The Circulatory System

Dr. Reem A.R. Alsaad
Medicine College of university Babylon
1.Summarize the structural layers of the walls of all four heart chambers .
3. Outline the histological features of the pericardium, endocardium.
4. Outline of the innate cardiac conducting system.
5.Differntiate between the layers of arteries .
6. summarize the differences between types of veins .
7. Differentiate between the types of capillaries.

The Circulatory System
The circulatory system comprises both the blood and lymphatic vascular system . The circulatory system pumps and directs blood cells and substances carried in blood to all tissues of the body. It includes both the blood and lymphatic vascular systems, and in an adult the total length of its vessels is estimated at between 100,000 and 150,000 kilometers
? The heart propels blood through the system.
? Arteries , a series of vessels efferent from the heart that become smaller as they branch into the various organs, carry blood to the tissues.
? Capillaries , the smallest vessels, are the sites of O 2 , CO 2 ,
nutrient, and waste product exchange between blood and tissues. Together with the smallest arterial and venous branches carrying blood to and from them, capillaries in almost every organ form a complex network of thin, anastomosing tubules called the microvasculature or
microvascular bed.
? Veins result from the convergence of venules into a system of larger channels that continue enlarging as they approach the heart, toward which they carry the blood to be pumped again, the blood vascular system is composed of the following structures:
The heart , an organ whose function is to pump the blood .The arteries , a series of efferent vessels that become smaller as they branch , and whose function is to carry the blood , with nutrients and oxygen to the tissues.
The capillaries , the smaller blood vessels , constituting a complex network of thin tubules that anastomose profusely and through whose walls the interchange between blood and tissues takes place.
The veins , which result from the convergence of the capillaries into a system of channels .These channels become larger as they approach the heart, toward which they convey the blood to the pumped again.
The lymphatic vascular system begins in the lymphatic capillaries , closed – ended tubules that anastomose to from vessels of steadily increasing size , this vessels terminate in the blood vascular system emptying into the large vein near the heart ,one of the function of the lymphatic system is to return the fluid of the tissue spaces to the blood .The internal surface of all components of the blood and lymphatic systems is lined by a single layer of a squamous epithelium , called endothelium. The internal surface of all components of the blood and lymphatic systems is lined by a single layer of a squamous epithelium, called endothelium . As the interface between blood and the organs, cardiovascular endothelial cells have crucial physiologic and medical importance. Not only must endothelial cells maintain a selectively permeable, antithrombogenic (inhibitory to clot formation) barrier, they also determine when and where white blood cells leave the circulation for the interstitial space of tissues and secrete a variety of paracrine factors for vessel dilation, constriction, and growth of adjacent cells.
it is customary to divide the circulatory system into the microvasculature , vessels that are more than 0.1 mm in diameter (large arterioles , muscular and elastic arteries , and muscular veins ), and the microvasculature (arterioles , capillaries , and post capillary venules ). Visible only with a microscope ,the microvasculature is particularly important as the site of interchanges between the blood and surrounding tissues under normal condition and in the event of inflammatory processes

Vascular Wall
The vascular wall is composed of three basic structural constituents :
1-the endothelium
2-muscular tissues
3-connective tissue(includes elastic elements)
The endothelium is a special type of epithelium interposed as a semipermeable barrier between two compartments of the internal medium ,the blood plasma and the interstitial fluid , endothelium is highly differentiated to actively mediate and monitor the extensive bidirectional exchange of small molecules and to restrict the transport of some macromolecules.


The endothelium has another functions ,conversion of angiotensin I into angiotensin II, and conversion of bradykinin; serotonin ;prostaglandins ;norepinephrine ; thrombin ; etc. …to biologically inter compounds and another function Lipolysis of lipoproteins and the last function the Production of vasoactive factors .
2- muscular tissues :vascular smooth muscle tissue is present in all vessels except capillary and pericytic venules .Smooth muscle cells are frequent and are arranged in helical layers in the tunica media of the blood vessel ,each muscle cell is enclosed by a basal lamina and by variable amounts of connective tissue both secreted by it. Vascular smooth muscle cells ,mainly of arterioles and small arteries , are frequently connected by communicating (gap) junctions.
Pericytes are multi-functional cells that wrap around the endothelial cells that line the capillaries and venules throughout the body Pericytes are embedded in basement membrane where they communicate with endothelial cells of the body s smallest blood vessels by means of both direct physical contact and paracrine signaling.

