10. Cardiovascular System

The cardiovascular system is a closed system consisting of:

1. Pipes: Blood and lymph vessels
2. Pump: The heart

The primary function of the cardiovascular system is to transport and exchange:

• Nutrients
• Gases (such as oxygen)
• Body fluids
• Waste material
• Heat
• Blood cells around the body.

The blood vascular system is a closed system of vessels through which blood passes to the tissues, facilitated by the continuous contraction of the heart.

The main components of the blood vascular system include:

1. Heart (right ventricle)
2. Pulmonary arteries
3. Arterioles
4. Capillaries
5. Venules
6. Pulmonary veins
7. Heart (left atrium)
8. Heart (left ventricle)
9. Arteries (elastic and muscular)
10. Small/medium/large veins
11. Heart (right atrium)

In the blood vascular system, the diameter of blood vessels varies as follows:

• d > 0.1 mm: arteries and veins
• d < 0.1 mm: arterioles, venules, and capillaries

The color coding in the blood vascular system illustration indicates:

• Red: Vessels transporting oxygenated blood
• Blue: Vessels transporting deoxygenated blood
• Purple: Vessels involved in gas exchange

Interstitial fluid diffuses through the capillaries into the space between cells, providing nourishment to tissues. Most of this fluid is reabsorbed into the capillaries, while the remaining fluid, known as lymph, is drained into lymphatic capillaries, which return it to the systemic blood circulation.

The interstitial fluid contains gases, proteins, minerals, nutrients, and other substances that nourish tissues, as well as damaged cells, cancer cells, and foreign particles such as bacteria and viruses that may have entered the tissue fluids.

The fluid that remains after reabsorption from blood capillaries is known as lymph, which is drained into lymphatic capillaries and eventually returned to the large veins and systemic blood circulation.

1. The blood vascular system: Responsible for the circulation of blood.

2. The lymph vascular system: Responsible for the circulation of lymph, returning lymph from tissues back to the blood cardiovascular system.

The lymphatic system consists of a complex network of vessels, tissues, and organs. Key components include:

• Lymph nodes (e.g., Axillary, Inguinal, Intestinal)
• Lymphatic vessels (e.g., Lymphatic capillaries, Lymphatic collecting vessels)
• Organs (e.g., Spleen, Thymus, Tonsils)
• Lymph ducts and trunks

Lymph flows through the lymphatic system in a cyclical manner:

1. Starts at Blood capillaries
2. Moves into Lymphatic capillaries
3. Travels through Lymphatic collecting vessels (with valves)
4. Passes through Lymph nodes
5. Enters Lymph trunks
6. Finally reaches the Lymph ducts and returns to the Heart

The tunica intima consists of:

1. Endothelium: A simple squamous epithelium surrounded by a basement membrane.
2. Subendothelium: Loose connective tissue.
3. Internal elastic lamina: Present in arteries, separating the tunica intima from the tunica media.

The tunica media is primarily composed of:

• Layers of smooth muscle: Arranged concentrically.
• External elastic lamina: Found in large arteries, separating the tunica media from the tunica externa (adventitia).

The tunica externa (adventitia) is the outermost layer of the vessel wall, characterized by:

• Dense connective tissue: Composed of collagen and elastic fibers that blend with surrounding connective tissue.
• Smooth muscle bundles: In large veins, these may run longitudinally in this layer.
• Vasa vasorum: Small blood vessels that nourish the adventitia and tunica media, typically found in the walls of large vessels.

The leaflets of vein valves consist of:

1. Endothelium - the inner lining of the leaflets.
2. Core of connective tissue - provides structural support to the leaflets.

The tunica media is thicker in arteries, providing them with more strength and elasticity, while in veins, it is thinner.

Veins tend to collapse because they have thinner walls and are typically larger in diameter, while arteries maintain a round shape due to their thicker walls and the presence of more smooth muscle and elastic fibers.

Valves in veins prevent backflow of blood, ensuring that it flows in one direction towards the heart, especially in the context of lower pressure in the venous system.

Elastic laminae are layers of elastic tissue found in arteries, specifically the internal and external elastic laminae, which help maintain the shape and elasticity of the artery walls.

The tunica externa (adventitia) is the thickest tunica in veins, providing structural support and protection to the vein, as well as anchoring it to surrounding tissues.

• Arteries:

  • Thick, layered wall
  • Small lumen

• Veins:

  • Thinner, more irregular wall
  • Larger lumen

• Tunica media: Contains many elastic fibers (lamellae) which appear as dark, wavy lines.
• Smooth muscle cells: Present but less prominent than elastic fibers.
• Examples: The aorta and its large branches are classified as elastic arteries.

• Tunica intima: Innermost layer composed of a single layer of endothelial cells.
• Internal elastic lamina: Prominent layer between tunica intima and tunica media.
• Tunica media: Composed of smooth muscle cells.
• External elastic lamina: May be prominent, located between tunica media and tunica adventitia.
• Tunica adventitia: Outermost layer made of connective tissue.

Capillaries allow for the exchange of metabolites and waste between blood and surrounding tissues or cells.

