Circulatory System Overview

Circulatory System

• The circulatory system is responsible for pumping blood and its contents throughout the body.
• The cardiovascular system is divided into two functional parts: blood vascular and lymph vascular systems.
• The blood vascular system circulates the blood, while the lymph vascular system circulates lymph.
• The circulatory system transports oxygen and nutrients to tissues, removes waste products, regulates body temperature, transports hormones, and fights diseases.
• The circulatory or cardiovascular system includes the heart, arteries, capillaries, and veins.

Learning Objectives

• The circulatory system comprises the heart, blood vessels, and the conduction system of the heart.
• The heart has histological characteristics including epicardium, myocardium, and endocardium.
• Arteries and veins have three histological layers: tunica intima, tunica media, and tunica adventitia.
• The system differentiates between arteries, veins, capillaries, and lymph vessels.

Functions of Endothelial Cells

• Endothelial cells form a non-thrombogenic surface, preventing blood clotting.
• They secrete factors that regulate blood clot formation (e.g., heparin, tissue plasminogen activator, von Willebrand factor).
• Cells regulate blood flow and vascular tone by releasing factors that stimulate smooth muscle contraction.
• They play a role in inflammation and local immune responses.
• They stimulate white blood cell migration and proliferation at injury or infection sites.

Blood Brain Barrier

• Brain capillaries have tight junctions between endothelial cells, creating a tight seal.
• Astrocytes in the brain contribute to this tight junction formation.
• Brain capillaries have very few pinocytotic vesicles, further limiting permeability.

General Structure of Blood Vessels

• Blood vessels (excluding capillaries) have three layers: tunica externa, tunica media, and tunica intima.
• The tunica intima lines the interior of the blood vessel.
• The tunica media is the middle layer, composed of smooth muscle and elastic fibers.
• The tunica externa is the outermost layer composed of connective tissue.

Tunica Intima (Inner Layer)

• The innermost layer touching the blood.
• Composed of endothelium, a simple squamous epithelium.
• Contains a subendothelial layer (loose areolar connective tissue), sometimes muscle fibers.
• In arteries and large veins, the tunica intima has an internal elastic lamina that separates it from the tunica media.
• An underlying basement membrane supports the tunica intima.

Atherosclerosis

• Atherosclerosis is hardening of the arteries.
• Plaque buildup of calcium and fatty material thickens the artery walls.
• Yellow plaques develop primarily in the tunica intima.
• The tunica intima thickens, while the tunica media thins in this condition.
• This can lead to vessel occlusion and aneurysm formation.

Coronary Heart Disease

• Coronary heart disease affects about 14 million people in the U.S.
• It arises when calcium and scar tissue build up in the coronary arteries, causing atherosclerosis.
• This buildup narrows the coronary arteries, hindering blood flow to the heart muscle.

Heads Up! Questions

• Arteries are vessels that transport blood away from the heart; they become progressively smaller as they branch into various organs to deliver blood to tissues.
• The heart pumps blood through the circulatory system.

Endothelial Cells in Capillaries

• The endothelium acts as a semipermeable barrier between blood and interstitial fluid.
• Capillary endothelial cells are flattened, squamous cells with their long axis aligned with blood flow directions.

Blood Brain Barrier (cont'd)

• The capillary junctions play a crucial role by blocking large molecules.
• Tight junctions are vital components.

Heads Up! Questions (cont'd)

• The tunica intima and tunica media encounter degenerative changes in pathologies like arteriosclerosis and atherosclerosis.

Tunica Media (Middle Layer)

• The tunica media consists of smooth muscle cells organized in a circular pattern, elastic fibers, and connective tissue fibers (type I and III collagen).
• The thickness of the tunica media decreases as the vessel diameter decreases. A well-developed tunica media contains multiple elastic laminae.

Tunica Adventitia (Outermost Layer)

• The tunica adventitia is chiefly composed primarily of dense irregular connective tissues, elastic fibers, and the vasa vasorum (small blood vessels in the vessel walls).
• The tunica adventitia is thicker in veins than in arteries. It provides blood supply to the walls of larger arteries and veins.

Why are Vasa Vasorum More Frequent in Veins Than Arteries?

• Venous blood contains lower oxygen and nutrient levels than arterial blood.
• Lower partial oxygen and osmotic pressures in veins necessitate a greater requirement for vasa vasorum for proper blood vessel nourishment.

Heads Up! Questions (cont'd)

• The three layers in a blood vessel are tunica intima, tunica media, tunica adventitia, respectively, from innermost to outermost.
• The tunica intima is composed primarily of simple squamous epithelium.

True/False Question

• Arteries and veins possess tunica intima, tunica media, and tunica adventitia. (True)

Small Arteries

• Small arteries have diameters of 0.3 to 1 mm.
• They boast a well-defined internal elastic lamina within their tunica intima.
• Their tunica media contains three or four layers of smooth muscle cells.
• The tunica adventitia lacks an external elastic membrane, in comparison to larger arteries.
• The function of small arteries is to distribute blood to arterioles, controlling blood flow via vasodilation and constriction mechanisms.

