Blood Vessels

∞ generated and posted on 2020.03.15 ∞

Circulation through our bodies is the movement of blood within blood vessels and plasma is the extracellular fluid portion of blood.

Blood flows through arteries then arterioles then capillaries then venules then veins, and fluid that doesn't make it all the way through capillaries instead is collected as lymph which flows within lymphatic vessels. The acellular, fluid portion of blood, called plasma, in turn consists of a combination of water, salts, proteins, and various multimolecular complexes.

This page contains the following terms: Blood, Blood vessel, Endothelium, Artery, Arteriole, Capillary, Precapillary sphincter, Capillary bed, Vein, Venules, Pulmonary circulation, Systemic circulation, Lymph, Lymphatic vessel, Plasma, Fibrinogen, Albumins, Globulins, Heparin, Serum


Oxygen, nutrient, waste and also heat-carrying circulatory fluid associated with closed circulatory systems in animals.
Blood consists of water, salts, proteins, glucose, lipids, cells known as blood cells, and also other "formed elements", particularly the platelets that are involved in blood clotting.

Blood as found in vertebrates transports oxygen acquired in association with oxygen-acquisition organs (such as lungs in us and gills in fish) and then circulates that oxygen throughout the body.

Blood also picks up nutrients as well as water during absorption and then circulates those nutrients throughout the body (with an ample role for the liver, which serves as the first stop for those nutrients coming from the small intestine via the blood).

Blood is pumped through the body using the heart, and movement occurs within a circulatory system that is split into two distinct parts called pulmonary circulation (circulating through the lungs) and systemic circulation (everything else).

Links to terms of possible interest: Blood, Buffy coat, Formed elements, Hematocrit, Plasma, RBC, Red blood cells, White blood cells

The above video introduces us especially to blood including clotting and blood types.

The above video introduces us especially to the ability of blood to carry oxygen.

Blood vessel

Cell-enclosed tubes within which circulating fluids pass both from and to an associated heart.
Blood vessels can be distinguished into arteries (and arterioles) versus veins (and venules), with capillaries in the middle.

At a minimum, blood vessels are lined with or, in the case of capillaries, consist of epithelial tissue that is named/described as endothelium.

Blood vessels that are other than capillaries possess additional layers, including layers consisting of smooth musculature. The latter is thicker in arteries than it is in veins. Between the epithelial lining and the smooth muscle is elastic connective tissue.

The muscle allows constriction of vessels, a process known as vasoconstriction. This is a reduction in their diameters, i.e., so that blood can be shunted particularly to other, not vasoconstricted locations where it is needed more.

The elastic connective tissue by contrast allows arteries to hold blood pressure over longer time periods, just as an expanded balloon holds air pressure, or as well tanks, important components of rural plumbing, hold pressurized water as generated by well pumps.

Links to terms of possible interest: Artery, Blood vessel, Capillary, Capillary network, Connective tissue, Endothelium, Epithelium, Lumen, Subendothelium, Tunica adventitia, Tunica intima, Tunica media, Vein

The above video provides a nice, fast introduction to arteries, capillaries, and veins as associated with systemic circulation, but stumbles a bit when including pulmonary circulation as well, that is, blood flow to the lungs.

The above video provides an introductive especially to blood vessels.


Cellular lining of blood vessels.
Endothelium consists of epithelial tissue. It is a single layer of cells and is unbroken throughout the circulatory system, providing a continuous if only somewhat selective barrier between the blood and the rest of the body. In particular, it is especially large things that are unable to pass through this epithelium, particularly proteins and formed elements (that is, blood cells and cell-like things).

It is especially across the endothelium – as that endothelium is found making up the walls of capillaries – that movement of smaller-sized blood components out of blood occurs. Note though that movement across endothelium works both ways, with nutrients and oxygen moving out of the blood, wastes including carbon dioxide moving into the blood, and water as well as salts moving in both directions.

