Histology of CVS in a Nutshell

Histology of CVS

• Lymphatic system: has blind ended lymphatic capillaries that collect interstitial fluid.  Vessels converge into larger vessels until they dump excess fluid back into subclavian vein→ vena cava.  Larger vessels do have valves.  Lymph nodes work to clean the fluid.
• Cardiovascular system: Heart is endocrine organ- secretes important peptide.  Blood also important to transport hormones / nutrients.
• Pulmonary circuit:  Blood from RV into pulmonary artery→ lungs
• Systemic circuit: LV→ aorta→ rest of body
• Heart: Visceral layer of serous pericardium AKA epicardium.

Endocardium:

• Single layer of squamous epithelium with a basal lamina. Inner most layer of the heart. Orient with direction of blood flow. Many vasoactive substances contained.
• Beneath basal lamina = have subendothelial CT layer (loose) with nerves, adipose cells, Purkinje fibers.
• Is continuous with intima of all vessels.

Myocardium:

• Thickest layer, is the cardiac muscle itself. Cardiomyocytes are arranged in layered sheets. They wrap the heart in a curving fashion.
• Layers insert on fibrous cardiac skeleton
• Endomysium = CT between fibers
• Perimysium = CT between layers of fibers

Myocytes:

• Single nuclei, have intercalated disks: Add structural support through tight junctions (desmosomes) and allow communication via cap junctions.
• Tight junctions at ends, gap junctions lateral portion.
• In the SR membrane, have IP3 receptor.
• T tubules = allow Ca receptors to be in close proximity to SR by invaginating. In atrial myocytes = T tubule system is reduced. More popular in Ventricles.
• Numerous mitochondria / glycogen.
• Endocrine function: through Atrial myocytes = have perinuclear secretory granules with peptides ANP / BNP = important in blood volume homeostasis. These will be elevated in blood after heart attack.
• After heart attack, get fibrous tissue invasion = scarring of heart where necrotic tissue was replaced. Myocytes are replaced by CT    Puts more stress on exisiting myocytesà hypertrophy. Get cascade effect.

Cardiac skeleton:

• Structural framework formed from fibrocollagen, chondroid (cartilage like).
• Attachment sites for cardiac muscle. Sheets of muscle insert onto it.
• Separtes atria from ventricles = isolates electrical conduction.
• 3 components:
  • Annuli fibrosi = at base of aorta, pulmonary artery, and AV openings.
  • Trigone fibrosum = at cusps of aortic valves, are the support base of valve.
  • Septum membranaceum = interventricular septum. Is perpendicular to other components.
  • Conus ligament: CT that binds fibrous ring of aorta with fibrous ring of pulmonary artery.

Conduction system: Heart has intrinsic rhythm.

SA node: Fires AP 70 times a minuteà goes through atriumà activate AV nodeà fires AP through bundle of Hisà divides into purkinje fiber network all throughout the subendocardium which stimulates muscles in ventricles.

Thus, atria contract first.

Epicardium:

• Outermost layer
• Has subepicardial CT layer with nerves, ganglia, coronary blood vessels, and andipose tissue
• Also has mesothelium (simple squamous) forming the visceral layer of pericardium.
• Heart all = visceral layer of serous pericardium + myocardium + endocardium
• Parietal layer of pericardium up against outer fibrous layer. Separated from visceral layer by pericardial space.

Arteries / Veins

• Similar structure. Both have 3 layers
• Tunica intima lines lumen. Lined with endothelium which has basal lamina. Beyond basal lamina, have subendothelial CT, little longitudinal smooth muscleà deeper, have internal elastic lamina = network of elastic fibers, more prevalent in arteries. Provides integrity and elasticity. Holes in network = finestraà allow substances to transport of nutrients / gasses.
• Tunica media = primarily Smooth muscle. These cells are in circular rings around vessel. Most prominent layer in arteries. Rather small in veins. Has elastic fibers, type III collagen reticular fibers, and proteoglycans secreted by smooth muscle cells. Have external elastic lamina in large muscular arteries which is boundary between tunica media and tunica adventitia. Capillaries / small venules have pericytes = undifferentiated cells which create rings around capillaries, effectively replace tunica media.
• Tunica adventitia, has irregular CT and other smooth muscle. Has elastic fibers and longitudinal muscle fibers. Have fibroblasts with Type I collage.  Contains blood vessels / nerves.  Nerves control the contraction of smooth muscle cells to modulate flow. The BVs in larger vessels supports them whereas passive diffusion from blood content would not.  These vessels within vessels = vasa vasorum = more numerous in veins (carries deoxy blood).  Tunica adventitia also has lymphoid cells. In arteries = demarcated by an external elastic lamina.  Tunica adventitia is more prominent in veins.

