Circulatory System: Professor Meng Yunlian

CIRCULATORY SYSTEM

Professor Yunlian

Meng

Organs Several different kinds of tissues are organised in particular ways to form organs. From the structure view there are two kinds of organs in human body: cavity organs and parenchyma organs.

Cavity organs The wall of cavity organs can be divided into three or four layers.

The wall of blood vessels is made up of three layers: tunica intima , tunica media and tunica adventitia.

The parenchyma organs include three parts: ① capsule: connective tissue ② parenchyma: cortex and medulla ③interstitial substance: connective tissue, blood vessels, lymphatic vessels and nervous fibers.

Circulatory system We need a system to transport ① Metabolism substances: the nutrition and the oxygen to the tissues and cells, and waste products of metabolism and carbon dioxide out of tissues and cells. ② Hormone: distribute it to every part of our body . Circulatory system has this function.

Closed tubular system Cardiovascular system: heart , arteries, veins , capillaries Lymphatic vascular system: lymphatic capillaries, lymphatic vessels and lymphatic ducts

Cardiovascular System The cardiovascular system is the major circulatory system. It consists of the heart and of blood vessels. The heart as a pump propels the blood into arteries. The arteries is blood vessels that take blood from the heart to capillaries. The smallest arteries are called arterioles. Arterioles open into a network of capillaries. Exchanges of various substances between the blood and the tissues take place through the walls of capillaries. Blood from capillaries is collected by small venules which join to form veins. The veins return blood to the heart.

Lymphatic vascular system The lymphatic vascular system is the assistant circulatory system. It begins in the lymphatic capillaries which are closed-ended tubules that anastomose to form lymphatic vessels. Lymphatic vessels anastomose to form lymphatic ducts which terminate in the large vein near the heart. The function of lymphatic vascular system is to return the fluid of tissue spaces to the blood.

Diagram of Circulatory system

Heart As a pump

Veins Lymphatic ducts Lymphatic vessels Lymphatic capillaris

Arteries Capillaries

Waste products of metabolism Carbon dioxide

Nutriments Oxygen Tissue fluid Tissues and cells

General structure of wall of blood vessels Except for capillaries the wall of blood vessels is made up of three layers: tunica intima , tunica media and tunica adventitia. blood vessel -model.ppt Tunica intima It consists of endothelium, subendothelial layer and internal elastic lamina.

Endothelium The inner surface of circulatory system are lined by flattened endothelial cells. On surface view the cells are polygonal, and elongated along the length of the vessel. The cytoplasm is sparse.

EM:

There are some projections in the free surface of endothelial cells. The cytoplasm contains endoplasmic reticulum, mitochondria, pinocytic vesicles, microfilaments and intermediate filaments.

The pinocytic vesicles are formed by 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. These features are seen in situations where vessels are highly permeable. Adjoining endothelial cells are linked by tight junctions, and also by gap junctions.

Endothelial cells provide a smooth internal lining to blood vessels and to the heart. Also they mediate and monitor the bidirectional exchange of small molecules and to restrict the transport of some macromolecules. In addition to the functions mentioned above endothelial cells perform a number of other functions as follows.

①Endothelial cells are sensitive to alterations in blood pressure, in blood flow, and in oxygen tension in blood. ② They secrete various substances that can produce vasodilation by influencing the tone of muscle in the vessel wall. ③ They produce factors that control coagulation of blood. Under normal conditions clotting inhibited. When required, coagulation can be facilitated.

④ Under the influence of adverse stimuli endothelial cells undergo changes that facilitate passage of lymphocytes through the vessel wall. In acute inflammation, endothelium allows neutrophils to pass from blood into surrounding tissues. ⑤Under the influence of histamine (produced in allergic states by mast cells) endothelium becomes highly permeable, allowing proteins and fluid to diffuse from blood into tissues. The resultant accumulation of fluid in tissues is called oedema. Changes in properties of endothelium described above take place rapidly (within minutes).

Subendothelial layer A thin layer of connection tissue containing collagen fibers, elastic fibers and a few of smooth muscles. Internal elastic lamina wave-liked, pink-colored band formed by elastin. blood vessel -model.ppt medium-sized artery-1.ppt

Tunica media blood vessel -model.ppt

The tunica media may consist predominantly of elastic tissue(large artery) or of smooth muscle. Collagen fibers and elastic fibers are usually present. But there is no fibroblast. Vascular smooth muscle is present in all vessels except capillaries and postcapillary venules. Each muscle cells is enclosed by a basal lamina and by connective tissue both secreted by it. There are gap junctions between smooth muscle cells.

