CIRCULATORY SYSTEM

Kristina C. Erasmo, M.D.

Function

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

• Immune defense

CIRCULATORY SYSTEM

• Cardiovascular systemo Hearto Two systems of blood vessels:

o Systemic circulationo 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:o Left ventricle – systemic circulationo Right ventricle – pulmonary o Left atriumo Right atrium

Pericardium

• Connective tissue sac covering the hearto Outer sac (fibrous pericardium)o Inner sac (serous pericardium)

o Parietal pericardiumo Visceral pericardiumo 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 septumo 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:o Arterieso Veinso 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:o Lining blood vessels to facilitate

blood flowo Secretes components of

underlying CT

Endothelium

• Functions:o Secrete prostacyclin – inhibits

platelet aggregation, promotes vasodilation

o Nitric oxide and endothelin I – vasodilator

o 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: o 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:

o Internal elastic lamina/membrane – demarcates tunica intima from tunica media

Arteries

• Classification:o Small (arteriole)o Medium (muscular or distributing)o 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 layerso 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: o Polygonal endothelial cells, o Subendothelial layer – loose CTo Indistinct IEL

Large Arteries

• Tunica media: o 40-60 layers of elastic laminae

arranged concentricallyo Indistinct EEL

• Tunica adventitia: o Thin, merges with surrounding CTo Contains vasa vasorum

Sensory Organs Associated with Arteries

• Carotid bodies and aortic bodies o Sensitive to changes in oxygen and

CO2 tension of blood• Carotid sinus

o Sensitive to changes in arterial blood pressure

Veins

• Classification:o Small (venules)o Mediumo 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: o Polygonal endothelial cellso 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 zoneso Internal zone – dense CTo Middle zone – longitudinally

arranged muscle fiberso 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

Lymph Vascular System

• Network of conduits that carry lymph

• Returns the extracellular liquid to the bloodstream (“drainage” system)

• Lymph circulates in one direction

Lymph Vascular System

• Functions:o Return the fluid of the tissue

spaces to the bloodo Upon entering the lymphatic

capillaries, this fluid contributes to the formation of the liquid part of the lymph

Lymph Vascular System

• Functions:o By passing through the lymphoid

organs, contributes to the circulation of lymphocytes and other immunological factors

Lymphatic Vessels

• Lymphatic capillaries – absent in cartilage, bone, thymus, teeth, placenta, CNS

• Lymphatic vessels• Lymphatic ducts (2) – drain into the

veins at the base of neck

Lymphatic Capillaries

• Also lined by endothelium (thin due to incomplete basal lamina)

• Branch and anastomose more freely than blood capillaries

• More variable in shape and caliber• Not associated with pericytes

Lymphatic Vessels

• Large lumen in relation to the thickness of their walls

• Endothelial lining + connective tissue layer (collagenous and elastic fibers, occasional smooth muscle)

• Large vessels: 3 layers (TI, TM, TA)• Have valves

Lymphatic Ducts

• Right lymphatic duct

• Thoracic duct

Right Lymphatic Duct

• Smaller, shorter• Right side of body above the

diaphragm• Drains into the right brachiocephalic

vein (junction of internal jugular and subclavian vein)

Thoracic Duct

• Collects lymph from the other parts of the body not served by the right lymphatic duct

• Drains at the junction of left jugular and subclavian veins