You are a multicellular organism, which means that your cells have differentiated to perform specific tasks helpful to the entire body. It also means that most of your cells don’t have a way of obtaining resources from the environment and instead must rely on the other cells of the organism to deliver the resources to them.
The transport cells of the body are called blood and they exist in a solution of water. Red blood cells, the main type, are actually your only cells without a nucleus to increase surface area for more efficient transport. The main purpose of blood is to deliver oxygen to the body cells and remove the carbon dioxide. Your blood basically removes carbon from cells with oxygen as a way to carry it.
To carry the oxygen, blood cells developed a large protein structure called hemoglobin with four proteins surrounding a central iron atom. That iron atom has vacant and accessible orbitals for the electrons of oxygen to bind to it.
Depending on the concentration of the surroundings, the iron-oxygen bond will break to release an oxygen atom and pick up another one. If hemoglobin is attached to oxygen in an excess of carbon dioxide, the oxygen will diffuse away to equalize the surroundings. The iron quickly replaces the oxygen with the oxygen atom of a carbon dioxide molecule. The opposite concentrations occur within the lungs, and it is how the blood removes carbon dioxide and picks up new oxygen.
The first multicellular organisms got by with a fairly inefficient transport system. Most of them had a cavity filled with blood cells and water called a hemocoel meaning “blood cavity.” The hemocoel, with the eventual aid of a pump, pushed the water and blood cells around allowing for gas exchange with body tissues. Most of those organisms were small, slow moving, and lived in water, so this open system was an acceptably efficient method of nutrient distribution.
As organisms developed and the need for quicker movements became highly advantageous, those cavities began to shrink and branch out. Again, this was not a product of choice but by mutations over generations. Some organisms still have open circulatory systems because they were able to efficiently survive using a hemocoel, many sea creatures and insects fall into this category.
The organisms that needed both size and speed to survive were better suited to do so with a closed system. The blood could be pushed, under pressure, to the specific areas of the body that needed it the most within blood vessels. Vessels in more centralized areas developed more protective layers. The thick blood vessels branched out to thin layered vessels called capillaries. After delivering nutrients to body cells, the capillaries merge together into thicker vessels before repeating the process.
The image above shows the red vessels with high oxygen concentrations and the blue with high carbon dioxide concentration. With the adaptation of lungs, a separate system of looping cavities allowed the blood to flow directly to and from the lungs before being sent back to the pump to go to the rest of the body.
The pumping system, usually called a heart, is the reason that any of the transport can happen. All organisms that have internal transport have also developed a method of pressurizing the blood to force it to move through the vessel. Organisms with simple cavities might have multiple pumps to keep the blood moving throughout. Organisms with more complex, branching vessels use a single pump but with separate sections. The most complex organisms with lungs have a separate pair of sections as part of the pump.
The pump’s sections work antagonistically, like all your muscles, to pull in blood from the cavity and then push it back out. Your heart has four sections that allow it to pull in and push blood out to the body and lungs separately. The separate chambers keep the oxygen rich blood (red) away from the carbon dioxide rich blood (blue).
The heart is one of the most incredible adaptations that life on our planet has ever seen! Communicative cells have been automated to signal the heart to constantly pump. Those signals can change the pace of the pumping depending on the environmental situation, without the need for conscious attention. Those signals are also sent to the muscle surrounding the branching vessels, and it’s the reason you can check your heart rate by simply touching a vein.
Naturally, a transport system that reaches all parts of the body is going to deliver more than just nutrients. Your communicative cells utilize the transport system by secreting chemical signals into it, we call those chemicals hormones. The transport is also going to pick up waste from cells to remove from the organism. You have a specific hormone that indicates to your kidneys, one of your blood-filtration organs, to hold onto or remove more water. When you have an excess of water, your body removes it by urinating.
Urinating doesn’t remove all waste and we know that sometimes invaders get into the transport system. There’s no need to worry, specialized web-like areas catch invaders and junk as the blood passes through. The defense cells, also hanging out in the transport system, can then go to work. We call those web-like structures lymph nodes and the reason they swell when you’re sick is because they are literally catching invaders and defense cells.
Your transport system is vital for you staying alive for several different reasons. It’s the road system of your body with over 25 trillion blood cells traveling throughout picking up and dropping off nutrients and waste. Next time you stop to listen to your heartbeat, I want you to think about how intricate and complex your circulatory system is. It’s easy to forget about since it will still work even when it’s not on your mind, but doesn’t that make even more amazing?