Circulatory System Histology

Tunica intima

single layer of endothelial cells sitting on a basal lamina

Elastic arteries

- (conducting arteries)

- the biggest arteries closest to the heart (includes the aorta and its major branches and pulmonary arteries)

- Need to be elastic to handle the high pressure of blood coming from the heart

Muscular arteries

- (distributing) arteries

- The next biggest arteries, come after the elastic arteries

- have an external elastic lamina or membrane, which separates tunica media from tunica adventitia - The identifying characteristic of muscular arteries is a relatively thick tunica media composed mostly of smooth muscle cells

Arterioles

- includes metarterioles

- The smallest arteries

- The terminal arterial vessels that regulate blood flow into the capillary beds

- no internal elastic lamina

- no external Elastic lamina

- 1 - 2 layers of smooth muscle

- very thin adventitia

- Constriction and relaxation of these smooth muscle cells regulate blood pressure

Metarterioles

branched from arterioles

- surrounded by discontinuous layer of smooth musle

Gap Junction

- connect smooth muscle in both elastic and muscular arteries

- allows muscles to contract together

- allows nutrient and oxygen exchange

Dissecting Aorta

tear develops in the tunic intima of aorta

- blood surges through tear to the tunic media causing intima and media to separate

Marfan's syndrome

results from a mutation in fibrillin, the major component of elastic fibers.

internal elastic lamina

-ONLY IN ARTERIES, NOT IN VEINS

-separating the tunica intima from the tunica media is an internal elastic lamina,

- composed of elastin, that is particularly well developed in muscular arteries

Vein vs. Artery

1) Typically, veins have thinner walls than arteries

2) The lumen of the vein is much larger than the lumen of an artery relative to the size of its tunics

3) veins have valves ; arteries never have valve

Vein Valves

- prevent backflow of blood

- portion of the intima jutting into the lumen

- reinforced with collagen & elastic fibers

- one way

Deep Vein Thrombosis

- local coagulation or clotting of the blood in a part of the circulatory system

- happens with skeletal muscle pump is not working

Varicose veins

- valves fail to close properly

- blood pools in the vein

- pooling force wall of vein outward causing bulging and rope like appearance

- most common in legs and feet

Capillaries

- major place for gas exchange between blood stream and extra vesicular tissue

- walls one cell thick - diameter < RBC - don't have tunica media

Precapillary sphincters

- rings of smooth muscle which allow for constriction to cut off blood flow to capillaries or relaxation to allow flow into capillary bed

- Only 3-5% of the capillaries are open to blood flow at any one time

Metarteriole-thoroughfare channel

- shunt to bypass the true capillaries

Pericytes

- present in some capillaries and post-capillary venules

- have contractile function (replace tunica media)

- help move blood through capillary

- contain actin, myosin, tropomyosin

- important in growth of new blood vessels and regeneration of injured blood vessels.

Continuous capillaries

- one continues layer of endothelial cells attached by tight junctions

- basal lamina continuous

- absence of fenestrae

- found in all kind of muscle tissue, connective tissue, exocrine glands, and nervous tissue

- numerous pinocytotic vesicles (transport in both directions)

Adluminal

the side near or facing the lumen

Abluminal

the side directed away from the luman

Fenestrated Capillaries

- have pores called fenestrae - more permeable to water and small solutes

- found in tissue where rapid interchange is required (kidney, intestine, endocrine glands)

- pores bridged by ultrathin diaphragm (None in renal glomerulus -> faster filtration)

Sinusoidal Capillaries

- fenestrated

- devoid of diaphragms

- endothelial cells form a discontinuous layer separated by wide spaces

- discontinuous basil lamina

- tortuous path, enlarged diameter that slow circulation of blood

- found mainly in the liver, bone marrow, spleen

- whole cells can pass through the large holes

- megakaryocytes "pinch off" through sinusoidal capillaries

blood-brain barrier (BBB)

lets essential metabolites, such as oxygen and glucose, pass from the blood to the brain and central nervous system (CNS) but blocks most molecules that are bigger than about 500 Daltons.

