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).

SEE HANDOUT OF VESSEL CROSS-SECTION

ARTERIES - customarily divided into 3 groups, but actually get continuous reduction
in size away from heart.
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
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

ARTERY-VEIN COMPARISON
Artery Vein
Lumen = 1/2 wall thickness, oval Lumen larger relative to wall, oblong
Maintains rigidity 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
Classified as:
- 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:
- 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:
- 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

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:
- 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
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:
- Lymph enters via afferent vessels on convex side of node; exits via efferent vessels at
hilum ("dent" in kidney)
-Functions:
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

SEE HANDOUT ON SPLEEN STRUCTURE

- Functions of Spleen:
- 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.


To Lecture 21