Capillaries have the smallest diameter of all blood vessels and are composed of endothelium (simple squamous epithelium) resting on a basal lamina. They do not have a tunica media or tunica externa.

Capillaries have pericytes that contain processes and contractile proteins, which share the basal lamina of the endothelium, contributing to the regulation of blood flow and capillary stability.

Key features of sinusoidal capillaries in the spleen include:

• Complex network of cells and spaces
• Presence of macrophages scattered throughout
• Big fenestrations (small holes) in the cell walls
• Larger spaces known as sinusoids highlighted in the structure
• Red pulp cord visible in the image

Lymphatic capillaries collect interstitial fluid (lymph) that remains after reabsorption from blood capillaries and return the lymph to systemic blood circulation.

The flow in lymphatic capillaries is unidirectional, meaning it only moves in one direction towards the lymphatic vessels and eventually to the heart.

Lymphatic capillaries consist of one layer of endothelial cells and a discontinuous basal lamina with openings that allow interstitial fluid to enter.

Lymphatic capillaries lead to lymphatic vessels, which then connect to collecting ducts, trunks, and ultimately the lymph duct that drains into the superior vena cava, returning lymph to the heart.

Lymphatic vessels have very thin walls compared to venules, which have thicker walls. This structural difference allows lymphatic vessels to be more permeable, facilitating the absorption of interstitial fluid and immune cells.

The three tunics of the heart are:

1. Endocardium: Composed of endothelium and subendothelial connective tissue.
2. Myocardium: Made up of cardiac muscle cells.
3. Epicardium: Contains connective tissue and coronary arteries.

The three tunics of blood vessels are:

1. Tunica Intima: Composed of endothelium and subendothelial connective tissue.
2. Tunica Media: Made up of smooth muscle cells.
3. Tunica Externa (Adventitia): Contains connective tissue and vasa vasorum.

The endocardium is the innermost layer of the heart that lines the chambers and extends over valves and papillary muscles. Its main components include:

• Endothelium and basal lamina/basement membrane
• Subendothelium: loose connective tissue containing nerves, blood vessels, and branches of the conducting system of the heart.
• Valves: composed of leaflets of endocardium (endothelium and core of connective tissue).

• Short, branching cells
• 1 nucleus (maximum of 2 nuclei) centrally located
• Obvious cross striation due to sarcomeres
• Intercalated discs, which are unique strong junctions
• Dyads, consisting of T tubule and 1 terminal cisterna

The myocardium is responsible for involuntary rapid, rhythmic contraction, allowing the heart to contract and propel blood into the circulatory system.

The three types of junctions found in intercalated discs of cardiac muscle cells are adherens junctions, desmosomes, and gap junctions.

Fibroblasts in cardiac muscle tissue are responsible for producing and maintaining the extracellular matrix, providing structural support and playing a role in tissue repair.

Cardiac muscle cells are short, branching, and have interdigitating processes between them, connected by intercalated discs that contain adherens junctions, desmosomes, and gap junctions.

Cardiac muscle fibers are arranged in a spiral pattern around the chambers of the heart. This arrangement allows the heart to wring out blood effectively from the chambers during contraction, enhancing the efficiency of blood ejection.

The dense fibrous connective tissue skeleton of the heart provides structural support for the cardiac muscle fibers and helps maintain the integrity of the heart's shape during contraction and relaxation.

In addition to cardiac muscle fibers, the myocardium contains conducting cells, which are essential for the heart's electrical conduction system, facilitating coordinated contractions.

The outer layer of the heart is called the epicardium. It contains loose connective tissue, large amounts of adipose tissue, nerves, and coronary vessels. Its external surface is lined by mesothelium, which is a type of simple squamous epithelium.

The main components of the heart's conducting system include:

1. Sinoatrial Node (S.A. Node) - Acts as the heart's natural pacemaker.
2. Atrioventricular Node (A.V. Node) - Receives impulses from the S.A. node and transmits them to the ventricles.
3. Atrioventricular Bundle (Bundle of His) - Conducts impulses from the A.V. node to the ventricles.
4. Right and Left Bundle Branches - Carry impulses to the right and left ventricles, respectively.
5. Purkinje Fibers - Distribute the electrical impulse throughout the ventricles, causing them to contract.

Purkinje fibers are specialized cardiac muscle cells that belong to the heart conduction system. They are usually found in the sub-endothelium connective tissue of the endocardium.

Purkinje fibers are characterized by:

1. Pale, swollen appearance resembling cotton wool balls.
2. Few myofibrils.
3. Abundant glycogen and mitochondria.
4. Absence of T-tubules.
5. Presence of gap junctions and desmosomes instead of intercalated discs.

The main components of the blood vascular system include:

1. Blood vessels
2. Basic structure of their wall
3. Types of blood vessels
4. Types of capillaries

The key features of the lymph vascular system include:

1. Lymphatic vessels
2. Basic structure
3. Types of lymphatic vessels

The three layers of the heart wall are:

1. Epicardium (outer layer)
2. Myocardium (middle layer, muscular)
3. Endocardium (inner layer)

Cardiac muscle tissue is characterized by:

• Striations (striped appearance)
• Intercalated discs (connections between cells)
• Involuntary control
• Single central nucleus in each cell