1. CIRCULATORY SYSTEM
Kristina C. Erasmo, M.D.

2. Function
Transport – oxygen, carbon dioxide, nutrients, hormones, waste products
Immune defense

3. CIRCULATORY SYSTEM Cardiovascular system Lymph vascular system Heart
2 systems of blood vessels:
Systemic circulation
Pulmonary circulation
Lymph vascular system

4. Heart
Muscular pump that propels blood to the arteries of both systemic and pulmonary circulation
Systemic circulation – brings blood from the heart to the rest of the body then back to the heart
Pulmonary circulation – brings blood from the heart to the lungs and then back to the heart

5. Heart Located in the central mediastinum of the thoracic cavity
Four chambers:
Left ventricle – systemic circulation
Right ventricle – pulmonary
Left atrium
Right atrium

7. Pericardium Connective tissue sac covering the heart
Outer sac (fibrous pericardium)
Inner sac (serous pericardium)
Parietal pericardium
Visceral pericardium
Pericardial cavity – with fluid

8. Pericardium
Fibrous and parietal pericardia – dense CT (collagen and elastic fibers, fibroblasts, other CT elements)
Inner aspect of parietal pericardium – simple squamous epithelium (mesothelium)
Visceral pericardium a.k.a. epicardium

9. Histologic Layers of Heart Wall
Epicardium
Myocardium
Endocardium

11. Epicardium a.k.a. visceral pericardium Outermost layer of heart wall
Lining epithelium of external surface: mesothelium

12. Myocardium Thickest (thickness varies for different parts)
Thickest: left ventricle
Thinnest: atria
Mainly cardiac muscle fibers

13. Purkinje Fibers Modified cardiac muscle cells
Initiate and conduct the electrical impulse that controls the contraction of the heart
Form the impulse-conducting system of the heart
Larger, contain more glycogen than ordinary cardiac muscle cells

16. Myocardium
Atrial natriuretic peptide (ANP) – polypeptide hormone secreted by cardiac muscle cells in atria and interventricular septum
Causes natriuresis – excretion of sodium by the kidneys

17. Endocardium Thinnest Lining epithelium: endothelium
Continuous with innermost layer of the great blood vessels
Lines all internal surfaces of heart

18. Endocardium Subendothelial layer – loose CT
External to subendothelial layer – dense CT, thickest portion of endocardium

20. Blood Vascular System
Refers to the system of blood vessels within which blood circulates
3 types of blood vessels:
Arteries
Veins
Capillaries

21. Blood Vascular System
Arteries – carry blood from the heart to the capillaries
Veins – carry blood from the capillaries to the heart

22. Endothelium Lining of the luminal surface of all blood vessels
Functions:
Lining blood vessels to facilitate blood flow
Secretes components of underlying CT

23. Endothelium Functions:
Secrete prostacyclin – inhibits platelet aggregation, promotes vasodilation
Nitric oxide and endothelin I – vasodilator
Von Willebrand factor – blood coagulation

24. Vasa Vasorum
Network of small blood vessels that supply large blood vessels
Veins: present in the tunica adventitia and media
Arteries: confined to adventitia

27. Capillaries Smallest blood vessels (5-10 um)
Connect arterioles and venules
Function: Allow exchange of gases and materials between blood and surrounding tissue

28. Capillaries
Wall consists of single layer of endothelial cells and its associated basal lamina
Surrounding connective tissue elements

29. Pericytes
Slender, elongated, perivascular cells that usually accompany capillaries
Look like fibroblasts
Not part of the endothelium
Contain tropomyosin
Function: contractile cells that influence the luminal size of capillaries

31. Types of Capillaries Continuous (Type I) Fenestrated (Type II)
Sinusoids

32. Continuous Capillary Found in muscles, lungs, CNS, skin
The endothelial cell and the basal lamina do not form openings (which would allow substances to pass the capillary wall without passing through both the endothelial cell and the basal lamina)

35. Fenestrated Capillary
Mucous membranes of GIT, renal glomerulus, pancreas, some endocrine glands
Endothelial cell body forms small openings (fenestrations) which allow components of the blood and interstitial fluid to bypass the endothelial cells

