THE BLOOD Is made up of: » The heart. » & . » Connecting vessels ( )a & v.: » Blood . » Blood sinusoids. » Arteries venous anastomosis.

» These vessels distribute blood to every part of the body and return it to the heart for redistribution.

General Structure of Blood Vessels (the inner most coat): it consist of: • Endothelial lining: formed of simple squamous epithelium with flat nuclei rest on a basal lamina. Have a turnover rate of 1% per day. • Subendothelial layer: consists of loose C.T. rich in elastic fibers, may contain occasional smooth muscle cells. • Internal elastic lamina: it is seen as a wavy pink composed of elastin. . Tunica media : • smooth muscle cells. • elastic fibres. • Fine collagenous and reticular fibers. • external elastic lamina.

In capillaries and post- , the media is replaced by cells called . Tunica adventitia : • Fine collagenous fibres ,collagen type I. • Elastic fibers. • Tiny blood vessels called (vessels of vessels) Lymphatics are also present in the adventitia of arteries only but in veins they can penetrate the media.

• L.M of The : The Intima: - Forms about 1/6 of the thickness of the wall . -The subendothelial layer is thick, the C. T. fibers in it are longitudinally arranged. -The internal elastic lamina is present but not be easily discerned, since it is similar to M.elastic laminae layer. The Media: very thick and forms the main thickness of wall. -fenestrated elastic laminae concentrically arranged -smooth muscle cells, reticular fibers, collagenous fibers -ground substance consisting of chondroitin sulfate. The Adventitia: • Is relatively thin (collagenous and elastic fibres). • C. T. cells, vasa vasorum, lymphatics and nerves. Function of Large Elastic Arteries: • Transport blood away from the heart. • Maintain diastolic pressure.

Medium sized

A&V

• Medium Sized Arteries With Specialized Characters: • Basilar (Cerebral ) : • Site: supply the brain. • Character: • Has thin wall and wide lumen. • Its internal elastic lamina is very thick. • It has no elastic fibers in the media. • Its media and adventitia are thin. • Coronary Artery : • Site: supply the heart with blood like medium sized artery but it differs in the subendothelial layer of intima which contains : • A morphous intercellular substance. • Smooth muscle fibers longitudinally arranged. • Fat droplets and monocytes. • There is external elastic lamina between the media and adventitia. • Umbilical Artery : • Site: in the umbilical cord. • Characters: • Carries venous blood. • Has no internal elastic lamina. • The media is formed of inner longitudinal and outer circular layer of smooth muscle fibers. • Adentitia is formed of mucoid C. T. called Wharton’s Jelly. • III- : • I- Venules : Definition: a) Post capillary venules : • Are the small branches of the arteries. Their • Definition: Small venules that collect capillaries. diameter is less than 0.1mm. • Size: Their diameter is 8-30 m. Structure: • L. M Structure :as capillary: • In small arterioles. • : simple squamorus epithelium . • The intima consists only of endothelium and its • Basement membrane: which supports the basement membrane. endothelium. • The adventitia is very thin and contains collagen • Pericytes lie between the endothelium and fibers only. basement membrane. • In larger arterioles • They can differentiate to smooth muscle cells. • There is also internal elastic lamina. • N.B. : • The media consists of only 1 or 2 circular layers • In the thymus gland and node, the post- of smooth muscle capillary venules are lined with simple cubical • External elastic lamina is present. epithelium, which shows wide intercellular spaces • The adventitia may be as thick as the media and allowing migration of the T. lymphocytes. it contains collagen and elastic fibers. • b) Collecting Venules : Function: • Definition: Are venules which collect the post- • They supply the tissues and organs with arterial capillary ones. blood. • Size : 30 – 50 m in diameter. • They control the blood pressure.: • Structure: In addition to endothelium and The meta-arterioles: Are the terminal parts of the pericytes, they possesses an adventitia consisting before their connection with the blood of fibroblasts and collagen fibers. capillaries. They have sphincters at their • c) Muscular Venules : terminal. • Definition: Large sized venules into which the • Rod shaped granules about 3 m, containing collecting venules empty. protein of the blood coagulation are present • Size: 50-100 m in diameter . normally in the endothelial cell of the arterioles. (Known as Factor VIII). Deficiency of this group • Structure: They have one or two layers of smooth of protein results in impaired adhesion of muscle cells in their media. The adventitia is platelets to injured endothelium prolonged relatively well developed. bleeding and is among the causes of hemophilia. • N.B.: Venules are very much involved in  inflammation • VD. • migration of the leucocytes to the site of inflammation

