Human_Histology

138 Textbook of Human Histology

Fig. 11.12: Parts of a nail as seen in a longitudinal section

(Schematic representation)

look pink in color.

When we view a nail in longitudinal section (Fig. 11.12) it is seen that the nail rests on the cells of the germinative zone (stratum spinosum and stratum basale). The germinative zone is particularly thick near the root of the nail where it forms the germinal matrix. The nail substance is formed mainly by proliferation of cells in the germinal matrix. However, the superficial layers of the nail are derived from the proximal nail fold.

When viewed from the surface (i.e. through the nail substance) the area of the germinal matrix appears white (in comparison to the pink color of the rest of the nail). Most of this white area is overlapped by the fold of skin (proximal nail fold) covering the root of the nail, but just distal to the nail fold a small semilunar white area called the lunule is seen (Fig. 11.13). The lunule is most conspicuous in the thumb nail. The germinal matrix is connected to the underlying bone (distal phalanx) by fibrous tissue.

The germinative zone underlying the body of the nail (i.e. the nail bed) is much thinner than the germinal matrix. It does not contribute to the growth of the nail; and is, therefore, called the sterile matrix. As the nail grows it slides distally over the sterile matrix. The dermis that lies deep to the sterile matrix does not show the usual dermal papillae. Instead it shows a number of parallel, longitudinal ridges. These ridges look like very regularly arranged papillae in transverse sections through a nail.

The root of the nail is overlapped by a fold of skin called the proximal nail fold. The greater part of each lateral margin of the nail is also overlapped by a skin fold called the lateral nail fold. The groove between the lateral nail

Fig. 11.13: Lunule of a nail (Schematic representation)

fold and the nail bed (in which the lateral margin of the nail lies) is called the lateral nail groove.

The stratum corneum lining the deep surface of the proximal nail fold extends for a short distance on to the surface of the nail. This extension of the stratum corneum is called the eponychium. The stratum corneum lining the skin of the finger tip is also reflected onto the undersurface of the free distal edge of the nail: this reflection is called the hyponychium.

The dermis underlying the nail bed is firmly attached to the distal phalanx. It is highly vascular and contains arteriovenous anastomoses. It also contains numerous sensory nerve endings.

Growth of Nails

Nails undergo constant growth by proliferation of cells in the germinal matrix. Growth is faster in hot weather than in cold. Finger nails grow faster than toe nails. Nail growth can be disturbed by serious illness or by injury over the nail root, resulting in transverse grooves or white patches in the nails. These grooves or patches slowly grow toward the free edge of the nail. If a nail is lost by injury a new one grows out of the germinal matrix if the latter is intact.

Pathlogical Correlation

•Onychia: It is the inflammation of nail folds and shedding of nail resulting due to the introduction of microscopic pathogens through small wounds.

•Onycholysis: It is characterized by the loosening of exposed portion of nail from nail bed. It usually begins at the free edge and continues to lunula.

•Paronychia: It is caused due to bacterial or fungal infection producing change in the shape of nail plate.

•Koilonychia: It is caused due to iron deficiency or Vit B12 deficiency and is characterized by abnormal thinness and concavity (spoon-shape) of the nails.

Chapter

12

The Cardiovascular System

The cardiovascular system consists of the heart and blood vessels. The blood vessels that take blood from the heart to various tissues are called arteries. The smallest arteries are called arterioles. Arterioles open into a network of capillaries that pervade the tissues. Exchanges of various substances between the blood and the tissues take place through the walls of capillaries. In some situations, capil­ laries are replaced by slightly different vessels called sinusoids. Blood from capillaries (or from sinusoids) is collected by small venules that join to form veins. The veins return blood to the heart.

Blood vessels deliver nutrients, oxygen, and hormones to the cells of the body and remove metabolic base products and carbon dioxide from them.