Arteries:

• Elastic arteries = Conducting arteries, largest.  Have layers of elastic fibers intermixed with layers of SM within the tunica media.  Thickest = Aortic arch.  When blood goes through, they distend, thus need elastic recoil.
• Nuscular arteries = Distributing arteries, modulate blood. These go to organs.  Contain prominent internal elastic lamina.  See corrugated lumen.
• Arterioles = Diameter less than 100 microm.  Width is equal to diameter of lumen.  In tunica intima = Weibel-Palade bodies = contain secretory granules with Von Willebrand’s clotting factor.
• Subendothelial layer has collagen III. Internal elastic lamina only seen in largest arterioles.

• • Tunica media = 1-3 layers of smooth muscle, NO external elastic lamina.
  • Tunica adventitia = If present, is fibroelastic CT, not prominent.
  • Metarterioles = Supply capillary beds.  Have discontinuous smooth muscle (not a complete sheet)  which controls blood flow to capillary beds

Capillary bed:

Terminal arteriole→ metarteriole branches off toward veinule.  Connection = central channel. Toward venule, connection = throughfare channel.  Have sphincters on arteriole side to control flow, reduce / increase. Precapillary sphincters are higher up, can shut off flow completely.

Capillaries: single layer of squamous endothelial cells, oriented with the flow. Have no tunica media, do have basal lamina.

Are 5-10 microns in diameter.  Cells are joined by occludentes junctions.

Pericytes = On outside of capillaries and veinules, form gap junctions with endothelium and act to aid contraction to control blood flow.

Three classes of capillaries:

• Continuous – most common in muscle, connective tissue proper, and gut. Has thin cytoplasm, connected by tight junctions. Basal lamina continuous all around endothelium. No pores / fenestrations. Seen in muscle / nerve / CT.
• Fenestrated – single endothelial cell. Has Holes (fenestrations) which are brided by diaphragms. Located in endocrine sweat glands, intestinal villi, exocrine pancrease tissue. Do not have diaphragms in the renal glomeruli.
• Discontinuous capillary AKA sinusoid = larger diameter, irregular shape due to molding by surrounding tissue.  Large gaps between endothelial cells.  Have tight junctions.  Endothelial cells are much larger.  Have a discontinuous basal lamina which does not have cross gaps between cells.  Located in the liver, spleen, bone marrow, and lymph nodes.  These are blood pools (cisternae-like), flattened sac like structures.

Capillary endothelial cell function:

• Permeability = called exchange vessels. Have small pores  (pinocytotic vesicles, fenestrae) and large pores (intercellular junctions, paracellular).
• Metabolic function = activate angiotensin II, inactivate vasoactive Bradykinin, biogenic amines, prostaglandin. Involved in lipolysis (via ectoesterases) and produce vasoactive factors ET and NO.
• Antithrombogenic = prevent contact between cubendothelial CT and platelets.

Veins: Always larger than companion artery.

• Venules = small vessels that collect the blood from the capillary beds, companion to arterioles. More irregular lumen than arterioles and are larger.  Thinner wall, no media. Are preferred place of exit for most WBCs.
• Medium sized = companion to muscular arteries, thinner wall, larger vessel. Irregular shapedlumen.  Adventitia is the most prominent layer.  No internal elastic lamina, very reduced tunica media. No external elastic lamina. Do have valves that stick out from intima = keep blood flowing, made of leaflets. The internal portion is the CT from the intima and covered by endothelium.

Valve sinus – Dilation of wall behind leaflets. Usually found in extremeties. Excess pressure = varicose veins.

• Large veins – thin wall compared to diameter. Thick subendothelial CT, little tunica media except for in the legs.  Have large tunica adventitia with longtintudinal SMC.  Also have deep vasovasorum.

Lymphatic system:

Open system, no pump, no circulation. Vessels empty into lymphatic ducts.

Structure:

1. Capillaries = layer of endothelial cells with incomplete basal lamina.  Anchoring filaments keep vessels open.
2. Small and medium vessels = have valves, elastic fibers and smooth muscle.
3. Ducts = thin endothelium with elastic / collagen fibers, have internal elastic lamina.  Tunica media has 2 SMC layers. Tunica adventitia has smooth muscle and collagen.

2 Main ducts: main lymphatic duct = drains 1/3 of body, thoracic duct drains 2/3 of body.  Collect fluid and dump into subclavian vein.