1. Topic 8: Circulatory System
Animal Histology BIOL 241
Topic 8:
Circulatory System
Dr. Issa Al-Amri
Department of Biological Sciences & Chemistry
College of Arts & Sciences

2. Introduction
The circulatory system consists of cardiovascular and lymphatic vascular systems.
The cardiovascular or blood vascular system consists of the heart, arteries, veins, and capillaries.
The lymphatic vascular system consists of lymphatic capillaries and lymphatic ducts of different sizes.

3. Blood vascular system Blood Vascular System
Consists of: heart, arteries, veins, and capillaries.
The system functions include:
Transports oxygen and nutrients to tissues.
Carries carbon dioxide and waste products from the tissues.
Circulates hormones from the site of synthesis to their target cells.

4. Cardiovascular system: heart and arteries (arterial = red, venous = red)

5. Cardiovascular system:
blood circulation to various
organs

6. Heart
A. The heart
A four-chambered pump composed of two atria and two ventricles , surrounded by a fibroserous sac called the pericardium.
Receives sympathetic and parasympathetic nerve fibers, which modulate rate of the heartbeat but do not initiate it.
Produces hormone (atrial natriuretic peptide) which increase secretion of sodium and water by the kidneys, inhibits renin release, and decreases blood pressure.

7. Diagram of Heart

8. Heart layers 1. Cardiac layers Endocardium:
lines lumen of the heart, composed of:
Simple squamous epithelium (endothelium)
Thin layer of loose connective tissue.
Subendocardium: connective tissue layer contains veins, nerves, and Purkinje fibers, underlies it.
Myocardium:
Consists of layers of cardiac muscle cells.
Contracts to propel blood into arteries for distribution to the body.
Specialized cardiac muscle cells in the atria produce several peptides that maintain fluid and electrolyte balance and decrease blood pressure.

10. Heart layers Fibrous skeleton of the heart C. Epicardium :
Outermost layer of the heart, constitutes visceral layer of pericardium.
Composed of simple squamous epithelium (mesothelium) on external surface.
Beneath the mesothelium lies fibroelastic connective tissue, containing nerves and coronary vessels, and adipose tissue.
Fibrous skeleton of the heart
Consists of thick bundles of collagen fiber rings oriented around the heart valves.
Contains occasional foci of fibrocartilage.

12. Heart valves Heart valves a. Atrioventricular (AV) valves.
b. Semilunar valves (tricuspid, pulmonary, aortic and mitral).
Valves are folds of endocardium, composed of two or three leaflets; each of which consists of central core of dense connective tissue of collagen and elastic fibers and lined on both sides by endothelium.
Valve bases attached to annuli fibrosi of cardiac skeleton and their ventricular surface is connected to chordae tendineae of papillary muscles.

15. Cardiac conducting system
This system consists of modified cardiac muscle fibers to initiate impulses and conduct them rapidly throughout the heart (initiate and regulate the heartbeat).
It is composed of S–A node, A–V node, A–V bundle of His and Purkinje fibers.
Sinoatrial (SA) node:
Pacemaker of the heart: composed of specialized cardiac cells located within the wall of the right atrium.
Generates impulses that initiate contraction of atrial muscle cells; which then conducted to AV node.

16. Cardiac conducting system
AV nodeis: located in the wall of right atrium, adjacent to the tricuspid valve.
AV bundle of His: a band of conducting tissue radiating from AV node into interventricular septum, where it divides into two branches and continues as Purkinje fibers.
Purkinje fibers: large, modified cardiac muscle cells that contact with cardiac muscle cells at apex of the heart via gap junctions, desmosomes, and fasciae adherentes.
Autonomic nervous system: modulates the heart rate and stroke volume. Sympathetic innervation accelerates the heart rate, whereas parasympathetic stimulation slows the heart rate.