Links to terms of possible interest: Basement membrane, Continuous endothelium, Endothelium, Fenestrated endothelium, Fenestration, Sinusoid endothelium, Tunica intima

The above video provides quite the detailed overview of what endothelium is all about.


Blood vessel carrying blood away from the heart.
In systemic circulation, that which flows to all locations in the body except directly towards the lungs, arteries carry oxygenated blood. In pulmonary circulation, that which flows toward the lungs from the heart, arteries instead carry unoxygenated blood. What distinguishes an artery from a vein therefore is not oxygen levels within the blood but instead directional flow to or instead from the heart. Indeed the anatomy of blood vessels versus veins is a function specifically of that latter distinction since blood flowing away from the heart possesses a much higher pressure (i.e., blood pressure) than blood flowing towards the heart.

Given that blood circulates, what then distinguishes that flowing away from the heart from that flowing towards the heart? The answer, for the most part, is that capillaries are found at the dividing line between these two flows. Thus, arteries carry blood towards capillary beds while veins carry blood away from capillary beds.

The exception to this "rule" of only arteries carrying blood towards capillary beds is seen with what are known as portal veins, which carry blood between capillary beds and therefore away from as well as towards capillary beds. Portal veins nevertheless still carry blood away from capillary beds that explicitly have been fed by arteries or, more precisely, fed by arterioles. It is just that portal veins also carry blood towards capillary beds.

Links to terms of possible interest: Arteriole, Artery, Elastic artery, Muscular artery, Smooth muscle, Tunica extrema, Tunica interna, Tunica media

The above two videos walk through major arteries of the body. This perspective is very much anatomical, i.e., different parts of the body need blood and that blood is supplied by arteries that have different names. One job of a student of anatomy is the learning of these names.


Small diameter blood vessels that immediately precede capillaries in circulation.
The arterioles are the smallest diameter arteries as well as the last arteries found prior to the branching of vessels to form capillaries. It is from the arterioles that the precapillary sphincters are found, controlling the movement of blood into the capillaries.

Links to terms of possible interest: Arteriole, Capillary, Lumen, Pericyte

Short, pretty visualization of an actual arteriole as well as vein, with blood moving very quickly through them. Note that "Leukocyte", as mentioned, is another name for white blood cell.

The above video distinguishes between cross sections of arterioles, venules, and post-capillary venules.


Smallest of blood vessels from or to which the oxygen, nutrients, wastes, carbon dioxide, and the water solution making up blood can readily move.
The purpose of circulatory systems, in animals possessing what are known as closed circulatory systems – that is, in which the circulating fluid never leaves the blood vessels, versus open circulatory systems in which it does leave vessels – is basically to refresh the blood that is found in capillaries. This is because capillaries are the point of transaction between blood and surrounding tissues. Thus, everything that is carried by blood, whether to or from capillaries, is there either because it is being carried towards capillaries or because it has been deposited from body tissues back into capillaries.

Another way of thinking about capillaries is that they are the blood vessels that are thin enough in their walls that movement of substances into and out of them, from as well as towards surrounding tissues, is relatively easy. Capillaries nonetheless are microscopic. Thus we have a plethora of macroscopic components of the circulatory system – the heart, the arteries, and the veins – that all exist basically for the sake of feeding as well as draining a huge number of components of the circulatory system, the capillaries, where capillaries in fact cannot be seen with the naked eye.

Links to terms of possible interest: Capillary, Capillary wall, Osmosis, Osmotic pressure

The above video does a really nice job describing the physiology of capillaries.

Three types of capillaries.

Precapillary sphincter

Valves found at the start of the smallest of blood vessels that control blood flow particularly in response to oxygen needs of affected tissues.
Precapillary sphincters lie at the entrance to capillary networks and when closed block blood flow to those networks. This blockage of blood flow occurs basically when oxygen is not (yet) required by the tissues that are vascularized by those capillaries.