3-Connective Tissue: the vascular connective tissue component from connective tissue are present in the walls of blood vessels in amounts and proportions that vary based on local functional requirements. Collagen fibers , a ubiquitous element in the vascular system wall , are found between muscle cells , in adventitia and in some sub endothelial layers , elastic fibers guarantee the resilient shrinkage of the expanded vascular wall, these fibers predominate in large arteries where they are organized in parallel lamellae regularly distributed between the muscle cells throughout the entire media ,Ground substance forms a heterogeneous gel in the extracellular spaces of the vessel wall .It contributes to the physical properties of the walls of the vessels and probably affects the diffusion and permeability across the wall. The concentration of glycosaminoglycan is higher in arterial than in venous tissue.

Structural Plan of Blood Vessels
1-Tunca Intima
The intima consists of one layer of endothelial cells supported by a sub endothelial layer of loose connective tissue containing occasional smooth muscle cells .In arteries the intima is separated from the media by an internal elastic lamina , the most external component of the intima .this lamina composed of elastin , has gaps (fenestrae) that allow the diffusion of substances to nourish cells deep in the vessel wall .As a result of the absence of blood pressure and the contraction of the vessel at death , the tunica intima of the arteries generally has an undulating appearance in tissue sections


2-Tunca Media
The media consists primarily of concentric layers of helically arranged smooth muscle cells .Interposed among these cells are variable amounts of elastic fibers and lamellae reticular fibers (collgen type III), and glycoproteins , smooth muscle cells are the cellular source of this extracellular matrix .In arteries , the media has a thinner external elastica lamina , which separates it from the yunica adventitia.
3- Tunica Adventitia
The adventitia consists principally of collagen and elastic fibers collagen in the adventitia is type I the adventitia layer gradually becomes continuous with the connective tissue of the organ through which the vessel runs
Vasa Vasorum
Large vessels usually have vasa vasorum which are arterioles , capillaries and venules that branch profusely in the adventitia and the outer part of the media , the vasa vasorum provide metabolites to the adventitia and the media , since in largrvessels the layers are too thick to be nourished solely by diffusion from the blood in the lumen .



Large Elastic Arteries
Large elastic arteries help to stabilize the blood flow . the elastic arteries include the aorta and its large branches they have a yellowish color from the accumulation of elastin in the media .
Carotid Sinuses
Carotid sinuses are slight dilatation of the internal carotid arteries . these sinuses contain baroreceptors that detect changes in blood pressure and relay the information to the central nervous system .The arterial media layer of the sinus is thinner to allow it to respond to changes in blood pressure , the intima and impulses are processed in the brain to control vasoconstriction and maintain normal blood pressure.
Arterioles
The arterioles are generally less than 0.5 mm in diameter and have relatively narrow lumens , the subendothelial layer is very thin. In the very small arterioles , the internal elastic lamina is absent , and the media is generally composed of one or two circularly arranged layers of smooth muscle cells , it shows no external elastic lamina .Above the arterioles are small arteries in which the tunica media is more developed , and the lumens are larger than those of the arterioles .In both arterioles and small arteries , the tunica adventitia is very thin.

