General Properties of the BBB 3 страница

The tunica intima of elastic arteries is relatively thick and consists of:

-an endothelial lining with its basal lamina

-a subendothelial layer of connective tissue

-the internal elastic membrane (lamina)

In elastic arteries, the internal elastic membrane is not conspicuous, because it is one of many elastic layers in the wall of the vessel.

The endothelium is a simple squamous epithelium that functions is the control of the movement of substances from and to the vascular lumen.

The main cell type of this layer is the smooth muscle cell.

In the larger elastic arteries, the subendothelial layer consists of connective tissue with both collagen and elastic fibers.

Tunica media

The tunica media is the thickest of the tree layers of elastic arteries and consists of:

-elastin in the form of fenestrated sheets or lamellae between the muscle layers

-smooth muscle cells arranged in layers

-collagen fibers and ground substance

The elastic lamellae are arranged in concentric layers. Fenestrations in the lamellae facilitate the diffusion of substances through the arterial wall.

Tunica adventitia

In elastic arteries, the tunica adventitia is a relatively thin connective tissue layer that is usually less than half the thickness of the tunica media.

It consists of:

-collagen fibers

-elastic fibers

-fibroblasts and macrophages

The tunica adventitia contains blood vessels (vasa vasorum) and nerves (nervi vascularis).

The fibers help in preventing the expansion of the arterial wall beyond physiologic limits during the systolic period of the cardiac cycle.

Muscular arteries

Figure 30. Schematic diagram of a muscular artery

Muscular arteries have more smooth muscle and less elastin in the tunica media than do elastic arteries. Prominent internal and external elastic membranes help to distinguish muscular arteries from elastic arteries. The muscular arteries are the brachial, radial, ulnar, femoral and tibial arteries.

Tunica intima

This tunica is thinner in muscular arteries than in elastic arteries and consists of:

- an endothelial lining with its basal lamina

- a sparse subendothelial layer of connective tissue

- and a prominent elastic membrane

Tunica media

This tunica consists of smooth muscle cells between collagen fibers and relatively little elastic material. The smooth muscle cells are arranged in a spiral fashion in the arterial wall. Their contraction assists in maintaining blood pressure.

Tunica adventitia

This tunica consists of collagen fibers, elastic fibers and some fibroblasts and adipose cells. Compared with elastic arteries the tunica adventitia of muscularis arteries is relatively about the same thickness as the tunica media. Collagen fibers are principal extra cellular component.

However, a concentration of elastic material immediately adjacent to the tunica media constitutes the external elastic lamina typical of muscular arteries.

Small arteries and arterioles

Small arteries and arterioles are distinguished from one another by the number of smooth muscle layers in the tunica media.

Arterioles have only one or two layers of smooth muscle in the tunica media, a small artery may have up to about eight layers.

Typically, the tunica intima of a small artery has an internal elastic membrane, whereas this layer may or may not be present in the arteriole.

The tunica adventitia is a thin, sheath of connective tissue that blends with the connective tissue in which these vessels travel.

The arterioles have two important functions;

1). Maintain the blood pressure inside the arterial system

2). Control blood flow to the capillary networks.

The arterio-venous connections

The arterio-venous connections are classified into two main types, namely;

1). Blood capillaries and the vessels regulating their blood flow

2). Arterio-venous anastomosis (A-V shunts)

Figure 31. Diagram of microcirculation showing a metarteriole giving rise to capillaries

Capillaries

Capillaries are the smallest diameter blood vessels, often smaller than diameter of an erythrocyte.

Capillaries form blood vessel networks that allow fluids containing gases, waste products to move through their thin wall. They consist of a single layer of endothelial cells and their basal lamina. The endothelial cells form a tube just large enough to allow the passage of red blood cells one a time.

Capillaries are described as:

1). Continuous or somatic capillaries typically found in muscle, lung, and central nervous system (CNS). They appear in cross sections as two plasma membranes enclosing a ribbon of cytoplasm that may include the nucleus. Numerous pinocytotic vesicles underlie both the luminal and basal plasma surfaces. The vesicles function in transport of materials between the lumen and the connective tissue. In certain continuous capillaries, pericytes may be found in association with the endothelium. The pericytes is derived from the same precursor cell that forms endothelial cells in new vessels and can give rise to smooth muscle cells during vessel growth.

Figure 32. EM and diagram of continuous capillary (left) and fenestrated capillaries (right)

2). Fenestrated or visceral capillaries, typically found in endocrine glands and sites of fluid and metabolite absorption such as the gallbladder and intestinal tract.