Tunica adventitia This coat consists of connective tissue in which collagen fibres are prominent. In arteries, especially in middle sized arteries, these is an external elastic lamina between tunica media and adventitia.

It is of interest to note that the fibrous elements and smooth muscle in the intima and the adventitia run longitudinally (i.e., along the length of the vessel), whereas those in the media run circularly. The internal and external elastic lamina are present in the form of fenestrated sheets.

Vasa vasorum in the wall of blood vessles The walls of large and medium sized vessels are supplied by vasa vasorum which are arterioles, capillaries and venules. These vessels supply the adventitia and the outer part of the media. These layers of the vessel wall also contain many lymphatic vessels.

Innervation Blood vessels have a 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 adventitia.

Arteries On the basis of the kind of tissue that predominates in the tunica media, the arteries are often divided into large elastic arteries, medium muscular arteries, small arteries and arterioles. Elastic arteries include the aorta and the large arteries supplying the head and neck (carotids) and limbs (subclavian, axillary, iliac). The remaining arteries are muscular.

Large elastic arteries Structural features Tunica intima The subendothelial layer is thick and internal elastic lamina is not prominent. Tunica media consists of 40 ～ 70 layers of elastic laminae. Between the elastic laminae there are some smooth muscle cells and reticular fibers, proteoglycans and glycoproteins. Tunica adventitia consists of connective tissue and the external elastic lamina is not prominent. large artery.ppt

Function Assistant pump Although all arteries carry blood to peripheral tissues, elastic and muscular arteries play different additional roles. When the left ventricle of the heart contracts, and blood enters the large elastic arteries with considerable force, these arteries distend significantly 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.

Medium-sized arteries( muscular arteries) Structural features Tunica intima The internal elastic lamina is prominent. Tunica media consists of 10 ～ 40 layers of smooth muscle arranged circularly. Between groups of muscle fibres some connective tissue is present, which may contain some elastic fibres. Tunica adventitia consists of connective tissue and the external elastic lamina is prominent. medium-sized artery-1.ppt

Function Distributing arteries The muscular arteries have the ability to alter the size of their lumen by contraction or relaxation of smooth muscle in their walls. Muscular arteries can, therefore, regulate the amount of blood flowing into the regions supplied by them. The transition from elastic to muscular arteries is not abrupt. In proceeding distally along the artery there is a gradual reduction in elastic fibres and increase in smooth muscle content in the media.

Small Arteries Structural features Diameter 0.5 ～ 1mm Internal elastic lamina is present. 3 ～ 9 layers smooth muscle in the tunica media. no external elastic lamina. -

small artery.ppt

Arterioles Structural features When traced distally, small arteries progressively decrease in caliber till they have a diameter less than 500μm, which are called Arterioles. They have no internal and external elastic laminae and have 1 ～ 2 layers of smooth muscle in their media. The adventitia of arterioles is formed by a thin network of collagen fibres. arterioles-venule.ppt

The terminal arterioles are the most thin branches of arterioles . The initial segment of each such branch is surrounded by a few smooth muscle cells that constitute a precapillary sphincter. Blood flow through any part of the capillary bed can be controlled by the precapillary sphincter. microcirculation-1.ppt

Carotid body and aortic body The carotid bodies are near the bifurcation of the common carotid artery. The carotid body consists of cell cords and fenestrated capillaries. The cells can be divided into typeⅠand type Ⅱcells. The typeⅠcells contain numerous dense-core vesicles that store dopamine, serotonin, and adrenaline. Some afferent nerve fibers end on the surface of typeⅠcells The type Ⅱ cells are supporting cells. carotid body.ppt

The carotid bodies as chemoreceptors are sensitive to low oxygen tension, high carbon dioxide concentraction, and low blood pH. Aortic bodies located on the arch of the aorta are similar in structure to carotid bodies and are believed to have a similar function.