VASCULATURE
Large blood vessels and those of the microvasculature branch frequently and undergo gradual transitions into structures with different histologic features and functions.
Elastic Arteries
Elastic arteries are the aorta, the pulmonary artery, and their largest branches; these large vessels are also called conducting arteries because their major role is to carry blood to smaller arteries. the most prominent feature of elastic arteries is the thick media in which elastic lamellae, each about 10 µm thick, alternate with layers of smooth muscle fibers. The adult aorta has about 50 elastic lamellae (more if the individual is hypertensive).
The intima: is well developed, with many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood pressure and contraction of the vessel at death . The internal elastic lamina is not easily discerned because it is similar to the elastic laminae of the next layer . The adventitia is much thinner than the media. The numerous elastic laminae of these arteries contribute to their important function of making blood flow more uniform. During ventricular contraction (systole), blood is moved through the arteries forcefully and the elastin is stretched, distending the wall within the limit set by the wall´s collagen. When the ventricles relax (diastole), ventricular pressure drops to a low level, but the elastin rebounds passively, helping to maintain arterial pressure. The aortic and pulmonary valves prevent backflow of blood into the heart, so the rebound continues the blood flow away from the heart. Arterial blood pressure and blood velocity decrease and become less variable as the distance from the heart increases.
Arterial Sensory Structures:
Carotid sinuses are slight dilations of the bilateral internal carotid arteries where they branch from the (elastic) common carotid arteries; they act as important baroreceptors monitoring arterial blood pressure. At these sinuses the media is thinner, allowing greater distension when blood pressure rises, and the adventitia contains many sensory nerve endings from cranial nerve IX, the glossopharyngeal nerve. The brain’s vasomotor centers process these afferent impulses and adjust vasoconstriction, maintaining normal blood pressure. Functionally
similar baroreceptors are also present in the aortic arch.
Histologically more complex chemoreceptors that monitor blood levels of CO2 and O2, as well as its hydrogen ion concentration (pH), are found in the carotid bodies and aortic bodies, located in the walls of the carotid sinuses and aortic arch, respectively. These structures are parts of
the autonomic nervous system called paraganglia with rich capillary networks. The capillaries are closely surrounded by numerous, large, neural crest-derived glomus (type I) cells filled with dense-core vesicles containing dopamine, acetylcholine, and other neurotransmitters, which are supported by smaller satellite (type II) cells. Appropriate ion channels in the glomus cell membranes respond to stimuli in the arterial blood, primarily hypoxia (low O2), hypercapnia (excess CO2), or acidosis, by activating release of neurotransmitters. Sensory fibers branching from the glossopharyngeal nerve form synapses with the glomus cells and signal brain centers to initiate cardiovascular and respiratory adjustments that correct the condition.
Muscular Arteries
The muscular arteries distribute blood to the organs and help regulate blood pressure by contracting or relaxing the smooth muscle in the media. The intima has a very thin subendothelial layer and a prominent internal elastic lamina. The media may contain up to 40 layers of large smooth muscle cells interspersed with a variable number of elastic lamellae (depending on the size of the vessel). An external elastic lamina, the last component of the media, is present only in the larger muscular arteries. The adventitia consists of connective tissue. Lymphatic capillaries, vasa vasorum, and nerves are also found in the adventitia, and these structures may penetrate to the outer part of the media.


Arterioles
Muscular arteries branch repeatedly into smaller and smaller arteries, until reaching a size with three or four medial layers of smooth muscle. The smallest arteries branch as arterioles, which have only one or two smooth muscle layers; these indicate the beginning of an organ´s microvasculature where exchanges between blood and tissue fluid occur. Arterioles are generally less than 0.1 mm in diameter, with lumens approximately as wide as the wall is thick. The subendothelial layer is very thin, elastic laminae are absent, and the media consists of the circularly arranged smooth muscle cells. In both small arteries and arterioles, the adventitia is very thin and unobtrusive.
Arterioles almost always branch to form anastomosing networks or beds of capillaries that surround the parenchymal cells of the organ. Smooth muscle fibers act as sphincters closing arterioles and producing periodic blood flow into capillaries . Acting as “resistance vessels,” muscle tone usually keeps arterioles partially closed and makes these vessels the major determinants of systemic blood pressure.

