Systemic circulation is a process where oxygenated blood is carried away from the heart, towards all the parts of the body and returns with deoxygenated blood back to the heart. As the blood begins to flow, it leaves the heart through the left ventricle and goes through the aorta.
The artery aorta is the largest artery in the body, therefore, the pressure inside it is greater than the pressure within other arteries. Also, when leaving the heart, the aorta gets segmented into three branches, allowing blood to reach capillaries from all over the body. When blood finally reaches its destination, delivering oxygen to
all the cells, it comes back to the heart through the vena cava and it is, obviously, deoxygenated.
The pulmonary circulation is the one following the order where blood goes through the heart, followed by lungs and back to the heart again. In other words, as its name mentions, "pulmonary" (from Latin, pulmonarius) refers to lungs, therefore, it is the process involving such organs. As the systemic circulation ends, the pulmonary one starts by leaving the heart with deoxygenated blood through the pulmonary artery and towards the lungs, where an exchange will happen between oxygen and carbon dioxide.
An essential element of the circulatory system is the presence of blood.
Blood itself is composed essentially of three parts; plasma, which is the fluid; red blood cells that transport O2 and CO2; white blood cells belonging to the immune system; and platelets, cell fragments that help blood clotting.
Additionally, the transport system is also highly dependent on the heart. Having four different chambers, the heart is mostly divided into left and right; left side being the one responsible for the systemic transport, and the right one for the pulmonary circulation. As seen in the right diagram, the heart has many vessels, those of which receive a different name.
However, although they serve different purposes, they can also be generally ca
tegorized into three main segments; arteries, veins and capillaries. The first one, arteries, carry blood away from the heart through their strong thick walls, smooth muscle and small lumen, which increase the pressure. Veins on the other hand, carry blood to the heart, thing of which I personally relate to the portuguese word "vem" (comes), through their thin wall and large lumen. Also, given that the blood is coming to the heart, pressure is lost during the process (it loses through the segmentations of arteries/veins, distance traveled...), resulting in a smaller pressure within the vein itself. Finally, capillaries are what connect arteries and veins. They also have a wall of one-cell thickness, which allows a faster diffusion of materials across the membrane.
Finally, in order for the blood to reach every cell in the body, it has to be pumped by the heart with enough pressure and force that will allow its travel. In other words, the heart must beat. The pacemaker is located in the wall of the right atrium at the Sinoatrial node (SAN). Each time the pacemaker sends out a signal, the heart carries out a contraction or a beat. The heart beats by itself (myogenic). Nerves and hormones can transmit messages to the pacemaker: Sympathetic nerve secretes adrenaline (also known as "epinephrine") which carries messages from the brain to the pacemaker telling the pacemaker to speed up the beating of the heart. Another nerve tells it to slow it down, and Adrenalin, carried to the pacemaker by the bloodstream tells it to speed up the beating of the heart.