18. Heart Figure 1 : Endocardium -connective tissue (CT)
-connective tissue endothelium (En)
-connective tissue (CT)
-Purkinji fibers (PF)
-myocardium (My)
Figure 2: Heart valve
-red bloos cells (RBC)
-cardiac muscle cells (CM)
-nucleus (N)
-boxed area: Purkinji fibers:
-myofibrilis (m); nucleus (N)
Figure 3: Purkinji fibers
-valve leaflet
-endocardium (EC)
-lumen (L)
-endothelial lining (En)
-smooth muscle cells (SM)
-blood vessels (BV)

19. Arteries B. The Arteries
Arteries conduct blood away from the heart to the organs and tissues.
Arterial walls composed of three layers (tunicae):
Tunica intima (inner)
Tunica media (middle)
Tunica adventitia (outer)

21. Arteries Types of arteries:
Elastic arteries (conducting arteries) are large. They include the aorta and its major branches.
Elastic arteries help reduce changes in blood pressure associated with the heartbeat.
Small vessels (vasa vasorum) and nerves are located in their tunicae adventitia and media. The vasa vasorum vascularize the walls of the elastic arteries.
Thick, concentric sheaths of elastic membranes, known as fenestrated membranes, are located in the tunica media.

24. Elastic artery
Figure 1 : Elastic artery (L.S). Aorta
-tunica intima (TI)
-epithelium nuclei (arrowheads)
-tunica media(TM)
-smooth muscle cells nuclei(N)
-fenestrated membranes(FM)
-external elastic lamina(xEL)
-tunica adventitia(TA)
-vasavasorum(VV)
Figure 2: Elastic artery (C.S)
- endothelium nuclei (arrowheads)
-lumen(L) , elastic fibers(EF)
-smooth muscle cells(SM)
-nuclei corkscrew-shaped(arrows)
Figure 3: Elastic artery (C.S)
- tunica media(TM)
-elastic lamina(xEL)
-collagen fibers(CF)
-fibroblasts(F)
-vasa vasorum(VV)
Figure 4: Elastic artery (C.S)
-nuclei(N), tunica adventitia(TA)
-collagenous fiber bundles (CF)
-elastic fibers(EF)

25. Arteries
b. Muscular arteries (distributing arteries) distribute blood to various organs.
They include most of the named arteries of human body.
These medium-sized arteries are smaller than elastic arteries but larger than arterioles.
Tunica adventitia contains vasa vasorum.
Tunica media is thick, composed of layers of smooth muscle cells. Larger muscular arteries possess an external elastic lamina separating their boundary with the tunica adventitia.
The tunica intima is characterized by its endothelium and a prominent subendothelial internal elastic lamina

27. Arteries
Arterioles regulate blood pressure and are the terminal arterial vessels.
They are the smallest arteries, with diameters less than 0.1 mm and a narrow lumen; their luminal diameter usually equals the wall thickness.
The tunica adventitia is scant, whereas the tunica media consists of up to two layers of smooth muscle.
The tunica intima consists of an endothelium, basal lamina, and scant connective tissue.

31. Capillaries C. Capillaries:
Small vessels (about 8–10 µm in diameter and usually less than 1 mm long).
Exhibit selective permeability, permitting exchange of O2, CO2, metabolites, nutrients, metabolic wastes, signaling molecules, hormones, and other sub-stances between the blood and tissues.

33. Veins
Veins
Conduct blood away from the organs and tissues and return it to the heart.
Veins contain about 70% of the body’s total blood volume at any given time.
Their walls are composed of three layers: the tunica intima (inner), tunica media (middle), and tunica adventitia (outer),the thickest and most prominent.
Vasa vasorum are more numerous in veins than arteries. A distinct internal elastic lamina is also absent in veins.
The components of these layers and the variations among different types of veins are summarized in Table 11.2.