Carotid sinuses Carotid sinuses contain baroreceptors that detect the changes in blood pressure and relay the information to the central nervous system. Carotid sinuses locate on the internal carotid arteries where the wall is thinner to allow it to respond to changes in blood pressure. The intima and the adventitia are very rich in nerve endings. The afferent nerve impulses are processed in the brain to control vasoconstriction and maintain normal blood pressure.

Atheroma The most common disease of arteries is atheroma, in which the intima becomes infiltrated with fat and collagen. Atheroma leads to narrowing of the arterial lumen, and consequently to reduce 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). In the brain (cerebral thrombosis) it leads to a stroke and paralysis. An artery weakened by atheroma may undergo dilation (aneurysm), or may even rupture.

Capillaries The most thin vessels in the body.

Form the capillary plexus , the total length of capillaries in human body: 10,000 km

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

Structure LM Endothelium Basement membrane Pericyte flattened with processes functions i. supporting ii. contraction ⅲ. undifferentiated cell

Pericyte

Scanning electron micrograph of a capillary.

EM: Classification and Structure Three types of capillaries

Fenestrated capillary

Continuous capillary

Sinusoid

Continuous capillaries Structure features Continuous endothelium containing more pinocytotic vesicles. Tight junctions. Continuous basement membrane. capillary1.ppt

Distribution–

skin, connective tissue, muscle, lungs and brain.

Exchange of substance through pinocytotic vesicles. –

Fenestrated capillaries Structure features Endothelial cells have fenestrae or pores. Each pore is obliterated by a diaphragm that is thinner than a cell membrane. Tight junction. Continuous basement membrane. capillary2.ppt Distribution Gastrointestinal tract, endocrine gland and renal glomerulus. Some big molecular substance can pass the wall of this kind of capillaries more easily.

Discontinous sinusoid capillaries Structural features Enlarged capillaries, 30 ～ 40µm in diameter. Endothelial cells have pores without diaphragms. There are wide spaces between the cells. Basal lamina is incomplete or absent. capillary3.ppt

Distribution Liver, spleen, hypophysis, adrenal cortex and bone marrow. The interchange of substances between blood and tissue is greatly facilitated by the structure of this type of capillaries.

Veins Structural features The veins can also be divided into large veins, mediumsized veins, small veins and venules. 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. ① The wall of a vein is distinctly thinner than that of an artery having the same sized lumen. middle sized A-V.ppt

②The tunica media contains a much larger quantity of collagen than in arteries. The amount of elastic tissue or of muscle is much less. So the wall of a vein is easily compressed. Medium-sized vein

③ In arteries the tunica media is usually thicker than the adventitia. In contrast the adventitia of veins is thicker than the media. In some large veins (e.g., the inferior vena cava) the adventitia contains a considerable amount of elastic and muscle fibres . These fibres facilitate elongation and shortening of the vena cava with respiration. large vein.ppt

④ Valves of vein

Most medium and small sized veins contain valves that allow the flow of blood towards 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 fibres. vein valves.ppt Valves are especially numerous in veins of the limbs, but are absent in very small veins; in veins within the cranial cavity, or within the vertebral canal.

Microcirculation 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. Microcirculation adjust blood flow through capillaries to fit the tissue requirement .

What is microcirculation? The blood circulation takes place among the microvasculature which includes arterioles, capillaries and postcapillary venules. microcirculation.ppt Types of microcirculation ①The usual sequence of arteriole metarteriole capillary bed venule vein. The interchange of substances between blood and tissues and cells takes place throughly. microcirculation-1.ppt

②Arteriovenous anastomoses In many parts of the body small arteries and veins are connected by direct channels that constitute arteriovenous anastomoses. This helps the blood to retune to the heart rapidly. Arteriovenous anastomoses in the skin help in regulating body temperature, by increasing blood flow through capillaries in warm weather; and decreasing it in cold weather to prevent hear loss. microcirculation.ppt microcirculation-1.ppt

③An arterial portal system This is a special kind of microcirculation presenting in the kidney glomerulus. The blood flow from an afferent arteriole several coiled capillaries (glomerulus) efferent arteriole capillaries venule. microcirculation.ppt ④ A venous portal system This kind of microcirculation is present in the live. The blood flow from a venule capillaries a venule.