ENDOTHELIUM

The inner surfaces of the heart, and of all blood vessels are lined by flattened endothelial cells (also called endotheliocytes). On surface view the cells are polygonal, and elongated along the length of the vessel. Cytoplasm is sparse.

The cytoplasm contains endoplasmic reticulum and mitochondria. Microfilaments and intermediate filaments are also present, and these provide mechanical support to the cell. Many endothelial cells show invaginations of cell membrane (on both internal and external surfaces). Sometimes the inner and outer invaginations meet to form channels passing right across the cell (seen typically in small arterioles). These features are seen in situations where vessels are highly permeable.

Adjoining endothelial cells are linked by tight junctions, and also by gap junctions. Externally, they are supported by a basal lamina.

Functions of Endothelium

Apart from providing a smooth internal lining to blood vessels and to the heart, endothelial cells perform a number of other functions as follows:

Endothelial cells are sensitive to alterations in blood pressure, blood flow, and in oxygen tension in blood.

They secrete various substances that can produce vasodilation by influencing the tone of muscle in the vessel wall.

They produce factors that control coagulation of blood. Under normal conditions clotting is inhibited. When required, coagulation can be facilitated.

Under the influence of adverse stimuli (e.g., by cytokines) endothelial cells undergo changes that facilitate passage of lymphocytes through the vessel wall. In acute inflam­ mation, endothelium allows neutrophils to pass from blood into surrounding tissues.

Under the influence of histamine (produced in allergic states) endothelium becomes highly permeable, allow­ ing proteins and fluid to diffuse from blood into tissues. The resultant accumulation of fluid in tissues is called edema.

Note: Changes in properties of endothelium described above take place rapidly (within minutes).

ARTERIES

Basic Structure of Arteries

The histological structure of an artery varies considerably with its diameter. However, all arteries have some features in common which are as follows (Fig. 12.1):

The wall of an artery is made up of three layers

The innermost layer is called the tunica intima (tunica = coat). It consists of:

•An endothelial lining

•A thin layer of glycoprotein which lines the external aspect of the endothelium and is called the basal lamina

•A delicate layer of subendothelial connective tissue

•A membrane formed by elastic fibers called the internal elastic lamina.

140 Textbook of Human Histology

Outside the tunica intima there is the tunica media or middle layer. The media may consist predomi­ nantly of elastic tissue or of smooth muscle. Some connective tissue is usually present. On the outside the media is limited by a membrane formed by elastic fibers, this is the external elastic lamina.

The outermost layer is called the tunica adventitia. This coat consists of connective tissue in which collagen fibers are prominent. This layer prevents undue stretching or distension of the artery.

The fibrous elements in the intima and the adventitia (mainly collagen) run longitudinally (i.e. along the length of the vessel), whereas those in the media (elastic tissue or muscle) run circularly. Elastic fibers, including those of the internal and external elastic laminae are often in the form of fenestrated sheets (fenestrated = having holes in it).

Elastic and Muscular Arteries

On the basis of the kind of tissue that predominates in the tunica media, arteries are often divided into:

Elastic arteries (large or conducting arteries)

Muscular arteries (medium arteries)

Elastic arteries include the aorta and the large arteries

supplying the head and neck (carotids) and limbs (sub­ clavian, axillary, iliac). The remaining arteries are muscular (Table 12.1).

Table 12.1: Comparison between elastic artery and muscular artery

Layers	Elastic artery	Muscular artery
Adventitia	It is relatively thin with	It consists of thin layer of
	greater proportion of	fibroelastic tissue.
	elastic fibers.
Media	Made up mainly of elastic	Made up mainly of
	tissue in the form of	smooth muscles
	fenestrated concentric	arranged circularly
	membranes. There may
	be as many as fifty layers
	of elastic membranes.
Intima	It is made up of	Intima is well developed,
	endothelium,	specially internal elastic
	subendothelial	lamina which stands out
	connective tissue and	prominently.
	internal elastic lamina.
	The subendothelial
	connective tissue
	contains more elastic
	fibers. The internal elastic
	lamina is not distinct.

