(Transcribed from Dr. Rodney’s lecture, 29 Feb 2000 by Brian Buschman)
Return to Semester One Goodies
Return to The Unofficial Ross Page
Blood vessels are divided into three tunics which are circumferential layers that are divided based on composition and location.
1) Tunica intima is the innermost layer of veins and arteries and is made of a single layer of endothelial cells which rest on a basal lamina. Deep to the intima is a subendothelial layer of loose connective tissue and smooth muscle arranged longitudinally.
2) Tunica media is in the middle and composed of helically arranged layers of smooth muscle with reticular fibers (collagen III), elastin and proteoglycans between then. Arteries have a very large media and have internal and external elastic membranes on either side of the tunica media which have fenestrae to allow passage of nutrients and waste into and out of the media.
3) Tunica adventitia is the outer layer which is made of collagen type I with a few elastic fibers which merges with the connective tissue around the vessels. In veins it is usually the largest tunic.
Vasa vasorum are small veins and arteries that supply the large vessels since the are too think for nutrients to diffuse directly from the lumen to the outer layers. They are found in the media and adventitia in veins and in the adventitia only in arteries. It is believed that they are not in the media of arteries because they would be crushed by the arterial pressures and that they are unnecessary due to the very high nutrient content in arteries.
Lymphatics are found in the adventitia of arteries and in the media and adventitia of veins. Again they are not in the media of arteries due to the high arterial blood pressure.
Nervi vascularis area network of sympathetic nerve fibers that release norepinephrine (NE) to cause vasoconstriction. The nerve fibers, like lymphatics and vessels, enter the media and adventitia in veins but only the adventitia in arteries. In arteries the response is somewhat delayed because NE must diffuse into the media to cause contraction of the smooth muscle.
Capillaries are made of a single layer of endothelial cells that rest on a basal lamina except for some endothelial cells in discontinuous capillaries. The nucleuses of the endothelial cells bulge into the lumen. The endothelial cells are held together by zonula occludens, desmosomes and gap junctions.
Pericytes surround capillary walls and are cells that can form smooth muscle, actin, myosin and tropomysin to make repairs when the capillaries are damaged. It is thought that they may play a contractile role due to the presence of the fibers usually found in muscle cells.
There are four main classes of capillaries:
1) Continuous (somatic) capillaries have no fenestrae and transport material in both directions by pinocytotic vessels. They are found in muscle, lung, connective tissue, exocrine glands and the CNS. In the CNS they have very few pinocytotic vesicles because of the blood brain barrier. Notice that they are found in exocrine and not endocrine glands that have fenestrated capillaries.
2) Fenestrated (visceral) capillaries have fenestrae for the passage of materials and diaphragms to regulate the size of the material passed.
3) Fenestrated without diaphragm capillaries have a very thick basal lamina to separate the endothelium from the overlying podocytes. They are found in the renal glomerulus and the size of the fenestrae is regulated by the podocytes as will be discussed when we talk about the kidney.
4) Discontinuous capillaries follow a tortuous path and have an enlarged diameter to slow the flow of blood and has many fenestrae without diaphragms. It has a discontinuous basal lamina. They have macrophages among the endothelial cells and are found in hemopoetic tissues (liver, spleen and bone marrow).
Capillaries function in many ways:
1) They form a selectively permeable barrier between the circulatory system and the tissues supplied.
2) Play a metabolic role as they make PgI2, growth factors for blood cells, fibroblast GF, platelet GF, and in the lungs have angiotensin converting enzyme (ACE).
3) Inactivation of intercellular messengers (hormones, seretonin, NE, thrombin, etc).
4) Antithrombogenic function as they form a barrier between connective tissue and platelets so that platelets cannon bind to the sides to form a clot.
Metarterioles are vessels that connect between arteries and veins which give off capillaries. They have a muscular band, the precapillary sphincter, which regulates the flow of blood through a given capillary bed. This is important as there is not enough blood in the body to fill all the capillaries at any given time so they must be turned off and on as needed.