Fenestrated capillaries are characterized by the presence of fenestrae in the walls of endothelial cells. That provides channels across the capillary wall. A continuous basal lamina is present. Fenestrated capillaries also have pinocytotic vesicles.

3). Discontinuous or sinusoidal capillaries, typically found in the liver, spleen and bone marrow. They are larger and more irregularly shaped than other capillaries. Structural features of these capillaries vary and include:

-presence of unusually wide gaps (fenestrae) between endothelial cells, as in the liver and spleen

-partial of total absence of basal lamina underlying the endothelium

-presence of specialized cells, such as the stellate sinusoidal macrophages (Kupffer cells) and vitamin A storage cells (of Ito) in the liver, among the lining endothelial cells

Capillaries have three essential functions:

1. Selective permeability

The movement of large molecules from the blood to the tissues and from the tissues to the blood is related to the size and charge of the molecules. The intercellular junctions allow passage of water and hydrophilic molecules with a diameter of less than 1,5nm. The pinocytotic vesicles and the fenestrations of the endothelial cells allow the passage of large molecules through the endothelium.

2. Synthetic and metabolic activities

Endothelial cells are involved in a number of synthetic activities, such as production of prostacyclin, plasminogen activator and others.

3. Antithrombogenic function

Endothelial cells produce anticoagulants and antithrombogenic agents. They are also form a barrier between platelets of the blood and subendothelial tissue.

Arteriovenous shunts

Arteriovenous shunts allow blood to bypass capillaries by providing direct routes between arteries and veins. In many tissues, there are direct routes between the arteries and veins that can divert blood from the capillaries. These routes are called arteriovenous anastomoses or shunts.

Arteriovenous shunts (AV) are commonly found in the skin of the fingertips, nose and lips and in the erectile tissue of the penis and clitoris. Contrary to the ordinary precapillary sphincter, contraction of the arteriole smooth muscle of the AV shuntsends blood to a capillary bed; relaxation of the smooth muscle sends blood to avenule, bypassing the capillary bed.

AV shunts serve in thermoregulation at the body surface. Closing an AV shunt in the skin causes blood flow through the capillary bed, enhancing heat loss. Opening an AV shunt in the skin reduces the blood flow to the skin capillaries, thereby conserving body heat.

In addition, preferential thoroughfares, the proximal segment of which is called a metarteriole, also exist that allow some blood to pass more directly from artery to vein. Capillaries arise from both arterioles and metarterioles. Although capillaries themselves have no smooth muscle in their wall, there is a sphincter of smooth muscle, called precapillary sphincter, at their origin from either an arteriole or a metarteriole. These sphincters control the amount of blood passing through the capillary bed.

Veins

The tunics of veins are not as distinct (well defined) as the tunics of arteries. Traditionally, veins are divided into three types on the basis of size.

-small veins or venules, further subclassified as postcapillary and muscular venules

-medium veins

-large veins

Difference between veins and arteries

1. Typically, veins have thinner walls than their accompanying arteries.

2. The lumen of the vein is usually larger than that of the artery.

3. The lumen of the vein is often collapsed, whereas the lumen of the artery is often patent.

4. Many veins have valves.

5. Adventitia is the thickest tunica in the veins and media in the arteries.

6. The veins have more collagen fibers than elastic.

7. Shape of the wall in the vein is irregular and in the artery is regular.

Venules

Muscular venules are distinguished from postcapillary venules by the presence of a tunica media.

Postcapillary venules receive blood from capillaries and possess an endothelial lining with its basal lamina and pericytes.

Muscular venules are located distal to the postcapillary venules in the returning venous network. Whereas postcapillary venules have no true tunica media, the muscular venules have one or two layers of smooth muscle that constitute a tunica media and have a thin tunica adventitia.

Medium veins

Figure 33. Schematic diagram of a medium-sized veins

The three tunics of the venous wall are most evident in medium-sized veins.

The tunica intima consists of an endothelium with its basal lamina, a thin subendothelial layer with some smooth muscle cells, scattered among connective tissue elements, and, in some cases, a thin internal elastic membrane.

The tunica media of medium sized veins is much thinner than the same layer in medium sized arteries and contains circularly arranged smooth muscle cells and collagen fibers.

The tunica adventitia is usually thicker than the tunica media and consists of longitudinally oriented bundles of smooth muscle cells, collagen fibers, and networks of elastic fibers.

Large veins

Figure 34. Schematic diagram of a large veins

In large veins, the tunica media is relatively thin, and the tunica adventitia is relatively thick.