Although all arteries carry blood to peripheral tissues, elastic and muscular arteries play differing additional roles.

Elastic Arteries

When the left ventricle of the heart contracts, and blood enters the large elastic arteries with considerable force,

Fig. 12.1: Layers in the wall of a typical artery (Schematic representation)

Fig. 12.2: Elastic artery (Schematic representation). The left half of the figure shows the appearance in a section stained with hema­ toxylin and eosin. The right half shows the appearance in a section stained by a special method that makes elastic fibers evident. (With this method the elastic fibers are stained black, muscle fibers are yellow, and collagen is pink). 1–tunica intima; 2–tunica media con­ taining abundant elastic tissue arranged in the form of a number of membranes; 3–tunica adventitia

these arteries distend significantly. They are able to do so because of much elastic tissue in their walls. During diastole (i.e. relaxation of the left ventricle) the walls of the arteries come back to their original size because of the elastic recoil of their walls. This recoil acts as an additional force that pushes the blood into smaller arteries. It is because of this fact that blood flows continuously through arteries (but with fluctuation of pressure during systole and diastole).

The elastic arteries are also called as conducting vessels as their main function is to conduct the blood from heart to muscular arteries.

Structure of Elastic Arteries (Fig. 12.2 and Plate 12.1)

Tunica intima: It is made up of endothelium, subendo­ thelial connective tissue and internal elastic lamina. The subendothelial connective tissue contains more elastic fibers in the elastic arteries. The internal elastic lamina is not distinct from the media as it has the same structure as the elastic membranes of the media.

Tunica media: The media is made up mainly of elastic tissue. The elastic tissue is in the form of a series of concentric membranes that are frequently fenestrated (Plate 12.1). In the aorta (which is the largest elastic artery) there may be as many as fifty layers of elastic mem­ branes. Between the elastic membranes there is some loose connective tissue. Some smooth muscle cells may be present.

Tunica adventitia: It is relatively thin in large arteries, in which a greater proportion of elastic fibers are present. These fibers merge with the external elastic lamina.

Chapter 12 The Cardiovascular System 141

Muscular Arteries

A muscular artery has the ability to alter the size of its lumen by contraction or relaxation of smooth muscle in its wall. Muscular arteries can, therefore, regulate the amount of blood flowing into the regions supplied by them, hence they are also called as distributing arteries.

Structure of Muscular Arteries

The muscular arteries differ from elastic arteries in having more smooth muscle fibers than elastic fibers. The transi­ tion from elastic to muscular arteries is not abrupt. In proceeding distally along the artery there is a gradual reduction in elastic fibers and increase in smooth muscle content in the media.

Tunica intima: The internal elastic lamina in the mus­ cular arteries stands out distinctly from the muscular media of smaller arteries.

Tunica media: It is made up mainly of smooth muscles (Plate 12.2). This muscle is arranged circularly. Between groups of muscle fibers some connective tissue is present, which may contain some elastic fibers. Longitu­ dinally arranged muscle is present in the media of arteries that undergo repeated stretching or bending. Examples of such arteries are the coronary, carotid, axillary, and palmar arteries.

Tunica adventitia.

Clinical Correlation

Atheroma

The most common disease of arteries is atheroma, in which the intimabecomesinfiltratedwithfatandcollagen.Thethickenings formed are atheromatous plaques.Atheromaleadstonarrowing of the arterial lumen, and consequently to reduced blood flow. Damage to endothelium can induce coagulation of blood forming a thrombus which can completely obstruct the artery. This leads to death of the tissue supplied. When this happens in an artery supplying the myocardium (coronary thrombosis) it leads to myocardial infarction (manifesting as a heart attack).Inthebrain (cerebral thrombosis) it leads to a stroke and paralysis. An arteryweakenedbyatheromamayundergodilation(aneurysm), or may even rupture.

