Circulatory and Lymphatic Systems, and Thoracic Cavity

Review and Key Concepts

• Reinforcement of Concepts: The instructor emphasizes the importance of reviewing certain concepts for the exam.

• Eliminating Confusion: Acknowledges that the sheer volume of information can be confusing and aims to simplify it.

• Clinically Relevant Information: Focus on what is clinically relevant, such as common uses for medications or injection sites.

Circulatory System

• Arteries (Overview):

  • Initially, word slides will be presented but the instructor will skip through most specific artery branch details, as they will not be on the test in excessive detail.

  • The aim is to provide completeness for those interested, but the focus will be on primary routes.

• Components of the Circulatory System:

  • Cardiovascular System: Heart, arteries, veins, capillaries, blood.

  • Lymphatic System: Lymph nodes, lymph vessels, lymph.

• Vessels - General Definition: A 'vessel' is a tube that carries some kind of fluid (e.g., artery, vein, lymph vessel).

• Arteries - Characteristics:

  • Generally thick-walled and very elastic, containing more elastic connective tissue than veins.

  • Lack valves.

  • Contain smooth muscle in their walls.

  • Always carry blood away from the heart.

  • Blood flows from large arteries (e.g., aorta) to smaller arteries (e.g., axillary artery), then to arterioles, and finally to capillaries.

• Arterioles: Smaller and thinner-walled than arteries.

• Capillaries: Single-cell layer thick, forming the connection between the arterial and venous systems.

  • Found throughout the body in 'capillary beds'.

  • Important Capillary Beds: Lungs, liver, and intestines (for nutrient and water absorption).

  • They are the primary site of nutrient and waste exchange.

• Venules: Smaller and thinner-walled than veins.

• Veins - Characteristics:

  • Typically contain valves to ensure unidirectional blood flow back to the heart, counteracting the decrease in pressure as blood moves away from the heart.

  • Fascia (connective tissue) and muscle contractions (e.g., during walking) compress veins, helping to squirt blood forward and prevent retrograde flow.

  • Always carry blood toward the heart.

• Anastomoses (AV Shunts):

  • Also known as arterial-venous shunts, or AV shunts.

  • These are connections within capillary beds that allow for the constriction or dilation of blood flow via smooth muscle walls.

  • Function: Direct blood flow to areas of higher demand (e.g., digestive system after a meal, muscles during exercise) by opening or closing capillary beds or shunting blood away.

  • Temperature Regulation: In cold conditions, peripheral circulation constricts to prioritize blood flow to vital internal organs for survival.

• Vasa Vasorum: Medium to large blood vessels have their own blood vessels to supply their outer walls; these are called vasa vasorum.

• Innervation of Vessels:

  • Smooth muscle in vessel walls is innervated.

  • Vasoconstriction (narrowing of blood vessels) of most blood vessels in the body is primarily controlled by sympathetic nervous system input.

  • Other organs may also receive parasympathetic input.

• Redundancy in Circulation: The body has numerous interconnections between vessels, providing redundancy.

  • Importance: If one vessel is damaged, collateral circulation can still supply the tissue, preventing necrosis.

  • Exercise can help build up or improve collateral circulation.

Lymphatic System

• Components and Organs:

  • Tissues and Cells: Reticular cells, B lymphocytes, T lymphocytes.

  • Organs: Spleen, thymus, Peyer's patches, lymph nodes, tonsils, bone marrow.

  • Vessels: Lymphatic capillaries, lymph collecting ducts, lymph vessels (afferent and efferent).

• Lymph Vessels:

  • Afferent vessels: Carry lymph into the lymph node.

  • Efferent vessels: Carry lymph out of the lymph node.

  • Some common lymph vessels include the jugular trunk and thoracic duct.

• Lymph Fluid:

  • Tissue fluid picked up and returned to the blood.

  • Often described as a dialysate of blood, filtered of red blood cells (RBCs) and most white blood cells (WBCs).

  • From abdominal viscera, especially after a fatty meal, lymph may contain chylomicrons (lipid materials), giving it a white/milky or dark brown/black appearance.

• Cisterna Chyli:

  • A large sac located in the dorsal part of the lumbar area, inside the body, lateral to the aorta.

