Blood Vessel Histology

The combined lining of all your blood vessels is called the endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f and is continuous with the lining of the heart.  This creates a closed circuit through which blood flows.  If your blood is not in your vessels, you should see a doctor.

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Tunica media

The tunica mediaThe middle layer of a blood vessel, composed of smooth muscle and elastic tissue, responsible for va is the layer of smooth muscle cells that surround all vessels except capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues..  As you travel from the heart to systemic capillaries, the tunica media decreases in thickness compared to the relative decrease in diameter.  As you travel from capillaries back to the heart, the tunica media increases again, but only to a very small thickness (nothing like the thickness of the aorta that leaves the heart).

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Tunica externa

Like all organs, blood vessels have a connective tissue wrapping that separates and secures them to nearby organs.  Some vessels have no tunica externaThe outermost layer of a blood vessel, made of connective tissue, providing support and anchoring. and some have such a large tunica externa that a vaso vasorumSmall blood vessels that supply the outer layers of large blood vessels with nutrients and oxygen. (literally meaning vessel of the vessels) is needed to vascularize the tissue of the tunica external. 

Arteries

Elastic Arteries

Elastic arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr are found close to the heart where the contractive force of the heart is large.  Elastic arteries don’t like to contract and make the diameter of their lumen smaller, but they will contract to maintain the lumen diameter.  

Muscular Arteries

These arteries tend to hold their circular shape quite well in histological pictures.  They are reliably circular with a very distinct layer of smooth muscle around them

Arteriole

These are the most important vessels in controlling blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange..  Although they are small, they are found in large numbers throughout the body, giving them the most influence over blood pressure – even more influence than the aorta itself.  Arterioles are often overlooked in histology.  They are smaller than expected and can hold only a few red blood cells in their lumen.  They are often overlooked because of how small they are and the factA statement based on direct observation that is repeatedly confirmed. that they really just look like a little circle or oval of a few smooth muscle cells.

Capillaries (3 types)

Continuous Capillaries

Continuous capillaries are the most common and least permeable of capillaries.  The prefix inter-means between and a cleft is a channel-like space between two structures.  This term is used in reference to the spaces between the cells of capillaries.  Intercellular clefts can be of varying sizes, and allow only small moleculesGroups of atoms bonded together. such as waterThe universal solvent essential for life., ionsCharged atoms or molecules., and glucoseA simple sugar that is the main source of energy for cells. to squeeze between the cells of the tunica intima.

Sinusoidal Capillaries

Sinusoidal capillaries are quite porous and are found in places such as the bone marrow.  Red and white blood cells are generated in bone marrow and the capillaries have to be at least big enough for them to get into them. 

Fenestrated Capillaries

Fenestrated capillaries are somewhere in between sinusoidal and continuous.  The word fenestration means window and these capillaries have holes or windows that are about as big as the protein albuminA plasma protein that helps maintain osmotic pressure and transport substances..  Albumin and other plasmaThe liquid component of blood. proteinsLarge molecules made of amino acids with various functions in the body. are made by the liverA large organ that produces bile, detoxifies blood, and stores nutrients. and play an important role in the blood colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries.  This is the solute level in your blood plasma, giving it the ability to keep your fluids in your blood vessels.  When your fluids are not in your blood vessels, but are still in your body, you have edemaExcess fluid in interstitial spaces. (defined as the swelling of a tissue).

Veins

Venules

Venules are the vessels that drain capillary beds.  They are capable of some reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p in order to reclaim all the fluids that have moved from the plasma to the interstitial fluids of the tissue.

Veins

Veins are much larger in diameter than their companion arteries.  Veins also hold a majority of the blood held is your vessels.  This reservoir of blood can be mobilized into circulation by contraction of the smooth muscle in the tunica media of large veins.  This gives veins the nickname of capacitance vessels since they have an enormous capacity to hold this blood reservoir.  Large veins of the legs really have to work against gravity to circulate blood back to the heart.  To help aid in this process are infoldings of the tunica intima called venous valves.  These valves prevent the backflow of blood.  For people who might work long hours on concrete floors (such as nurses, teachers, food service, and retail workers), returning blood to the heart and preventing backflow can become problematic as these valves start to fail later in life.  Varicose veins, or the reflection of the pooling blood through the skinThe body’s largest organ, providing protection and regulation., are often the result of such professions).  Early and often use of compression stockings can hold off the progression to varicose veins.