ARTERIOLES

When traced distally, muscular arteries progressively decrease in calibre till they have a diameter of about 100 µm. They then become continuous with arterioles (Fig. 12.3). The larger or muscular arterioles are 100 to 50 µm in diameter. Arterioles less than 50 µm in diameter are called terminal arterioles. All the three layers, i.e. tunica adven­ titia, tunica media and tunica intima are thin as compared

142 Textbook of Human Histology

Plate 12.1: Elastic Artery

Elastic artery is characterized by presence of:

Tunicaintimaconsistingofendothelium,subendothelialconnec­ tive tissue, and internal elastic lamina

Thefirstlayerofelasticfibersiscalledtheinternalelasticlamina. The internal elastic lamina is not distinct from the elastic fibers of media

Well developed subendothelial layer in tunica intima

Thick tunica media with many elastic fibers and some smooth muscle fibers

Tunica adventitia containing collagen fibers with several elastic fibers

Vasa vasorum in the tunica adventitia (Not seen in this slide).

A

Key

1. Endothelium

2. Subendothelial connective tissue Tunica intima

3. Internal elastic lamina

4. Tunica media

5. Tunica adventitia

B

Elastic artery. A. As seen in drawing; B. Photomicrograph

to arteries. In arterioles, the adventitia is made up of a thin network of collagen fibers.

Arterioles are the main regulators of peripheral vascular resistance. Contraction and relaxation of the smooth mus­ cles present in the walls of the arterioles can alter the peripheral vascular resistance (or blood pressure) and the blood flow.

Muscular arterioles can be distinguished from true arteries:

By their small diameter

They do not have an internal elastic lamina. They have a few layers of smooth muscle in their media.

Terminal arterioles can be distinguished from muscular

arterioles as follows:

Contd...

Contd...

They have a diameter less than 50 µm, the smallest terminal arterioles having a diameter as small as 12 µm.

They have only a thin layer of muscle in their walls.

They give off lateral branches (called meta­arterioles) to the capillary bed.

The initial segment of each lateral branch is surrounded by a few smooth muscle cells. These muscle cells constitute the precapillary sphincter. This sphincter regulate the flow of blood to the capillaries.

CAPILLARIES

Terminal arterioles are continued into a capillary plexus that pervades the tissue supplied. Capillaries are the smallest blood vessels. The average diameter of a capillary

Chapter 12 The Cardiovascular System 143

Plate 12.2: Muscular (Medium Size) Artery

In musculararteries,thetunicaintimaismade up of endothelium and internal elastic lamina (arrow), which is thrown into wavy folds due to contraction of smooth muscle in the mediaTunica media is composed mainly of smooth

muscle fibers arranged circularly

Tunica adventitia contains collagen fibers and

few elastic fibers.

A

Key

1. Tunica intima

2. Tunica media

3. Tunica adventitia

B

Muscular (medium size) artery. A. As seen in drawing; B. Photomicrograph

Fig. 12.3: Photomicrograph showing an arteriole and a venule

is 8 µm. Exchanges (of oxygen, carbon dioxide, fluids, and various molecules) between blood and tissue take place through the walls of the capillary plexus (and through postcapillary venules). The arrangement of the capillary plexus and its density varies from tissue to tissue, the density being greatest in tissues having high metabolic activity.

Structure of Capillaries

The wall of a capillary is formed essentially by endothelial cells that are lined on the outside by a basal lamina (glyco­ protein). Overlying the basal lamina there may be isolated branching perivascular cells (pericytes), and a delicate net­ work of reticular fibers and cells. Pericyte or adventitial cells contain contractile filaments in the cytoplasm and can transform into other cells.

Types of Capillaries

There are two types of capillaries:

1.Continuous

2.Fenestrated

Continuous Capillaries

Typically, the edges of endothelial cells fuse completely with those of adjoining cells to form a continuous wall. Such capillaries are called continuous capillaries (Figs. 12.4A and B).

