Circulatory System

BIO 408

Histology

Dr. D. L. Daley

Components of Circulatory System

n Cardiovascular system - heart and vessels

n Carries blood around the body to all the tissues

n Two-way system

n Lymphatic system - lymph vessel, lymph nodes and lymphatic organs (spleen & thymus)

n Collects excess tissue fluid and returns it to venous circulation

n One-way system

Cardiovascular System

n Heart - pumps blood into two separate circuits

n Pulmonary circuit - carries blood to the lungs

n Systemic circuit - distributes blood to and from all the organs and tissues of the body

Vessels

n Arteries - vessels that transport blood away from the heart

n Capillaries - smallest vessels only one cell layer thick walls

n Form vast networks called capillary beds

n Veins - vessels that transport blood back to the heart

n Drain the capillary beds

Vessel Tunics

n Three separate concentric layers form the walls of a typical blood vessel

n Tunica Intima - innermost layer - composed of a single layer of flattened, squamous endothelial cells

n Form a lining for all vessels and is continuous with the lining of the heart and the cells that form the walls of capillaries

Vessel Tunics

n Tunica media is the middle layer - composed mostly of smooth muscle cells oriented concentrically around the lumen

n Tunica adventitia is the outermost layer - composed mostly of fibroelastic connective tissue arranged longitudinally

Tunica Intima

n The endothelial cells are simple squamous epithelium that rest atop a basal lamina

n Arranged in a flattened sheet of elongate cells - more or less parallel to the long axis of the vessel

n Form exceptionally smooth surface

n Secrete type II, IV & V collagen, laminin, nitric oxide & von Willebrand factor

Tunica Intima

n Membrane-bound enzymes such as angiotensin-converting enzyme (ACE) that converts Angiotensin I to angiotensin II which is a powerful vasoconstrictor

n Mostly in the capillaries of the lungs

n Also enzymes that inactivate bradykinin, prostaglandins, thrombin and norepinephrine

n Also bind lipoprotein lipase that degrades lipoproteins

Tunica Intima

n Immediately below is subendothelial layer of loose connective tissue and a few scattered smooth muscle cells

n Beneath the subendothelial layer is an internal elastic lamina layer - well developed in muscular arteries

n Composed of elastin - a fenestrated sheet that permits the diffusion of substances in to the deeper regions of the arterial wall

Tunica Media

n Thickest layer of a vessel

n Mostly helically arranged smooth muscle

n Interspersed within the layers of smooth muscle are elastic fibers, type III collagen and proteoglycans

n Larger muscular arteries have an external elastic lamina, that separates the tunica media from overlying tunica adventitia

n Capillaries do not have a tunica media but rather have overlying pericytes

Tunica Adventitia

n Covering the vessels on their outside surface is the tunica adventitia

n Composed mostly of fibroblasts, type I collagen fibers and longitudinally oriented elastic fibers

Vasa Vasorum

n In large vessels, the thickness and muscularity prevents the cells from being nourished by the diffusion from the lumen

n Deeper cells of the tunica media and tunica adventitia are nourished by the vasa vasorum

n Small arteries that enter the vessel walls and branch profusely

n More prevalent in veins than arteries because the the blood of veins contains less oxygen and nutrients than arterial blood

Nerve Supply to Vessels

n A network of vasomotor nerves of the sympathetic NS supplies the smooth muscle of blood vessels

n Responsible for vasoconstriction of vessels walls

n The neurotransmitter norepinephrine is used and is released outside the tunica media and diffuses into the smooth muscle

n The arteries supplying skeletal muscle also receive cholinergic (parasympathetic) nerves to bring about vasodilation

Arteries

n Two major arteries of the heart

n Pulmonary trunk leaves the right side of the heart - divides into right & left pulmonary arteries

n Aorta - leaves the left side of the heart - quickly forms a sharp arch and descends toward the abdomen

n The first arteries that branch off the aorta are the rt & lt coronary arteries

n The descending aorta terminates by bifurcating into right & left iliac arteries

n Three major trunks leave the aortic arch

n Right brachiocephalic artery, left common carotid artery and the left subclavian artery - supply the superior extremities and the head and neck

Types of Arteries

n Elastic (conducting) arteries

n Muscular (distributing) arteries

n Arterioles

Elastic Arteries

n Aorta, right brachiocephalic, left common carotid, left subclavian, pulmonary trunk and the iliac arteries are elastic arteries

n The walls of of these may be yellow in the fresh state because of the abundance of elastin

n Tunica intima - endothelium with Weibel-Palade bodies (inclusions that contain von Willebrand factor - helps coagulation of platelets), basal lamina, subendothelial layer & incomplete internal elastic lamina

Elastic Arteries

n Tunica Media - 40 to 70 fenestrated elastic membranes (fenestrated lamellae of elastin) alternating with smooth muscle, thin external elastic lamina and vasa vasorum in outer half

n Tunica Adventitia - thin layer of fibroelastic connective tissue, vaso vasorum, lymph vessels & nerve fibers

Muscular Arteries

n Tunica intima - endothelium with Weibel-Palade bodies, basal lamina, subendothelial layer & thick internal elastic lamina

n Tunica media - up to 40 layers of smooth muscles, thick external elastic lamina

n Tunica Adventitia - thin layer of fibroelastic connective tissue, vasa vasorum not very prominent, lymph vessels and nerve fibers

Arterioles

n Terminal arterial vessels - less than 0.1 mm

n Tunica intima - endothelium with Weibel-Palade bodies, basal lamina, subendothelial layer not very prominent, some elastic fibers instead of a well defined internal elastic lamina

n Tunica media - 1 or 2 layers of smooth muscle

Arterioles

n Tunica adventitia - loose connective tissue & nerve fibers

n Metarterioles - arterioles that supply blood to capillary beds - smooth muscle layer is not continuous - individual smooth muscle cells spaced apart & encircle the endothelium of the capillary arising from the metarteriole - act as sphincter for the capillary bed