Arterioles

• Arterioles, the smallest arteries, exhibit very thin tunica intima featuring only one or two smooth muscle layers.
• The subendothelial layer is not significant.
• The smooth muscle content of the tunica media is organized into a circular structure.
• The arteriole walls have thinner tunica adventitia compared to small arteries.
• Arterioles are "resistance vessels" regulating blood flow to capillaries by muscle tone.

Capillaries

• Capillaries are minute, pervasive vessels facilitating metabolic exchange between blood and surrounding tissues.
• Their thin walls accommodate rapid diffusion of gases (oxygen, carbon dioxide), metabolites, nutrients, and waste products.
• The tunica intima constitutes the single layer of flattened endothelium cells constituting the walls of this vessel.

Pericytes

• Pericytes are cells surrounding capillaries and venules.
• They produce the extracellular matrix components and fuse the basement membranes of the endothelium and the pericytes.

Continuous Capillaries

• Continuous capillaries are the most common type, characterized by tight junctions between endothelial cells that reduce permeability.
• Found in muscle, nervous tissue, lungs, and exocrine glands,
• The uninterrupted endothelial lining provides tight control over permeability.

Fenestrated Capillaries

• Fenestrated capillaries have numerous openings (fenestrations) in the endothelium, facilitating higher permeability.
• Found in organs where rapid interchange of substances (e.g., kidneys, endocrine glands) is essential.

Sinusoidal Capillaries

• Sinusoidal capillaries possess large perforations and discontinuous endothelia, enabling extensive permeability.
• Found in crucial locales for cell and macromolecule exchange.

Venules

• Small venules transition from capillaries and increase in size.
• The primary role of venules includes collecting blood from capillaries and as a site where white blood cells can leave blood circulation for infection sites.
• Venules respond to vasoactive agents.
• Venules exhibit permeability changes in response to vasoactive compounds.

Veins

• Veins transport blood back to the heart.
• Veins typically contain a significantly higher volume of blood than arteries.
• They usually feature a thinner tunica media relative to arteries and thicker tunica adventitia.
• Veins often have valves preventing backflow of blood.

Large Veins

• Large veins include the vena cava and pulmonary veins.
• Large veins display cardiac muscle within the tunica adventitia.
• Large veins have vasa vasorum and nerves within their structure.

Medium-Sized Veins

• Medium-sized veins are thinner-walled than accompanying arteries.
• Typically have valves and a thinner tunica media.
• Tunica media contains fewer smooth muscle cells and more reticular collagen.

Varicose Veins

• Varicose veins are enlarged, tortuous superficial veins,
• Typically influenced by age or weakening and degeneration of vein walls influencing their ability to prevent backflow.
• Common in legs and feet.

Heart

• The heart is a muscular pump residing between the lungs, leaning slightly to the left
• Composed of two pumps circulating blood systemically.
• Atria are upper chambers.
• Ventricles are lower chambers
• The heart has valves at entrances and exits of ventricles.

Heart Chambers' Walls

• The walls of the four heart chambers exhibit three layers: endocardium (inner), myocardium (middle), and epicardium (outer).

Endocardium

• The endocardium is the innermost layer of the heart.
• Lines the heart's chambers and covers the heart valves.
• Composed of endothelial cells and a subepicardial supportive connective tissue layer.

Myocardium

• The myocardium is the largest and thickest cardiac tissue.
• This thick layer is mostly composed of cardiac muscle and features a dense irregular connective tissue framework.
• Cardiac muscle is vital for effective heart contractions influencing the strength and force of cardiac pumping.

Epicardium

• The epicardium is the outermost layer of the heart.
• It's also known as the visceral pericardium.
• Composed of a single layer of squamous mesothelium.
• Supported by a subepicardial layer, connective tissue.
• Contains blood vessels, nerves, and adipose tissue.

Cardiac Skeleton

• The cardiac skeleton consists of dense irregular connective tissue.
• Ring-shaped structures surround heart valves
• Facilitates anchoring, support, and the electrical insulation between the atria and ventricles.

Conducting System

• The heart's specialized conducting system includes the SA (sinoatrial) node. This region acts as a pacemaker producing electrical impulses spread throughout the heart.
• The AV node (atrioventricular node) is another component, regulating the transmission of these impulses to the ventricles.
• Conductive pathways facilitate coordinated heart contractions.

Cardiac Cycle

• The cardiac cycle encompasses both contraction (systole) and relaxation (diastole) phases.
• Electrical impulses are crucial to produce rhythmic contraction and relaxation in the heart muscle.

Fibrous Connective Tissue

• Fibrous connective tissue is a critical structural component in the heart.
• Reactive interstitial fibrosis or replacement fibrosis forms during instances of pathological cardiac events.

Heads Up! Questions (cont'd)

• Purkinje fibers reside in the subendocardial layer and are characterized by a substantial glycogen content, which results in a clear appearance under microscopy.