Where would we be without blood? That red stuff that carries vital oxygen from our lungs to our muscles, and helps move our body’s chemical waste to where it can be recycled or disposed of? Blood is vital for life in humans, but did you know that not all animals have blood, and that some have blood that is very different to our own?

In fact, many small creatures, such as amoebas, sponges and corals, don’t need blood. Most of their body makes direct contact with the outside environment, which means they can survive by directly absorbing nutrients from their surroundings. Furthermore, their waste can “leak” out by a process known as diffusion. For these animals, a heart, veins, arteries and blood would be an unnecessary investment.

The innards of larger animals, like humans, don’t have such a direct link to the outside world, so absorption and diffusion are not an option. Rather, they need a transport system in their bodies that can help stuff to get in, out and around. Specifically, our bodies need to move oxygen from our lungs to our cells to produce energy by burning the sugars, fats and proteins we eat in a controlled way. To solve the transport problem, many animals have developed some kind of circulatory system, made of an oxygen- and nutrient-carrying fluid, some plumbing, and sometimes one or more pumps to keep things moving. In humans, the fluid is blood, and the plumbing is our veins, arteries and capillaries. The oxygen carrier in our blood is hemoglobin.

What is hemoglobin, and how does it work?

Hemoglobin is an incredible molecule, combining four long protein chains, four flat ring structures (known as heme) and four iron atoms, each held in the center of each heme ring. Blood’s characteristic red color comes from the combination of iron and heme. For hemoglobin to do its important work of transporting oxygen, it needs to not only pick up oxygen in the lungs, but also let go of it in the parts of the body where it is needed. It uses iron to bind the oxygen, and the rest of the hemoglobin molecule works as an efficient molecular machine to make sure the oxygen is bound loosely enough to be able to be dropped off at its destination.

The shape of the protein chains in hemoglobin control how much oxygen it binds or releases. When oxygen binds to the iron in one heme ring, it changes the shape of the four protein chains, making it easier for the other three heme groups to pick up oxygen atoms of their own (here’s a neat video that shows how this works). The protein chains are also sensitive to changes in pH and carbon dioxide concentrations, which allows oxygen to be released in actively metabolizing tissues, where the pH is low and the carbon dioxide concentration is high (like in active muscles).

Iron rusts by taking in oxygen, but hemoglobin is not like any iron nail. If you want to get the oxygen (or iron) out of the rust, you need to do some pretty fancy chemistry. But the hemoglobin molecule, on the other hand, is nature’s way of holding the iron in a delicate balance, where it is able to bind oxygen loosely and reversibly, without “rusting” permanently.

Hemoglobin is packaged into red blood cells, which are specialised delivery cells that, unlike most other cells in your body, do not contain any DNA! Human red blood cells get rid of their nucleus (where the DNA is) as they mature, which gives them more space for hemoglobin, and makes them small enough to fit through the smallest blood vessels. Red blood cells are shaped like a donut without a hole. Their shape gives them a high surface area, allowing lots of space for oxygen to pass through. They are very flexible, and can squeeze through tiny blood vessels that are roughly half their diameter.

Pretty much all animals that have backbones (vertebrates) use hemoglobin to carry oxygen around. But what about invertebrates? Let’s take a look at some alternative solutions they’ve developed for the problem of carrying oxygen. Spiders, crustaceans, octopuses, and squid, for instance, carry oxygen using a copper-based pigment known as hemocyanin, which makes their blood blue. This may seem weird, but it’s really not so surprising once you consider that creatures that look so different from us on the outside have different chemistry, too – hemoglobin is not the only answer! Unlike hemoglobin, hemocyanin is able to operate in low-oxygen environments and in the cold, making it perfect for deep sea creatures.

Some leeches and segmented worms use an iron-carrying pigment called chlorocrourin, which is particularly fun because it appears green when its diluted, and red when it is concentrated!

One more special case deserves a mention. There is, in fact, one creature that does have a backbone but doesn’t have hemoglobin. It’s the Antarctic icefish, which survives in icy cold water by simply dissolving oxygen in its blood without an oxygen carrier. This is only possible because cold water can carry a lot of dissolved oxygen, making it easier for the fish to capture it without an oxygen carrier like hemoglobin.

Our Blood Carries More Than Oxygen

Our blood carries lots of cells and molecules that perform functions, such as fighting infection, repairing blood vessels, and transporting chemical messages, such as hormones, around the body. Here are just a few of the different cells and molecules floating around in your blood right now:

White blood cells. These cells, also known as leukocytes, work on many fronts to destroy and repel invaders, keeping our bodies clear of infection. Some white blood cells produce specialized molecules called antibodies that tag viruses and bacteria for destruction.

Neutrophils. These are generally the first molecules to reach a site of infection. Along with macrophages (the “Pac-Men” of your blood) neutrophils engulf and digest bacteria, fungi and parasites. If you’ve ever looked at pus or seen that cloudy stuff that comes out of your nose when you’re sick, you’ve seen dead neutrophils: the aftermath of a bloody war against a foreign invader.

Platelets and clotting factors. When the circulatory system springs a leak (such as when you cut yourself) your body needs to patch the hole to stop you losing too much blood. This repair job belongs to platelets, which gather at any sites of damage and start the process of forming a clot. The blood also contains clotting factors that continue the process and ultimately form a scab.

All these cells and molecules travel around in a straw-colored liquid called plasma. Plasma makes up more than half your blood volume, helping the cells and molecules get around, a bit like how water gets you moving on a water slide. Donated plasma is used to make treatments for a number for immune deficiencies (by replacing antibodies), bleeding disorders (by replacing clotting factors) and other conditions.

If you’d like to donate blood, check out one of the websites below to find a location in your part of the planet:

USA:

Canada: https://blood.ca/en

Australia: http://www.donateblood.com.au/

UK: https://www.nhsbt.nhs.uk/donate/

New Zealand: https://www.nzblood.co.nz/

Author(s) Alison Gould Alison Gould (BSc (Hons) PhD FRACI CChem) is a Scientific Communications Specialist in the Research and Development Team at the Australian Red Cross Blood Service. You can follow her on Twitter @A2ali.