Histology: Circulatory System and Heart

Question 1

Layers of Heart Wall

Answer 1

• Epicardium: outermost; includes visceral layer of serous pericardium
• Myocardium: middle layer
• Endocardium:

Question 2

Epicardium

Answer 2

• Visceral layer of serous pericardium​
  
  • CT and adipose tissue deep to mesothelium
• Adheres to outer surface of heart
• Blood vessels and nerves within the adipose tissue

Question 3

Myocardium

Answer 3

• Cardiac muscle
  
  • Principle component of heart wall
• Thinner in atria than in ventricles (atria lower pressure)

Question 4

Endocardium

Answer 4

3 Sub-Layers

• Endothelium and Subendothelial CT (En)
• Connective Tissue and Smooth Muscle Cells
• Subendocardial Layer (SEn)
  
  • Continuous with CT of myocardium
  • Conducting system (P is purkinje fibers)

Question 5

Fibrous Skeleton of Heart

Answer 5

• Dense irregular CT
• Provides attachments for valve cusps (4 rings) and cardiac muscle
  
  • 2 trigones connect rings
• Insulates conducting system between atria and ventricles

Question 6

Valve Cusps

Answer 6

• Fibrosa: forms core of the valve cusp
  
  • Extensions of fibrous skeleton
• Songiosa: loose CT (shock absorber) at top of valve cusp
  
  • Arterialis - Semilunar valves
  • Auricularis - AV valves
• Ventricularis: dense CT on bottom
  
  • Covered by endothelium
  • Continuous with chordae tendinae

Question 7

Conducting System of Heart

Answer 7

• Located in sunendocardial layer
• SA Node
  
  • Pacemaker (70 bpm)
• AV Node
• Bundle of His
  
  • Passes through fibrous skeleton
• R/L Bundle Branches
• Subendocardial fibers (purkinje fibers)
  
  • Stimulates ventricular contraction

Question 8

Purkinje Fibers

Answer 8

• Located in subendocardial layer of ventricles
• Larger than cardiac muscle cells
  
  • Large, round nuclei
  • Sparce myofibrils
• Contain large amounts of glycogen (resistant to hypoxia)

Question 9

Blood Vessels

Answer 9

Tunica Intima

• Simple Squamous Endothelium
• Subendothelial layer (CT)
• Internal elastic lamina (in arteries)

Tunica Media

• Thickest layer (arteries)
• Smooth muscle
• External elastic lamine (in arteries)

Tunica Adventitia (Externa)

• CT
• Thickest layer (veins)
• Vaso vasorum (vessels of the vessels)
• Nervi vasorum (nerves of the vessels

Question 10

Endothelium

Answer 10

• Simple squamous (selectively permeable)
• Maintains non-thrombogenic surface (prevents blood clotting)
  
  • Does secrete clotting factors in response to injury
• Regulates vascular tone
• Regulates inflammation and immune responses

Question 11

Large Arteries

Answer 11

• “Elastic Arteries”
• Tunica Intima
  
  • Wavy due to contraction following cell death
• Tunica Media
  
  • ​Very thick
  • 50 layers of elastic lamellae
• Tunica Adventitia
  
  • ​CT
• Maintain uniform blood pressure in response to ventricular contraction/relaxation

Question 12

Medium Arteries

Answer 12

• Most named arteries
• Tunica Intima:
  
  • Folded after vessel death
  • Prominant internal elastic lamina
• Tunica Media:
  
  • 40 layers of smooth muscle
  • Few elastic lamellae
• Tunica Adventitia
  
  • ​Prominant external elastic lamina

Question 13

Arterioles

Answer 13

• Smallest arteries
• Lumen as wide as wall
• Tunica Intima:
  
  • ​NO internal elastic lamina
• Tunica Media:
  
  • 1-2 layers of smooth muscles
• Tunica Adventitia
  
  • ​Thin, inconspicuous
• Major determinant of systemic blood pressue
  
  • ​Regulate capillary flow

Question 14

Capillaries

Answer 14

• Single layer of endothelial cells surrounded by basal lamina
  
  • Attached to each other by tight junctions
• Site of exchange between blood and tissue fluid
• Supplied by metarterioles (contains precapillary sphinctors to prevent blood flow to capillary beds)

Question 15

Continuous Capillaries

Answer 15

• Tight junctions between endothelial cells
• Continuous basal lamina
  
  • Vesicle-mediated transcytosis
• Muscle, lungs, exocrine glands, nervous tissue

Question 16

Fenestrated Capillaries

Answer 16

• Openings in endothelial cells (fenestrations)
  
  • Rapid exchange between blood and tissues
• Continuous basal lamina
• Kidneys, intestine, endocrine glands

Question 17

Sinusoidal Capillaires

Answer 17

• Spaces between cells and large fenestrations
• Discontinuous basal lamina
• Larger diameter
  
  • Spleen (filter old red blood cells out of blood), liver, bone marrow

Question 18

Pericytes

Answer 18

• Undifferentiated mesenchymal cells
  
  • Form new blood vessels after an injury
  • Located along capillaries and post-capillary venules
• Larger nucleus than endothelial cell
• Long cytoplasmic processes surround endothelial layer

Question 19

Venules

Answer 19

Postcapillary Venules

• Covered by pericytes
• Site of action for histamine/serotonin during allergic reactions and inflammation
• High Endothelial Venules (HEVs)
  
  • Found in lymphoid tissue
  • Can recruit lymphocytes
  • Cuboidal endothelial cells

Muscular Venules

• Distal to postcapillary venules
• 1-2 layers smooth muscle
• No pericytes

Question 20

Medium Veins

Answer 20

• Most of deep veins that accompany arteries
• Contain valves

Tunica Intima:

• Subendothelial layer containing scattered smooth muscle cells

Tunica Media:

• Thinner than arteries
• Circular smooth muscle

Tunica Adventitia:

• Most prominant layer
• Scattered longitudinal smooth muscle

Question 21

Large Veins

Answer 21

• SVC, IVC, Brachiocephalic vv.
• Contain valves

Tunica Intima:

• Subendothelial layer with smooth muscle

Tunica Media:

• Circular smooth muscle

Tunica Adventitia:

• Thickest layer
• Lots of longitudinal smooth muscle

Question 22

Arteriovenous Shunts

Answer 22

• Anastomoses between arterioles and venules
  
  • Thicker tunica media (innervated by ANS)
• Thermoregulation: bypass capillary bed to conserve heat

Question 23

Venous Portal System

Answer 23

• Blood flows through 2 successive capillary beds connected by a portal vein (connects the two capillary beds)
• Allows substances to be absorbed into blood in 1st capillary bed
• Substances delivered to tissues in 2nd capillary bed
• E.g. Hepatic Portal System

Question 24

Lymphatic Capillaries

Answer 24

• Blind ended tubes
• Collect excess interstitial fluid (lymph)
  
  • Valves facilitate fluid entry and prevent escape
• More permeable than blood capillaries
• Anchoring filaments maintain patency

Question 25

Lymphatic Vessels

Answer 25

• Thin walls with CT and smooth muscle (no discernable tunics)
• Tight junctions prevent lymph from leaking out
• Valves prevent backflow
• Interrupted by lymph nodes
• Converge to form lymphatic trunks/ducts