CIRCULATORY SYSTEM II

VIII. Structure of vessels 6 - Venous vessels A. post capillary venules (those venules directly connected to capillaries)  m  m in diameter (bigger than most capillaries) 2. consist of endothelium with incomplete layer of pericytes that are located between the endothelial cells and the basal lamina of the endothelium.

VIII. Structure of vessels 6 - Venous vessels A. post capillary venules 3. Wall lacks smooth muscle cells and instead has longitudinally arranged fibroblasts and associated reticular fibers. These do not form a complete layer around the vessel. 4. Note the difference between the pre-capillary arteriole and post- capillary venule in this photograph. The nuclei of circumferential smooth muscle cells can be seen in the arteriole, while they are lacking in the venule.

B. venules (beyond the post-capillary venules) mm in diameter 2. intima usually lacks an obvious subendothelial layer of connective tissue 3. thin media consisting of a few smooth muscle cells. As venule size increases, the amount of smooth muscle increases.

4. well developed adventitia rich in collagen fibers. 5. despite tunic investments, blood cells such as monocytes can still pass through the walls of venules a. the process utilized by monocytes to pass through capillary or venule walls is called diapedesis - the act of a leukocyte squeezing through small spaces between cells B. venules

C. small to medium sized veins mm in diameter 2. intima usually has a thin sub-endothelial layer of connective tissue associated with endothelium. Sub-endothelial layer of C.T. is absent in some cases.

4. well developed adventitia rich in collagen fibers. 3. thin media C. small to medium sized veins

C. small to medium sized veins

D. large veins 4. this arrangement of longitudinal muscle in adventitia and circular in media helps to strengthen wall and help prevent settling of blood in limb veins due to gravity. 1. intima is well developed 2. media is muscular, but thin relative to that of muscular arteries with similar lumen size. A few circular layers of smooth muscle are present amongst abundant connective tissue. 3. adventitia is thickest and best developed tunic. Comprises most of wall of vessel. May contain longitudinal bundles of smooth muscle and vasa vasorum.

D. large veins Human vena cava Intima thin - not resolvable at this magnification Media thin Adventitia - thick with bands of longitudinal smooth muscle, vasa vasorum

5. in addition to this muscular arrangement, valves are also present in large veins. These also help prevent back flow of blood.

Cardiovascular Syste 6. skeletal muscle (in conjunction with the valves) is also helpful in preventing backflow by propelling blood through veins in the correct direction as the muscles contract.

IX. Heart A. The heart is composed of 2. Three layers or tunics a. endocardium b. myocardium c. epicardium 1. a fibrous connective tissue skeleton 3. valves 4. an electrical impulse conducting system

B. Tunics of the heart 1. endocardium - in a sense, simply a continuation of tunica intima a. consists of 3 parts * endothelium resting on a basal lamina and associated thin layer of collangenous fibers * Beneath the endothelium is the sub-endothelial layer of connective tissue containing elastic fibers and a few smooth muscle cells (some texts call this loose C.T. and others dense C.T.) * Beneath that lies the subendocardial layer of loose connective tissue that contains small blood vessels and nerves. In the ventricle, the Purkinje fibers are associated with the subendocardial zone.

1. endocardium - simply a continuation of tunica intima

2. Myocardium - consists of 2 components a. fascicles of cardiac muscle cells that connect to the fibrous connective tissue skeleton of the heart

b. Noncontactile, modified muscle cells that form the impulse (action potential) generating and conducting system of the heart * this system is composed of the atrioventricular and sinoatrial nodes, and the Purkinje fibers * Purkinje fibers - cardiac muscle cells specialized for impulse (action potential) generation and impulse conduction * the cells of this system generate and conduct action potentials that synchronize the heartbeat a. autonomic nerve b. Purkinje fibers c. cardiac muscle

* Purkinje fibers are scattered along the innermost portion of the myocardium in the ventricle, next to the endocardium

3. Epicardium - epithelium and connective tissue covering of heart. Also called the visceral pericardium. a. squamous to cuboidal epithelial external lining - a continuation of the mesothelium that lines the pericardial cavity. b. below this is a layer of connective tissue with high concentration of elastic fibers - elastic layer

c. between the elastic layer and myocardium is the subepicardial layer which consists of loose connective tissue containing nerves, arteries, veins, and adipose tissue. C. Valves - read in text, p. 117 (not much there).

X. A few words about the lymphatic vessels A. System of thin walled vessels that runs throughout the body. B. These vessels are lined with endothelium. C. Larger lymphatic vessels are similar in structure to veins except with thinner walls and with no clear cut separation between the 3 tunics (intima, media, adventitia). iname=40a15&oid=348

X. A few words about the lymphatic vessels C. Lymph vessels begin with blind-ended capillaries that originate in connective tissue of nearly all parts of body. D. These capillaries and associated small lymph vessels converge into larger vessels that return fluid expressed from arteries and veins into tissues to the venous component of the blood circulatory system.

E. All lymphatic vessels eventually drain into 2 large ducts, the thoracic duct (cisterna chyli) and the less extensive right lymphatic duct that, respectively, connect to the left and right subclavian veins in the neck. F. The lymph vessels are also a pathway by which various cells of the immune system can enter or leave the blood circulatory system and tissues.

Thoracic duct (cisterna chyli, on left side, connects to left subclavian vein)

Lacteal - lymph vssel in lamina propria of small intestine that extends into a villus.