- Created by extensive tight junctions with very few pinocytotic vesicles

- develops early in embryo when astrocyte foot processes wrap around endothelial cells.

- Astrocytes secrete factors that induce extensive tight junctions

Usual arteriole sequence

—> metarteriole —> capillary —> venule and vein.

Arteriovenous anastomosis

(direct communication between arterioles and venules-no capillaries).

Arterial portal system

present in the kidney glomerulus (arteriole —> capillary —>arteriole)

Venous portal system

present in the liver (venule —> capillary —> venule).

Endocardium

- faces the heart chamber/blood

- continuous with the tunica intima of the blood vessels entering and leaving the heart

1) endothelium-this layer of connective tissue lined by simple squamous endothelium

2) Between the endocardium and myocardium is a layer of variable thickness called the subendocardial layer : dense connective tissue that contains small blood vessels, nerves, and Purkinje fibers from the conduction system of the heart. Attaches to the connective tissue around the cardiac muscle

Myocardium

- middle and thickest of the three layers,

- contains cardiac muscle cells arranged in complex spirals around the orifices of the chamber

- separated by loose connective tissue (CT).

- contains lots of capillaries mainly supplying lots of oxygen

Epicardium

- outer most layer aka the visceral layer of the serous perichardium

- thin fibrous connective tissue covered by think mesothelium (simple squamous)

- Subepicardial layer of loose connective tissue contains the coronary vessels, nerves, ganglia, and fat.

- Contains coronary arteries & veins

Fibrous skeleton

masses of dense connective tissue in endocardium which anchors valves and surround the two AV valves and gives them their shape

Heart Valves

-composed of connective tissue with overlying endocardium on all sides

- attach to a complex of dense irregular connective tissue that forms fibrous ring

- spongiosa layer contains lots of proteoglycans that act as shock absorbers

- collagen and elastic fibers are found throughout the valve

Heart Damage

dead muscle cells are replaced by fibrous connective tissue which cannot contract and therefor weakens the heart

Purkinge fibers

located in the inner ventricular walls of the heart in sub-endocardium

- larger than cardiac muscle cells with fewer myfibrils and no t-tubules

- contains lots of glycogen and mitochondria

Risk factors for atherosclerosis

-leads to endothelial injury

1. Hyperlipidemia

2. Smoking

3. Hypertension

4. Turbulence

Hyperlipidemia

- is the circulation of high levels of lipid in the blood.

- LDL gets modified over time and may become oxidized and this oxidized and modified LDL is preferentially taken up by macrophages.

Foam cells

- from macrophages and smooth muscle cells

engorged macrophages that unsuccessfully tried to remove LDL cholesterol particles from artery walls. Trigger oxidation and inflammation that attract more immune scanners to scene.

Smoking

- may damage the endothelial cells by production of carbon monoxide.

- nicotine is a potent vasoconstrictor and can cause vasospasm.

- may promote the oxidation of LDL.

Hypertension

may physically injure the endothelial cells

Turbulence

may also physically injure the endothelial cells.

- plaques tend to form in areas of high turbulence in vessels, specifically branch points.

Lymph circulation

- lymph dumps back into veins at thoracic duct and right lymphatic duct

- all lymph flows in one direction toward the heart

- slow because there is no pump

- contraction of skeletal muscle, pulsation of nearby arteries, valves, pressure changes, & rhythmic contraction of lymphatic ducts aid in circulation

Anchoring fibrils

hold open lymphatic capillaries & anchoring them to surrounding connective tissue

Lymph capillary structure

- overlapping free borders of endothelial cells

- discontinuous basal lamina

- attachment of anchoring fibrils

- Fragmented, indistinct basal lamina distinguishes lymphatic capillaries from blood capillaries