Blood Vessels: Arteries and Veins

Classification of Blood Vessels

• The blood vascular system includes the heart, arteries, veins, and capillaries
• The heart is a four-chambered muscular organ that pumps blood throughout the body.
• Each half of the heart contains a receiving chamber(atrium) and pumping chamber (ventricle).

Types and Features of Arteries

• Arteries are large blood vessels that carry oxygenated blood away from the heart.
• Arteries have thick walls, a smaller lumen, and lack valves.
• Arteries are usually accompanied by veins and nerves, forming a neurovascular bundle.
• Arterioles are the minute branches of arteries visible to the naked eye.
• Large sized arteries (Elastic arteries) examples: Pulmonary trunk, Aorta & its branches (brachiocephalic, subclavian and common carotid artery)
• Medium sized arteries (Muscular arteries) examples: Radial artery, popliteal artery, temporal artery
• Smaller arteries are called arterioles.

Veins and Venules

• Veins transport deoxygenated blood from the body back to the heart.
• Veins have thin walls, a larger lumen, and contain valves for unidirectional blood flow against gravity.
• Valves are absent in venae cavae, hepatic, renal, uterine, ovarian, cerebral, spinal, pulmonary, and umbilical veins.
• Venules are smaller veins.
• Large sized veins example: vena cava and portal vein and their tributaries
• Small sized veins are called venules

Capillaries and Sinusoids

• Capillaries are networks of microscopic vessels connecting arterioles and venules.
• Capillaries facilitate nutrient exchange with tissues.
• Sinusoids are large, irregular vascular spaces surrounded by organ parenchyma.
• Sinusoids are different from capillaries due to a wider, irregular lumen and thinner, incomplete walls.
• Sinusoids are located in the liver, spleen, and bone marrow.

Microscopic Structure of Blood Vessels

• Blood vessels have an endothelial layer.
• The three coats (tunics) of blood vessels are: tunica intima, tunica media, and tunica adventitia.

Endothelium

• The endothelium is a special type of epithelium lining the inner surface of blood and lymphatic vessels.
• It forms an interface between circulating blood or lymph and the vessel wall.
• The endothelium is composed of a simple squamous epithelium.

Tunica Intima

• Innermost layer
• Composed of endothelium(simple squamous epithelium), basal lamina, subendothelial connective tissue, and internal elastic lamina(made of elastic material)

Tunica Media

• Intermediate layer surrounding the tunica intima
• Composed of elastic fibers and smooth muscles

Tunica Adventitia/Externa

• Outermost layer composed mainly of connective tissue
• It is separated from the tunica media by an external elastic lamina.

Microscopic Structure of Veins

• Veins have three layers like arteries, but these layers are not well-defined.

Large Elastic Arteries

• The tunica intima has 4 components.
• The tunica media is the thickest layer
• The tunica media contains a high proportion of elastic fibers, with smooth muscle cells between elastic lamellae.
• The tunica adventitia consists of connective tissues, is relatively thinner and contains collagen fibers.

Medium-Sized Muscular Arteries

• The tunica intima contains all 4 components with a clearly visible internal elastic lamina
• The tunica media includes a high proportion of smooth muscle cells.
• Tunica adventitia are made of connective tissue
• It is thicker than that of elastic artery

Large Veins

• The tunica intima includes endothelial cells, subendothelial connective tissues, and few smooth muscle cells.
• The tunica media are composed of smooth muscle cells, collagen fibers, and fibroblasts, and is thinner than in arteries
• It's hard to distinguish between the two.
• The tunica adventitia are thicker and made of smooth muscle cells, collagen, elastic fibers, and fibroblasts.

Medium-Sized Veins

• Tunica intima has little or no subendothelial connective tissue.
• Tunica media consists of few layers of smooth muscle cells and collagen and elastic fibres.
• Tunica adventitia are thicker than the media and contains collagen and elastic fibers.