The tunica intima of large veins consists of an endothelial lining with its basal lamina, a small amount of subendothelial connective tissue, and some smooth muscle cells. Often, the boundary between the tunica intima and tunica media is not clear, and it is not always easy to decide if the smooth muscle cells close to intimal endothelium should be classified as belonging to the tunica intima or to the tunica media.

The tunica media is relatively thin and contains smooth muscle cells, collagen fibers, and some fibroblasts. Cardiac muscle cells extend into the tunica media of the largest veins, the venae cavae and the pulmonary veins, near junction with the heart.

The tunica adventitia is the thickest layer of the wall of a vein. In the tunica adventitia of the largest veins, bundles of longitudinally disposed smooth muscle cells are found along with the usual collagen fibers, elastic fibers, and fibroblasts.

LYMPHATIC VESSELS

Lymphatic vessels convey fluids from the tissues to the bloodstream

In addition to blood vessels, there is a set of vessels that circulates fluid, called lymph, through certain parts of the body. These lymphatic vesselsare an adjunct to the blood vessels. Unlike the blood vessels, which convey blood to and from tissues, the lymphatic vessels are unidirectional, conveying fluid only from tissues. The smallest lymphatic vessels are called lymphatic capillaries. They are espe­cially numerous in the loose connective tissues under the epithelium of the skin and mucous membranes. The lym­phatic capillaries begin as "blind-ending" tubes in the microcapillary beds. Lymphatic capillaries converge into increasingly larger vessels, called lymphatic vessels that ultimately unite to form two main channels that empty into the blood vascular system by draining into the large veins in the base of the neck. They enter the vas­cular system at the junctions of the internal jugular and subclavian veins. The largest lymphatic vessel, draining most of the body and emptying into the veins on the left side, is the thoracic duct. The other main channel is the right lym­phatic duct.

Lymph capillary	Blood capillary
- Wider irregular lumen.	- Narrower regular lumen.
- Blind ended, i.e. it begins by a blind end.	- Has arterial and venous ends.
- The endothelium is not fenestrated.	- The endothelium may be fenestrated.
- It lacks a well-developed basement	- It has a well-developed basement
• membrane.	membrane.
- Lacks pericytes.	- Has pericytes.
- The outer surface of the endothelium is	- No anchoring fibres.
attached to the surrounding C.T.
intercellular substance by anchoring
fibres.

Lymphatic Capillaries

Lymphatic Capillaries Are More Permeable Than Blood Capillaries

Due to their greater permeability, lymphatic capillaries are more effective than blood capillaries in removing pro­tein-rich fluid from the intercellular spaces. Lymphatic ves­sels also serve to convey large proteins and lipids that do not get across the fenestrae of the absorptive capillaries in the small intestine.

Before lymph is returned to the blood, it passes through lymph nodes, where it is exposed to the cells of the im­mune system. Thus, the lymphatic vessels serve not only as an adjunct to the blood vascular system but also as an integral component of the immune system.

Lymphatic capillaries are essentially tubes of endothelium that, unlike the typical blood capillary, lack a contin­uous basal lamina. The incomplete basal lamina can be cor­related with their high permeability. Anchoring filaments extend between the incomplete basal lamina and the perivascular collagen. These filaments may help maintain the patency of the vessels during times of increased tissue pres­sure, as in inflammation.

As lymphatic vessels become larger, the wall becomes thicker. The increasing thickness is due to connective tissue and bundles of smooth muscle. Lymphatic vessels possess valves that prevent backflow of the lymph, thus aiding un­idirectional flow. There is no central pump in the lymphatic system. Lymph moves sluggishly, driven primarily by the compression of the lymphatic vessels by adjacent skeletal muscles.

These vessels remove substances and fluid from the extracellular spaces of the connective tissues, thus produc­ing lymph. Because the walls of the lymphatic capillaries are more permeable than the walls of blood capillaries, large molecules includ­ing foreign substances and cells gain entry more readily into the lymphatic capillaries than into blood capillaries.

As the lymph circulates through the lymphatic vessels, it passes through lymph nodes. Here, foreign substances (antigens) conveyed in the lymph are concentrated by the dendritic cells and presented to the lymphocytes. This leads to the cascade of steps that constitute the immune response.

Heart

The heart is a pump with four chambers with valves that maintain a one-way flow of blood.

Figure 35. Fibrous skeleton of the heart as seen as two atria removed

The heart contains 4 chambers (two atria and two ventricles) through which blood is pump. Valves guard the exits of chambers, preventing the backflow of blood.

An interatrial septum and an interventricular septum separate the right and left sides of the heart.

