CIRCULATORY SYSTEM - Divided into 3 major components:
1) Blood vascular system
2) Lymphatic system
3) Hemopoietic Organs/Tissues
I. CIRCULATION OF BLOOD - (Tetrapods)
1) Heart pumps blood to (a) lungs for oxygenation (pulmonary circulation - right side) and (b) the rest of the body (systemic circulation - left side).
2) Arteries take oxygenated blood from heart to rest of body, branch to become smaller distally.
3) Capillaries are small thin-walled vessels, generally receive blood from arteries, empty to veins; gas and nutrient exchange occur across walls.
4) Veins return blood from the tissues to the heart, merge to become larger as they approach heart.
BASIC C.S. STRUCTURE OF CIRCULATORY VESSELS (Inside to Outside)
1) Tunica Intima (interna) = consists of endothelium + subendothelial CT (areolar with reticular fibers) + internal elastic membrane
2) Tunica Media = composed of concentric layers of smooth muscle (cardiac muscle in aorta as it exits heart) and fenestrated elastic membranes. Some small vessels may lack this layer.
3) Tunica Adventitia = consists of outer elastic membrane surrounded by lamellated CT. Contains blood vessels in larger arteries and veins (vasa vasorum).
ARTERIES - customarily divided into 3 groups, but actually get continuous reduction in size away from heart.
1. Large Elastic Arteries
2. Muscular Arteries (small to medium-sized)
Muscular Artery - relatively thick Tunica Media, lumen relatively small and oval Arterioles - everything reduced; stress is less, therefore less elasticity necessary Elastic Arteries - large increase in elastic tissue in walls since have high stress due to high blood pressure; forms flat sheets of fibers with pores (= fenestrated elastic membranes)
VEINS - also customarily divided into 3 groups
1. Sinus = expansion of veins, covers wide area
2. Medium-sized Veins
Medium Vein - thinner-walled than artery; lumen larger and usually oblong Venules - everything reduced; thin T. adventitia; may lack T. media Large Vein/Sinus - T. media is virtually pure smooth muscle; very little CT; T. adventitia is proportionately thicker
Lumen = 1/2 wall thickness, oval
Lumen larger relative to wall, oblong
Collapses without blood
Inner Elastic Membrane is heavy
IEM slight and not distinct
CAPILLARIES = thin-walled; lumen = 8-10 _m generally, up to 30 _m
1) Continuous = endothelial cells provide uninterrupted lining with well- developed basal lamina
- Tight junctions don't extend all around periphery, allows passage of small molecules. Found in lung, muscle, skin, etc.
2) Fenestrated = endothelial cell cytoplasm contains pores, closed by a thin diaphragm except in glomerulus of kidney
- Found in intestinal mucosa, endocrine glands, glomerulus
3) Sinusoidal = larger lumen; irregular walls with wide gaps between endothelial cells
- Found in liver, bone marrow, spleen
- Walls consist of endothelium + basal lamina+ thin sheath of delicate collagen and reticular fibers in larger capillaries
II. LYMPH VESSELS (LYMPHATICS) + LYMPH NODES = LYMPHATIC SYSTEM Lymph Vessels = look like small to medium-sized veins without RBCs; Tunics somewhat less distinct Lymph Capillaries = resemble continuous capillaries but:
1) Basal lamina is incomplete or lacking
2) One end terminates blindly, fluids enter via diffusion + some hydrostatic pressure
3) Generally larger lumens
4) Anchoring filaments extend from outer surface into surrounding CT to hold vessels open
- Lymph vessels with valves to prevent backflow in amniotes. Lymph movement is facilitated by skeletal muscle contraction. Lymph hearts propel lymph in amphibians and to a lesser extent in reptiles.
III. HEMOPOIETIC TISSUES
A. Myeloid Tissue = Red Bone Marrow = active bone marrow; present in cavities of sternum, ribs, vertebrae, skull, proximal ends of some long bones
- Components include:
1) Stroma = vascular CT supported by a network of collagen and reticular fibers. Also present are phagocytic reticular cells and singular adipose cells.
2) Developing Blood Cells = many types and stages present
3) Sinusoids = associated with outer surface are phagocytic reticular cells; newly formed blood cells gain access to circulation by passing through sinusoid wall
- Inactive bone marrow = Yellow Bone Marrow = most of hemopoietic tissue replaced by adipose. Under certain conditions, yellow marrow may be converted to active state -- undifferentiated mesenchyme cells proliferate and form myeloid cells.