• . Varicose

ARTERIO-VENOUS CONNECTION

• 1-Blood Capillaries

• II- Blood Sinusoid

• III- Avterio-venous anastomosis (A-V shunts) 1-Blood Capillaries Definition: • Thin walled tubular blood channels which conduct the blood from the terminal arterioles to the venuloes. • Diameter: 8-10m. • Network of these vessels are known as capillary beds. L M: • Single layer of endothelial cells rolled up in the form of a tube. • Rest on a basal lamina. • Pericytes which lie between endothelium and the basal lamina. E M: • Contain all the usual organelles. • Abundant microfilaments and intermediate filaments. • Endothelial cells show endocytotic pits and vesicles. • The cells are hold together by zona occludens and gap junction. • Pericytes with long cytoplasmic processes surround the endothelial cells along capillaries, enclosed in their own basal lamina. Their cytoplasm contains myosin, actin and tropomysin which suggest their contractile function. The continuous or somatic capillary: Character: No fenestrae May have numerous pinocytotic v. as in muscle  few or no pinocytotic v. as in nervous system. Site : muscle tissue, C. T. exocrine glands nervous tissue. Fenestrated or visceral capillary: Character: ♣ Large fenestrae present in the walls of endothelial cells. ♣ The fenestrae are closed by a diaphragm that is thinner than a cell membrane. ♣A continuous basal lamina is present. Site: Kidney intestine endocrine gland. Fenestrated capillary without diaphragm: Character: Has a very thick basal lamina which separate the endothelium from the overlying epithelial cells. Site: Renal glomerulus cap. Discontinuous sinusoidal capillary: Character: ☻Has tortuous pathway ☻ Enlarged diameter 30-40m ☻ The basal lamina and endothelial cells are discontinuous. ☻ Multiple fenestrae without diaphragm are present. Function of capillaries: 1-Selective exchange of materials from blood to tissue and vice versa. 2-Metabolic functions as: Activationconvert angiotensin I to angiotensin II.  Inactivationconversion of serotonin,prostaglandins,  norepinephrine  biologically inert compound.  Lipolysis breakdown of lipoproteins to yield triglycerides and cholesterol.  Production of vasoactive factors e.g. endothelin and nitric oxide. 3-Antithrombogenic function: ♠Endothelial cells if desquamate uncovered subendothelial C.T. aggregation of blood platelets-fibrin coagulation thrombi obstruct vascular flow. ♠ Endothelial cells, when present contact of platelet with the subendothelial C.T antithrombogenic effect. II- Blood Sinusoid • Definition: These are dilated irregular blood spaces, which measure about 5-30m

• Structure: • Perforated endothelial cells of simple squamous epithelium. Their walls contain pores not covered by diaphragms. • The endothelium rests on a thin layer of reticular C. T. No basement membrane is present. • Macrophages are present outside the sinusoidal wall, extend their pseudo- podia inside the wall to phagocytose any foreign body. • Site and function: In bone marrow:- • Allow migration of blood cells, to the general circulation. • Have slow circulation O2 tension stimulate erythropoiesis. In spleen: store and filter the blood. In the liver: allow the blood to come into contact with the hepatocytes. In endocrine glands: allow migration of hormones to general circulation.

III- Avterio-venous anastomosis (A-V shunts): • Definition: Are blood channels, which allow the blood to pass directly from the arteries and arterioles to the veins or venules without its having to go through capillaries, metarterioles, or preferential channels. • Types: • Direct connections: between an arteriole and a by side branch as found in the placenta. • Glomus or complicated side branch present in skin, ear pinna, toes and nail bed. • Structure: • Are similar to the arterioles on their arterial side and similar to the venules on the venous side. • The intermediate segment of A-V shunt: • The lumen decreases gradually towards the venous side. • The internal elastic lamina disappears towards the venous side. • The media is well-developed, rich in longitudinal smooth muscles and contains myoepithelial cells which act as sphincter. • The adventitia becomes thicker at the venous side. • Sites: Exposed parts • tips of fingers, • toes, • external ear, • nose, • lip and tongue. Internal organs stomach,  intestine,  liver,  endocrine gland,  uterus,  placenta  sympathetic ganglia. • Function: • Conserve the body temperature; dilate in cold weather while constrict in hot weather. • Regulate the blood flow to the organs according to their need. • When dilate the venous return is increased and vice versa.