This reduces the amount of blood that must flow through the body's capillaries at any one time, which in turn reduces the amount of blood that the body must possess at any given time as well as helps to reduce the necessary output of the heart also at any given time. Indeed, too much blood flow to too much of the body at once can result in dangerous reductions in blood pressure, but due to precapillary sphincters, blood flow is apportioned only to those tissues that at a given moment require that blood flow.

Links to terms of possible interest: Arteriole, Arteriovenous anastomoses, Capillary, Capillary bed, Metarterioles, Precapillary sphincter, Thoroughfare channel, Venule

The above video provides a quick illustration of what precapillary sphincters accomplish, though does not delve into the physiology controlling their actions.

Capillary bed

Localized networks of very small blood vessels from which exchange of gasses and other substances occurs.
A capillary bed is a series of capillaries that contribute to the functioning of the tissues they are present within. This can be simply the maintenance or even growth of those tissues via their provisioning with nutrients and oxygen (while removing wastes) or instead can represent larger roles within the body including gas exchange and nutrient absorption from the gastrointestinal tract.

Links to terms of possible interest: Arteriole, Capillary, Capillary bed, Interstitial fluid, Lymphatic vessel, Plasma, Venule

The above video discusses especially the structures of capillary beds.


Blood vessel carrying blood towards the heart.
Veins possess less muscle in their walls than arteries and also have thinner walls than arteries, since the blood carried by veins possesses much lower pressure (i.e., blood pressure) than does the blood that is found within arteries. Veins also flow closer to the skin than arteries, which the body can 'get away with' again because the blood pressure within veins is lower. That is, cuts into veins (i.e., lacerations) do not result in as profuse bleeding as cuts into arteries.

Veins in addition possess one-way valves that allow blood – as veins are squeezed especially associated skeletal muscles – to move in the direction of the heart. This squeezing occurs in the course of body movement, including breathing, as well as via simply muscle contraction.

Links to terms of possible interest: One-way valve, Skeletal muscle, Vein

The above video considers the structural and functional differences between arteries, veins, and capillaries.


The small diameter blood vessels that immediately follow capillaries in circulation.
The venules are the smallest diameter veins as well as the first veins found in circulation following the capillaries. Blood thus flows from arteries to arterioles to capillaries to venules and then to veins. Capillary beds thus drain into venules while venules collect together to form veins.

Links to terms of possible interest: Arteriole, Arteriovenous anastomosis, Capillary, Capillary bed, Collateral arteries, Metarterioles, Precapillary sphincter, Thoroughfare channel, Vein, Venule

The above video is not quite about venules but instead is a description of what is going on within capillaries just prior to their emptying into venules.

Pulmonary circulation

Blood flow that goes from the heart to the lungs and then back to the heart.
The flow of blood in our bodies is broken up into two somewhat separate systems of circulation, pulmonary circulation versus systemic circulation. Pulmonary circulation is to the lungs exclusively and is driven by the right side of the heart. This is the weaker of the two paths of circulation owing to the smaller volume of vascularization through which blood is being pumped (just the lungs versus the entire rest of the body).

This blood leaves the heart unoxygenated (i.e., relatively depleted of oxygen) but loaded with carbon dioxide. The blood then gains oxygen and loses carbon dioxide in the lungs before returning to the heart, on its left side.

Pulmonary circulation thus is driven by the right side of the heart and is particularly involved, following birth, in the oxygenation of blood (as well as excretion of carbon dioxide) while systemic circulation is driven by the left side of the heart and is particularly involved in the delivery of oxygen to body tissues as well as the picking up of carbon dioxide from those same tissues.

Links to terms of possible interest: Heart, Lungs, Pulmonary circulation, Systemic circulation

The above video does a nice job focusing particularly on pulmonary circulation.

Systemic circulation

Blood flow that goes from the heart to the body other than the lungs and then back to the heart.
Systemic circulation is the stronger of the two body circulatory sub-systems – pulmonary circulation versus systemic circulation – and is powered by the left side of the heart. The blood leaves the heart for systemic circulation carrying oxygenated blood that has been depleted of carbon dioxide and returns to the heart, but on its right side, with blood that has been depleted in oxygen but which is carrying increased amounts of carbon dioxide.

