Download CARDIOVASCULAR AND LYMPHATIC SYSTEMS Roger J. Bick

CARDIOVASCULAR AND LYMPHATIC SYSTEMS
Roger J. Bick, PhD, MMEd
Objectives:-After this lecture and lab you should be able to distinguish and know
 the layers and cell types of the heart and vasculature
 The differences between large arteries and large veins
 The differences between arterioles and small veins
 The different types of capillaries; anastomoses
 The origination, termination and architecture of lymphatics
Key Words:- Tunicae Intima, Media, Adventitia. Myo-, epi-, endocardium. Valves, Elastic, Veins,
Arteries, venules, arterioles, metarterioles, capillaries (fenestrated, continuous, sinusoidal). Lymphatic.
Vasa vasorum. Pericyte. Purkinje
Cardiovascular system is made of 3 layers throughout. The heart has 3 layers - endocardium (inside - a
thin layer of endothelium), myocardium (middle - a thick layer of striated muscle), and epicardium (outer
- a thin layer of squamous endothelium), attached to each other by connective tissue. The vessels (veins,
arteries and capillaries) have 3 layers:- a tunica intima (inner, akin to the endocardium: tunica [L] means
coat), a tunic media (middle, akin to the myocardium) and tunica adventitia (outer, akin to the
epicardium).
The heart: is a pump for blood and the bulk of the tissue is striated, involuntary cardiac muscle, which
can undergo hypertrophy (thickening), atrophy (thinning), necrosis (damage-induced cell death) and
apoptosis (programmed cell death). In the lab you will have a section of heart that will look like this:-
There are 4 valves in the heart, tricuspid (right AV), mitral (left AV), and semilunar (pulmonary artery
and aorta). Valves are folds of endocardium covered by endothelium; Tricuspid and mitral valves thicker
than semilunar and all contain, lymphatics, blood vessels and a central connective tissue core. The
pericardium is probably not present in your lab specimen.
Pericardium (Outer, fluid filled sac covering the heart): Parietal pericardium (outer-nearest the skin) – thin mesothelium that adheres to fat and
connective tissue of pericardial cavity.
 Pericardial Fluid – Plasma ultrafiltrate from visceral pericardium; Proteinaceous in nature;
lubricant for the contraction/relaxation of the heart; 15-50cc.
 Visceral pericardium – thin mesothelium, sometimes cuboidal, that adheres to epicardium.
Epicardium – Thin layer of flat to cuboidal cells, covering fibrous and adipose connective tissue;
Contains nerves and vasa vasorum (vessels) of both heart and of coronary vessels found in this
connective tissue mass (equivalent to tunica adventitia of the vessels).
Myocardium – thickest layer of the heart (usually), composed of bundles of cardiac muscle of varying
thickness, recognized by striations, intercalated discs, branched fibers and centrally located nuclei.
Strands of connective tissue and some vascular tissue course through it (equivalent to the tunica media of
the vessels).
Endocardium – Simple squamous epithelium, sometimes cuboidal, over a CT layer of variable thickness,
the subendocardial layer (thicker in the atria than in the ventricles), containing collagen fibers, elastic
fibers and smooth muscle (equivalent to the tunica intima of the vessels). Purkinje fibers associated with
the Conduction System are found here.
Heart is supported in chest cavity by dense collagenous and elastic connective tissue fibrous skeleton
(composed of the annuli fibrosi at the atrioventricular orifices, the arterial foramina, the trigonal fibrosa
and the septum membranaceum).
Impulse generating and Conducting System Pathway - SA node (pacemaker) to AV node via Bundle
of His, then out to left and right anterior and posterior bundle branches, which goes to Purkinje
fibers and Purkinje cells. (You won’t be able to discern any of the features of these areas in your slides,
except maybe some vacuous cells (empty-looking) that might be part of this system (Purkinje) so here’s a
diagram and a picture of Purkinje cells.
GENERAL STRUCTURE OF BLOOD VESSELS
Blood vessels have 3 layers, Tunicae Intima, Media, Adventitia. When large veins are compared with
large arteries, major differences are seen in the thickness of, and cell types in, these layers. The largest
artery is the elastic artery, such as the aorta and its major branches, so called because the tunica media
(middle layer) contains lots of elastic fibers (made of Elastin, black squiggles in the following image) as
well as connective tissue and smooth muscle.
Compare this with the largest type of vein such as the Pulmonary vein, (which does NOT have lots of
elastic in it, just a few here and there for some flexibility) which is supported against collapse by lots of
smooth muscle. This is important, the elastic in the artery is in the media(circular), while the muscle in
the vein is in the adventitia (longitudinal).
Compare the artery (lower vessel) with the vein (upper
vessel) in this image:- Note the layers
Next in size is the muscular artery, in which the elastic fibers have mostly disappeared and the media
contains mostly smooth muscle Compare and contrast, a medium sized vein and note the thicknesses of
the tunics and less muscle in the vein
Summarize the vessels so far:
 Thickest layer (tunica) of the larger arteries is the media; the thickest layer of the larger veins is
the adventitia.