Arteries vs. Veins

• Arteries have a smaller overall diameter and smaller lumen.
• Artery walls are thicker and prevents collapsing after death.
• Blood flow is more rapid and has a high oxygen content.
• The tunica intima is relatively thicker.
• Internal and external elastic laminae are well developed.
• The tunica media is muscular and considerably thicker.
• Tunica adventitia is about half the thickness of media and has high elastin content.
• Arteries have no valves
• Veins have a large overall diameter and larger lumen.
• Vein walls are thinner and collapses after death.
• Blood flow of slower with a low oxygen content.
• The tunica intima is relatively thinner.
• Internal and external elastic laminae are less developed.
• The tunica media is generally a thin muscular layer.
• The tunica adventitia is the thickest coat of the wall and is composed chiefly of collagen fibers.
• Valves are present.

Arterial and Venous Supply Terms

• Arterial anastomosis: The communication between arteries or their branches.
• Collateral supply: Circulation through anastomosis and supply of blood through smaller vessels when a major vessel is obstructed.
• End arteries: Arteries that do not form anastomoses with neighboring vessels, central artery of retina, central branches of cerebral arteries, arteries of spleen and kidney.
• Deep veins are deep in the body and are almost beside an artery with the same name: Example: femoral vein with the femoral artery.
• Superficial veins are closer to the body surface.
• Venous valves maintain unidirectional blood flow against gravity
• Venous pressure is low (7mmHg), and the valves utmost important in venous return.
• Valves prevent backflow during venous return.

Arteriovenous Anastomosis

• Arteriovenous anastomosis (shunt) is the communication between an artery and a vein.
• When the organ is active, the shunts are closed, and when the organ is at rest, blood bypasses the capillary bed.
• Shunt vessels are straight or coiled possessing a thick muscular coat and under sympathetic influence.
• Shunts are in the skin of the nose, lips, erectile tissue of sexual organs, and the thyroid gland.

Vasa Vasorum

• The network of small blood vessels which supply the large blood vessels

Lymphoid Tissues

• Lymphoid tissues are a drainage system accessory to the venous system.
• This system removes larger particles (proteins and particulate matter) from tissue fluid.
• It is considered a 'drainage system of coarse type’.
• The venous system is considered a 'drainage system of fine type’.
• Lymph is the tissue fluid flowing in the lymphatic system.

Lymphoid Tissues in the Body

• Central lymphoid tissue: bone marrow, thymus
• Peripheral lymphoid organs
• Circulating lymphocytes
• Lymphatic follicle (nodule)
• Bone marrow helps differentiate B-lymphocytes, which can synthesize antibodies after transforming into plasma cells.
• The thymus helps differentiate immunologically competent T-lymphocytes.

Functions of Lymph Nodes

• Filters lymph and removes particulate matter like carbon, bacteria, dust, and cancer cells via phagocytosis
• Produces lymphocytes.
• Plasma cells produce antibodies.

Spleen Functions

• Filters blood by removing worn-out RBCs, WBCs, and platelets.

Major Lymphatic Ducts

• Filtered lymph passes through larger lymphatics and collects into two large trunks: the thoracic duct and the right lymphatic duct.

Hemodynamics and Blood Flow

• Blood flow equals the volume of blood that flows through tissue per unit of time (mL/min).
• Total blood flow is equivalent to cardiac output (CO).
• CO equals heart rate (HR) multiplied by stroke volume (SV).
• The distribution of CO is determined by pressure differences and resistance.

Blood Pressure

• Contraction of the ventricles generates blood pressure.
• Systolic BP is the highest pressure in arteries during systole.
• Diastolic BP is the lowest arterial pressure during diastole.
• Pressure progressively falls with distance from the left ventricle.
• Blood pressure is dependent on total blood volume.

Vascular Resistance

• Resistance in blood flow is due to friction between blood and vessel walls.
• Vascular resistance depends on lumen size, vasoconstriction, blood viscosity, and total vessel length.
• An additional 400 miles of blood vessels for each 2.2lb. of fat

Venous Return

• The volume of blood is flowing back to the heart through systemic veins.
• Occurs due to pressure generated by the constriction of the left ventricle
• A small pressure difference from venule (16 mmHg) to the right ventricle (0 mmHg) is sufficient

Skeletal Muscle Pump

• Skeletal muscle pump helps the blood flow in one direction due to the valves.
• Respiratory pump is due to the pressure changes in the thoracic and abdominal cavities