In any case, all blood moving away from the heart does so within arteries and towards capillaries while all blood moving towards the heart and away from capillaries does so within veins, whether this is in systemic or instead is within pulmonary circulation. In addition, per round of circulation all blood passes through the heart twice, and the lungs once plus some other part of the body once.

Links to terms of possible interest: Aorta, Carotid artery, CO2, Digestive tract, Heart, Hepatic portal vein, Hepatic vein, Iliac artery, Iliac vein, Inferior vena cava, Mesenteric arteries, Jugular vein, Kidneys, Liver, Lungs, O2, Pulmonary artery, Pulmonary vein, Renal artery, Renal vein, Subclavian artery, Subclavian vein, Superior vena cava

The above video does a nice job focusing particularly on systemic circulation.

The lymphatic system is another key but somewhat separate component of systemic circulation.


Drained interstitial fluid on its way to return via vessels to the blood.
When blood enters capillaries it is its smaller, non-protein, and non-formed-element components that are forced by blood pressure out of the blood and into the interstitial fluid. Indeed, to a large extent this process is responsible for giving rise to this interstitial fluid, with our cells bathed by a near-constant flow of this blood-derived fluid.

The water of the interstitial fluid is then drawn back into the blood, but not completely. The remaining fluid therefore accumulates within the tissues and literally has to go somewhere. That somewhere is into lymphatic vessels, as lymph.

Lymph thus basically is interstitial fluid that is drained from tissues through vessels. These vessels combine to form larger vessels that ultimately drain back into blood, thus completing a loop that begins in the blood at the capillaries and which ends with a return to blood, though now in a location that typically is quite far from where the lymph is first formed.

Links to terms of possible interest: Arteriole, Capillary, Endothelium, Interstitial fluid, Loose connective tissue, Lymph, Lymphatic capillary, Smooth muscle

The lymphatic system and the formation of lymph.

The above video provides a good introduction to the lymphatic system. Note, though, that the narrator misspeaks when stating that the "tonsils produce antigens".

Lymphatic vessel

Route by which interstitial fluid drains from tissues to return to blood.
In addition to blood vessels, the body has an entire other system of vessels. These serve to drain the fluid in tissues that is found outside of the blood, the interstitial fluid.

This drained fluid is then channeled, though lymphatic vessels, back to the blood. The path taken is rather long and involves numerous vessels, though begins as drainage from tissues into open-ended vessels.

Like veins, lymphatic vessels too possess one-way valves that allow the lymph to move forward, unidirectionally, in the course of the squeezing of lymph vessels during body movement. Lymphatic vessels additionally pass through structures, known as lymph nodes, where interactions of immune system cells (i.e., white blood cells, a.k.a., leukocytes) can occur with foreign substances, which is a component of the generation of immune responses.

Those lymph vessels that drain into lymph nodes are described as afferent lymph vessels whereas those lymph vessels that drain out of lymph nodes are described as efferent lymph vessels. After the lymph nodes the lymph vessels combine into larger lymph vessels, ultimately draining into the left or right subclavian veins.

Links to terms of possible interest: Endothelium, Lymph, Lymphatic capillaries, Lymphatic nodes, Lymphatic vessel, One-way valves

The above video provides a quick overview of lymphatic vessels and how they function.


The liquid portion of blood.
Blood consists of a combination of fluid (plasma) and formed elements (cells and cell-like things). The plasma consists first and primarily of water but also any substances that are dissolved in blood including glucose, amino acids, salts, and pH buffers as well as suspended (colloidal) substances including numerous proteins. This liquid component of blood, which is a soft yellow in color, makes up a bit more than half of blood volume when blood is separated, via centrifugation, into plasma versus formed elements.

Plasma differs somewhat from interstitial fluid in terms of plasma's relatively high protein content. Contrast plasma as well with serum, which is the liquid portion of blood following blood clotting and therefore the removal of the various clotting factors from blood.