 Bigger arteries contain lots of elastic and a large amount of smooth muscle (but less than large
veins); the larger veins contain lots of smooth muscle and some (but not much) elastic fibers. Here
is a rough comparison of layer thickness, with the artery on the left, the vein on the right:Both the artery (left) and the vein have an internal elastic lamina (IEL), a structure very
evident in big, muscular arteries that have to accept large, pulsatile volumes of blood.
IEL is also present in elastic arteries, but difficult to distinguish due to all the elastic fibers.
Large veins are supported against total collapse by smooth muscle in their tunica adventitia.
Largest arteries are elastic arteries, which gradually lose elastic, retain an internal elastic lamina, and
become muscular arteries.
The largest veins are large muscular veins that gradually lose their muscle component and become
smaller muscular veins.
Both large arteries and large veins possess a Vasa Vasorum (“Vessels of the vessel”), a network of small
blood vessels in the tunica adventitia.
Fortunately, arteries and veins usually travel in close proximity to each other, so immediate comparisons
can be made and as luck would have it, a nerve or a lymphatic, and sometimes both, are also usually close
by.
The three tunics thin out, the elastic disappears and the component smooth muscle becomes less.
As the veins become smaller they become less round, while arteries usually maintain a nice circular.
Small muscular arteries have 5-10 concentric layers of smooth muscle in their tunica media ( seen above),
while veins have a couple of layers of smooth muscle cells in the tunica media and a large, wispy, illdefined adventitia. Compare the small muscular artery on the left with the small vein on the right. The
artery has a number of concentric layers of smooth muscle, The vein on the right still has a few concentric
layers of smooth muscle, and a much bigger lumen than the artery, which is usually the case, and when
more smooth muscle is lost, the veins tend to lose their roundness. Note the nerve in the bottom right
hand corner.
The final stages of the ‘big’ vessels are venules and arterioles. Venules have a very thin, low cuboidal to
squamous endothelium (wall), minimal tunica media (maybe a few smooth muscle cells and a few wisps
of connective tissue), and an ill-defined adventitia that blends with the surrounding fibrous connective
tissue.
Arterioles still maintain a round profile, with 2-5 concentric layers of smooth muscle cells, a low
cuboidal to squamous endothelium and sparse adventitia.
Note: 1) The circular profile of the arteriole
2) The larger size of the lumen of the venule
3) The 2 layers of smooth muscle cells surrounding the arteriole
The open arrowhead is pointing to a pericyte, which we’ll dicuss later.
On to the tiny vessel, the capillary, of which there are 3 different types (logical thinking should allow
you to make an educated guess as to in which tissue they are found).
 Continuous (no holes in the endothelium)
 Fenestrated (has holes in the endothelium)
 Sinusoidal or sinusoids (large with big holes in the endothelium)
Logically, continuous in tissue where contamination not wanted, from blood reaching tissue or vice
versa, Fenestrated where we want select products getting into the blood, and Sinusoids in tissue where
large things, even cells such as macrophages, pass in and out of the blood. Can you, even now, think of
where you would find these particular types of capillaries?
 Continuous capillary - characterized by absence of fenestrae (holes). Common in muscle,
connective tissue, exocrine glands and nervous tissue. Normal to see pinocytotic vesicles on both
surfaces of endothelial cells, responsible for transport of macromolecules.
 Fenestrated with diaphragms are found in tissue where specific products are required to pass
from synthesizing organs into the blood. The thyroid is one example where transport of thyroid
hormones occurs. Capillaries are nestled between follicles.
 Fenestrated without diaphragms, such as are found in the glomeruli of kidneys, and allow
passage of larger products i.e. proteins.
 Sinusoids are very large capillaries (diameter >30 m), and have no diaphragms, numerous
fenestrae (openings) and a discontinuous basal lamina. Large compounds and even cells pass
through and can move in and out of these vessels. Found in liver (shown in the following
example), bone marrow and the spleen.
Capillaries are formed from tiny branches of the arterioles, the metarterioles, which are
surrounded by a discontinuous layer of smooth muscle, and constriction of the metarterioles helps
regulate blood flow in tissues.
In some tissues there are arteriovenous anastomoses (connections) that allow blood to empty
directly into venules from the arterial side, so that when vessels in these anastomoses contract, all the
blood passes directly and rapidly straight through. The number of anastomoses and richness of capillary
network is directly related to the metabolic rate of the tissue or organ. For example, the liver, a highly
synthetic and detoxification organ, has a vast capillary network.
To summarize: Size of vessels diminishes through arterial circulation, and then increases back up
through venous return. Elastic arteries to muscular arteries (with internal elastic lamina) to muscular
arteries to arterioles to metarterioles to capillaries to venules to large venules to small muscular veinto–
large muscular vein.
