuous cartilage plates that become reduced in size as the bronchial diameter diminishes.

Adventitia is dense connective tissue that is continuous with that of adjacent structures.

Figure 80. Divisions of the bronchial tree

BRONCHIOLES

Bronchiolar Structure

Bronchioles are air-conducting ducts. The larger bronchioles represent branches of the segmental bronchi. These ducts branch repeatedly, giving rise to the smaller terminal bronchioles that also branch. They finally give rise to the respiratory bron­chioles.

Cartilage and Glands Are Not Present in Bronchioles

The larger diameter bronchioles initially have a ciliated, pseudostratified columnar epithelium that gradually trans­forms to a simple ciliated columnar epithelium as the duct narrows. A relatively thick layer of smooth muscle is present in the wall of all bronchioles. Small bronchioles have a simple cuboidal epithelium. The smallest conducting bronchioles, the terminal bronchioles, are lined with a simple cuboidal epithelium in which Clara cells (see below) are found among the ciliated cells. A small amount of connective tissue underlies the epithelium, and a circumferential layer of smooth mus­cle underlies the connective tissue in the conducting por­tions.

Figure 81. Diagram of Clara cell between bronchial ciliated epithelial cells

Clara cells are nonciliated cells secrete a surface-active agent, a lipoprotein. The functional role of this agent is to prevent luminal adhesion should the wall of the airway fold on it­self, particularly during expiration.

Respiratory Bronchioles Are the First Part of the Bronchial Tree That Allows Gas Exchange to Occur

Figure 82. Photomicrograph showing the respiratory portion of the bronchial tree. TB, terminal bronchiole; RB, respiratory bronchioles, AD, alveolar ducts; AS, alveolar sacs

Respiratory bronchioles constitute a transitional zone in the respiratory system concerned with both air conduction and gas exchange between air and blood. They have a nar­row diameter and are lined by a cuboidal epithelium. Scattered, thin-walled outpocketings, alveoli, extend from the lumen of the re­spiratory bronchioles.

ALVEOLI

Alveoli Are the Site of Gas Exchange

Alveoli are the terminal air spaces of the respiratory sys­tem and are the actual site of gas exchange between the air and the blood. At some point, each alveolus is confluent with a respiratory bronchiole, by means of an alveolar duct, and an alveolar sac.

Alveolar ducts are elongate airways that have almost no walls, only alveoli, as their peripheral boundary with rings of smooth muscle in the knob-like interalveolar septa.

Figure 83. Photomicrograph showing an alveolar sac with adjacent alveoli.

AS, alveolar sac; A, alveoli

Alveolar sacs are spaces surrounded by clusters of al­veoli. The surrounding alveoli open into these spaces. Al­veoli are surrounded and separated from one another by a thin connective tissue layer that contains numerous blood capillaries. The tissue between adjacent alveolar air spaces is called the alveolar septum.

Alveolar Epithelium Is Composed of Type I and II Alveolar Cells and Occasional Brush Cells

Type I alveolar cells, also known as type I pneumocytes, are extremely thin squamous cells that line most of the surface of the alveoli.

Figure 84. Diagram of a type II alveolar cell

Type II alveolar cells are secretory cells. They are cuboidal cells interspersed among the type I cells. Their apical cytoplasm is filled with granules which rich in phospholipids, among which is the surface active agent surfactant. Without adequate secretions of surfactant, the alveoli would collapse on exhalation.

The Alveolar Septum Is the Site of the Air-Blood Barrier

Figure 85. Diagram of the alveolar septum

The components of the alveolar septum are

-Alveolar epithelial cells

-Basal lamina of the alveolar epithelium

-Basal laminaof the capillary endothelium

-Endothelial cellsof the rich capillary network

-Other connective tissue elements, including fibroblasts, macrophages, collagen fibers, and elastic fibers

Most of the basal surface of the alveolar epithelium is in intimate association with the capillaries of the septum. Any one capillary may be shared by two or more alveoli.

The air-blood barrier refers to the cells and cell products across which gases must diffuse between the alveolar com­partment and capillary compartment. The thinnest air-blood barrier consists of a monomolecular layer of surfactant, a type I epithelial cell and its basal lamina, and a capillary endothelial cell and its basal lamina. Often, these two basal laminae are fused.