  • Located approximately at the level of the diaphragm's curve, around $L3-L4$.

  • Collects lymph before it enters the thoracic duct.

• Thoracic Duct: A major lymphatic vessel that collects lymph from the cisterna chyli and other regions, eventually returning it to the venous system (e.g., cephalic vein, external jugular vein).

• Lymph Nodes - Structure and Function:

  • Are organs, surrounded by a connective tissue capsule.

  • Have afferent lymphatic vessels bringing lymph in and efferent vessels carrying it out.

  • Regions: Cortex (outer part) and Medulla (inner part).

  • Hilus: Where afferent vessels come in, efferent vessels leave, and blood vessels supply the node.

  • Function: Immune surveillance – filtering lymph for cancer cells, bacteria, viruses, and other foreign material.

• Lymphocenters and Lymph Nodes:

  • Lymphocenter: A regional cluster of lymph nodes (e.g., superficial cervical lymphocenter).

  • Lymph Node: An individual organ within a lymphocenter (e.g., superficial cervical lymph node).

  • Lymph nodes generally drain tissues and fluid from the surrounding regions.

• Examples of Lymph Nodes:

  • Superficial Cervical Lymphocenter: Contains the superficial cervical lymph node, which drains the head, neck, and shoulder regions.

  • Axillary Lymphocenters: Contain the axillary lymph node (found in the armpit/axilla) and, in cats, accessory axillary lymph nodes (caudal along lateral thoracic veins).

  • Tracheobronchial Lymph Nodes and Cranial Mediastinal Lymph Nodes: Found in the thoracic cavity, typically drain into the thoracic duct.

Blood Flow to the Forelimb (Dog vs. Cat)

• Dog - Primary Arterial Route to the Forepaw:

  • Subclavian artery

  • Axillary artery

  • Brachial artery

  • Median artery

  • Radial artery

• Cat - Primary Arterial Route to the Forepaw:

  • Subclavian artery

  • Axillary artery

  • Brachial artery

  • Median artery (its continuation is the radial artery)

  • Radial artery (main artery to the paw)

• Artery Renaming: A single vessel is renamed as it passes through different anatomical regions (e.g., subclavian in thoracic cavity, becomes axillary after the first rib, then brachial after axillary branches are given off).

• Axillary Artery Branches (Dog): Includes external thoracic artery, lateral thoracic artery, subscapular artery, and cranial circumflex humeral artery.

• Brachial Artery Branches: Includes the deep brachial artery and superficial collateral ulnar artery.

• Common Interosseous Artery: A vessel that comes off the brachial artery, gives off branches like cranial and caudal interosseous arteries, and helps supply deep structures between the radius and ulna, highlighting circulatory redundancy.

Thoracic Cavity and Respiration

• Muscles of Inspiration (Inhaling - generally pull ribs cranially and outward, and contract diaphragm to increase thoracic volume)

  • Extralumbar constrictors

  • Scalenus

  • Serratus dorsalis cranialis

  • Rectus thoracis

  • Diaphragm (contracts, pushing abdominal organs caudally)

• Muscles of Expiration (Exhaling - generally pull ribs caudally and inward, and relax diaphragm to decrease thoracic volume)

  • Transversus thoracis

  • Internal intercostals

  • Serratus dorsalis caudalis

• Mechanics of Breathing:

  • Inspiration: Diaphragm contracts, along with other inspiratory muscles, increasing the volume of the thoracic cavity.

    • This creates negative pressure within the thoracic cavity relative to positive external atmospheric pressure.

    • Air rushes in from high to low pressure.

  • Expiration: Diaphragm relaxes; expiratory muscles contract, decreasing the volume of the thoracic cavity.

    • This increases pressure, forcing air out.

• Thoracic Cavity Contents: Under normal circumstances, houses organs such as the thymus, heart, and lungs. Organs are only found here unless pathology is present.

• Endothoracic Fascia: A layer of deep fascia lining the entire inside of the thoracic cavity.

• Mediastinum: A connective tissue partition that divides the thoracic cavity into right and left sides.

  • Contains the heart, trachea, esophagus, blood vessels, and nerves.

  • Compared to a