Veins
Veins carry blood back to the heart from microvasculature all over the body. Blood entering veins is under very low pressure and moves toward the heart by contraction of smooth muscle fibers in the media and by external compressions from surrounding muscles and other organs. Valves project from the tunica intima to prevent backflow of blood. Most veins are small or medium veins , with diameters of 10
mm or less . Such veins are usually located close and parallel to corresponding muscular arteries.
The intima usually has a thin subendothelial layer, and the media consists of small bundles of smooth muscle cells intermixed with reticular fibers and a delicate network of elastic fibers.
The collagenous adventitial layer is well developed. The big venous trunks, paired with elastic arteries close to the heart, are the large veins Large veins have a well-developed intima.
The media: is relatively thin, with alternating layers of smooth muscle and connective tissue.
The adventitial layer: is thicker than the media in large veins and frequently contains longitudinal bundles of smooth muscle. Both the media and adventitia contain elastic fibers, but internal and external elastic laminae like those of arteries are not present.
Medium and large veins have valves consisting of paired folds of the intima projecting across the lumen . They are rich in elastic fibers and are lined on both sides by endothelium. The valves, which are especially numerous in veins of the legs, help keep the flow of venous blood directed toward the heart
Muscular Veins
Most venues are muscular , with at least a few smooth muscle cells in their walls .These vessels usually accompany arterioles from which they are easily distinguished in sectioned tissues because their thinner wall and irregular and collapsed lumen , these venues may also influence blood flow in the arterioles by producing and secreting diffusible vasoactive substances.
Capillaries
Capillaries have structural variations to permit different levels of metabolic , exchange between blood and surrounding tissues , they are composed of a single layer of endothelial cells rolled up in the form of a tube. they can be grouped into three types , depending on the continuity of both the endothelial sheet and the basal lamina

1-The continuous or somatic capillaries :are characterized by the absence of fenestrae in their wall .they are found in all types of muscle tissue , connective tissue , exocrine glands and nervous tissue .In some places , but not in the nervous system , numerous pinocytotic vesicles are present on both surfaces of endothelial cells .Pinocytotic vesicles appear as isolated vesicles in the cytoplasm of these cells .They can also fuse forming transendothelial channels , responsible for the transport of macromolecules in both directions across the endothelial cytoplasm.
2-The fenestrated , or visceral , capillaries : are characterized by the presence of several circular trans cellular opening in the endothelium membrane called fenestrae ,fenestrae are limited by the cell membrane resulting in a continuous cell membrane channel from the blood front to the tissue front
3-The discontinuous sinusoidal capillaries : the third type have the following characteristics
a-it have a tortuous path and greatly enlarged diameter which slows the circulation of blood
b-the endothelial cells form a discontinuous layer and are separated from one another by wide spaces .
c-the cytoplasm of the endothelial cells has multiple fenestrations without diaphragms
d-Macrophages are located either among or outside the cells of the endothelium
e-the basal lamina is discontinuous.
Sinusoidal capillaries are found mainly in the liver and in bone marrow and spleen.



