Transcript 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. Veins
Arteries
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