37. Figure 1 : Artirioles and venules - large arteriole (A)
Arterioles, Venules, Capillaries, Lumph vessels
Figure 1 : Artirioles and venules
- large arteriole (A)
-companion venule (Ve)
-duct (D), lumen (L)
-endothelial cell nuclei (N)
-smooth muscle cells (SM)
-internal elastic lamina ( iEL)
-tunica adventitia (TA)
-serous units (SU)
-serous demilunes (SD).
Figure 2: Artirioles and venules
-small arteriole (A)
- venule (Ve), lumina (L)
-tunica media ( TM) of arteriole
-nuclei (N) of endothelial cells
-connective tissue (CT)
Figure 3: Capillary
-endothelial cell nuclei(N)
-Red blood cells (arrows)
-collagen fibers (CF), capillary (C)
-nuclei connective cells (arrow)
Figure 4: Lymphatic vessel
-epithelium (E)
-goblet cells (GC)
-plasma cells (PC)
-mast cells(MC), lymphocytes(Ly)
-muscle fibers(SM)
-lumen (L), endothelium (En)

38. Figure 1 : Artery and Vein (C.S) -muscular artery (MA)
Muscular Artery, Vein
Figure 1 : Artery and Vein (C.S)
-muscular artery (MA)
-vein (V), tunica intima (TI)
-endothelial layer (En)
-elastic lamina (iEL)
-tunica media(TM)
-smooth muscle cells (SM)
-tunica adventitia (TA)
-chiefly of collagen (CF)
-elastic (EF) fibers
-intima (TI), media( TM)
Figure 2: Artery and Vein (C.S)
-tunica media (TM)
-external elastic lamina (xEL)
-collagenous (CF) ,elastic( EF) fibers
-vasa vasorum (VV)
Figure 3: Artery (C.S)
-higher mag. of box in fig.2
- endothelium (En)
-subendothelial connective (arrow)
-internal elastic lamina (iEL)
-numerouselastic fibers (EF)
Figure 4: Large vein (C.S)
- tunica intima (TI), endothelium (EN)
-collagen fibers (CF)

39. Veins Comparison with arteries: Types of veins:
Veins have thinner walls and larger, more irregular lumina than arteries. They have valves in their lumina that prevent retrograde flow of the blood.
Types of veins:
Large veins include vena cava and pulmonary veins. These veins possess cardiac muscle in the tunica adventitia for a short distance as they enter the heart. This layer also contains vasa vasorum and nerves.
Small and medium-sized veins include the external jugular vein. These veins have a diameter of 1 to 9 mm.
Venules have a diameter of 0.2 to 1 mm and are involved in exchange of metabolites with tissues and in diapedesis (exiting of blood cells through vessel walls).

42. Lymphatic vascular system
II. Lymphatic Vascular System
This system consists of peripheral lymphatic capillaries, lymphatic vessels of gradually increasing size, and lymphatic ducts.
The lymphatic vascular system collects excess tissue fluid (lymph) and returns it to the venous portion of the cardiovascular system.
It drains most tissues with the exception of the nervous system and bone marrow.

43. Lymphatic vascular system
Lymphatic capillaries are thin-walled vessels that begin as blind-ended channels (e.g., lacteals) adjacent to capillary beds where they collect lymph.
They are composed of a single layer of attenuated endothelial cells that lack fenestrae and fasciae occludentes. They possess a sparse basal lamina.
Lymph enters these leaky capillaries via spaces between overlapping endothelial cells.
3. Small lymphatic anchoring filaments between the surrounding connective tissue and the abluminal plasma membrane assist in maintaining luminal patency in these delicate vessels.

44. Lymphatic vascular system
Large lymphatic vessels possess valves and are similar in structure to small veins, except that they have larger lumina and thinner walls.
Lymph nodes that filter the lymph are interposed along their routes.
These vessels converge to form the thoracic duct and right lymphatic duct. The thoracic duct empties into the venous system at the junction of the left internal jugular vein with the subclavian vein, whereas the right lymphatic duct empties into the venous system at a similar location on the right side of the neck.