Topic 3 Circulation

Objective 1 Blood Vessel Structure/Function General Histological Features of Blood Vessels: Arteries/Veins  arteries and veins have three layers (tunics)  the central blood filled opening is called the lumen

Intima (Interna) : simple squamous epithelium which lines the lumen Subendothelial layer: loose areolar CT located external to the endothelium in vessels with a diameter > 1 mm Endothelial cell products: Nitric oxide: relaxes vascular smooth muscle Prostacyclin: relaxes vascular smooth muscle and repels platelets Endothelin: contracts vascular smooth muscle causing vasoconstriction Endothelium

Smooth muscle cells arranged in a circular fashion Elastic fibers organized into sheets Muscle fibers cause vasoconstriction (contracted cells) and vasodilation (relaxed cells)  they are controlled by SNS vasomotor nerves and chemical factors Elastic fibers provide elasticity (stretch and recoil) Media Aorta Small Artery

Vasa Vasorum Adventitia (Externa) Collagen fibers with nerve fibers, lymphatic vessels and elastic fibers in large vessels : small blood vessels that nourish the wall of larger blood vessels Protect, reinforce vessels and attach them to surrounding structures Vasa Vasorum Blue arrow: vasa vasorum Green arrow: tunica media

Comparing Arteries and Veins Thick tunica media Thin tunica adventitia No valves Round lumen Veins Thin tunica media Thick tunica adventitia Valves present Irregular lumen

Recall the vessels of the cardiovascular system ………

Comparing Blood Vessels Internal diameter is 2.5 cm – 1 cm Well developed tunica media rich in elastic fibers; adventitia is thin conducting artery: drives the circulation of blood into smaller vessels during systole, the artery fills with a stroke volume and is stretched during diastole the artery maintains pressure as it recoils on the blood, propelling it further along Elastic Artery

Distributing Artery elastic arteries supply pressure Muscular Artery External diameter is 1 cm to 0.3 mm Well developed tunica media rich in smooth muscle fibers Internal elastic lamina and external elastic lamina are sheets of elastic fibers that separate the tunica intima/tunica media and tunica media/tunica adventitia dilate and constrict to control the distribution of blood to organs vasoconstriction reduces the diameter of the artery and decreases the flow of blood to an organ vasodilation increases the diameter of the artery and increases the flow of blood to an organ Distributing Artery elastic arteries supply pressure stomach skin kidney heart Muscular arteries dilate and constrict to direct blood flow

Arteriole External diameter is < 0.3 mm Internal elastic lamina present in most cases Tunica media is a single layer of smooth muscle cells vasoconstriction and vasodilation controls the distribution of blood to capillary beds Arteriole

Arterioles deliver blood to capillary beds

Pericytes Capillary External diameter averages 8-10 μm Single layer of endothelium (simple squamous epithelium) resting on a basement membrane No elastic laminae are present are connective tissue cells that stabilize the capillary wall Thin walls permit the exchange of gases, nutrients and wastes with interstitial fluid Pericytes

Precapillary sphincters Capillary Beds:  smooth muscle sphincters that open and close to regulate the distribution of blood through capillary beds Precapillary sphincters

Structural Types of Capillaries: : endothelial cells are joined by tight junctions but may be interrupted in some places by intercellular clefts Continuous Capillaries Fenestrated Capillaries like continuous capillaries, but the endothelial cells have pores (fenestrations) covered by thin membranes; these capillaries are more permeable than continuous capillaries

Sinusoids : leaky capillaries that have large lumens and are usually fenestrated; few tight junctions and intracellular clefts; found in liver, bone marrow, lymphoid tissues and some endocrine organs Spleen sinusoids and liver sinusoids are lined with macrophages (called Kupffer Cells)

External diameter ranges from 8-100 m Postcapillary venule (small): consists of endothelium with a few pericytes; are extremely porous Larger venules: Contain endothelium, a thin tunica media and a thin tunica adventitia Collect blood from tissue capillaries and carry it to veins Venule

Vein Capacitance Vessel Jugular Vein External diameters vary, but they are larger than their corresponding arteries Lumen has an irregular shape All three tunics are present  tunica externa is thick  tunica media is generally thin Valves, formed from endothelium, are present in some places to prevent the backflow of blood Collect blood from venules and carry it to the heart Veins hold up to 50% of the total blood volume Capacitance Vessel Jugular Vein

Valves prevent the backflow of blood……. Venous vales are formed from folds of the: They are abundant in veins of the limbs and are absent in veins of the ventral body cavity endothelium

Venous Sinuses large diameter veins endothelium They are made of Examples of venous sinuses: Coronary Sinus Cranial Sinuses

The Distribution of Blood Volume In the Vascular Tree: Location % of Total Blood Volume Aorta 2 Other Systemic Arteries 8 Systemic Arterioles 1 Systemic Capillaries 5 Systemic Veins 50 Heart Chambers 12 Pulmonary Circulation 18 Sinusoids 4

Objective 2 Vascular Disorders Arteriosclerosis Causes: aging, hypertension, diabetes, smoking, hereditary factors, elevated cholesterol Characteristics: abnormal thickening and abnormal hardening of the vessel walls in the arterial system  smooth muscle cells and collagen fibers migrate into the tunica media  lumen becomes narrowed Hyperplastic arteriosclerosis

Atherosclerosis Most common cause of arteriosclerosis: Endothelial cells are injured; inflammation occurs Macrophages release free radicals and enzymes that further damage the vessel wall LDL becomes oxidized and ingested by macrophages (foam cells) Foam cells invade the wall and accumulate (fatty streak) Smooth muscle cells proliferate, secrete collagen and form a fibrous plaque Vessel becomes narrowed and obstructed; platelets adhere and lead to thrombus formation Atherosclerosis

Varicose Veins Damage to one or more valves in a vein Venous distention and pooling of blood; veins become tortuous and palpable Surrounding tissue may become edematous Varicose Veins

Localized dilation or outpouching of a blood vessel or cardiac chamber Arterial walls become weakened and may rupture; common sites are the abdominal aorta, renal arteries and cerebral arteries Aneurysm

Aneurysm in a Basilar Artery Aortic Aneurysm

Phlebitis Thrombophlebitis Inflammation of a vein usually due to Infection or trauma May lead to pooled blood and edema Inflammation of a vein associated with a blood clot Redness, heat, swelling and pain in the affected area; the clot may break off and become an embolus that blocks smaller vessels in other areas Phlebitis Thrombophlebitis Superficial thrombophlebitis is an inflammation of a vein just below the surface of the skin, which results from a blood clot. This condition may occur after recently using a intravenous (IV) line, or after trauma to the vein. It involves an inflammatory response associated with a clot in the vein. Some symptoms can include pain and tenderness along the vein and hardening and feeling cord-like. Superficial thrombophlebitis is usually a benign and short-term condition. Symptoms generally subside in 1 to 2 weeks, but hardness of the vein may remain for much longer.