Alveolar Macrophages Remove Inhaled Particulate Matter From the Air Spaces and Red Blood Cells From the Septum.

Urinary system

The urinary system consists of the paired kidneys, the paired ureters, which lead from the kidneys to the bladder, and the urethra, which leads from the bladder to the ex­terior of the body.

Kidneys Conserve Body Fluid and Electrolytes and Remove Metabolic Waste

The kidneys are highly vascular, large, reddish, bean-shaped organs.

The kidneys pro­duce urine, initially an ultrafiltrate of the blood that is then modified by selective resorption and specific secretion by the cells of the kidney.

Kidneys Also Function as Endocrine Organs

The endocrine activities of the kidneys include

-Synthesis and secretion of erythropoietin, a growth fac­tor regulating red blood cell formation

-Synthesis and secretion of renin, a hormone involved in control of blood pressure and blood volume

-Hydroxylation of vitamin D, a steroid prohormone, to produce its active form

KIDNEY STRUCTURE

Figure 86. Diagram of a kidney seen from behind

1. Capsule

The kidney surface is covered by a thin con­nective tissue capsule. The capsule consists of an outer layer of fibroblasts and collagen fibers and an inner layer of myofibroblasts. 2. Cortex and Medulla

Substance of kidney can be divided into two distinct regions:

-Cortex, the outer reddish brown-colored part

-Medulla, the much lighter-colored inner part

Approxi­mately 90-95% of the blood passing through the kidney is in the cortex; 5-10% is in the medulla.

The Cortex Is Characterized by Renal Corpusclesand Their Tubules

The cortex consists of renal corpuscles, along with the convoluted and straight tubules of the nephron, the col­lecting tubules, and an extensive vascular supply. The renal corpuscles comprise the beginning segment of the nephron and contain a unique capillary network called glomerulus.

The surface of the kidney reveals a series of vertical striations that appear to emanate from the medulla. These are the medullary rays.

Each medullary ray contains straight collecting tubulesand straight tubule components of the nephrons.

Figure 87.Diagram of adult human kidney

The Medulla Is Characterized by Straight Tubules, Collecting Ducts, and a Special Capillary Network

The straight tubular segments of the nephrons and the collecting ducts continue from the cortex into the medulla. They are accompanied by a capillary network, the vasa recta, that runs in parallel with the various tubules.

The tubules in the medulla, because of their arrangement and because of differences in length, collectively form a number of conical structures called pyramids. Usually 8— 12 pyramids may occur in the human kidney. The bases of the pyramids face the cortex and the apices face the renal sinus.

Figure 88. Light micrograph of human kidney showing the capsule (Cap) and underlying cortex; outer layer of the capsule (OLC) and the inner layer of the capsule (ILC)

The Renal Columns Represent Cortical Tissue Contained Within the Medulla

The caps of cortical tissue that lie over the pyramids form the renal col­umns.

The apical portion of each pyramid, which is known as the papilla, projects into a minor calyx, a cup-shaped struc­ture that represents an extension of the renal pelvis. The tip of the papilla is perforated by the openings of the collecting ducts. The minor calyces are branches of the two or three major calyces that, in turn, are major divisions of the renal pelvis.

Kidney Lobes and Lobules

The Number of Lobes in a Kidney Equals the Number of Medullary Pyramids

The Lobule Consists of a Collecting Duct and All the Nephrons That It Drains

Nephron

The nephron is the basic functional and structural unit of the kidney.

In the human there are about: 2 million nephrons in each kidney. They are responsible for the production of urine and correspond to the secretory part of other glands. It is the collecting tubule that is re­sponsible for the final concentration of the urine.

The renal corpuscle represents the beginning of the nephron. It consists of the glomerulus, a tuft of capillaries composed of 10-20 capillary loops, surrounded by a dou­ble-layered epithelial cup, the renal or Bowman's capsule. Bowman's capsule is the initial portion of the nephron where blood flowing through the glomerular capillaries undergoes a filtration process to produce the initial urine filtrate.

The glomerular capillaries are supplied by an afferent arteriole and are drained by an efferent arteriole that then branches, forming a new capillary network to supply the kidney tubules. This is then an arterial portal system. Where the afferent and efferent arterioles penetrate and exit from the parietal layer of Bowman's capsule is called the vas­cular pole. Opposite this site is the urinary pole of the renal corpuscle, where the proximal convoluted tubule be­gins.