⑤ Thoroughfare Channels In many situations arterioles and venules are connected by thoroughfare channels. These channels resemble capillaries, but have a larger caliber. Through thoroughfare channels blood flow run a relatively direct course between the arteriole and venule. A thoroughfare channel and the capillaries associated with it are sometimes referred to as a microcirculatory unit. microcirculation-1.ppt

HEART The heart is a muscular pump that contracts rhythmical, pumping the blood through the circulatory system. There are three layers in the wall of the heart: endocardium, myocardium and epicardium.

Endocardium It corresponds to the tunica intima of blood vessels and contains three layers also. Endothelium Subendothelial layer A thin layer of delicate connective tissue containing fibroblast, CF, EF, SM. Subendocardial layer A layer of connective tissue containing veins, nerves, and branches of the impulseconducting system of the heart(Purkinje cells). heart wall.ppt

Myocardium It is the thickest of the tunics of the heart and consists of cardiac muscle cells arranged in layers that surround the heart chambers in a complex spiral. heart wall.ppt It has been shown that atrial myocardial fibres secrete an atrial natriuretic factor which increases renal excretion of water, sodium and potassium, thus reducing blood pressure.

Epicardium Epicardium is the visceral layer of pericardium. It consists of a layer of connective tissue which is covered, on the free surface, by a layer of flattened mesothelial cells. epicardium.ppt Pericardium It is a serous membrane in which the heart lies. It includes two layers: visceral layer(epicardium) and parietal layer. Between these two layers is a small amount of fluid that facilitates the movements of the heart.

Cardiac fibrous skeleton At the junction of the atria and ventricles, and around the openings of large blood vessels there are rings of dense fibrous tissue. Similar dense fibrous tissue is also present in the interventricular septum. These masses of dense fibrous tissue constitute the cardiac fibrous skeleton. The principal components are the septum membranaceum, the trigona fibrosa, and the annuli fibrosi. They give attachment to fasciculi of heart muscle.

Cardiac valve The valves of the heart are folds of endocardium that consist of a central core of dense fibrous connective tissue lined on both sides by endothelial layers. The base of the valves are attached to the annuli fibrosi of the fibrous skeleton. heart valve.ppt

Conducting system The heart has a specialized system to generate a rhythmic stimulus that is spread to the entire myocardium. This specialized system is conducting system of the heart which includes sinoatriol node(SA

node), atrioventricular node(AV node ) and AV bundle. conducting system.ppt The special cardiac muscle cells made up of conducting system of the heart. These special cardiac muscle cells can be divided into three types: nodal myocytes, transitional myocytes and Purkinje fibres.

Nodal myocytes are present in the AV node and SA node and are narrow, rounded, cylindrical or polygonal cells with single nucleus. They are responsible for pace-maker functions. So the nodal myocytes are also called pacemaker cells. Transitional myocytes are present in the nodes, and in the stem and main branches of the AV bundle. They are similar to cardiac myocytes except that they are narrower. Conduction through them is slow.

Purkinje fibres are chains of cells which are united by desmosomes. These cells have a larger diameter, and are shorter than typical cardiac myocytes. A Purkinje fibre has a central nucleus surrounded by clear cytoplasm containing abundant glycogen. Myofibrils are inconspicuous and are confined to the periphery of the fibres. Mitochondria are numerous and the sarcoplasmic reticulum is prominent. Purkinje cells.ppt heart wall. ppt

Lymphatic vascular system The lymphatic vascular system returns the extracellular liquid (tissue fluid)to the bloodstream. When the tissue fluid goes into the lymphatic vessels it is called lymph. The structures of lymphatic capillaries are similar to that of capillaries except that they have thinner walls and larger caliber. L-Capillary.ppt

The lymphatic vessels have a structure similar to that of small veins except they have thinner walls and larger caliber. They also have more numerous internal valves. lymphatic vessel.ppt The lymphatic ducts is similar to that of large vein. In the middle layer the muscle bundles are longitudinally and circularly arranged, with longitudinal fibers predominating. The adventitia is relatively underdeveloped. Like the arteries and veins the large lymphatic ducts contain vasa vasorum and a rich neural network.

questions 1. Compare the structure and function of large artery and medium-sized artery. 2. How many types of capillaries are there? Describe their structures. 3. What are the structural differences between the arteries and their accompanying veins? 4. What is microcirculation? Describe the typers of microcirculation and their primary functions. 5. what is conducting system of the heart?