The right atrium receives blood returning from the body via the inferior and superior venae cavae, the two largest veins of the body. The right ventricle receives blood from the right atrium and pumps it to the lungs via the pulmonary arteries.

The atrium receives the oxygenated blood returning from the lungs via the four pulmonary veins. The left ventricle receives blood from the left atrium and pumps it into the aorta for distribution into the systemic circulation.

The wall of the heart includes:

-a musculature of cardiac muscle for contraction to propel the blood

-a fibrous “skeleton” for attachment of the valves

-an internal conducting system for synchronization of muscle contraction

The fibrous skeleton, comprised of dense connective tissue, encircles the base of two arteries leaving the heart (aorta and pulmonary trunk) and the openings between the atria and the ventricles (right and left atrioventricular (A-V) orifices).

The fibrous skeleton also includes an extension into the interventricular septum. (the membranous portion) is devoid of cardiac muscle; it contains a short length of unbranched A-V bundle of the cardiac conduction system.

The heart wall contains cardiac muscle. The wall of the heart is organized in essentially the same manner in the atria and ventricles.

It consists of three layers.

Epicardium consists of a layer of mesothelial cells on the outer surface of the heart and its underlying connective tissue. The blood vessels and nerves that supply the heart lie in epicardium and are surrounded by adipose tissue that cushions the heart in the pericardial cavity.

Myocardium, the cardiac muscle, is the principal component of the heart.

Cardiac muscle

Cardiac muscle is the type of striated muscle found in the wall of the heart and in the base of the large veins that empty into the heart. It consists of long fibers that appear to branch and anastomose with neighboring fibers. Unlike skeletal muscle fibers, the fibers of cardiac muscle are formed by individual, mononucleated cardiac muscle cells that are joined to one another in linear array.

Cardiac muscle has the same types and arrangement of contractile filaments as skeletal mus­cle. The cardiac muscle cells as well as the fibers they form exhibit cross-striations. In addition, cardiac muscle fibers ex­hibit densely staining cross-bands, called intercalated discs, that cross the fibers in a linear fashion or in a fashion that resembles the risers of a set of steps. The in­tercalated discs represent the site of attachment of a cardiac muscle cell to its neighbors.

Nuclei of Cardiac Muscle Cells Are Located in the Center of the Cell

The central location of the nucleus in cardiac muscle cells helps to distinguish them from multinucleated skeletal mus­cle fibers, whose nuclei lie immediately under the sarcolemma. Cardiac muscle cells are characterized by very large mitochondria that are densely packed between the myofibrils and juxtanuclear mitochondria. There are, also, associated concentrations of glycogen granules between the myofibrils.

Intercalated Discs

The intercalated discs represent the major site of attach­ment between cardiac muscle cells. The step-like appearance of the intercalated discs appears to be due to the presence of a trans­verse component that crosses the fibers at a right angle to the myofibrils and a lateral component that runs parallel to the myofibrils

Smooth Endoplasmic Reticulum and the T System

The smooth endoplasmic reticulum (sER) of cardiac muscle is not as well organized as that of skeletal muscle. It does not usually separate bundles of myofilaments into discrete cylindrical myofibrils.

The Smooth Endoplasmic Reticulum in Cardiac Muscle Is Organized Into a Single Network per Sarcomere, Extending From Z Disc to Z Disc

The T tubules in cardiac muscle penetrate into the myofilament bundles at the level of the Z disc, between the ends the sER network. Thus, there is only one T tubule per sarcomere in cardiac muscle. The small terminal cisternae f the sER at the level of the Z disc interact with the T tubules to form a diad. The basal lamina adheres to the invaginated plasma membrane of the T tubule as it penetrates into the cytoplasm of the muscle cell. The T tubules are larger and more numerous in mammalian cardiac ventricular muscle than in mammalian skeletal muscle. They are less numerous, however, in cardiac atrial muscle.

Spontaneous Contraction Versus Neural Control

Cardiac Muscle Cells Exhibit a Spontaneous Rhythmic Contraction or Beat

In the heart, this beat is initiated, locally regulated, and coordinated by specialized, modified cardiac muscle cells that are organized into nodes and bundles to transmit the contractile impulse to various parts of the myo­cardium in a precise sequence. These cells, called cardiac conducting cells (Purkinje cells), and their functions.

The myocardium of the ventricles is substantially thicker than that of the atria because of the large amount of cardiac muscle in the walls of the two pumping chambers.

Endocardium consists of an inner layer of endothelium and subendothelial connective tissue, a middle layer of connective tissue and smooth muscle cells, and a deeper layer, also called the subendocardial layer of connective tissue that is continuous with the connective tissue of the myocardium.

