Cardiovascular System

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Parts of the Cardiovascular System

• Heart to pump blood.

• Blood vascular system.

• Arteries - carry oxygen & nutrients to tissues.

• Capillaries -effect interchange between blood & tissue.

• Veins - convey products of metabolism toward heart.

• Lymphatics return tissue fluid to circulatory system.

Types of Arteries

A muscular artery is considered a "generic" vessel because it has most all the layers possible. We categorize vessels by noting the components they demonstrate.

Medium muscular artery All large & medium vessels have the same components & 3 basic layers or tunics, ie. intima, media & adventitia.

Muscular artery & accompanying vein have different amounts of the same components.

Tunics of a Small Artery Tunica Intima (TI)

• Thinnest.

Consist of:

• Endothelium.

• Basal lamina.

• Thin layer of collagen & occasional elastic fibers.

• Also present in lymphatics.

• Internal elastic lamina (IEL) may be present.

Tunica Media (TM)

• Thickest layer.

• Primarily a smooth muscle layer arrayed in a circular fashion.

• Network of elastic fibers can form elastic laminae.

• Not present in capillaries.

Tunica Adventitia (TA)

• Mostly collagen in loose CT.

• May have external elastic lamina (EEL).

• Most prominent in veins.

• Some elastic fibers.

• Contains blood & lymphatic vessels & nerves.

Vasa Vasorum

• Vessels of vessels.

• Small blood vessels vascularizing TA & external part of TM of large arteries & veins.

Elastic Arteries

• Large elastic, conducting arteries: conduct blood away from heart to muscular arteries.

• Characterized by high content of elastic fibers & fenestrated elastic laminae in TM.

• Elastic laminae (50 or more) alternate with layers of smooth muscle, collagen & elastic fibers.

• Elastic recoil is responsible for maintaining arterial blood pressure during diastole.

• EEL is the outermost layer of TM.

• TA: Thin.

Muscular Arteries

• Distribute blood to the organs.

• Most of the named arteries in the body are of this type.

• Lose most of the medial elastic sheets but maintain the IEL & EEL.

• Regulate the distribution of blood flow to the different regions of the body using the thick muscular TM.

• Round lumen

• IEL is prominent & scalloped.

• Up to 40 layers of smooth muscle 7-15 layers of muscle in smallest peripheral arteries.

• EEL less distinct & more diffuse than IEL.

• TA greater than or equal to thickness of TM.

Examples of Muscular Arteries

• Brachial.

• Ulnar.

• Radial.

• Proper palmar digital.

Small Arteries

• Microvasculature is visible only with the LM & includes: arterioles, capillaries & venules.

• Functions to exchange nutrients & wastes between blood & tissues.


• 2-6 smooth muscle layers.

• IEL present.

• No EEL.

• Smallest, terminal arterioles have 1 layer of smooth muscle & no IEL.

Weibel-Palade Bodies

• Membrane limited endothelial vesicles that contain a protein of the blood coagulation scheme known as van Willebrand's factor (factor VIII).

• Deficiency results in impaired platelet adhesion to injured endothelium & prolonged bleeding.

• One cause of hemophilia.

Terminal Arterioles

Feed into:

• True capillary.

• Metarteriole.

• Glomus.

• Arteriovenous shunt.

True capillary

Consists of:

• TI = endothelial cell & BL.

• TM = pericytes.

• TA = wafer thin "wisps" of CL.

• Precapillary sphincter (a single smooth muscle cell) regulates flow to the capillary.

• An ex. of local control.

• Capillaries are the major site of gaseous exchange.

• Continuous & fenestrated.


• Oxygen, carbon dioxide, water & glucose cross.

• Leukocytes leave the vascular system by passing through the junctions via diapedesis.

• Secrete types II, IV & V collagen, fibronectin & laminin.

Categories of endothelial cells

• Continuous (somatic) most common.

• Fenestrated with diaphragm (visceral).

• Fenestrated without diaphragms (kidney).

• Sinusoid (discontinuous).

• Rich in enzymes & bear receptors for angiotensin.

Endothelial Cell Adhesion:

• Endothelial cells are joined by zonulae adherens, nexus (old term macula communicans) & zonulae occludens.


• Are believed to represent reserve cells capable of differentiating into macrophages.

• They wrap around capillaries & postcapillary venules.

• Share BL with endothelial cell.

Continuous Capillary

• Somatic.

• Most common.

• Continuous BL.

• Tightness of tight junctions depends on the tissue.

• Ex. muscle, skin, CT.

