Download Microscopic Anatomy: Arteries, Veins and Capillaries

Physiology 2
Name
Redwood High School
Class Period
Microscopic Anatomy: Arteries, Veins and Capillaries
Background
The blood and heart are not sufficient by themselves to form a circulatory system. A network of tube-like
structures functions to carry blood away from the heart, to body tissues and back to the heart. Therefore, the blood
vessels contribute to the circular pattern of blood flow throughout the body. Vessels supply the “plumbing” of the
circulatory system. They accept blood, subject to the pumping pressure of the heart, and distribute it to various body
regions. Five different kinds of blood vessels compose the vascular network of the circulatory system: arteries,
arterioles, capillaries, venules, and veins.
Arteries transport pumped blood away from the heart. A major artery supplying a body region eventually
subdivides into smaller arteries, which subdivide into smaller and more numerous arterioles.
Arterioles subdivide into even more numerous, microscopic capillaries. The capillaries are so small and numerous
that (1) cubic inch of skeletal muscle has well over a million vessels. A body cell is usually near a group of capillaries.
At the capillary, oxygen and nutrients leave the blood and enter themselves of carbon dioxide and other waste
products. These cellular products of metabolism enter the blood at capillary sites. Thus, a true two-way transfer (or
exchange) of substances takes place here between capillaries and body cells. Capillary walls consist of only one layer
of epithelial cells, and this thinness is integral to the capillary’s function.
In a given body region, capillaries collect into larger, less numerous vessels called venules. These in turn collect into
larger, less numerous veins. The venule-vein route returns blood to the heart from various body regions. Thus, the
following sequence describes the general direction that blood flows after leaving the heart: artery; arteriole; capillary;
venule and vein.
The arteries-arterioles-capillaries leading from the heart to a body region appear much as a tree limb that
subdivides into progressively smaller and more numerous branches as it leads from the tree trunk. The capillariesvenules-veins leading back to the heart resemble a recollection of limb branches back toward the trunk. This pattern,
which you can visualize, is the general systematic circulation for supplying and draining blood from most body regions.
The systemic circulation includes the blood flow that leaves the left ventricle of the heart through the aorta and travels
back to the right atrium after supplying all body regions along the way. The pulmonary circulation, sending blood from
the right ventricle of the heart to the lungs returning it to the left atrium of the heart, is also composed of this sequence
of vessels.
As blood moves through a sequence of vessels, a pressure difference determines the direction of blood flow. Blood
flows from a region of higher pressure to a region of lower pressure. Pressure is highest in the major arteries closest to the
pressure source, the heart. The pressure diminishes in vessels that are progressively farther from the heart’s influence.
While in the capillary, the blood is in the only type of vessel that exchanges substances with cells and thus serves their
needs.
In this laboratory you will study the microscopic/tissue structure of arteries and veins.
Focus Questions
•
What are the important differences in microscopic structure between an artery, vein and capillary?
Procedure
Microscopic Study of Arteries and Veins
1. The following slide are available for this study: artery-vein-capillaries, mammal
2. Obtain and observe the ‘artery-vein-capillaries-mammal’ slide under the compound light microscope. This slide
represents a typical, medium-sized systemic artery and vein (most commonly from a cat). You will use these
slides, along with available illustrations and the descriptions below, to make observations about the differences
between these structures.
In transverse section, you can see that each artery and vein has an inner space, or lumen, through which the
blood flows and a multilayered vessel wall.
The inner coat is the tunica interna. Its simple squamous epithelium is the tissue that provides a relatively
smooth trip for blood as it flows along this inside, free surface. This surface reduces friction as blood flows
through the vessel. It is usually termed endothelium because it lines the inside surface of the blood vessel. The
midlayer, the tunica media, is the thickest layer in either vessel. However, it is proportionately thicker in the artery.
Smooth muscle cells, mostly arranged circularly, and reinforcing collagen and elastic fibers constitute its makeup.
The outermost layer, the tunica externa, is mainly a wrapping of dense connective tissue and loose connective
tissue. This layer prevents the vessel from collapsing.
The main properties of arteries can be listed as follows: 1)they are elastic; 2)they are contractile; 3) they
contain blood flowing under high pressure; and 4) they allow for fluctuating blood pressure.
The first two descriptions result from the substantial thickness of the middle layer of smooth and elastic fibers.
Nerves supplying the smooth muscle here can stimulate contraction of the circularly arrayed muscle fibers.
Contraction of these muscles can produce a reduction of the vessel lumen, or vasoconstriction. Relaxation of this
muscle leads to vasodilation.
The last two descriptions originate not only from structure but from the vessel location as well. Arteries are
relatively close the source of pressure, the heart. The pressure fluctuates up and down, reflecting the systole
(contraction) and diastole (relaxation) of the heart’s left ventricle. Thus, this is a changing pressure.
The main properties of veins are the opposite. They have a comparatively thin middle layer and thus smaller
capacity for contraction (from muscle) and stretching (from elastic fibers). Pressures have dropped off in systemic
veins because the blood has traveled a greater distance from the powerful left ventricle, compared with the trip
through arteries. This increased distance, plus the friction produced from blood running through vessels
(especially the capillaries), dampens the pulsatile nature of the pressure from the pumping heart. Although the
endothelium greatly reduces friction, it does not eliminate it. This pulsatile pressure has largely disappeared at the
level of the arterioles.
Capillaries are simple vessels in which nutrients and gases can diffuse into/from the tissues. The walls of
capillaries consist just of a thin tunica interna, allowing easy diffusion of gases across through the vessel wall. You
should observe capillary structure, even though you won’t include this in your drawing.
3. Make a microscopic drawing of an artery and vein. In your drawing, you should label each vessel and each layer in the vessel
wall, including lumen. You do not need to include a capillary in your drawing.
2. Use the Venn diagram below to organize the following terms related to arteries and veins:
• carries blood away from heart
• valves present
• carries blood
• tunica media
• venules
• thin walls
• controls blood pressure
• additional term- your choice
Arteries
• carries blood toward heart
• arterioles
• tunica interna
• lacks valves
• thick walls
• low pressure
• high pressure
• additional term- your choice
Veins
• high pressure
• oxygenated blood
• deoxygenated blood
• tunica externa
• elastic walls
• contractile
• lumen
• additional term- your choice
Physiology 2
Name
Redwood High School
Class Period
Microscopic Anatomy: Arteries, Veins and Capillaries
Analysis
1. Use your observations to complete the table below.
Structure
Artery
Vein
Capillary
Function