Figure 89.Schematic diagram of a renal gromerulus and the structures associated at the vascular pole (top) and urinary pole (bottom)

Continuing from Bowman's capsule, the remaining seg­ments of the nephron, namely, the tubular parts, are

-Proximal thick segment, consisting of the proximal convoluted tubule (pars convoluta) and the proximal straight tubule (pars recta)

-Thin segment, which constitutes the thin limb of the loop of Henle (the loop of Henle is the entire U-shaped portion of a nephron)

-Distal thick segment, consisting of the distal straight segment (pars recta) and the distal convoluted tubule (pars convoluta)

The distal convoluted tubule connects to the collecting tubule, often through a connecting tubule, thus forming the uriniferous tubule, i.e., the nephron plus collecting tubule.

Types of Nephrons

Figure 90.Diagram of two types of nephrons in the kidney and their associated collecting ducts system

The types of nephrons are based on where their renal corpuscles are located in the cortex:

• Cortical or subcapsular nephrons have their renal cor­puscles located in the outer part of the cortex. They have short loops of Henle, extending only into the outer re­gion of the pyramid.

• Juxtamedullary nephrons have their renal corpuscles in proximity to the base of a medullary pyramid. They have long loops of Henle and long thin segments that extend well into the inner region of the pyramid.

• Intermediate nephrons have their renal corpuscles in the midregion of the cortex. Their loops of Henle are of intermediate length.

Renal (Malpighian) Corpuscle

The Renal Corpuscle Contains the Filtration Apparatus of the Kidney

The renal corpuscle consists of the glomerular capillary tuft and the surrounding visceral and parietal epithelial layers of Bowman’s capsule. The filtration apparatus, enclosed by the parietal layer of Bowman’s capsule, consists of three components:

-endothelium of the glomerular capillaries has nu­merous fenestrations. -visceral layer of Bowman’s capsule -basal lamina lying between these two cellular elements

Figure 91.Schematic diagram of Mesangial cells and capillaries

Podocytes Constitute the Visceral Layer of Bowman's Capsule

The podocytes, the cells of the inner or visceral layer of Bowman's capsule, extend processes around the glomerular capillaries. Each process, in turn, has numerous secondary processes called pedicelsor foot processes. The elongated spaces between the interdigitating foot processes, called filtration slits, and allow the filtrate from the blood to enter the Bowman's space. The foot processes contain numerous microfilaments (actin) that are thought to have a role in regulating the size and patency of the filtration slits. The filtration slit membrane is thin membrane, spans the slits.

The thick basal lamina is the joint product of the endothelium and the podocytes. It is the principal component of the filtration barrier. It is called the glomerular basement membrane (GBM).

The Glomerular Basement Membrane Acts as a Physical Barrier and an Ion-Selective Filter

The components tend to lo­calize in particular portions of the GBM:

The lamina rara interna, adjacent to the capillary en­dothelium

The lamina rara externa, adjacent to the podocyte pro­cesses

The lamina densa, the fused portion of the basal lam­inae, sandwiched between the laminae rarae

Type IV collagen is concentrated in the lamina densa, which is organized into a feltwork that acts as a physical filter. The GBM restricts the movement of particles, such as albumin or hemoglobin. The narrow slit pores formed by the pedicels and the filtration slit membranes also act as physical barriers to bulk flow and free diffusion.

The outer layer of Bowman's capsule, called the parietal layer, is a simple squamous epithelium. At the urinary pole of the renal corpuscle, it is continuous with the cuboidal epithelium of the proximal convoluted tubule.(figure 4)

The space between the visceral and parietal layers of Bowman's capsule is called the urinary or Bowman's space. It is the receptacle for the plasma filtrate produced by the filtration apparatus of the renal corpuscle.

Mesangium

The renal corpuscle contains an additional group of cells called mesangial cells. These cells and their extracellular matrix constitute the mesangium. Some Mesangial cells are located outside of the corpuscles along the vascular pole where they are also designated as lacis cells and form part of what is called the juxtaglomerular apparatus.

The functions of mesangial cells are:

-Mesangial cells are phagocytic; they can remove trapped residues and aggregated proteins from the GBM.

-Mesangial cells and their matrix provide structural sup­port for the podocytes where the epithelial basement membrane is absent or incomplete.