In continuous capillaries exchanges of material between blood and tissue take place through the cytoplasm of

144 Textbook of Human Histology

A		A

B

Figs. 12.4A and B: Structure of continuous capillary. (A) Circular section; (B) Longitudinal section (Schematic representation)

B

Figs. 12.5A and B: Structure of fenestrated capillary. (A) Circular section; (B) Longitudinal section (Schematic representation)

A

B

Figs. 12.6A and B: Structure of sinusoid. (A) Circular section;

(B) Longitudinal section (Schematic representation)

endothelial cells. This is suggested by the presence of numerous pinocytotic vesicles in the cytoplasm; and by the presence of numerous depressions (caveolae) on the cell surfaces, which may represent pinocytotic vesicles in the process of formation. Apart from transport through the cytoplasm, substances may also pass through the inter­ cellular material separating adjoining endothelial cells.

Continuous capillaries are seen in the skin, connective tissue, muscle, lungs, and brain.

Fenestrated Capillaries

In some organs the walls of capillaries appear to have apertures in their endothelial lining, these are, therefore,

called fenestrated capillaries (Figs. 12.5A and B). The “apertures” are, however, always closed by a thin diaph­ ragm (which may represent greatly thinned out cytoplasm of an endothelial cell, or only the basal lamina).

Some fenestrations represent areas where endothelial cell cytoplasm has pores passing through the entire thickness of the cell.

In the case of fenestrated capillaries diffusion of sub­ stances takes place through the numerous fenestrae in the capillary wall.

Fenestrated capillaries are seen in renal glomeruli, intestinal villi, endocrine glands, and pancreas.

SINUSOIDS

In some tissues the “exchange” network is made up of vessels that are somewhat different from capillaries, and are called sinusoids (Figs. 12.6A and B).

Sinusoids are found typically in organs that are made up of cords or plates of cells. These include the liver, the adrenal cortex, the hypophysis cerebri, and the parathyroid glands. Sinusoids are also present in the spleen, in the bone marrow, and in the carotid body.

The wall of a sinusoid consists only of endothelium supported by a thin layer of connective tissue. The wall may be incomplete at places, so that blood may come into direct contact with tissue cells. Deficiency in the wall may be in the form of fenestrations (fenestrated sinusoids) or in the form of long slits (discontinuous sinusoids, as in the spleen).

At some places the wall of the sinusoid consists of phagocytic cells instead of endothelial cells.

Chapter 12 The Cardiovascular System 145

Plate 12.3: Vein

A

C

Vein. A. As seen in drawing; B. Photomicrograph

(low magnification); C. Photomicrograph (high magnification)

B

The vein has a thinner wall and a larger lumen than the artery

Thetunicaintima,media,andadventitiacanbemadeout,buttheyare not sharply demarcated

Themediaisthinandcontainsamuchlargerquantityofcollagenfibers than arteries. The amount of elastic tissue or of muscle is much less

The adventitia is relatively thick and contains considerable amount of elastic and muscle fibers.

Note: Theluminalsurfaceappearsasadarkline,withanoccasionalnucleus alongit.

Key

1.Tunica intima

2.Tunica media

3.Tunica adventitia Cf. Collagen fibers

Sm. Smooth muscles

Sinusoids have a broader lumen (about 20 µm) than capillaries. The lumen may be irregular. Because of this fact blood flow through them is relatively sluggish.

VEINS

The basic structure of veins is similar to that of arteries. The tunica intima, media, and adventitia can be distinguished, specially in large veins. The structure of veins differs from that of arteries in the following respects (Fig. 12.7 and Plate 12.3):

The wall of a vein is distinctly thinner than that of an artery having the same sized lumen.

The tunica media contains a much larger quantity of collagen than in arteries. The amount of elastic tissue or of muscle is much less.

