Cardiovascular System

Cardiovascular System

Heart

VeinsArteries

Blood vessels Lyphatics

Capillaries

Heart

▪ 3 layers:a) Endocardium—inner layerb) Myocardium—middle layerc) Epicardium (pericardium)—outer layer

▪ Large mass of muscle

▪ Cardiac muscle: variant form of striated muscle– Small cells; butted together at ends– Irregularly shaped– Numerous blood vessels between them– Network of fibers

▪ “Myocardium”—greek ”myos” (muscle); ”kardio” (heart)– The mass of muscle that comprises the organ– “stringy” look compared to skeletal muscle

Intercalated disks

▪ Most prominent feature

▪ Specialized cell-to-cell adhesion or communication site

▪ Demarcates the beginning of one myocyte and the end of the next

Intercalated disks

▪ Represent the interface between adjacent muscle cells where many junction complexes are present

desmosomes

facia adherentes

gap junctions—ionic continuity between adjacent cells

—”electrical synapses”

—allow heart muscles to act as in a multinucleated syncitium

bind cardiac cells firmly together to prevent their pulling apart under constant contractile act

Endocardium

▪ Inner lining

▪ Among the most metabolically active tissue

▪ Also covers the valve cusps

▪ Simple squamous epithelial covering + subendothelial CT

+ middle layer: connective tissue + smooth muscle

+ subendocardial layer: continuous to myocardium

: conducting system of the heart

Myocardium

▪ Principal component of the heart

▪ Thickest layer

▪ Arranged in layers that surround the heart chambers in complex spiral

▪ Thicker in ventrices than in atria

Epicardium

▪ Visceral part of serous pericardium

▪ Outer surface of the heart

▪ Single layer of mesothelial cells (simple squamous)+ CT and adipose tissue

Parietal: line the inner surface of pericardium: surrounds the heart and roots of great vessels

*15-50 mL= amount of pericardial fluid

Blood Vessels

Artery: vessel which carries oxygenated blood

Vein: carries deoxygenated blood

Capillary: channel which connects arterial and venous system

General Features

1. Tunica intima: innermost layer

a. Endothelium: simple squamous epithelium

b. Subendothelial layer: loose CT with occasional smooth muscle cells

c. Internal elastic lamina/ membrane (arteries): has elastin

: holes (fenestrae)—allow diffusion of substances to nourish cells deep in the vessel wall

General Features

2. Tunica media: middle layer

▪ Made up of either smooth muscles or elastic fibers

a. External elastic lamina—separates media from adventitia

General Features3. Tunica adventitia/ tunica externa: outermost layer

▪ Made up of dense irregular CT▪ Type I collagen and elastic

fibersa. Vasa vasorum: “vessels of the

vessel”**supplies blood to the vascular walls too far from the lumen to be nourished by blood

b. Nervi vascularis: vasomotor nerves

** unmyelinated sympathetic nerve fibers (ANS)*neurotransmitter:

norepinephrine>>discharge of norepinephrine from these nerves produces vasoconstriction

1. Large arteries/ large elastic arteries

▪ Help stabilize blood flow

▪ Carries oxygenated blood in it

▪ Usually thickness of large sized arterial walls is less than lumen diameter

▪ Rich in elastic fibers

▪ Aorta, pulmonary artery, brachiocephalic trunk, common carotid, subclavian, & common iliac arteries

2. MUSCULAR ARTERIES

▪ Include medium-sized and small diameter arteries.

▪ Their tunica media contains more smooth muscle and fewer elastic fibers.

▪ The large amount of smooth muscle makes the walls of muscular arteries relatively thick

3. SMALL ARTERIES

• The same structure as the medium-sized arteries, EXCEPT that small arteries have a smaller diameter and their walls are thinner.

• Have only up to 8 layers of smooth muscle in their walls.

• Adapted for vasoconstriction and vasodilation.

4. ARTERIOLES▪ Is a very small artery that

delivers blood to capillaries.

▪ The smallest arteries in which the 3 tunics can be identified.

▪ The tunica media consists of only one or 2 layers of circular smooth muscle cells.

▪ Play a key role in regulating blood flow from arteries into capillaries by regulating resistance.

