Blood Vessels: Arteries and Veins

Classification of Blood Vessels

• The blood vascular system consists of the heart, arteries, veins, and capillaries.
• The heart is a four-chambered muscular organ which pumps the blood.
• Each half of the heart has an atrium for receiving blood and a ventricle for pumping blood.

Arteries

• Arteries are large blood vessels that carry oxygenated blood away from the heart.
• Characteristically, arteries are thick-walled, have a smaller lumen, and do not have valves.
• Arteries are usually accompanied by veins and nerves, forming a neurovascular bundle.
• Arterioles are minute branches of arteries visible to the naked eye.

Types of Arteries

• Large sized arteries (Elastic arteries) include the pulmonary trunk, aorta, and their branches like brachiocephalic, subclavian, and common carotid artery
• Medium sized arteries (Muscular arteries) include the radial, popliteal, and temporal arteries.
• Smaller arteries are called arterioles.

Veins

• Veins are blood vessels that carry deoxygenated blood back to the heart.
• Veins have thin walls, a larger lumen, and valves to maintain unidirectional blood flow against gravity.
• Valves are absent in the venae cavae, hepatic, renal, uterine, ovarian, cerebral, spinal, pulmonary, and umbilical veins.
• Venules are smaller veins.

Types of Veins

• Large-sized veins include the vena cava and portal vein with their tributaries.
• Medium-sized and small-sized veins, also known as venules, are other types of veins.

Capillaries and Sinusoids

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

Microscopic Structure of Arteries and Veins

• Both arteries and veins have three coats or tunics, which include the tunica intima (innermost), tunica media (middle), and tunica adventitia (outermost), along with an endothelial layer.

Endothelium

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

Microscopic Structure of Arteries: Tunica Intima

• The tunica intima, the innermost layer, is composed of the endothelium (simple squamous epithelium), basal lamina, subendothelial connective tissue, and internal elastic lamina made of elastic material.

Microscopic Structure of Arteries: Tunica Media

• The tunica media surrounds the tunica intima.
• It consists of elastic fibers and smooth muscles.

Microscopic Structure of Arteries: Tunica Adventitia/Externa

• The tunica adventitia/externa, the outermost layer, is composed mainly of connective tissue.
• It is separated from the tunica media by the external elastic lamina.

Microscopic Structure of Veins

• Veins also have tunics similar to that of arteries.
• The three layers of veins are not well defined.

Microscopic Structure of A Large (Elastic) artery

• Tunica intima consists of all 4 components
• Tunica media is the thickest of the three layers, contains a high proportion of elastic fibers, and has layers of smooth muscle cells between elastic lamellae
• Tunica adventitia is made up of connective tissues, is relatively thinner, and contains collagen fibers

Microscopic Structure of A Medium Sized (Muscular) Artery

• Tunica intima contains all 4 components
• Internal elastic lamina is more clearly visible
• Tunica media contains high proportion of smooth muscle cells
• Tunica adventitia is made of connective tissue, and is thicker than that of elastic artery

Microscopic Structure of a Large Vein

• Tunica intima consists of endothelial cells, subendothelial connective tissues, and few smooth muscle cells
• Tunica media consists of smooth muscle cells, collagen fibers and fibroblasts, and is thinner than that of in arteries
• Tunica adventitia is thicker, consists of smooth muscle cells, collagen, elastic fibers & fibroblasts

Microscopic Structure of a Medium Sized Vein

• Tunica intima consists of little or no sub endothelial connective tissue
• Tunica media consists of few layers of smooth muscle cells and collagen fibres and elastic fibers
• Tunica adventitia is thicker than the media, and consists of collagen and elastic fibers

Differences Between Artery and Vein

• Arteries have a smaller overall diameter and smaller lumen.
• Artery walls are thicker and do not collapse after death, and there is more rapid blood flow.
• Arteries carry blood with a high oxygen content, and their tunica intima is relatively thicker.
• Arteries have well-developed internal and external elastic laminae.
• The tunica media in arteries is muscular and considerably thicker.
• The tunica adventitia in arteries is about half the thickness of the media and has a high elastin content, and they have no valves.
• Veins have a large overall diameter and larger lumen.
• Vein walls are thinner and collapse after death, with slower blood flow, and have a low oxygen content.
• The tunica intima in veins is relatively thinner.
• Veins have less developed internal and external elastic laminae, and their tunica media is generally a thin muscular layer.
• The tunica adventitia is the thickest coat of wall and is composed chiefly of collagen fibers, and they have valves.

