Cardiovascular System Histology and Physiology Study Guide

Overview and Functions of the Circulatory System

• The circulatory system is defined as a complex network consisting of blood, a central pump (the heart), and two primary vessel networks: an arterial distribution network and a venous collection network.
• A specialized exchange system (microcirculation) exists to facilitate the movement of nutrients and waste products between the blood and extravascular tissues.
• The lymphatic system runs parallel to the veins; it is a network of vessels that drains fluid from extravascular spaces back into the blood vascular system.
• Core functions of the Cardiovascular System (CVS) include:
  • Maintenance of adequate blood flow, also known as cardiac output.
  • Delivery of oxygen ($O_2$), nutrients, hormones, electrolytes, and water to peripheral tissues.
  • Removal of carbon dioxide ($CO_2$) and various metabolic waste products.
  • Maintenance of normal thermoregulation.
  • Maintenance of normal glomerular filtration rate (GFR) and subsequent urine output.

Heart Anatomy and Chambers

• The heart is the first organ to form during embryonic development.
• In mammals and birds, the heart consists of four distinct chambers:
  • Right Atrium (RA).
  • Right Ventricle (RV).
  • Left Atrium (LA).
  • Left Ventricle (LV).
• The right heart serves the pulmonary circulation, propelling unoxygenated blood to the lungs.
• The left heart serves the systemic circulation, propelling oxygenated blood through the body.
• Left ventricular myocardial thickness is typically $2 \times$ to $4 \times$ thicker than that of the right ventricle in adults, which is necessary to accommodate the higher pressures required for systemic circulation.

Histological Layers of the Heart (Heart Tunics)

• The heart is composed of three primary layers, listed from the inside to the outside:

Endocardium

• This layer forms the inner lining of the heart and the valves.
• It is the equivalent of the tunica intima in blood vessels.
• Components of the Endocardium:
  • Endothelium: The innermost surface composed of endothelial cells in direct contact with the blood; these cells play a vital role in hemostasis.
  • Basal Lamina: A layer of connective tissue supporting the endothelium.
  • Subendocardium (Subendocardial Layer): Subendothelial connective tissue that often contains part of the conduction system, small nerves, and (in the ventricles) Purkinje fibers.

Myocardium

• This is the middle layer and consists of heart muscle mass.
• It is equivalent to the tunica media of blood vessels.
• Histological features of cardiomyocytes:
  • Involuntary striated muscle cells arranged in sarcomeres.
  • Usually contains a single, centrally located nucleus.
  • Rich in sarcoplasmic reticulum and mitochondria (mitochondria can make up to $20 \%$ of the cell volume due to high oxygen requirements).
  • Cells are branched and connected via intercalated discs.
  • Low capacity for regeneration via mitosis.
  • Intercalated discs contain gap junctions (for electrical signaling/synapses) and anchoring junctions like abundant desmosomes and adherent junctions.
  • Lipofuscin (wear-and-tear pigment) may be present in aging cells.

Epicardium and Pericardium

• The epicardium is the outer surface of the heart and is also known as the visceral layer of the serous pericardium.
• It is equivalent to the tunica adventitia/externa of blood vessels.
• Composition of the Epicardium:
  • Mesothelium: A surface layer of simple squamous epithelium.
  • A thin layer of dense connective tissue.
  • A variably thick layer of adipose tissue (epicardial fat), which contains the coronary arteries and veins.
• The epicardium is contiguous with the endocardium at the level of the endocardial cushion.
• The pericardial sac (parietal pericardium) consists of a fibrous pericardium and a serous parietal layer. The pericardial space between the visceral and parietal layers contains a small amount of serous fluid secreted by mesothelial cells for lubrication.

The Cardiac Conduction System

• Contraction impulses are generated and transmitted by specialized cardiac muscle fibers rather than nerves.
• Sinoatrial Node (SA Node):
  • Located in the wall of the right atrium.
  • Generates the first impulses, sending a wave of depolarization around the atria via gap junctions.
• Atrioventricular Node (AV Node):
  • Located in the interatrial septum.
  • Initiates impulse generation for the ventricles.
• AV Bundle (Bundle of His):
  • Branches from the AV node to form the Purkinje fibers.
• Purkinje Fibers:
  • Located in the subendocardial layer (deepest layer of endocardium).
  • Modified cardiomyocytes specialized for impulse conduction rather than contraction.
  • Distinctive histological features: Larger than contractile cardiac muscle cells, fewer myofibrils (located at the periphery), lack T-tubules, and contain large amounts of glycogen and mitochondria.
  • Connected by desmosomes and gap junctions; intercalated disc appearance varies by location.
• Sequence of contraction: The apex of the heart contracts first, followed by the papillary muscles, and then the depolarization wave spreads upward through the ventricular walls.

The Cardiac Skeleton and Valves

• The cardiac skeleton consists of four dense bands of fibrous connective tissue encircling the bases of the pulmonary trunk, aorta, and atrioventricular (AV) valves, providing structural support.
• Fibrous Trigon: A triangular mass of fibrous connective tissue connecting the aortic ring and the right and left AV rings.
• Os Cordis: An area of the cardiac skeleton that undergoes osseous differentiation, primarily seen in cattle.
• Valve Leaflets:
  • Consist mainly of dense connective tissue (collagen-rich) covered by a thin layer of endothelium.
  • Anchored by the fibrous skeleton at the base.
  • Chordae Tendineae: Small strands of connective tissue that bind the distal parts of the valve leaflets to the papillary muscles.

