CIRCULATORY SYSTEM

Kristina C. Erasmo, M.D.

Function

• Transport – oxygen, carbon dioxide, nutrients, hormones, waste products

• Immune defense

CIRCULATORY SYSTEM

• Cardiovascular system– Heart– 2 systems of blood vessels:• Systemic circulation• Pulmonary circulation

• Lymph vascular system

Heart

• Muscular pump that propels blood to the arteries of both systemic and pulmonary circulation

• Systemic circulation – brings blood from the heart to the rest of the body then back to the heart

• Pulmonary circulation – brings blood from the heart to the lungs and then back to the heart

Heart

• Located in the central mediastinum of the thoracic cavity

• Four chambers:– Left ventricle – systemic circulation– Right ventricle – pulmonary – Left atrium– Right atrium

Pericardium

• Connective tissue sac covering the heart– Outer sac (fibrous pericardium)– Inner sac (serous pericardium)• Parietal pericardium• Visceral pericardium• Pericardial cavity – with fluid

Pericardium

• Fibrous and parietal pericardia – dense CT (collagen and elastic fibers, fibroblasts, other CT elements)

• Inner aspect of parietal pericardium – simple squamous epithelium (mesothelium)

• Visceral pericardium a.k.a. epicardium

Histologic Layers of Heart Wall

• Epicardium• Myocardium• Endocardium

Epicardium

• a.k.a. visceral pericardium• Outermost layer of heart wall• Lining epithelium of external surface:

mesothelium

Myocardium

• Thickest (thickness varies for different parts)• Thickest: left ventricle• Thinnest: atria• Mainly cardiac muscle fibers

Purkinje Fibers

• Modified cardiac muscle cells• Initiate and conduct the electrical impulse that

controls the contraction of the heart• Form the impulse-conducting system of the

heart• Larger, contain more glycogen than ordinary

cardiac muscle cells

Myocardium

• Atrial natriuretic peptide (ANP) – polypeptide hormone secreted by cardiac muscle cells in atria and interventricular septum– Causes natriuresis – excretion of sodium by the

kidneys

Endocardium

• Thinnest• Lining epithelium: endothelium• Continuous with innermost layer of the great

blood vessels• Lines all internal surfaces of heart

Endocardium

• Subendothelial layer – loose CT• External to subendothelial layer – dense CT,

thickest portion of endocardium

Blood Vascular System

• Refers to the system of blood vessels within which blood circulates

• 3 types of blood vessels:– Arteries– Veins– Capillaries

Blood Vascular System

• Arteries – carry blood from the heart to the capillaries

• Veins – carry blood from the capillaries to the heart

Endothelium

• Lining of the luminal surface of all blood vessels

• Functions:– Lining blood vessels to facilitate blood flow– Secretes components of underlying CT

Endothelium

• Functions:– Secrete prostacyclin – inhibits platelet

aggregation, promotes vasodilation– Nitric oxide and endothelin I – vasodilator– Von Willebrand factor – blood coagulation

Vasa Vasorum

• Network of small blood vessels that supply large blood vessels

• Veins: present in the tunica adventitia and media

• Arteries: confined to adventitia

Capillaries

• Smallest blood vessels (5-10 um)• Connect arterioles and venules • Function: Allow exchange of gases and

materials between blood and surrounding tissue

Capillaries

• Wall consists of single layer of endothelial cells and its associated basal lamina

• Surrounding connective tissue elements

Pericytes

• Slender, elongated, perivascular cells that usually accompany capillaries

• Look like fibroblasts• Not part of the endothelium• Contain tropomyosin • Function: contractile cells that influence the

luminal size of capillaries

Types of Capillaries

• Continuous (Type I)• Fenestrated (Type II)• Sinusoids

Continuous Capillary

• Found in muscles, lungs, CNS, skin• The endothelial cell and the basal lamina do

not form openings (which would allow substances to pass the capillary wall without passing through both the endothelial cell and the basal lamina)

Fenestrated Capillary

• Mucous membranes of GIT, renal glomerulus, pancreas, some endocrine glands

• Endothelial cell body forms small openings (fenestrations) which allow components of the blood and interstitial fluid to bypass the endothelial cells