Links to terms of possible interest: Agranulocytes, Albumins, Amino acids, Basophil, Blood, Eosinophil, Erythrocyte, Fibrinogen, Formed elements, Gases, Globulins, Granulocyte, Leukocyte, Lymphocyte, Monocyte, Nitrogenous waste, Nutrients, Neutrophil, Plasma, Platelet, Proteins, Whole blood

The above video provides a nice introduction to blood plasma.


Blood protein involved structurally in blood clotting.
Contrasting plasma, serum is lacking in fibrinogen, along with other clotting factors, since serum is formed via blood clotting and constitutes the liquid portion of blood that is present within a blood clot.

The reason that fibrinogen is lost during clotting is that it is modified and thereby used up during the clotting cascade (or coagulation cascade with the blood clotting process actually called coagulation as well as thrombogenesis). Fibrinogen thus is a precursor to the protein that actually makes up the blood clot, called fibrin, and it is via the action of an additional protein, called thrombin, that the conversion of fibrinogen to fibrin occurs.

Links to terms of possible interest: Blood clot, Coagulation, Fibrin, Fibrinogen

The above video describes the formation of a blood clot and the role of fibrinogen in the process.


Blood proteins that serve to maintain the osmotic balance of blood.
The reason that blood requires proteins to maintain its osmotic balance is that the process of pumping blood results in the non-protein and non-formed element portion of blood being squeezed out of the blood into the interstitial fluid (which is that fluid that is found outside of the blood but not inside of body cells). This fluid that is lost from the blood, as it is pushed across capillary walls by blood pressure, must be recovered by the blood, and effecting this is the role of albumins.

The albumins attract water, but are too large to be squeezed out of blood across (most) capillary walls. Blood pressure thus pushes water away from albumins, but as blood pressure declines across capillary beds, the water is then pulled back into blood via their attraction to albumins.

(Technically this is blood pressure pushing fluid out of the blood despite countering osmotic pressure which instead would pull the fluid back into the blood, but as blood pressure declines with distance from the heart its ability to counter osmotic pressure declines as well, and thus water is pulled back into the blood.)

Albumins also serve as carrier proteins within blood, such as of hydrophobic hormones.


Proteins found in blood involved in transport as well as immune functions as antibodies.
Antibodies are also known as gamma globulins (γ globulins) while the other globulins are described either as alpha globulins (α globulins) or beta globulins (β globulins). The latter are involved in the transport of lipid-soluble substances within the blood, which otherwise are not able to dissolve directly in large amounts within the water making up plasma.

These lipid-globulin complexes include the famous (or infamous) HDLs and LDLs, which stand for high-density lipoprotein and low-density lipoprotein, respectively, where the proteins here are globulins while the lipids include cholesterol. We'll get back to gamma globulins during our subsequent discussion of antibodies and immunity.

The above video provides a quick description of the globulin portion of plasma.


Naturally occurring non-proteinaceous anticoagulant added to blood to prevent clotting.
Heparin is added to whole blood to prevent its clotting, which otherwise would naturally occur, following blood collection, upon blood exposure to oxygen. Heparin also can be injected into the blood circulating within living bodies (i.e., ourselves) to interfere with blood clotting within the body. Heparin is also produced and secreted by white blood cells that are known as basophils and mast cells.

The above video discusses the pharmacology of heparin and other anticoagulants; note the production of fibrin at the end of the coagulation pathway.


The liquid portion of blood minus fibrinogen.
Serum is the liquid portion of a blood clot rather the liquid portion of blood itself (which instead is called plasma). Plasma thus is blood minus its formed elements while serum is blood minus its formed elements as well as minus its clotting factors, particularly minus fibrinogen.

Serum is rich in globulins such as gamma globulins, which are antibodies. The study of antibodies thus is a component of the study of serum, the study of which is called serology.

Serum is formed by allowing blood to clot and then centrifuging the clotted blood such that solid is separated from liquid. The so-collected liquid is the serum.