The impulse-conducting system of the heart is located in the subendothelial layer of the endocardium.

The interventricular septum is the wall between the right and left ventricles. It contains cardiac muscle except in the membranous portion. Endocardium lines each surface of the interventricular septum.

The interatrial septum is much thinner than the interventricuolar septum. Except for certain localized areas that contain fibrous tissue, it has a center layer of cardiac muscle, with a lining of endocardium facing each chamber.

The valves of the heart have a center sheet of fibrous tissue.

The surfaces of the valve that are exposed to blood are covered with endothelium.

Fibrous, thread-like cords called the chordae tendineae, also covered with endothelium, extend from the free edge of the A-V valves to muscular projections from the wall of the ventricles, called papillary muscles.

Intrinsic regulation of heart rate

Contraction of the heart is synchronized by specialized cardiac muscle fibers. Cardiac muscle is capable of contracting in a rhythmic manner without any direct stimulates from the nervous system. The base of this beating action is initiated at the sinoatrial (SA) node, a group of specialized cardiac muscle cells located near the junction of the superior vena cava and the right atrium.

Because of this function, the SA node is referred to as the pacemaker. The SA node initiates an impulse that spreads along the cardiac muscle fibers of the atria and along internodal tracts composed of modified cardiac muscle fibers. The impulse is then picked up at the atrioventricular (AV) node and conducted across the fibrous skeleton to the ventricles by the AV bundle (of His).

Figure 36. Chambers of the heart and the impulse-conducting system

The bundle divides into smaller right and left bundle branches and then into Purkinje fibers.

The AV bundle, the bundle branches, and the Purkinje fibers are modified cardiac muscle cells that are specialized to conduct impulses.

The components of conducting system convey the impulse at a rate approximate 4 times faster than the cardiac muscle fibers. The conducting elements are the only elements that can convey impulses across the fibers skeleton and serve to coordinate the contractions of the atria and ventricles.

The contraction is initiated in the atria, forcing blood into the ventricles. Then, a wave of contraction in the ventricles begins at the apex of the heart, forcing blood from the heart through the aorta and pulmonary trunk.

Systemic regulation of heart function

The heart is innervated by both divisions of the autonomic nervous system.

Specialized receptors monitor heart function

Specialized sensory nerve receptors for physiologic reflexes are located in the walls of large blood vessels near the heart.

They function as:

1. Baroreceptors, which sense general blood pressure.

These receptors are located in the carotid sinus and aortic branch.

2. Chemoreceptors, which detect alterations in oxygen and carbon dioxide tension and in PH. These receptors are the carotid and aortic bodies located at the bifurcation of the carotid arteries and in the aortic arch, respectively.

Lymphoid system and organs of hematopoiesis and immune response

Organs of hematopoiesis and immune defense can be divided into central and peripheral organs.

Central organs of hematopoiesis and immune defense are thymus and red bone marrow.

Peripheral organs of hematopoiesis and immune defense are spleen, lymph nodes and solitary lymphatic nodules.

Thymus

The thymus is a bilobed organ located in the superior mediastinum anterior to the heart and great vessels.

It develops bilaterally from the third branchial pouch.

The thymus is where stem lymphocytes proliferate and differentiate into T lymphocytes.

Although many lymphocytes come to lie between the epithelial cells, the epithelial cells remain joined to one another by desmosomes at the end of long cytoplasmic processes.

The epithelial cells form a cytoplasmic reticulum within the parenchyma of the thymus and are designated as epithelioreticular cells. The epithelioreticular cells serve as a framework for the thymic lymphocytes. The structural and functional unit of the thymus is thymic lobule. Each of it contains cortex and medulla. A connective tissue capsule surrounds the cortex and extends trabeculae to the margin of the cortex and medulla.

Cortex is the outer portion of the parenchyma which consists of dense population of so-called thymocytes that are T lymphocytes, and scattered epithelial reticular cells that have multiple processes and partially compartmentalize thymocytes. The connective tissue here contains variable number of plasma cells, granulocytes, T lymphoblasts, mast cells, fat cells and macrophages. Numerous macrophages are present in the cortex for phagocytosis of generating lymphocytes that were programmed against “self” antigens and, thus, must be destroyed.

Medulla is the inner portion of the parenchyma that contains a lesser number of lymphocytes and T lymphoblasts and more epithelial-reticular cells than the cortex.

Thymic or Hassall’s corpuscles are a distinctive feature of the thymic medulla.

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