Sinusoids

• a.k.a. discontinuous capillaries or sinusoidal capillaries

• Formed by fenestrated endothelial cells, (which may not even form a complete layer of cells)

• Basal lamina is also incomplete• Found in liver, spleen, red bone marrow (free

exchange of substances or even cells between bloodstream and organ is advantageous)

Histologic Layers of Arteries and Veins

• Tunica adventitia• Tunica media• Tunica intima

Tunica Adventitia

• Outermost coat of arteries and veins• Chiefly made up of connective tissue with cells

and fibers arranged longitudinally

Tunica Media

• Middle coat of arteries and veins• Concentrically arranged smooth muscle fibers• Interspersed between the muscle cells are

connective tissue elements (collagen and elastic fibers)

Tunica Media

• In large arteries: – External elastic lamina/membrane – outer layer

of elastin which demarcates the tunica media from the tunica adventitia

Tunica Intima

• Innermost coat of arteries and veins• Consists of a layer of endothelium that rest on

a basal lamina• Subendothelial layer of loose CT• In arteries:– Internal elastic lamina/membrane – demarcates

tunica intima from tunica media

Arteries

• Classification:– Small (arteriole)– Medium (muscular or distributing)– Large (elastic or conducting)

• Decrease in size but increase in number as they go farther from the heart

Arterioles

• Smallest arteries (40 – 400 um)• Tunica intima: only endothelium, no IEL or

subendothelial CT• Tunica media: no EEL• Tunica adventitia: loose CT

Arterioles

• Relatively thick walls and narrow lumens – offer considerable resistance to blood flow (thus able to deliver blood to the capillary beds under greatly reduced pressure)

Medium Arteries

• Muscular arteries – well-developed tunica media

• Distributing arteries – distribute blood to the different parts of the body

• Biggest: brachial and femoral arteries

Medium Arteries

• Tunica intima: 3 layers– Endothelium, subendothelium, and IEL

• Tunica media: Up to 40 layers of circularly arranged smooth muscle fibers, EEL present

• Tunica adventitia: loose CT, vasa vasorum

Large Arteries

• Elastic arteries – high elastic tissue content• Conducting arteries – major conduits in the

transport of blood away from the heart• E.g. aorta• Recoil is responsible for the flow of blood

through the blood vessels during diastole

Large Arteries

• Tunica intima: – Polygonal endothelial cells, – Subendothelial layer – loose CT– Indistinct IEL

Large Arteries

• Tunica media: – 40-60 layers of elastic laminae arranged

concentrically– Indistinct EEL

• Tunica adventitia: – Thin, merges with surrounding CT– Contains vasa vasorum

Sensory Organs Associated with Arteries

• Carotid bodies and aortic bodies – Sensitive to changes in oxygen and CO2 tension of

blood• Carotid sinus– Sensitive to changes in arterial blood pressure

Veins

• Classification:– Small (venules)– Medium– Large

Veins

• Carries blood from the capillaries to the heart• More numerous than arteries• Bigger calibers, more irregular lumens, thinner

and less elastic walls• Increase in caliber and walls become thicker as

they travel toward the heart

Small Veins (Venules)

• The 3 histologic layers of the vessel wall can only be appreciated in venules with a luminal diameter of at least 50 um

• Like capillaries, they are sites for exchanage of gases and substances

Medium Veins

• Tunica intima: – Polygonal endothelial cells– Subendothelial layer

• Tunica media: small bundles of circularly arranged smooth muscle cells

• Tunica adventitia: bulk of the wall

Large Veins

• E.g. inferior and superior vena cava, portal vein

• Tunica intima: thicker• Tunica media: poorly developed, few smooth

muscle fibers

Large Veins

• Tunica adventitia: very thick, consists of 3 poorly-defined zones– Internal zone – dense CT– Middle zone – longitudinally arranged muscle

fibers– Outer zone – collagenous and elastic fibers

Arteries vs. VeinsArteries Veins

Lumen Patent, RBC not seen Lumen may be collapsed, RBC usually seen

Endothelial lining Well-defined Not so well-defined

Internal elastic lamina (IEL)

Distinct Poorly defined

Tunica media Forms 2/3 of thickness of wall

1/3

External elastic lamina (EEL)

Distinct Poorly defined

Tunica adventitia Forms 1/3 of thickness 2/3