CAPILLARIES ▪ Are microscopic vessels that connect arterioles to

venules.

▪ Consist of endothelium which is a layer of simple squamous epithelium surrounded by a delicate loose connective tissue.

▪ MICROCIRCULATION – the flow of blood from arterioles to venules through capillaries.

▪ Capillaries branch without changing their diameter.

TYPES OF CAPILLARIES

1. CONTINUOUS CAPILLARIES – most common type

▪ Have tight, occluding junctions sealing the intercellular clefts between all the endothelial cells to produce minimal fluid leakage.

▪ Characterized by the distinct continuity of the endothelial cells in its walls.

TYPES OF CAPILLARIES

2. FENESTRATED CAPILLARIES – characterized by the presence of small circular fenestrae through the very thin squamous endothelial cells.

▪ Found in tissues where rapid interchange of substances occurs between the tissues and the blood --- kidney, intestine, choroid plexus and endocrine glands.

TYPES OF CAPILLARIES

3. SINUSOID CAPILLARIES – DISCONTINUOUS CAPILLARY

▪ Permits maximal exchange of macromolecules between tissues and blood

▪ Has the following characteristics:

a. Have large fenestrae without diaphragms

b. Cells form discontinuous layer and are separated from one another by wide spaces

c. Basal lamina is discontinuous.

Capillary with pericyte

Pericyte: • Rouget Cells• provide vascular

support and promote stability of capillaries and postcapillary venules through physical and chemical signaling with vascular endothelial cells

1. continuous capillaries with many tight junctions so that all exchange must occur through the cells;

2. fenestrated capillaries with small pores or fenestrations through the cells; and

3. discontinuous capillaries, or sinusoids, with larger lumens, large spaces between the endothelial cells, and a discontinuous basal lamina

TYPES OF VEINS1. Venules• collect blood from

capillaries and drain into veins.

• Composed of endothelium resting on a delicate connective tissue layer.

• The walls of venules consist of a tunica interna and a tunica media that has only a few scattered smooth muscle fibers and fibroblasts.

• Venules are further subclassified as postcapillary and muscular venules

▪ Postcapillary venules collect blood from the capillary network and are characterized by the presence of pericytes.

▪ Muscular venules are distinguished from postcapillary venules by the presence of a tunica media.

2. SMALL VEINS – are slightly larger in diameter than venules.• All 3 tunics are present in small veins.• The tunica media contains a continuous layer of smooth muscle cells, and the connective tissue of the tunica adventitia surrounds the tunica media.

3. MEDIUM-SIZED VEINS• Collect blood from small veins and deliver it

to large veins.• The 3 distinctive tunics make up the wall of

the medium-sized and large veins.• Tunica media contains some circular smooth

muscle and sparsely scattered elastic fibers.

• Tunica adventitia is the predominant layer consist of dense collagen fibers that determines the degree to which they can be extended.

• Have many valves.

4. LARGE VEINS

• There are many valves in veins of the LOWER LIMBS than in veins of upper limbs, prevents the flow of blood toward the feet in response to the pull of gravity.

Wall of large vein with valve

SMALL VEIN

MEDIUM-SIZED VEIN

Clinical Histology (arteries)

1. Inflammation of the artery is known as arteritis

2. Atherosclerosis is characterized by the formation of atheromatous plaque in the tunica intima of larger blood vessels reducing its lumen size

3. Endothelial lining of an artery is anti-thrombotic in nature. Any breach or discontinuity in its layer leads to the formation of thrombus which in turn may lead to myocardial infarction or heart attack

4. An abnormal and permanent dilation of an artery is called aneurysm and is more common in large sized arteries like aorta

5. Monkeyberg’s medial sclerosis is a peripheral vascular disease characterized by the deposition of calcium in the tunica media of medium sized blood vessel.

Clinical Histology (veins)

1. Abnormally dilated and tortuous veins are called varicose veins

– Commonly seen in medium and small sized veins of the lower limb– Varicosities of the vein are due to weakening of the valves of the

lower limb

2. Thrombophlebitis is a condition where there is formation of a thrombus in vein with inflammatory changes