Classification and Structure of Blood Vessels

• The normal vascular pattern sequence is: Artery $→$ Arteriole $→$ Metarteriole $→$ Capillary $→$ Venule $→$ Vein.
• Most vessels (except capillaries) have three layers (tunics):
  1. Tunica Intima (Innermost): Contains the endothelium, subendothelial connective tissue, and an internal elastic membrane.
  2. Tunica Media (Middle): Contains smooth muscle cells and elastic lamellae/fibers. This layer is thicker in arteries than in veins.
  3. Tunica Adventitia/Externa (Outermost): Contains collagen, vasa vasorum, and nervi vasorum.

Specialized Vascular Structures

• Vasa Vasorum: Small blood vessels located in the tunica adventitia that supply nutrients and oxygen to the walls of large arteries and veins.
• Nervi Vasorum: Vascular nerves that innervate vessels to control vasodilation and vasoconstriction, regulating temperature and homeostasis.
• Vascular Endothelium: Acts as a physical barrier and an important mediator of hemostasis, fluid distribution, perfusion, and inflammation.

Arteries and Arterioles

• Elastic Arteries (e.g., Aorta, Pulmonary Artery, Carotid, Subclavian, Iliac):
  • Located closer to the heart.
  • Tunica media is rich in repeating elastic lamellae/sheets.
  • Function: Use elastic recoil to store energy from the heart's contraction and maintain steady blood flow.
• Muscular Arteries (e.g., Femoral, Brachial, Radial, Splenic):
  • Tunica media is the thickest layer and consists primarily of circumferentially arranged smooth muscle cells (often appearing like an "onion ring").
  • Regulate diameter and tone through vasodilation and vasoconstriction.
  • Distinguishable by a prominent internal elastic lamina.
• Arterioles:
  • Contain only $1 - 3$ layers of smooth muscle cells.
  • Exert the greatest effect on blood pressure.
  • Nuclei often bulge into the lumen.
  • Smallest arterioles lack an internal elastic membrane.
  • Metarterioles: Terminal vessels with precapillary sphincters that regulate flow into the capillary bed.

Capillaries and Microcirculation

• Capillaries are thin-walled tubules of mesenchymal origin, often made of a single endothelial cell rolled into a tube with a diameter of $7 - 9\,\mu m$.
• They are the primary site of exchange for $O_2$, $CO_2$, ions, glucose, water, amino acids, and hormones. They are generally impermeable to proteins and blood cells.
• Pericytes (Roujet cells):
  • Mesenchymal-like stem cells that wrap around the endothelium of capillaries and venules.
  • Communicate via paracrine signaling and physical contact.
  • Important for angiogenesis, homeostatic functions, and maintaining the blood-brain barrier.

Classification of Capillaries

1. Continuous Capillaries:
   • Most common type, found in muscle, brain, bone, and lungs.
   • Characterized by tight junctions and pinocytotic vesicles.
   • Found in blood-brain and blood-testis barriers.
2. Fenestrated Capillaries:
   • Contain gaps ($10 - 100\,nm$) called fenestrae.
   • Found where substantial fluid exchange occurs: intestinal villi, choroid plexus, ciliary process, and glomerular capillaries in the kidney.
   • In the kidney, podocytes with foot processes (pedicels) wrap around these capillaries to form filtration slits.
3. Discontinuous Capillaries (Sinusoids):
   • Have enlarged, irregular lumens ($20 - 40\,\mu m$) and a discontinuous or absent basal lamina.
   • Found in the liver, bone marrow, and spleen.
   • Allow large molecules and cells (like RBCs in the spleen) to exit.

Veins and Venules

• Postcapillary Venules:
  • Known as very "leaky" vessels.
  • They lack smooth muscle.
  • The primary site for leukocyte diapedesis (WBCs squeezing through vessel walls).
  • Internal pressure is approximately $5\,mm\,Hg$.
• Veins:
  • Feature a large, wide lumen and thinner walls compared to arteries of the same size.
  • Valves: Projections of the tunica intima covered by endothelium with an elastic fiber core, designed to prevent backflow.
  • Tunica adventitia is the thickest layer.
  • Tunica media is thin with irregularly oriented smooth muscle fibers.
  • A distinct internal elastic lamina is typically not present.

Lymphatic Vessels

• These vessels transport lymph and are complementary to the cardiovascular system.
• Lymphatic capillaries transition into larger vessels that freely anastomose.
• Histological features:
  • Very thin walls, lined by endothelial cells.
  • Overlap of endothelial cells creates one-way valves that allow fluid in but not out.
  • Contain a thin layer of smooth muscle and an adventitia that binds them to surrounding tissue.
  • Presence of valves to prevent backflow.
  • Contain clear fluid without RBCs, often appearing "empty" in histological sections.
  • Anchoring fibrils pull endothelial cells apart to allow fluid to enter the vessel from tissue spaces.