Because of the differences mentioned above, the wall of a vein is easily compressed. After death veins are usually collapsed. In contrast arteries retain their patency.

In arteries the tunica media is usually thicker than the adventitia. In contrast the adventitia of veins is thicker than the media (specially in large veins). In some large veins (e.g. the inferior vena cava) the adventitia contains a considerable amount of elastic and muscle fibers that run in a predominantly longitudinal direc­ tion. These fibers facilitate elongation and shortening of the vena cava with respiration. This is also facilita­ ted by the fact that collagen fibers in the adventitia form a meshwork that spirals around the vessel.

A clear distinction between the tunica intima, media and adventitia cannot be made out in small veins as

146 Textbook of Human Histology

Fig. 12.7: Mediu­sized artery (above) and vein (below). The left half of the figure shows the appearance as seen with hematoxylin and eosin staining. The right half shows appearance when elastic fibers are stained black.1–internal elastic lamina; 2–tunica media;

3–tunica adventitia, A–artery; V–vein (Schematic representation)

all these layers consist predominantly of fibrous tissue. Muscle is conspicuous by its complete absence in venous spaces of erectile tissue, in veins of cancellous bone, dural venous sinuses, retinal veins, and placental veins.

Valves of Veins

Most veins contain valves that allow the flow of blood toward the heart, but prevent its regurgitation in the opposite direction. Typically each valve is made up of two semilunar cusps. Each cusp is a fold of endothelium within which there is some connective tissue that is rich in elastic fibers. Valves are absent in very small veins; in veins within the cranial cavity, or within the vertebral canal; in the venae cavae; and in some other veins.

Flow of blood through veins is assisted by contractions of muscle in their walls. It is also assisted by contrac­ tion of surrounding muscles specially when the latter are enclosed in deep fascia.

Clinical Correlation

Varicose Veins

Varicose veins are permanently dilated and tortuous superficial veins of the lower extremities, especially the long saphenous vein and its tributaries. About 10–12% of the general population develops varicose veins of lower legs, with the peak incidence in 4th and 5th decades of life. Adult females are affected more commonly than the males, especially during pregnancy. This is attributed to venous stasis in the lower legs because of compression on the iliac veins by pregnant uterus.

VENULES

The smallest veins, into which capillaries drain, are called venules (Fig. 12.3). They are 20–30 µm in diameter. Their walls consist of endothelium, basal lamina, and a thin adventitia consisting of longitudinally running collagen fibers. Flattened or branching cells called pericytes may be present outside the basal laminae of small venules (called postcapillary venules), while some muscle may be present in larger vessels (muscular venules).

Functionally, venules have to be distinguished from true veins. The walls of venules (specially those of postcapillary venules) have considerable permeability and exchanges between blood and surrounding tissues can take place through them. In particular venules are the sites at which lymphocytes and other cells may pass out of (or into) the blood stream.

BLOOD VESSELS, LYMPHATICS AND NERVES SUPPLYING BLOOD VESSELS

The walls of small blood vessels receive adequate nutri­ tion by diffusion from blood in their lumina. However, the walls of large and medium sized vessels are supplied by small arteries called vasa vasorum (literally “vessels of vessels”; singular = vas vasis). These vessels supply the adventitia and the outer part of the media. These layers of the vessel wall also contain many lymphatic vessels.

Blood vessels have a fairly rich supply by autonomic nerves (sympathetic). The nerves are unmyelinated. Most of the nerves are vasomotor and supply smooth muscle. Their stimulation causes vasoconstriction in some arteries, and vasodilatation in others. Some myelinated sensory nerves are also present in the adventitia.

MECHANISMS CONTROLLING BLOOD FLOW THROUGH THE CAPILLARY BED

The requirements of blood flow through a tissue may vary considerably at different times. For example, a muscle needs much more blood when engaged in active contraction, than when relaxed. Blood flow through intestinal villi needs to be greatest when there is food to be absorbed. The mechanisms that adjust blood flow through capillaries are considered below.

