Patterns of Blood Vessels

Learning blood vessels can be overwhelming, which is why it might help to learn the general patterns of where they are. I guess it's a bit like learning your way around a city or something- first you get the general idea of where the main highways are, then you learn the details of the bits that you're interested in.

Arteries

The main "highway" of the arterial system is the aorta. The first branches of the aorta are the coronary arteries, as mentioned in my post about the heart and mediastinum. After that you have the arch of the aorta, which has the right brachiocephalic artery, the left common carotid and the left subclavian, as also mentioned in that post about the heart and mediastinum. Finally, you have the descending aorta, which has a thoracic part and an abdominal part (I'm sure you can guess where each part is).

The descending aorta has some paired branches and some unpaired branches. The thoracic and abdominal aorta both have paired body wall branches. In the thoracic aorta, these manifest as intervertebral arteries; in the abdominal aorta, these manifest as lumbar arteries. The thoracic and abdominal aorta also have unpaired visceral branches. In the thoracic aorta, these are the bronchial and oesophageal branches, whereas in the abdominal aorta these are the coeliac, superior mesenteric and inferior mesenteric, which I briefly mentioned in my post about coelomic cavities. The main difference between the abdominal and thoracic aorta is that the abdominal aorta has paired organ branches, whereas the thoracic aorta doesn't. These paired organ branches are paired (duh) and go to structures that are formed from the intermediate mesoderm, namely the kidneys, adrenal glands and gonads.

Further down the aorta, it bifurcates (splits into two). This results in the formation of the common iliac arteries (so called because the largest part of the pelvis is the iliac), which divide into the external and internal iliac, and so forth.

Anastomoses and End Arteries

Another important concept to talk about when talking about blood vessels is anastomoses. Anastomoses are areas where multiple blood vessels connect up to each other. This means that if one end is occluded (blocked), blood can flow in through the other side. If occlusion is gradual, then small vessels can enlarge to form extra anastomoses, but not if occlusion happens suddenly.

End arteries are the opposite: they are the only arteries supplying a particular area, so if one of them gets blocked, it's game over for that area of the body unless blood can be restored.

A lot of places have anastomoses, because they're pretty important backups for keeping us alive. The brain has an anastomosis called the Circle of Willis, the joints have periarticular anastomoses (as blood vessels might get squished if the joint moves in a particular way), and many parts of the digestive system also have anastomoses, which I'll probably go into greater detail in a later post.

Capillaries and Sinusoids

Arteries narrow down into arterioles, which narrow down into capillaries, which are just made up of a single layer of endothelial cells. In non-fenestrated capillaries (I think "fenestrated" means "has windows" or something weird like that), blood simply diffuses through the epithelial cells. In fenestrated capillaries, there are pores between the cells, allowing more things to get through. Sinusoids are like fenestrated capillaries, but they have even bigger holes so that sometimes whole cells can pass through.

Veins

Veins are sometimes called "capacitance vessels" as they can hold a lot of blood (if I remember correctly, they hold ~60% of the blood of the body). They usually run alongside arteries. Some arteries also have special veins of their own, called venae commitantes.

Most veins have valves in order to prevent backflow of blood. The main exception to this is the torso, which has no valves. The nearest valves to the torso are in the femoral veins, subclavian veins and jugular veins.

The vena cavae are the main veins of the body. Being torso veins, they do not have valves. The superior and inferior vena cavae drain into the right atrium of the heart. There is, however, an alternative way for blood to get between the superior and inferior vena cavae, and that is by the azygos veins in the torso, which join on to the lumbar veins in the abdomen. Another alternative way exists via the vertebral venous plexus, a series of valveless veins in the vertebral canal which has been referred to here. The significance of this is that it provides a route for both good and bad things to spread around the body.

Portal systems are slightly different systems of blood vessels. In most places in the body, you have arteries -> capillaries -> veins -> heart. The portal systems are somewhat different in that you have arteries -> capillaries -> veins -> capillaries -> more veins -> heart, or something to that effect. One of the most well-known portal systems in the body is in the GI tract, where veins draining the gut pass through the capillary bed in the liver before going to the heart.

If something is wrong with the liver, anastomoses can form to allow the blood to bypass the liver. These anastomoses are called portocaval anastomoses (I think porto refers to the portal system, whereas caval refers to the vena cavae, but I could be wrong). These usually appear around the beginning and end of the GI tract.

Lymph Vessels

Lymph vessels carry lymph, which is basically extracellular fluid plus other large stuff that can't get into the veins. They are thin-walled and have valves, like veins, but they also have leaky ends for things to get in. They also contain lymph nodes which are important in the function of the immune system.

The lymph vessels accompany the veins. The main lymph vessels are the thoracic duct, which dumps its crap into the left brachiocephalic vein, and the right lymph trunk, which dumps its crap into the right brachiocephalic vein. There are also other minor connections between the lymph nodes and the veins just in case anything goes wrong with the brachiocephalic veins.