Heart
Cardiac muscle in the four chambers of the heart wall contracts rhythmically, pumping the blood through the circulatory system . The right and left ventricles propel blood to the pulmonary and systemic circulation, respectively; right and left atria receive blood from the body and the pulmonary veins, respectively. The walls of all four heart chambers consist of three major layers: the internal endocardium; the middle myocardium; and the external epicardium.
The endocardium consists of a very thin inner layer of endothelium and supporting connective tissue, a middle myoelastic layer of smooth muscle fibers and connective tissue, and a deep layer of connective tissue called the subendocardial layer that merges with the myocardium, are also located in the subendocardial layer .
The myocardium, consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber. Because strong force is required to pump blood through the systemic and pulmonary circulations, the myocardium is much thicker in the walls of the ventricles, particularly the left, than in the atrial walls .
The epicardium is a simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves . The epicardium corresponds to the visceral layer of the pericardium, the membrane surrounding the heart. Where the large vessels enter and leave the heart, the epicardium is reflected back as the parietal layer lining the pericardium. During heart movements, underlying structures are cushioned by deposits of adipose tissue in the epicardium and friction within the pericardium is prevented by lubricant fluid produced by both layers of serous mesothelial cells.
Within these major layers the heart contains other structures important for its overall function of moving blood. Dense fibrous connective tissue of the cardiac skeleton forms part of the interventricular and interatrial septa, surrounds all valves of the heart, and extends into the valve cusps and the chordae tendineae to which they are attached .
These regions of dense irregular connective tissue perform the
following functions:
? Anchoring and supporting the heart valves
? Providing firm points of insertion for cardiac muscle
? Helping coordinate the heartbeat by acting as electrical insulation between atria and ventricles.
Within the subendocardial layer and adjacent myocardium, modified cardiac muscle cells make up the impulse conducting system of the heart, which generates and propagates waves of depolarization that spread through the myocardium to stimulate rhythmic contractions. This system (consists of two nodes of specialized myocardial tissue in the right atrium: the sinoatrial (SA) node (or pacemaker) and the atrioventricular (AV) node, followed by the AV bundle (of His) and the subendocardial conducting network.
Located in the right atrial wall near the superior vena cava, the SA node is a 6- to 7-mm3 mass of cardiac muscle cells with smaller size, fewer myofibrils, and fewer typical intercalated disks than the neighboring muscle fibers. Impulses initiated by these cells move along the myocardial fibers of both atria, stimulating their contraction. When the impulses reach the slightly smaller AV node, located in the floor of the right atrium near the AV valve and composed of cells similar to those of the SA node, they stimulate depolarization of those cells. Conducting muscle fibers from the AV node form the AV bundle, pass through an opening in the cardiac skeleton into the interventricular septum, and bifurcate into the wall of each ventricle.
At the apex of the heart, the bundles branch further into a subendocardial conducting network of myofibers, usually called Purkinje fibers. These are pale-staining fibers, larger than the adjacent contractile muscle fibers, with sparse, peripheral myofibrils and much glycogen . Purkinje fibers mingle distally with contractile fibers of
both ventricles and trigger waves of contraction through both ventricles simultaneously.
Both parasympathetic and sympathetic neural components innervate the heart. Ganglionic nerve cells and nerve fibers are present in the regions close to the SA and AV nodes, where they affect heart rate and rhythm, such as during physical exercise and emotional stress. Stimulation
of the parasympathetic division (vagus nerve) slows the heartbeat, whereas stimulation of the sympathetic nerve accelerates activity of the pacemaker. Between fibers of the myocardium are afferent free nerve endings that register pain, such as the discomfort called angina pectoris
that occurs when partially occluded coronary arteries cause local oxygen deprivation.

The atrioventricular bundle is formed by cells similar to those of the atrioventricular node .Distally , however , these cells become larger than ordinary cardiac muscle cells and acquire a distinctive appearance. These so-called Purkinje cells have one or two central nuclei and their cytoplasm is rich in mitochondria and glycogen .The myofibrils are sparse and are restricted to the periphery of the cytoplasm, after traveling in the subendocardic layer , they penetrate the ventricle and became intramyocardic .This arrangement is important because it allows the stimulus to get into the innermost layers of the ventricular musculature.

Both the parasympathetic and sympathetic divisions of the autonomic system contribute to innervation of the heart and form widespread plexuses at the base of the heart..Ganglionic nerve cells and nerve fibers are present in the regions close to the sinoatrial and atrioventricular nodes .Although these nerves do not affect generation of the heartbeat , a process attributed to the sinoatrial ( pacemaker) node, they do affect heart rhythum ,such as during physical exercise and emotional stress .Stimulation of the parasympathetic division ( vagus nerve) slows the heartbeat , whereas stimulation of the sympathetic nerve accelerates the rhythm of the pacemaker.
Between the muscular fibers of the myocardium are numerous afferent free nerve endings that are related to sensibility and pain ,partial obstruction of the coronary artiers reduces the supply of oxygen to the myocardium and causes pain ( angina pectoris ) the same sensorial enervation occurs during a heart attack , which is very paiful because many muscular fibers die as a result of the low levels of oxygen.


Lymphatic Vascular System
The lymphatic vascular system returns the extracellular liquid to the bloodstream. .In addition to blood vessels, the human body has a system of endothelium-lined thin-walled channels that collects fluid from the tissue spaces and returns it to the blood .This fluid is called lymph unlike blood ,it circulates in only one direction , toward the heart .The lymphatic capillaries originate in the various tissues as thin closed-ended vessels that consists of a single layer of endothelium and an incomplete basal lamina , lymphatic capillaries are held open by numerous micro fibrils of the elastic fiber system, which also bind them firmly to the surrounding connective tissue
The thin lymphatic vessels gradually converge and ultimately end up as two large trunks – the thoracic duct and the right lymphatic duct – that empty into the junction of the left internal jugular vein with the left subclavian vein and into the confluence of the right subclavian vein and the right internal jugular