B. Lymphoid Tissue = basic feature is heavy population of lymphocytes; functions in defense against infection and in immune response
- 2 Types of Lymphocytes; differ in cell surface proteins
1) T-lymphocytes = differentiate in Thymus; responsible for cell-mediated immune response; 4 subtypes with differing functions (cytotoxic, helper, memory, suppressor)
2) B-lymphocytes = differentiate in red bone marrow; responsible for humoral antibody response
COMPONENTS OF LYMPHOID TISSUE
1. Thymus = site of T-cell differentiation, located near base of neck
- Gross structure = bilobed; subdivided into 1000s of lobules; each lobe enclosed in CT capsule with inward extensions (trabeculae); each lobule divided into outer cortex (where lymphocyte production takes place) and inner medulla (less numerous lymphocytes and therefore pale-staining; presence of Hassall's Corpuscles = spherical eosinophilic structures formed by concentrically arranged reticular epithelial cells with marked tendency to keratinize)
- Elements of Thymic Morphology:
a) Epithelial Element = extensive loose network of epithelial reticular cells
b) Lymphatic Element = T-lymphocytes at various developmental stages
c) Connective Tissue Element = capsule + trabeculae of collagen
- No afferent (incoming) lymphatic vessels in thymus, lymphocytes enter bloodstream via thin-walled venules or efferent lymph vessels present in trabeculae.
SEE HANDOUT OF THYMUS STRUCTURE
2. Lymph Nodules = spherical aggregations of lymphocytes in loose CT underlying digestive, respiratory and urinary epithelium
- Lymph nodules are unencapsulated, most are small and solitary
- There are several sites where lymph nodules are large and multiple:
a) Tonsils = unencapsulated aggregates of lymphoid tissue forming incomplete ring circling entrance to digestive and respiratory tract
i) Palatine = posterior roof of mouth
ii) Lingual = base of tongue
iii) Pharyngeal = surrounding posterior nasopharynx
b) Peyer's Patches = aggregates in wall of ileum
c) Appendix = large nodules in wall
- Structurally lymph nodules may show pale center where lymphocytes are actively enlarging and dividing = Germinal Center
- Nodules act as first line of defense against microorganisms passing through "leaks" in epithelium
3. Lymph Nodes = found only in mammals; encapsulated kidney-shaped structures occurring as enlargements along lymph vessels
- Structural Elements:
a) Capsule = CT with inward extending trabeculae
b) Cortex = outer portion, contains nodules
c) Medulla = inner portion, branching lymphoid strands (lymph cords) enclosing a number of open lymph spaces
- Lymph enters via afferent vessels on convex side of node; exits via efferent vessels at hilum ("dent" in kidney)
a) Proliferation of B-cells during immune response
b) Recirculation of lymphocytes after phagocytic removal of degenerating cells and particulate matter = filtering function
4. Spleen = large elongate organ in abdominal cavity; lies across blood circulation to act as "blood filter", as lymph node acted as lymph filter.
- Structural Elements:
a) CT Capsule = covered by mesothelium, trabeculae extend inwardly; made up of dense collagen fibers + elastic fibers + some smooth muscle
b) Red Pulp = majority of internal spleen is this; composed of (i) numerous wide thin- walled sinusoids, and (ii) supporting network of reticular cells and fibers with numerous free RBCs, macrophages present
c) White Pulp = typical lymphatic tissue (B-cells mainly) surrounding and following arteries, forms pale islands of tissue (= splenic nodules)
- Arteries enter and veins exit at hilum
a) Antibody production via proliferation of B-cells in white pulp
b) Disposal of defective blood cells, especially RBCs
c) Storage of RBCs, when needed releases RBCs into circulation
- Does the spleen have closed or open circulation? - It is not completely known. Open = terminal arterial capillaries open into pulp reticulum and blood gradually filters into venous capillaries (sinusoids) Closed = arterial capillaries enter directly into venous sinuses, but capillaries with large hole (discontinuous capillaries) so RBCs can be exchanged freely
- It appears that open circulation predominates and that closed circulation serves only a minor role.