38. Sinusoids a.k.a. discontinuous capillaries or sinusoidal capillaries
Formed by fenestrated endothelial cells, (which may not even form a complete layer of cells)
Basal lamina is also incomplete
Found in liver, spleen, red bone marrow (free exchange of substances or even cells between bloodstream and organ is advantageous)

41. Histologic Layers of Arteries and Veins
Tunica adventitia
Tunica media
Tunica intima

43. Tunica Adventitia Outermost coat of arteries and veins
Chiefly made up of connective tissue with cells and fibers arranged longitudinally

44. Tunica Media Middle coat of arteries and veins
Concentrically arranged smooth muscle fibers
Interspersed between the muscle cells are connective tissue elements (collagen and elastic fibers)

45. Tunica Media In large arteries:
External elastic lamina/membrane – outer layer of elastin which demarcates the tunica media from the tunica adventitia

47. Tunica Intima Innermost coat of arteries and veins
Consists of a layer of endothelium that rest on a basal lamina
Subendothelial layer of loose CT
In arteries:
Internal elastic lamina/membrane – demarcates tunica intima from tunica media

49. Arteries Classification:
Small (arteriole)
Medium (muscular or distributing)
Large (elastic or conducting)
Decrease in size but increase in number as they go farther from the heart

50. Arterioles Smallest arteries (40 – 400 um)
Tunica intima: only endothelium, no IEL or subendothelial CT
Tunica media: no EEL
Tunica adventitia: loose CT

51. Arterioles
Relatively thick walls and narrow lumens – offer considerable resistance to blood flow (thus able to deliver blood to the capillary beds under greatly reduced pressure)

52. Medium Arteries Muscular arteries – well-developed tunica media
Distributing arteries – distribute blood to the different parts of the body
Biggest: brachial and femoral arteries

53. Medium Arteries Tunica intima: 3 layers
Endothelium, subendothelium, and IEL
Tunica media: Up to 40 layers of circularly arranged smooth muscle fibers, EEL present
Tunica adventitia: loose CT, vasa vasorum

55. Large Arteries Elastic arteries – high elastic tissue content
Conducting arteries – major conduits in the transport of blood away from the heart
E.g. aorta
Recoil is responsible for the flow of blood through the blood vessels during diastole

56. Large Arteries Tunica intima: Polygonal endothelial cells,
Subendothelial layer – loose CT
Indistinct IEL

57. Large Arteries Tunica media: Tunica adventitia:
40-60 layers of elastic laminae arranged concentrically
Indistinct EEL
Tunica adventitia:
Thin, merges with surrounding CT
Contains vasa vasorum

58. Sensory Organs Associated with Arteries
Carotid bodies and aortic bodies
Sensitive to changes in oxygen and CO2 tension of blood
Carotid sinus
Sensitive to changes in arterial blood pressure

59. Veins
Classification:
Small (venules)
Medium
Large

60. Veins Carries blood from the capillaries to the heart
More numerous than arteries
Bigger calibers, more irregular lumens, thinner and less elastic walls
Increase in caliber and walls become thicker as they travel toward the heart

61. Small Veins (Venules)
The 3 histologic layers of the vessel wall can only be appreciated in venules with a luminal diameter of at least 50 um
Like capillaries, they are sites for exchanage of gases and substances

62. Medium Veins Tunica intima:
Polygonal endothelial cells
Subendothelial layer
Tunica media: small bundles of circularly arranged smooth muscle cells
Tunica adventitia: bulk of the wall

63. Large Veins E.g. inferior and superior vena cava, portal vein
Tunica intima: thicker
Tunica media: poorly developed, few smooth muscle fibers

64. Large Veins
Tunica adventitia: very thick, consists of 3 poorly-defined zones
Internal zone – dense CT
Middle zone – longitudinally arranged muscle fibers
Outer zone – collagenous and elastic fibers

65. Arteries vs. Veins Arteries Veins Lumen Patent, RBC not seen
Lumen may be collapsed, RBC usually seen
Endothelial lining
Well-defined
Not so well-defined
Internal elastic lamina (IEL)
Distinct
Poorly defined
Tunica media
Forms 2/3 of thickness of wall
1/3
External elastic lamina (EEL)
Tunica adventitia
Forms 1/3 of thickness
2/3