Fenestrated Capillary with Diaphragms

• Fenestra pierce endothelium (role of diaphragms undetermined).

• Continuous BL.

• Found where rapid exchange of substances between blood & tissue is required.

• Ex. endocrine glands.

Fenestrated without Diaphragms

• Renal glomerulus.

• Very thick BL separate endothelium from overlying podocytes.

• BL consists of lamina lucida, lamina dense & another lamina lucida.


• Absent or sparse BL.

• Joined by nexus, & zonulae occludens.

• Large transcellular holes.

• Large lumen that slows blood flow.

• Ex. liver, Spleen, Bone marrow, Adrenal cortex.


• Wider than capillary.

• Has smooth muscle.

• Can drain to true capillary or capillary network.

• Can form a thoroughfare channel (preferrential channel), preferred because it is a little larger & accommodates blood cells more easily.

• Throughfare channels do not have smooth muscle.

• Constriction of metarterioles helps regulate but does not stop circulation to capillaries.

• Precapillary sphincter can completely stop flow.

Arteriovenous Shunt

• Wide lumen.

• A couple of muscle layers.

• Runs from arteriole to venule, bypassing capillary.

• Found in skin for temperature regulation.

• With exercise, muscle heat increases, transfer to blood, then to skin & heat loss.


• Complex highly coiled arteriovenous shunt in skin important in controlling circulation.

• Arterioles that are continuous with venules lose their IEL & acquire a thick muscle layer.

• Contraction can partially or completely close vessels.

Postcapillary Venule

• No smooth muscle.

• A few pericytes.

• Typically, postcapillary venules are the exit site for most of the wandering cells of the CT.

• High endothelial venules (HEV) are specialized postcapillary venules in lymphoid organs (a postcapillary venule does not have to be a HEV).

Muscular Venule

• Companion to arterioles.

• Larger than arteriole.

• Irregular lumen.

• Pericytes are progressively replaced by 1-2 layers of smooth muscle Drain into smallest veins.

Medium Vein

Veins have thin walls, are frequently collapsed & tunics are less well demarcated. Small to medium veins:

• Companion to muscular arteries & can occur in neurovascular bundles along with nerves.

• TA is thickest layer.

• Contain valves.

• Skeletal muscle "milks" veins.

• Patchy IEL.

Lymphatic Vessels

• Parallel venous channels except: CNS, Bone, Cartilage, Teeth, Placenta, Cartilage, Thymus, Bone marrow.

Lymph Flow

• Lymph is returned to general circulation.

• Right lymphatic duct returns fluid from upper right side at junction of internal jugular & subclavian

• Thoracic duct begins as a dilation, the cisterna chyli, and continues to enter at junction of left jugular & subclavian.

• Blind sacs, lacteals, collect fluid in the intestines.

Functions of Lymph

• Return tissue fluid & plasma proteins to general circulation.

• To add antibodies formed in lymph nodes to the blood.

• Lymph from lacteals in the intestine & lymphatic capillaries.

• No continuous BL.

• No pericytes.

• Anchoring filaments are thought to be related to elastic fibers & maintain the lumen in an open state.

Layers of the heart

Heart consists of 3 main layers:


• Analogous to TI Simple squamous.


• Analogous to TM Thickest layer.

• Typically 3-4 spiral layers of cells.

• Contains more capillaries per unit volume than any tissue in the body.


• Analogous to TA Adipose covering Contains coronary arteries.

Cardiac skeleton

• Dense fibrous CT (collagen & elastin) that provide for attachment of muscles.

• Annulus fibrosus - around aorta, pulmonary artery & AV orifices.

• Less well organized annulus fibrosus around aortic & pulmonary valves.

• Right & left trigone between AV valve & aortic valve.

• Membranous portion of interventricular septum.

Cardiac valves and attachments

• Cardiac valves are extensions of the endocardium with a core of dense collagenous fibers.

• Chordae tendineae emerge from papillary muscles and course to valve leaflets where they attach.

• Contain parallel bundles of collagen.

Heart conduction system

Made of modified cardiac cells.

1. Sino-atrial (SA) node contains pacemaker cells that initiate contraction & follower cells that transmit impulses to cause contraction of atria.

2. Atrio-ventricular (AV) node.

3. AV bundle of His.

4. Right & left bundle branches. Half way down they become Purkinje cells (no T-tubules, fewer myofibrils, 2-3X larger than cardiac cells, lighter staining, more gap junctions).

5. Subendocardial plexus.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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