-The cleaning of the GBM is believed to be the primary function of the mesangial cells.

Juxtaglomerular Apparatus

The Juxtaglomerular Apparatus Includes the Macula Densa, the Juxtaglomerular Cells, and the Extraglomerular Mesangial Cells

Lying directly adjacent to the afferent and efferent arterioles and adjacent to some extraglomerular mesangial cells at the vascular pole of the renal corpuscle is the terminal portion of the distal thick segment of the nephron. It con­tains, as part of its wall, cells that collectively are referred to as the macula densa. The cells of the macula densa are distinctive, in that they are narrower and usually taller than other distal tubule cells. The nuclei of these cells appear crowded, even to the extent that they appear partially superimposed over one another—thus, the name macula densa.

The smooth muscle cells of the ad­jacent afferent arteriole are modified. They contain secretory granules, and their nuclei are spherical. These smooth mus­cle cells are referred to as juxtaglomerularcells.

The Juxtaglomerular Apparatus Regulates Blood Pressure

The granules of the juxtaglomerular cells contain rennin,that is synthesized, stored, and released into the blood. In the blood, renin catalyzes the hydrolysis of circulating angiotensinogento produce angiotensin. Then,

-Angiotensin II increases blood pressure.

-Angiotensin II stimulates the release of the hormone aldosterone from the zona glomerulosa of the adrenal gland.

-Aldosterone, in turn, increases distal tubular resorption of sodium and concomitant resorption of water, thereby raising blood volume and pressure.

-Angiotensin II is also a potent vasoconstrictor that has a regulatory role in the control of renal and systemic vascular resistance.

Renin secretion stimulates by decrease in NaCl concentration. The cells of the macula densa monitor the NaCl concentration in the afferent arteriole and regulate the re­lease of renin by the juxtaglomerular cells.

KIDNEY TUBULE FUNCTION

As the glomerular filtrate passes through the uriniferous and collecting tubules of the kidney, it undergoes changes that involve both active and passive absorption, as well as secretion.

-Certain substances within the filtrate are reabsorbed, some partially, e.g., water, sodium, and bicarbonate and some completely, e.g., glucose.

-Other substances, e.g., creatinine and organic acids and bases, are added to the filtrate, i.e., the primary urine, by secretory activity of the tubule cells.

Thus, the volume of the filtrate is reduced substantially, and the urine is made hypertonic. The long loop of Henle and the collecting tubules that pass parallel to similarly arranged blood vessels, the vasa recta, serve as the basis the countercurrent multiplier mechanism that is instrumental in concentrating the urine and, thereby, making it hypertonic.

Proximal Thick Segment

The Proximal Thick Segment Is the Initial and Major Site of Reabsorption

The proximal convoluted tubule receives the primary filtrate from the urinary space of Bowman's capsule. The cuboidal cells of the tubule have the following features:

-A brush border, composed of numerous, relatively long microvilli

-A terminal bar apparatus, consisting of a narrow tight junction and a zonula adherens that seal off the inter­cellular space from the lumen of the tubule

-Large, flattened processes, plicaeor folds, on the lateral surfaces of the cells, which alternate with similar pro­cesses of adjacent cells

-Extensive interdigitation of basal processes of adjacent cells

-Basal striations, consisting of elongate mitochondria concentrated in the basal processes and oriented normal to the basal surface

The Proximal Convoluted Tubule Reabsorbs 80% of the Primary Filtrate

Water and Electrolyte Reabsorption

The proximal convoluted tubule reabsorbs about 150 liters of fluid per day. As in the intestinal and gallbladder epithelia, this resorption of fluid is driven by active transport of Na⁺ into the lateral intercellular space. Here, the fluid is reabsorbed into the vessels of the peritubular capillary network.

The proximal tubule reabsorbs amino acids, sugars, and proteins.

Proteins and large peptides are reabsorbed by endocytosis in the proximal tubule

Proximal Straight Segment

The cells of the straight segment of the proximal tubule, i.e., the thick descending limb of the loop of Henle, are not as specialized for absorption as are those of the con­voluted segment.

Thin Segment

The length of the thin segment of a nephron varies with its location in the cortex. The thin segment has four types of epithe­lial cells.

The cells possess few organelles. The specific functional roles of the four cell types in the thin segment are not yet clear.

Дата добавления: 2015-11-14