Circulatory System: Veins and Capillaries

Types of Veins

• Venules are small veins
• Large veins include the vena cava and portal vein with their tributaries
• Medium sized veins are categorized separately
• Small sized veins are called venules

Capillaries and Sinusoids

• Capillaries are microscopic vessels connecting arterioles and venules
• They are in close contact with tissues for nutrient exchange
• Sinusoids are large, irregular, vascular spaces surrounded by organ parenchyma
• Sinusoids are different from capillaries in the following ways:
  • Their lumen is wider and irregular
  • Their walls are thinner and may be incomplete
• Sinusoids are found in the liver, spleen, and bone marrow

Microscopic Structure of Arteries and Veins

• Endothelial layer is a special type of epithelium lining the interior surface of blood vessels and lymphatic vessels
• It forms an interface between circulating blood or lymph in the lumen and the rest of the vessel wall
• Three coats (tunics) are present in both arteries and veins:
  • Tunica intima (innermost)
  • Tunica media (middle)
  • Tunica adventitia (outermost)

Endothelium

• It's simple squamous epithelium

Microscopic Structure of Arteries

• Tunica intima (innermost layer) has four components:
  • Endothelium (simple squamous epithelium)
  • Basal lamina
  • Subendothelial connective tissue
  • Internal elastic lamina (made of elastic material)

Differences Between Arteries and Veins

• Arteries have a larger lumen and thicker walls that do not collapse after death
• Veins have thinner walls that collapse after death
• Blood flow is more rapid in arteries
• Blood flow is slower in veins
• Arteries have high oxygen content
• Veins have low oxygen content
• Tunica intima is relatively thicker in arteries
• Tunica intima is relatively thin in veins
• Internal and external elastic laminae are well-developed in arteries
• Internal and external elastic laminae are less developed in veins
• Tunica media is muscular and considerably thicker in arteries
• Tunica media is generally a thin muscular layer in veins
• Tunica adventitia is about half the thickness of media and has high elastin content in arteries
• Tunica adventitia is the thickest coat of the wall composed chiefly of collagen fibers in veins
• Arteries do not have valves
• Veins have valves

Terms Related to Arterial Supply and Venous Drainage

• Arterial anastomosis is the communication between arteries or branches of arteries
• Collateral supply is circulation through anastomosis
• End arteries do not anastomose with neighbors
  • Examples: Central artery of retina, central branches of cerebral arteries, arteries of spleen, and kidney
• Deep veins are located deep in the body and are usually alongside an artery with the same name
  • Example: Femoral vein with the femoral artery
• Superficial veins are closer to the body surface
• Venous valves are present in veins to maintain the unidirectional flow of blood against gravity
  • They are essential as venous pressure is low (7mmHg) and help prevent backflow during venous return
• Arterio-venous anastomosis (shunt) is the communication between an artery and a vein
  • When the organ is active, these shunts are closed, and blood circulates through the capillaries
  • When the organ is at rest, blood bypasses the capillary bed and is shunted back through the arterio-venous anastomosis
  • The shunt vessels can be straight or coiled and possess a thick muscular coat, and are under the influence of the sympathetic system
  • Shunts of simple structures are found in the skin of the nose, lips, erectile tissue of the sexual organ, and thyroid gland
• Vasa vasorum: The network of small blood vessels supplying large blood vessels

Lymphoid Tissues

• A drainage system accessory to the venous system
• Larger particles (proteins and particulate matter) are removed from tissue fluid by lymphatics
• Considered the 'drainage system of coarse type'
• The venous system is considered the 'drainage system of fine type'
• Tissue fluid flowing in the lymphatic system is called lymph
• Lymphoid tissues are categorized into:
  • Central lymphoid tissue
  • Peripheral lymphoid organs
  • Circulating lymphocytes
  • Lymphatic follicle (nodule)

Central Lymphoid Tissue

• Consists of:
  • Bone marrow
  • Thymus
• Functions:
  • Bone marrow helps differentiate B-lymphocytes, which can synthesize antibodies after transforming into plasma cells
  • Thymus helps differentiate immunologically competent T-lymphocytes

Peripheral Lymphoid Organs

• Consists of:
  • Lymph nodes
  • Spleen

Functions of Lymph Nodes

• Filter lymph and remove particulate matter like carbon, bacteria, dust, and cancer cells by phagocytic action
• Produce lymphocytes
• Plasma cells produce antibodies

Functions of Spleen

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

Major Lymphatic Ducts

• Filtered lymph passes through larger lymphatics and is collected into two large trunks:
  • Thoracic duct
  • Right lymphatic duct

Hemodynamics: Factors Affecting Blood Flow

• Blood flow is the volume of blood that flows through any tissue in a given period of time (in mL/min)
• Total blood flow is cardiac output (CO)
  • Volume of blood that circulates through the systemic (or pulmonary) blood vessels each minute
• CO = heart rate (HR) x stroke volume (SV)
• Distribution of CO depends on:
  • Pressure differences that drive blood through tissue - Flows from higher to lower pressure
  • Resistance to blood flow in specific blood vessels - Higher resistance means smaller blood flow

Explore the different types of veins, including venules and large veins such as the vena cava. Understand the role of capillaries in nutrient exchange and the unique features of sinusoids. This quiz covers the microscopic structure of arteries and veins.