Arterial Supply and Venous Drainage

• Arterial anastomosis is the communication between arteries or their branches.
• Collateral supply includes circulation through the anastomosis and supply of blood through a network of minor vessels joining adjacent vessels when a major vessel is obstructed.
• End arteries do not anastomose with neighbors, such as the central artery of the retina, central branches of cerebral arteries, and arteries of the spleen and kidney.

Arterio-Venous Anastomosis (Shunt)

• It is the communication between an artery and a vein
• When the organ is active these shunts are closed and the blood circulates through the capillaries
• When the organ is at rest, the blood bypasses the capillary bed and is shunted back through the arterio-vnous anastomosis
• The shunt vessels may be straight or coiled, possess a thick muscular coat, and are under the influence of the sympathetic system
• Shunts are found in simple structures found on the skin of the nose, lips, erectile tissue of the sexual organ, and thyroid gland.

Deep and Superficial Veins; Venous Valves

• Deep veins are located deep within the body, often near arteries with the same name, such as the femoral vein with the femoral artery.
• Superficial veins are closer to the body surface.
• Venous valves maintain unidirectional blood flow against gravity.
• Due to low venous pressure (7 mmHg), valves are important for venous return, and they prevent backflow.

Vasa Vasorum

• Vasa vasorum is the network of small blood vessels supplying large blood vessels.

Lymphoid Tissues

• Lymphoid tissues offer a drainage system accessory to the venous system.
• Lymphatics remove larger particles, including proteins and particulate matter.
• The lymphatics act as a 'drainage system of coarse type’.
• Lymph is the tissue fluid flowing in the lymphatic system.

Lymphoid Tissues in the Body

• Central lymphoid tissue includes bone marrow and the thymus.
• Peripheral lymphoid organs are lymph nodes and the spleen.
• Circulating lymphocytes, and lymphatic follicles (nodules)
• The bone marrow aids in B-lymphocyte differentiation into antibody-synthesizing plasma cells.
• The thymus facilitates T-lymphocyte differentiation.
• Lymph nodes filter the lymph and remove particulate matter via phagocytic action, and produce lymphocytes, and Plasma cells produce antibodies
• The spleen filters blood by removing worn-out RBCs, WBCs, and platelets.

Major Lymphatic Ducts

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

Hemodynamics

• Blood flow equals the volume of blood flowing through a tissue in a given period (mL/min)
• Total blood flow is the cardiac output (CO)
• CO is the volume of blood circulating through systemic (or pulmonary) blood vessels/minute
• Can be calculated by the formula : CO = heart rate (HR) x stroke volume (SV)
• The distribution of CO relies on pressure differences driving blood flow from higher to lower pressure and resistance to blood flow, as a higher resistance leads to smaller blood flow.

Blood pressure

• Contraction of the Ventricles generates the blood pressure
• The systolic blood pressure (BP) is the highest pressure attained during systole
• The diastolic blood pressure (DBP is the lowest arterial pressure during diastole
• The pressure falls progressively with distance from the left ventricle
• The blood pressure also depends on the total volume of blood.

Vascular resistance

• Opposition to blood flow due to friction between blood and walls of blood vessels
• Size of lumen – vasoconstriction makes the lumen smaller, causing greater resistance
• Blood viscosity - ratio of RBCs to plasma and protein concentration, higher viscosity means higher resistance
• Total blood vessel length - resistance is directly proportional to the length of the vessel since 400 miles of additional blood vessels are required for each 2.2lb. of fat

Venous return

• Volume of returning blood flow to the heart throughout the systemic veins
• Occurs due to pressure generated via constriction of the left ventricle
• A small amount of pressure the Venule (16 mmHg) to the right ventricle (0 mmHg) that is a sufficient amount