The circulatory system makes "large-scale" multicellularity possible. It provides a means of conveying nutrients and oxygen to and wastes out of the internal cells that make-up our bodies. Without a circulatory system, life, as we know it, would not exist.

I. There are two major components of the circulatory system.

A. The cardiovascular system

1. heart

2. arteries

3. arterioles

4. capillaries

5. venules

6. veins

B. The lymph vascular system

1. blind ended lymphatic capillaries that collect lymph fluid from tissues

2. larger lymphatic vessels that connect with one and other and finally empty collected lymph into large veins in the neck where the lymphatic and cardiovascular systems merge.


II. General cardiovascular circulation

A. Two major components

1. arterial system

2. venous system

a. portal systems

B. Together these two systems form two major circulatory loops

1. systemic loop

2. pulmonary loop

C. Diagram of general pattern of cardiovascular circulation


III. Structure of vessels 1 - General structure of larger blood vessels.

A. Blood vessels larger than capillaries are encircled by 2 or more of the following tissue layers

1. tunica intima (often referred to as the "intima")

a. layer of simple squamous epithelium called endothelium.

b. the endothelial layer is encircled by a subendothelial layer of loose connective tissue containing a few smooth muscle cells.

c. the intima often appears wrinkled in sections due to contraction of the smooth muscle cells.

2. tunica media (or "media")

a. this layer encircles the intima

b. consists of circumferential smooth muscle with extracellular matrix secreted by cells between themselves that consists of collagen fibers and elastin, as well as various proteoglycans

c. in most arteries a layer of elastin called the internal elastic lamina separates the intima and media.

d. in some larger arteries, an external elastic lamina separates the media from the outer adventitial layer.

e. in large blood vessels, the media receives nutrients from arterioles that branch off of arteries in the adventitia and extend into it.

3. tunica aventitia (or "adventitia")

a connective tissue layer containing many collagen and elastin fibers

b. this layer gradually becomes continuous with the connective tissue of the organ the vessel is in

c. in larger vessels a network of small blood vessels is present in the adventitia.

* these provide nutrients to cells in the adventitia and media.

* called the vasa vasorum


IV. Structure of vessels 2 - General relationship between tunics and arterial and venous vessels


V. Structure of vessels 3 - Arteries

A. Large elastic arteries (e.g. decending aorta and large branches thereof)

1. lots of elastin in media that give these vessels yellow color

2. very thick tunica intima consisting of the endothelium with a relatively thick loose connective tissue sub-layer. An elastic lamina may or may not be present.

3. sub-endothelial basal lamina may not be present

4. media consists of concentrically arranged, perforated elastic membranes with smooth muscle cells and collagen fibers between them. The number and thickness of these elastic layers increases with age

5. tunica adventitia is usually thin with a vasa vasorum (network of blood vessels)

B. Muscular arteries

1. essentially all the named arteries in the body, except for the large elastic arteries, are muscular arteries. Also the unnamed smaller arteries.

2. intima consists of endothelium and a flattened subendothelial layer of collagenous and elastic fibers. A predominant feature of the intima is an internal elastic lamina.

3. the media is the major identifying characteristic. It consists of a thick smooth muscular layer with as many as 40 layers of smooth muscle encircling the artery.

4. larger muscular arteries may have an external elastic lamina at the border of the media with the adventitia.

5. adventitia is well developed and usually thinner than media.

6. spasmodic contraction of the media smooth muscle helps prevent hemorrhaging during injury.

C. Arterioles

1. arterioles are small arteries, 0.04 - 0.4 mm in diameter

2. tunica intima has a very thin sub-endothelial layer of loose connective tissue

3. media is muscular and composed of 1-3 layers of smooth muscle

4. adventitia is fairly prominent

D. Metarterioles

1. metarterioles connect small arterioles to capillaries

2. less than 40 um in diameter

3. consist of endothelium surrounded by a few discontinuous smooth muscle fibers

4. act as sphincters that control the flow of blood through capillary beds


VI. Structure of vessels 4 - Capillaries

A. Capillaries

1. usually 7-9 um in diameter, but may be as small as 5 um or as large as 12 um in diameter (sinusoidal capillaries may be larger)

2. wall consists of a simple squamous epithelium called endothelium. This is just the continuation of the endothelium that lines the whole circulatory system, without the various additional connective tissue and muscular layers that we find surrounding larger blood vessels.

3. this simple squamous epithelium is surrounded by a basal lamina.

4. cells called pericytes may be sporadically found between the basal lamina and the endothelial cells. The cells are able to contract and thus constrict capillaries. Pericytes have their own basal lamina which may fuse with the basal lamina of the endothelial cells that form the capillary wall.

5. In transverse (cross-) sections we find that the circumferential wall of a capillary is generally formed by 2-3 squamous cells held together by occluding or gap junctions. Nuclei of the cells buldge into the capillary lumen.

6. The walls of certain types of capillaries may have pores (or fenestrae) penetrating the squamous endothelial cells, or there may be spaces between adjacent endothelial cells.

B. There are 3 types of capillaries.

1. continuous

a. no pores or spaces between cells in walls

b. most capillaries are of this type

2. fenestrated or perforated capillaries

a. pores penetrate the endothelial cells

b. found in tissues where rapid exchange of substances occurs (e.g., in the villi of the intestinal wall and glomeruli of the kidney).