Capillaries

n Arise from the terminal ends of arterioles

n Brach extensively into a network called a capillary bed between arterioles and venules

n Types

n Continuous

n Fenestrated

n Sinusoidal

n Formed from a single layer of squamous epithelial cells - 10 to 30 mm

n Rolled into a tube of 8 to 10 mm in diameter

Capillaries

n External surface - surrounded by basal lamina

n Pericytes are located along the outside of capillaries and small venules - appear to surround them

n May form gap junctions with the endothelial cells

Types of Capillaries

n Continuous capillaries - present in muscle, nervous and connective tissue

n In the brain labeled - modified continuous capillaries

n Intercellular junctions between endothelial cells are - fasciae occludentes - that prevent passage of many molecules

n Substances such as amino acids, glucose, nucleosides and purines move across the capillary wall via carrier mediated transport

Types of Capillaries

n Fenestrated capillaries - have pores (fenestrae) in their walls - 60 to 80 nm in diameter & covered by pore diaphragm (eight fibrils radiating out from a central area forming wedge-like channels each with an opening of about 5.5nm)

n Found in the pancreas,intestines and endocrine glands

n Fenestrated capillaries of the renal glomerulus lack any diaphragms

Types of Capillaries

n Sinusoidal capillaries - may possess discontinuous endothelial cells and basal lamina & large lumens

n Contains many large fenestrae without diaphragms - enhances exchange between blood and tissue

n Found in bone marrow, liver, spleen, lymphoid organs and certain endocrine organs

n Called sinusoids

Blood Flow in Capillary Beds

n Terminal of most arteries end in capillary beds that deliver the blood to venules for back to the venous side of circulation and the heart

n An artery that simply joins with a venous channel - arteriovenous anastomosis (AVA)

n When AVAs are closed the blood passes through the capillary bed

n When shunts are open - a large amount of blood bypasses the capillary bed & flows into the AVA

Blood Flow in Capillary Beds

n Blood flow through the capillary bed is controlled by either metarterioles with precapillary sphincters or by terminal arterioles

n Metarterioles from the proximal portion of a central channel - the distal portion is formed by the thoroughfare channel that empties into small venules

Classification of Veins

n Postcapillary venules

n 15-20 mm in diameter

n Receive blood from capillaries - their walls are similar to those of capillaries

n Site of leukocyte emigration

n See reticular fibers & pericytes surrounding the endothelium

n Pericytes are replaced by smooth muscle in larger venules (> 1mm)

Classification of Veins

n Postcapillary venules (small veins)

n The endothelial cells of venules of some lymphoid organs (nodes) have cuboidal rather than squamous epithelial cells - high-endothelial venules

n Function in lymphocyte recognition and segregation - so they end up in the correct lymphoid tissues

Classification of Veins

n Small & Medium Veins

n Tunica intima -endothelium, basal lamina, valves in some, subendothelial connective tissue

n Tunica media - reticular & elastic fibers, some smooth muscle cells

n Tunica adventitia - collagen layers with fibroblasts

Classification of Veins

n Large veins

n Tunica intima -endothelium, basal lamina, valves in some, subendothelial CT

n Tunica media - connective tissue: smooth muscle cells

n Tunica adventitia - smooth muscle cells oriented in longitudinal bundles, cardiac muscle cells near their entry into the heart; collagen layers with fibroblasts

Valves of Veins & Venules

n Composed of two leaflets

n Extensions of the intima

n Structurally reinforced by collagen and elastic fibers

n Block backward flow of blood

Heart

n Endocardium - simple squamous epithelium & underlying layer of fibroelastic connective tissue

n Subendocardial layer - loose CT with blood vessels, nerves and Purkinje fibers of the heart’s conduction system - attaches to the endomysium of the cardiac muscle

Heart

n Myocardium - middle layer - contains cardiac muscle cells

n Cells arranged in spirals around the orifices of the chambers

n Sinoatrial node - pacemaker - located at the junction of the superior vena cava and the right atrium - cells are spontaneously active

n Atrioventricular node - located in the septal wall just below the tricuspid valve - activity of these cells controlled by SA node

n Atriventricular bundle (bundle of His) - fibers course down the interventricular septum - terminate in small Purkinje fibers - subendocardial

Heart

n Epicardium - homologue of the tunica adventitia

n Outermost layer of the heart

n Composed of simple squamous epithelial cells

Heart

n Cardiac Skeleton

n Structural framework for the heart - fibrous CT - stops spread of electrical impulse between atria & ventricles

n Valves extend from this fibrous skeleton

n Annuli fibrosi - formed around the base of the aorta, pulmonary artery and atrioventricular orifices

n Trigonum fibrosum - formed primarily in the vicinity of the cuspal area of the aortic valve

n Septum membranaceum - constituting the upper portion of intraventricular septum

Lymphatic Vascular System

n Consists of a series of vessels that collect excess extracellular fluid formed at all capillary beds

n This excess fluid, called lymph in the vessels is returned to venous circulation

n System begins with lymphatic capillaries that drain off excess extracellular fluid - next are lymph vessels - eventually empty into two lymphatic ducts that empty into the internal jugular and subclavian veins

n Lymph nodes along the way - filter the lymph

Lymphatic Vascular System

n Lymphatic capillaries - single layer of attenuated endothelial cells with incomplete basal lamina

n Lymphatic ducts - similar to large veins

n Empty into the great veins of the neck - subclavian and internal jugular