Blood supply to relatively large areas of tissue is con­ trolled by contraction or relaxation of smooth muscle in the walls of muscular arteries and arterioles. Control of supply to smaller areas is effected through arteriovenous anastomoses, precapillary sphincters, and thoroughfare channels.

Chapter 12 The Cardiovascular System 147

Fig. 12.8: An arteriovenous anastomosis (glomus)

(Schematic representation)

Arteriovenous Anastomoses

In many parts of the body, small arteries and veins are connected by direct channels that constitute arteriovenous anastomoses. These channels may be straight or coiled. Their walls have a thick muscular coat that is richly supplied with sympathetic nerves. When the anastomoses are patent blood is short circuited from the artery to the vein so that very little blood passes through the capillary bed. However, when the muscle in the wall of the anastomosing channel contracts its lumen is occluded so that all blood now passes through the capillaries. Arteriovenous anastomoses are found in the skin specially in that of the nose, lips and external ear; and in the mucous membrane of the alimentary canal and nose. They are also seen in the tongue, in the thyroid, in sympathetic ganglia, and in the erectile tissues of sex organs.

Arteriovenous anastomoses in the skin help in regulat­ ing body temperature, by increasing blood flow through capillaries in warm weather; and decreasing it in cold weather to prevent heat loss.

In some regions we see arteriovenous anastomoses of a special kind. The vessels taking part in these anastomoses are in the form of a rounded bunch covered by connective tissue. This structure is called a glomus (Fig. 12.8). Each glomus consists of an afferent artery; one or more coiled (S­shaped) connecting vessels; and an efferent vein.

Blood flow through the glomus is controlled in two different ways:

Firstly, the wall of the afferent artery has a number of elevations that project into the lumen; and probably have a valvular function. These projections are pro­ duced partly by endothelium, and partly by muscle.

Secondly, the connecting vessels have thick muscular walls in which the muscle fibers are short and thick with central nuclei. These cells have some resemblance to epithelial cells and are, therefore, termed epithelioid cells (Fig. 12.9). They have similarities to pericytes

Fig. 12.9: Section across the connecting channel of an arteriovenous anastomosis showing epithelioid cells (Schematic representation)

present around capillaries. The lumen of the connecting channel can be occluded by contraction (or swelling) of epithelioid cells.

Glomera are found in the skin at the tips of the fingers and toes (specially in the digital pads and nailbeds); in the lips; the tip of the tongue; and in the nose. They are concerned with the regulation of the circulation in these areas in response to changes in temperature.

Added Information

Arteriovenous anastomoses are few and inefficient in the newborn.Inoldage,again,arteriovenousanastomosesofthe skin decrease considerably in number. These observations are to be correlated with the fact that temperature regulation is not efficient in the newborn as well as in old persons.

Precapillary Sphincters and Thoroughfare Channels

Arteriovenous anastomoses control blood flow through relatively large segments of the capillary bed. Much smaller segments can be individually controlled as follows.

Capillaries arise as side branches of terminal arterioles. The initial segment of each such branch is surrounded by a few smooth muscle cells that constitute a precapillary sphincter (Fig. 12.10). Blood flow, through any part of the capillary bed, can be controlled by the precapillary sphincter.

In many situations, arterioles and venules are connected (apart from capillaries) by some channels that resemble capillaries, but have a larger calibre. These channels run a relatively direct course between the arteriole and venule. Isolated smooth muscle fibers may be present on their walls. These are called thoroughfare channels (Fig. 12.10). At times when most of the precapillary sphincters in the region are contracted (restricting flow through capillaries), blood is short circuited from arteriole to venule through the thoroughfare channels. A thoroughfare channel and the capillaries associated with it are sometimes referred to as a microcirculatory unit.