3. sinusoidal capillaries

a. highly convoluted and with enlarged diameter (30-40 um)

b. open spaces are present between endothelial cells

c. in some sinusoidal capillaries, many small multiple fenestrations penetrate the endothelial cells

d. phagocytic cells are present in and around the endothelial layer

e. the endothelium lacks a continuous basal lamina

f. found mainly in liver and hematopoietic organs
such as bone marrow and spleen.

C. So, except for continuous capillaries, we can see that these vessels are constructed to allow for the passage of materials (macromolecules) and sometimes cells (e.g., monocytes, eosinophils) into and out of the circulatory system.

D. This can occur through pores (in cells) or spaces (between cells). In continuous capillaries, macromolecules can also be passed into or out of the circulatory system by what might be called transcellular pinocytosis. Cells can move into and out of continuous capillaries by diapedesis.


VII. Structure of vessels 5 – Sinusoids

A. These are circulatory structures that resemble capillaries in that they connect arterioles with venules (bone marrow, spleen, adrenyl cortex) or venules with venules (liver, adenohypophysis) and have just an endothelial wall, but they are different in a number of ways.

B. form large, uneven channals 30 um or more across

C. endothelium is incomplete and, in addition to endothelial cells, contains fixed phagocytic cells that bulge into the lumen


VIII. Structure of vessles 6 - Veins

A. post capillary venules (those venules directly connected to capillaries)

1. 30 um - 200 um in diameter

2. consist of endothelium with incomplete layer of pericytes and a thin connective tissue investment - similar to capillaries, primary site where leukocytes leave the circulatory system at sites of infection.

B. venules

1. 0.2 - 1 mm in diameter

2. intima lacks a subendothelial layer of connective tissue

3. thin media consisting of a few smooth muscle fibers (collecting venules). As venule size increases, the amount of muscle increases (muscular venules).

4. well developed adventitia rich in collagen fibers.

5. despite tunic investments, blood cells such as moncytes can still pass through the walls of venules

a. this process is called diapedesis - the act of a leucocyte squeezing between endothelial cells

C. small to medium sized veins

1. 1-9 mm in diameter

2. intima usually has a thin sub-endothelial layer of connective tissue associated with endothelium

3. thin media

4. well developed adventitia rich in collagen fibers.

D. large veins

1. intima is well developed, but still thin

2. Media is muscular, but thin relative to that of similar sized muscular arteries. A few circular layers of smooth muscle are present amongst abundant connective tissue.

3. Adventitia is thickest and best developed tunic. Comprises most of wall of vessel. May contain longitudinal bundles of smooth muscle.

4. This arrangement of longitudinal muscle in adventitia and circular in media helps to strengthen wall and help prevent settling of blood in limb veins due to gravity.

5. In addition to this muscular arrangement, valves are also present in large veins. These also help prevent back flow of blood.

6. Skeletal muscle in conjunction with valves is helpful in preventing backflow by propelling blood through veins in the correct direction as the muscles contract.


IX. Heart

A. The heart is composed of

1. 3 layers or tunics

a. endocardium

b. myocardium

c. epicardium

2. a fibrous connective tissue skeleton

3. valves

4. an electrical impulse conducting system

B. Tunics of the heart

1. endocardium - in a sense, simply a continuation of tunica intima

a. consists of 3 parts

* endothelium resting on a basal lamina and associated thin layer of collagen fibers

* a deeper, denser layer of connective tissue containing elastic fibers and a few smooth muscle cells

* the subendocardial zone of loose connective tissue that contains small blood vessels and nerves. In the ventricle, the subendocardial zone may contain scattered Purkinje fibers

2. Myocardium - consists of 2 components

a. fascicles of cardiac muscle cells that connect to the fibrous connective tissue skeleton of the heart

b. non-contactile, modified muscle cells that form the impulse (action potentil) generating and conducting system of the heart

* this system is composed of the atrioventricular and sinoatrial nodes, and the Purkinje fibers

* the cells of this system are muscle cells that are specialized for impulse (action potential) generation and impulse conduction

* these cells generate and conduct action potentials that synchronize the heartbeat

* (in the ventricle) Purkinje fibers are scattered along the innermost portion of the myocardium, next to the endocardium

3. Epicardium (visceral pericardium) – epithelium and connective tissue covering of heart

a. squamous to cuboidal epithelial external lining - a continuation of the epithelium that lines the pericardial cavity

b. below this is a layer of connective tissue with high concentration of elastic fibers

c. between the elastic layer and myocardium is the subepicardial layer that consists of loose connective tissue containing nerves, veins, and adipose tissue.

C. Valves - read in text

X. A few words about the lymphatic vessels

A. System of thin walled vessels that runs throughout the body.

B. These vessels are lined with endothelium.

C. They begin with blind-ended lymphatic capillaries that originate in connective tissue of nearly all parts of body.

D. These lymphatic capillaries and associated small lymph vessels converge into larger vessels that return fluid expressed from arteries and veins into tissues to the venous component of the blood circulatory system.

E. The lymph vessels carry fluid that eventually drains into 2 large ducts, the thoracic duct and the smaller right lymphatic duct that connect to large veins in the neck.

F. Larger lymphatic vessels are similar in structure to veins except with thinner walls and with no clear-cut separation between the 3 tunics (intima, media, adventitia).

G. The lymph vessels are also a pathway by which various cells of the immune system can enter or leave the blood circulatory system and tissues.