Finally, THE LYMPHATICS. These originate in tissues as thin, closed-end vessels, with a one cell
thick endothelium and an incomplete basal lamina. Often confused with venules, so the best way to
identify them with a light microscope is to look at the cells in the vessel which will be lymphocytes (as
opposed to RBC’s; black v orange/red). There is another point of confusion too;
BOTH VEINS AND LYMPHATICS CONTAIN VALVES
However, the lymphatic is VERY thin walled, held open against collapse by just a few smooth muscle
cells and a meager number of elastic fibers. The larger vein has a substantial tunica adventitia and amount
of smooth muscle cells. Thus, the vein is a much more substantial structure.
LYMPHATIC
VEIN
The thin lymphatics converge to form two large trunks, the thoracic duct and the right lymphatic duct,
which empty into the venous system. Lymph contained in the lymphatics is cleaned via passage through
the lymph nodes. Valves are more numerous in the lymphatic system and contraction of surrounding
skeletal muscle bundles aid in ensuring unidirectional flow of lymphatic fluid.
Lacteal – so called as it fills/absorbs fat. Found in center of intestinal villi.
CARDIOVASCULAR SYSTEM LABORATORY
Heart (slide #37) - Put slide on a sheet of white paper and compare to the picture shown on the first page
of this chapter. Note thicker ventricular wall, and thin atrial wall. You might be lucky and have a valve
hanging around.
Under the microscope look at: Outer adventitia, epicardial fat, small vessels (venules, arterioles, etc) and connective tissue.
 Middle media – myocardium – made of thick cardiac muscle. Look for striations, intercalated
discs and capillaries (what sort will these be?)
 Inner intima – thin. Squamous to low cuboidal epithelium on a connective tissue subendocardium. Continues around the inside of the ventricle and over the flap of the valve (tricuspid
in your section). There is a core connective tissue that ends at the top of the left ventricle, called
the annulus fibrosus. The endocardium continues into the atrial chamber where it thickens.
 Between the ventricle and the atrium is a large fat pad contianing many smaller vessels, nice
examples of unilocular fat and nerves.
Elastic Artery and large vein (slides 24 and 25)
24 is H & E stain, 25 is Verhoff-van Giesen stain. These are aorta showing a thin endothelial
intima, and concentric smooth muscle separated by wavy layers of elastic lamellae in the media. Move
the condenser up and down (slightly), and you’ll see pink bands come in and out of focus and they should
be easier to visualize. The adventitia is lighter pink and contains lots of blood vessels, the vasa vasorum.
The collapsed profile on these slides is the vena cava, a large vein. Note the thin tunica intima,
and the thinner (when compared to the aorta) tunica media with some smooth muscle that shows up
better as grayish-green in #25. The tunica adventitia is thick and contains prominent longitudinal
bundles of smooth muscle. There is also a nice vasa vasorum. (NOTE: longitudinal bundles in the
adventitia).
Muscular Artery (slide #10, 43, 6). Look for arterioles, medium sized veins and venules, particularly in
#10, the sole of the foot (or big toe). You’ll know you have a muscular artery because you’ll see a
prominent what? (Internal elastic lamina is what). Look around as veins and arteries often travel together,
and you can compare. Have a quick look in #43, a section of submandibular gland. What type of
capillaries would you likely find here? Slide #6 is small intestine, and here you will see many lovely
profiles of arteries, veins, arterioles, venules and capillaries and maybe lymphatics. As mentioned above,
veins and arteries often travel together and this is a great place to contrast and compare.
Capillaries. All the above slides will have examples of capillaries of one sort or another. Easiest way to
determine if a vessel is a capillary is to look for a hole surrounded by one cell, containing a single red
blood cell (RBC). As it is only one cell wide the profile will look like a “class ring”.
Continuous: Look in the myocardium in slide #37 and in tongue #38, just below the epidermis.
Fenestrated with diaphragms in #56, the thyroid. We can’t see the fenestrae at this magnification but the
capillaries are just outside the follicles. Muscular arteries in the CT.
Fenestrated without diaphragms in #7, the kidney. Look at the capillary loops in the cortical regions.
Sinusoidal #1 in Liver. Sinusoids contain RBC’s and macrophages (Kupffer cells) and run between
hepatocytes. Pick out flattened endothelial cell nuclei of the sinusoidal cells.
Lymphatics are often hard to identify as they are thin and flattened and look like venules (what do
lymphatics and veins have in common on a structural level?). However, no RBCs, but will contain
lymphocytes, leukocytes and eosinophilic lymph. Look for vessels in #6 that ares full of BB shot. On #6,
in the center of each villus, is a gap that is a lacteal. It has this name as after a fatty meal it will swell.
Thoracic Duct: lymphatic system does not circulate; a blind-ended system that arises in interstitium
and empties into thoracic duct, the smallest structure on slide #25.