The Blood Vascular System

• The blood vascular system consists of a central muscular pump, called the heart and blood vessels (arteries, veins, and capillaries).
• There are 2 circulatory systems: pulmonary and systemic.
• Large arteries are thick to withstand the heart's pumping force, while capillaries are thin for substance exchange.
• Vein thickness varies with gravity, with those against gravity having thicker walls, such as the inferior vena cava.
• Oxygen, hormones, nutrients, and macromolecules are transported to organs and tissues, while unwanted substances are removed.
• The right side receives blood from the body and pumps it to the lungs.
• While the left side receives blood from the lungs and pumps it to the body.
• The heart pumps about 6 liters of blood into major arteries and consists of two chambers: an atrium, and a ventricle.

Structure Of The Heart Wall:

• There are three layers of the heart wall: inner endocardium, middle myocardium, and outer epicardium.

The Endocardium

• The endocardium lines the chambers and valves of the heart.
• This consists of endothelium of simple squamous cells, a thin basal lamina layer, and a subendothelial layer of loose connective tissue.
• It contains blood vessels, nerves, and Purkinje fibers which are dense elastic membranes.

The Myocardium

• The myocardium is the heart's main substance, its thickest layer.
• Cardiac muscle groups are connected by intercalated discs.
• It is thicker in the ventricles relative to the atria.

The Epicardium

• A mesothelial membrane covers the surface of the epicardium.
• Serous membrane forms simple squamous epithelium and a connective tissue layer containing fat cells and branches of coronary vessels.

Valves of The Heart:

• There are four valves in total.
• A tricuspid valve exists between the right atrium and right ventricle.
• A pulmonary semilunar valve exists between the right ventricle and the pulmonary artery.
• A mitral valve is between the left atrium and left ventricle.
• An aortic semilunar valve is between the left ventricle and aorta.
• The valves are endocardium folds enclosing a dense collagenous and elastic fibers central core, covered by endothelium.

General Structure of Blood Vessel Walls:

• Blood and lymph vessel walls have 3 tunics (coats) with varying compositions and thicknesses: tunica intima, tunica media, and tunica adventitia.

Tunica Intima

• The tunica intima is the innermost layer of the blood vessel, nearest to the lumen.
• It consists of endothelium, squamous epithelium, and basal lamina. It provides a smooth surface for simple blood passage.
• It also has a subendothelial layer, which consists of loose connective tissue.
• This is also made up of an internal elastic lamina (lamina elastica enterna).

Tunica Media

• The tunica media is the muscular coat of blood vessels.
• The structure directly relates to the vessel's function and consists of circular smooth muscle fibers.
• The function is to produce elastin and collagen.
• In addition, propel blood to the end of the artery and manages blood entering the organ.
• Elastic and collagenic fibers are concentrically arranged in between muscle, with a fenestrated elastic membrane in large arteries.
• Collagen gives tensile strength and binds other vessel structures.
• Capillaries and postcapillary venules may lack an external elastic lamina.

Tunica Adventitia

• The tunica adventitia is outermost coat of blood vessels.
• The tissue is composed of loose C.T. with fibroblasts and few elastic fibers.
• Vasa vasorum are tiny blood vessels are found here which are present in larger vessels, since the thickness prevents diffusion from the lumen.

Arteries

• Types of arteries, including large, medium, and small arteries and arterioles.
• These all carry blood rich in oxygen.

Large Arteries

• These have thick walls as well as a wide lumen and wall helps maintain blood pressure during ventricular relaxation.
• The media is very rich in elastic fibers.
• An examples are the aorta, aortic arch, common iliac, pulmonary and subclavian.
• Aorta has extremely thick walls.
• The tunica intima inside is thick and rich in elastic fibers.
• Endothelial cells in the endothelium are joined by gap junctions.
• The subendothelium consists of loose C.T. with a number of fibroblast and elastic fibers present.
• The tunical adventitia is thin and contains collagen/elastic fibers, as well as nerves/vasa vasorum.
• The fibers allow expansion during systole and recoil during diastole, which maintains blood flow
• They have elastic fibers, so that they are able to stretch and take in more blood when the heart ejects it.

Medium-sized arteries:

• Known as muscular arteries are the most common type in the body and distribute blood to the organs.
• Consist of 3 layers or coats with different compositions.
• The tunica intema contains mostly simple squamous epithelium, a subendothelial layer of connective tissue, and internal elastic lamina.
• Tunica media consists of smooth muscle and collagenic fibers.
• The tunica adventitia is the outermost layer formed mostly of collegenic fibers and C.T. cells.
• The function is to transport blood to different regions and regulate blood flow via the muscular wall.

Arterioles

• Arterioles regulate blood flow into capillary beds and are small (0.1mm in diameter).
• The tunica intima contains endothelium with clear internal elastic lamina.
• Consists of 2-5 layers of circular smooth fibers with elastic fibers.
• The tunica adventitia is a layer containing collagen and elastic fibers.
• Their function is to decrease bp with capillaries that have impermeable walls, preventing interchange between blood and tissue fluid.
• Terminal and metarterioles are a type of arteriole.
• Arterial capillaries or metarterioles are between arterioles.

Veins

• Veins carry venous blood that lack oxygen from the body to the heart.
• Types of veins, including small venules, medium veins and large veins.
• Small venules are similar to arterioles.
• Starts at the capillary bed as post-capillary venules, which forms veins.

Small Post Capillary Venule:

• Has a diameter of 0.2 to 1 mm and smooth muscle fibers in the media increases as diameter increases.
• It has one simple squamous cell layer-outside it is a smooth muscle differentiate.
• Lack media and a very thin adventitia.
• They are the location of gas exchange from the blood to the tissue.

Large Muscular Venules

• They consist of layers of simple squamous epithelium rich in cell-supporting actin filaments and contain a basement membrane.
• Also consists of thin media including smooth muscles.
• And a thin adventitia consisting of areolar CT.

Medium-sized veins:

• These consist of a single layer of tunics.
• Unoxygenated blood can be ejected towards a heart via veins, distributed the blood to the organs.

Large Veins:

• Examples includes the superior/inferior vena cava and common iliac vein.
• There tunica intima is thicker than that of medium sized veins.
• Can have both elastic of non elastic fibers.
• The tunica adventitia contains bundles of smooth muscle and collegen bundles.
• They possess wide lumens that have a thick barrier against gravity including many valves.

Connections Between Arteries and Veins:

• The connections include blood capillaries, sinusoids, and anastomosis.

The Blood Capillaries

• Capillaries connect to all the veins and arteries where the cell wall of the capillary produces vessels through its endothelial.
• The function is to eject and circulate the blood inside the body.
• They exists in somatic, or in the presence of diaphram.
• Fenestrated capillaries contain endothelial cells with pores and share blood or bacterial toxins from nerves by preventing chemical intake.

Blood sinusoids

• Blood sinusoids are a connection between the veins and arteries.
• The walls have pores that aren't covered by diaphrams.
• The size have an irrregular lumen of 5-30 microns in diameter.
• The structure is lined with thin squamous cells, macrophages,
• Present in the bone marrow, the spleen (red pulp), the liver, and the endocrine cells.

Arteriovenous anastomosis

• Definition - arise outside both arterioles and arteries which connects with the vein cells or venules.
• Types consist both single side side branches found mostly in the placenta.
• Exist in exposed parts of the external parts of the body such as fingers, toes, ears, nose, lips, tongue and eyelids
• Can even exits in the stomachs, and the liver.