Cardiovascular System Histology

• Histology is the study of the cardiovascular system

Cardiovascular System Composition

• Consists of a central muscular pump, the heart
• Includes blood vessels: arteries, veins, and capillaries
• Has two circulation systems: pulmonary and systemic (or peripheral)

Circulation System Details

• Pulmonary systems carry blood to other tissues
• Large arteries are thick to withstand heart's pumping force
• Capillaries are thin for substance exchange with tissues
• Vein thickness varies with gravity; veins against gravity, like the inferior vena cava, have thick walls

Cardiovascular System Function

• Transports oxygen, hormones, nutrients, and macromolecules to organs and tissues
• Removes carbon dioxide and waste from cells

Heart Structure

• The heart has a right side, which receives blood from the body and pumps it to the lungs
• The heart also has a left side, which receives blood from the lungs and pumps it to the body
• The heart pumps 6 liters of blood in large arteries
• Each heart side has two chambers: an atrium for blood collection and a ventricle for ejection

Heart Wall Layers

• Inner endocardium
• Middle myocardium
• Outer epicardium

Endocardium Details

• Lines heart chambers and valves
• Made of:
  • Endothelium (simple squamous cells)
  • Thin basal lamina layer
  • Subendothelial layer of loose connective tissue, elastic fibers, blood vessels, nerves, and Purkinje fibers

Myocardium Details

• Forms the heart's main substance and thickest layer
• Made of cardiac muscle groups joined end-to-end at intercalated discs
• Myocardium is thicker in the ventricles compared to the atria

Epicardium Details

• The free surface is covered by visceral pericardium’s mesothelial membrane
• Serous membrane of simple squamous epithelium and connective tissue, containing fat cells and coronary vessel branches

Heart Valves

• There are four heart valves
  • Tricuspid: between right atrium and right ventricle
  • Pulmonary (semilunar): between right ventricle and pulmonary artery
  • Mitral: between left atrium and left ventricle
  • Aortic (semilunar): between left ventricle and aorta
• Valves are endocardium folds with a core of dense collagenous and elastic fibers, covered by endothelium

Blood Vessel Structure

• Walls have 3 coats/tunics with varying thickness and composition
  • Tunica intima
  • Tunica media
  • Tunica adventitia

Tunica Intima Details

• Innermost layer, close to the lumen
• Made of:
  • Endothelium (lamina endothelialis): simple squamous epithelium on a thin basal lamina, providing a smooth surface for blood passage
  • Subendothelial layer (lamina subendothelialis): loose connective tissue
  • Internal elastic lamina (lamina elastica enterna)

Tunica Media Details

• Muscular coat of blood vessels
• Structure is related to the vessel's function
• Made of circular smooth muscle fibers (in distributed arteries)
  • Function: produce elastin and collagen and propel blood, controlling blood flow to organs
• Elastic and collagenic fibers between muscle layers (large arteries)
  • Function: maintain forward blood movement during diastole, prevent closure during artery muscle contraction, and allow expansion during systole.
• External elastic lamina may be absent in capillary and postcapillary venule

Tunica Adventitia Details

• Outermost coat of blood vessels
• Loose connective tissue containing fibroblasts with few elastic and collagenic fibers
• Contains tiny blood vessels of the vessels called vasa vasorum
• Vasa vasorum are present in large vessels because the thickness of the vessel prevents the cells from being nourished by diffusion from the lumen.

Artery Types

• Large Artery
• Medium Sized Artery
• Small Artery & Arteriole

Large Arteries

• Also called elastic or conducting arteries because they have a thick wall and wide lumen
• Their walls maintain blood pressure during ventricular relaxation
• Their media are rich in elastic fibers
• Examples include the Aorta, aortic arch, common iliac, pulmonary, and subclavian arteries

Aorta Details

• It has a thick wall and wide lumen
• Tunica intima is thick and rich in elastic fibers
• Not apparent internal elastic lamina
• Made oF:
  • Endothelium that contains simple squamous epithelium with Endothelial cells joined by gap junctions
  • Subendothelium with loose connective tissue containing fibroblasts, collagen (type I, III and IV), and elastic fibers
  • Non-clear internal elastic lamina
• The tunica media is a very thick layer made of 40-70 laminae of fenestrated elastic membrane enclosing smooth muscles, collagenic and reticular fibers
• Tunica Adventitia is thin and narrow containing collagen, elastic fibers, nerves, and vasa vasorum, provides blood and nutrients to media and adventitia outer parts
• Elastic fibers allow expansion during systole; elastic recoil maintains blood flow and diastolic pressure during diastole
• They major elastic conducting arteries contain many elastic fibers and can stretch, taking the heart's ejected blood during ventricular conduction

Medium Sized Arteries

• Also called muscular arteries
• Are the most common arteries in the body, supplying blood to organs

Tunica Intima of Medium Sized Arteries

- Innermost layer
- Made oF:
  - Simple squamous epithelium
  - Subendothelial layer of connective tissue
  - Internal elastic lamina

Tunica Media of Medium Sized Arteries

• Includes circular smooth muscle fibers
• May have few scattered elastic fibers
• Includes fine collagenic fibers

Tunica Adventitia of Medium Sized Arteries

• Outermost layer composed of mostly collagenic fibers
• Contains some elastic fibers and connective tissue cells

Function Of Medium Sized Arteries

• Walls of fibers distribute blood to different body parts
• Regulate blood flow through contraction/relaxation

