2.1 - BLOOD VESSELS PART 1

Course Objectives

• Cardiovascular structures allow for gas and nutrient exchange throughout the body. This involves tracing blood flow and identifying related structures.

Lecture Learning Objectives

• Describe the composition and function of each layer (tunica) in blood vessels.
• Compare and contrast the structure of arteries, capillaries, and veins, using their layer names.
• List the differences between three types of arteries and two types of veins.
• Explain anatomical and functional differences between three types of capillaries.
• Describe three main microvascular pathways, focusing on the role and anatomy of capillary beds.
• Histologically identify and distinguish between arteries, veins, arterioles, venules, and capillaries, and their respective layers.
• Describe the composition of blood and explain its function in transport, regulation, and protection.
• Compare and contrast plasma and interstitial fluid, describing their interactions.
• Identify the structural and functional characteristics of erythrocytes.
• Classify leukocytes, detailing their function and how they reflect pathological conditions.
• Describe the structure and role of blood platelets.
• Define and outline the process of hemopoiesis, noting cell lineages.
• Visually identify erythrocytes, platelets, and the five types of leukocytes histologically.

Blood Vessels - Arteries, Veins, and Capillaries

• Arteries: Carry blood away from the heart; thick vessel walls.
• Veins: Carry blood to the heart; thin vessel walls.
• Capillaries: Site of exchange; capillary beds are where oxygen, nutrients, waste products, and other substances exchange with extracellular fluids.
• Anastomosis: Location where two or more blood vessels converge to supply the same body region. Vessels might share a name as "companion vessels."

Anatomy of Blood Vessels

• Tunica intima: Innermost layer, composed of endothelium (simple squamous epithelium lining blood vessels) and a subendothelial layer made of areolar connective tissue.
• Tunica media: Middle layer, composed of smooth muscle arranged circumferentially. It's responsible for vasoconstriction and vasodilation.
• Tunica externa: Outermost layer, composed of areolar connective tissue. It anchors the vessel and contains vasa vasorum (small blood vessels that supply the vessel).

Artery vs. Vein Anatomy

• Arteries: Thick tunica media, narrower lumen, more elastic fibers. These characteristics allow arteries to withstand and maintain a stable blood pressure. No valves. Backflow prevented by high pressure and shape.
• Veins: Thick tunica externa, wider lumen, thinner walls, contain valves to prevent backflow. Skeletal muscle and valves help propel blood back to the heart.

Arteries

• Elastic Arteries: Largest (1-2.5 cm); conducting arteries; high proportion of elastic fibers; equalize blood pressure to smaller arteries. Example: aorta.
• Muscular Arteries: Medium-sized (1-3 mm); distributing arteries; more smooth muscle enabling precise control of blood supply to different regions.
• Arterioles: Smallest (less than 3 mm); important in regulating blood pressure. Single layers of smooth muscle in the tunica media.

Veins

• Veins: Thin-walled vessels that return blood to the heart. Nearly all carry deoxygenated blood, except those of the pulmonary system. They function primarily as blood reservoirs.
• Venules: Smallest veins; always a companion vessel with arterioles. Venules are a primary location for white blood cell migration (diapedesis). Only the largest venules have all three tunics.

Capillaries

• Smallest blood vessels (just larger than average red blood cells); connection between arterioles and venules.
• Primarily composed of the tunica intima, allowing for optimal diffusion of nutrients and gases.
• Capillary beds allow for greater total exchange surface area compared to arteries and veins.
• Blood flows slower and continuously in capillary beds. Capillary beds are the functional unit of the cardiovascular system.

Capillary Beds

• Blood enters a capillary bed via a metarteriole, then travels through a thoroughfare channel, and leaves via a postcapillary venule.
• Precapillary sphincters control blood flow in true capillaries.
• Sphincters cycle via vasomotion to meet tissues' nutritional needs.

Capillary Types

• Continuous capillaries: Most common; form complete continuous vessels. Found in muscle, skin, CNS, etc.
• Fenestrated capillaries: Contain openings (fenestrations) in endothelial cells, allowing for increased permeability. Abundant in areas requiring fluid transport. Found in small intestines and kidney.
• Sinusoids: Large gaps in endothelial cells and discontinuous basement membranes, allowing transport of large materials. Found in bone marrow, liver, spleen, etc.

Microvascular Pathways

• Simple pathway: Blood flows from arteries, capillaries, and veins directly.
• Arteriovenous shunt: Allows blood to bypass capillary beds; arterioles connect directly to venules.
• Portal system: Blood flows through two capillary beds separated by a portal vein. This allows nutrients absorbed in the first capillary bed to be delivered directly to cells in the second capillary bed.

Blood

• Composed of liquid (plasma) and formed elements (erythrocytes, leukocytes, and platelets).
• Plasma: 55% of blood volume; mostly water, proteins, and other solutes; transport medium.
• Erythrocytes (RBCs): Transport oxygen and carbon dioxide; contain hemoglobin.
• Leukocytes (WBCs): Immune function; various types with specialized roles.
• Platelets: Involved in clot formation; small membrane fragments.

Blood: Function

• Transportation: Transports oxygen, nutrients, hormones, and waste products.
• Regulation: Controls body temperature, pH, and fluid balance.
• Protection: Provides immunity to pathogenic organisms through leukocytes and antibodies; platelets aid in clot formation.

Formed Elements: Erythrocytes

• Small, biconcave structure allows for efficient gas transport.
• Filled with hemoglobin.
• Gasses travel via diffusion.
• Form rouleaux in small vessels, which aids in their efficient passage.
• Mature RBCs lack organelles, and nucleus. Life span is 120 days.

Formed Elements: Leukocytes

• Larger than erythrocytes; various types with different roles in the immune response and the body.
• Granulocytes: Include neutrophils, eosinophils, and basophils (Neutrophils - most numerous; phagocytes. Eosinophils- rare; ingest antigen-antibody complexes, allergens, and parasites. Basophils- very rare; release histamine, heparin);
• Agranulocytes: Include lymphocytes and monocytes (Lymphocytes- common; part of the immune system; attack pathogens or produce antibodies. Monocytes- rare; become macrophages; phagocytes).

Formed Elements: Platelets

• Small, irregular fragments; produced in red bone marrow by cells called megakaryocytes (thrombopoiesis).
• Essential for clot formation; aid in stopping bleeding.

Hemopoiesis

• Blood cell formation; occurs in red bone marrow.
• Two different stem cells produce different types of blood cells.
• Hemocytoblasts (pluripotent stem cells): Two cell lines arise from hemocytoblasts.
  • Myeloid line: Erythrocytes, megakaryocytes/platelets, other granulocytes and monocytes.
  • Lymphoid line: Lymphocytes.

Blood Histology

• Microscopic examination of blood components shows different characteristics and structures for RBC, WBC (include all granulocytes plus lymphocytes and monocytes), platelets.

Vascular Anastomosis

• Arteries tend to have a lot of vascular anastomosis around body joints. This is beneficial because it provides multiple pathways to supply the same area. This is crucial since injuries can compromise blood supply and impede functionality.

Test your knowledge on the functions and components of blood with this quiz. Covering topics such as blood types, the structure of blood vessels, and the role of various blood cells, this quiz will challenge your understanding of human anatomy. Perfect for students studying biology or pre-med courses.