Arteriovenous anastomoses (shunts) are direct routes between arteries and veins that bypass capillary beds. They are located in the skin of fingertips, the nose and lips as well as erectile tissues. They function in thermoregulation in the skin and in maintaining the flaccid state of erectile tissues. When they are closed, they divert blood to capillaries in the dermis and the body looses heat. When open it bypasses the capillary beds and quickly returns to veins without loosing body heat.
Arterioles are the smallest class
of arteries. They have all three tunics
but the adventitia is very thin and the have a small media.
In general their media does not have internal and external elastic lamina and a
very thing subendothelial layer. Vessels of this size and larger contain
Weibel-Palade granules that contain von Willibrand’s factor (factor VIII) and
their deficiency is called hemophilia A.
Muscular arteries compose the majority of the named arteries of the body. They have all the tunics and parts of the tunics. Their adventitia has few elastic fibers as it’s saving that for the elastic arteries but it does have fibroblast cells. The media may have up to 40 layers of smooth muscle cells.
Elastic arteries are the largest and all their layers are thick. The media has the internal and external elastic lamina with fenestrae. The adventitia is an underdeveloped layer. In systole the arteries stretch to reduce blood pressure but in diastole they rebound to maintain the pressure.
An aneurysm is a weakness in an arterial wall (usually in aorta or at the base of the brain) where the wall balloons out. It’s etiology can be congenital or of another disease state.
The coronary arteries are the fastest aging vessels of the body that begin to show signs of age by the early twenties. Two types of aging are common:
1) Arteriosclerosis is a hardening of the arteries.
2) Athersclerosis involves the deposit of fatty acids in the intima of the artery that leads first to a fatty streak and then to a arterial plaque. This will lead to infarctions, blocked blood flow to a tissue area, usually in the kidneys, heart or cerebellum.
Veins are low pressure vessels that are considered capacitance vessels because they hold about 70% of the total blood volume. The adventitia is the thickest layer of the veins.
Venules are thin walled and have a relatively thick adventitia for it’s size.
Veins (small and medium sized) have a very thin intima.
Large veins have a well-developed intima, thin media and a thick adventitia.
Veins have valves, which are folds of intima, to prevent the backflow since their pressure is not as great as that in arteries. The valves function with the way veins are compressed by some skeletal muscle contractions to create a pump to get blood back to the heart.
Lymphatic vessels collect the extracellular fluid that does not make it back into the veins. It only contains efferent vessels and they are found in all tissues except for the bone marrow and nervous tissue.
Lymphatic capillaries are thin walled, blind-ended tubes with elastic fibers but NO fenestrations, basal lamina or zonula occludens. They are made of nothing more then a single layer of endothelial cells.
They converge into larger vessels which ultimately become lymph veins. The largest lymph veins are the right lymphatic duct and thoracic duct. They empty into the right and left junction of the internal jugular and brachiocephalic veins respectively. They flow through lymph nodes and the flow is assisted by numerous valves. The cisterna chyli is the beginning of the thoracic duct.
The heart has three layers which are very similar to the tunics of blood vessels.
1) The endocardium is a simple squamous epithelium with a layer of loose connective tissue.
2) The myocardium is the muscular layer of the heart.
3) The epicardium is a simple squamous epithelium and visceral pericardium what rests on the loose connective tissue of the subpericardial layer.
The heart also has fibrous pericardium that is made of dense connective tissue which encircles pulmonary artery and aorta, encircles the AV openings and serves as an attachment point for the valves of the heart.
The heart’s conduction system is based on a spontaneously depolarizing sinuatrial (SA) node which is the primary pacemaker of the heart. It transmits the signal to the atrioventricular (AV) node which is the secondary pacemaker and the on to the atrioventricular bundle (of His). Blood pressure is monitored by the carotid sinus and it’s chemical composition by the carotid body. Afferent fibers from these travel in the glossopharyngeal nerve to the brain and afferent fibers in the vagus nerve travel to the SA node to either speed up or slow down the heart as needed.
Return to Semester One Goodies
Return to The Unofficial Ross Page