Arterioles/Small Arteries

• Regulate blood flow into capillary beds
• They have a small diameter (0.1 mm) with thick walls related to the lumen.
• Tunica Intima has endothelium; thin, wavy internal elastic lamina; no subendothelial tissue
• Tunica Media has 2-5 smooth muscle layers and scattered elastic fibers without external elastic lamina Tunica Adventitia is thin and contains collagen and elastic fibers
• They decrease pressure in blood entering capillaries
• They have impermeable walls, and aren't involved in interchange between blood and tissue fluid

Terminal Arteriole

• Contians a single layer of smooth muscle fiber

Metarterioles

• They are vessels intermediate between arterioles and capillaries
• They have thin walls with endothelial cells on basal lamina, plus interrupted smooth muscle cells layer
• A band of circular smooth muscle fibers called precapillary sphincters are located at the junction of metarteriole with capillary
• precapillary sphincters control blood flow into the capillary

Veins

• Include small venules, medium sized veins, and large sized veins
• They carry venous blood from tissues
• Similar to arterioles
• Start at the capillary bed as small post-capillary venules, which join to create larger veins

Small Post Capillary Venule

• Have a diameter of 0.2-1mm
• Smooth muscle fibers appear in the media as diameter increases
• Lined by a simple squamous cells layer
• Outside of the squamous endothelial cells there is differentiate in to smooth muscle
• Thin adventitia while large ones have thin media; small venules do not have media
• Function similar to capillaries; sites for gas exchange from blood to tissue

Large Muscular Venules

• They are lined by simple squamous epithelium with rich actin filaments
• Presence of basement membrane outside endothelial lining
• A formed media with a thin layer of smooth muscles
• Contains a thin adventitia formed of areolar CT

Medium Sized/Muscular Veins

• Are made of 3 tunics/layers
• Unoxygenated blood is collected from venules towards the heart by muscular veins, blood only goes towards the heart

Large Veins

• Examples of large veins are the superior vena cava, inferior vena cava, and common iliac vein
• Thicker Tunica intema compared to medium veins
• Tunica media consists of circular smooth muscle fibers with collagen and a few or no elastic fibers
• Tunica adventitia is thick the thickest coat in large bein. contains bundles of smooth muscle and collagen. Vasa vasorum is present
• Wide lumen with many valves in the IVC and I.V.C. is thick
• Smooth muscle bundles help to strengthen the vessel wall against excessive extension

Connections Between Arteries And Veins

• The blood capillaries
• Sinusoids
• Arteriovenous anastomosis

Blood Capillaries

• They connect arteries and veins, found throughout the body
• Thin-walled vessels with 8-10um luminal diameter

Blood Capillary Size

• Diameter ranges from 7-9um and 0.25-1mm in length
• Have a complete regular lumen about 8 microns in diameter

Blood Capillary Function

• Conduct the blood from arteriole branches to venule branches
• Made of a single-layered wall with endothelial cells resting on basal lamina
• Endothelium is formed of simple squamous epithelium
• Pericytes are found around the endothelium

Types of Capillaries

• Continuous (somatic) capillaries
• Fenestrated capillaries with diaphragm
• Fenestrated capillaries without diaphragm

Continuous (Somatic) Capillaries

• Present all over the body; found in CT, skeletal muscles, brain, bone, lung and exocrine glands
• Found in the brain
  • Share in formation of blood brain barrier which prevent the passage of some substances as antibiotic, chemical and bacterial toxin from the blood to the nerve tissue

Fenestrated Capillaries with Diaphragm

• Present in endocrine glands, intestine and ciliary body of eye
• The endothelial cells contain pores (fenestrae) and rest on the basement membrane. Through these pours tissue fluid can pass

Fenestrated Capillaries Without Diaphragm

• Present in the glomeriolar blood capillaries of kidney
• Speed up the process of exchange

Blood Sinusoids

• Thin walled capillary like vessel
• A connection between arterieis and veins
• Irregular wide lumens from 5-30 microns in diameter
• They are lined with simple squamous endothelial cells
• Macrophages are present either in or along the outside of the sinusoidal wall
• Walls contain pores not covered by diaphragms

Locations

• Bone marrow allows for formed blood cells to carry
• Found in spleen to store blood and in their wall to filter the blood from any foreign body by phagocytic cells
• Liver allows contact with blood by liver cells and easy passage of protein from phagocytes into the blood stream
• Endocrine glands and endocrine glands to carry the secreted hormones and to increase the blood supply to the endocrine secretory cells

Arteriovenous Anastomosis

• Definition: Arteriovenous anastomoses arise as side branches from arteries and arterioles and pass as straight or tortuous vessels to be connected with the veins or venules

Arteriovenous Anastomosis Type

• Direct connection between an arteriole and a venule by a side branch, which is found in the placenta
• Indirect (or glomus) a complicated side branch, which is a specialized organ present in skin
• Present in exposed parts of the body such as fingers, toes, the external ear, the nose, the lip, tongue, eyelids, as well as liver, and stomach internal organs, intestine

Glomus Details

• On some sites (nail beds/auricle of ear), A-V anastomosis are branched and tortuous an form a specialized organ
• The arterial branch is coiled or branched
• The internal elastic lamina disappears
• The elastic tissue is reduced
• The smooth muscle of the media is replaced by myoepithelial cells (epitheloid like cells)
• The adventitia becomes thicker