Why is blood red?
Blood is red because heme is red, that's all. Nature simply works in such a way that complex compounds of transition metals with organic and inorganic substances usually have some color. For example, many complex compounds of divalent copper are colored dark blue; complex compound of ferric iron and cyanide in aqueous solution has a yellow color, and with thiocyanate it is red. And the complex compound of ferrous iron with porphyrin (heme) is colored red. This is how the distribution of valence electrons of this compound developed over energy levels. And it so happened that it is heme that is able to reversibly add molecular oxygen (without the formation of iron oxide!) and carbon oxides, and its red color is only indirectly related to this property. To convert heme iron into oxide, the heme must be irreversibly destroyed. Ferrous oxide is black, insoluble in water and incapable of giving up oxygen just like that. If BestFriend believes that by binding to oxygen, heme iron is oxidized to trivalent iron, then this is also not true. Ferric oxide has a brown-red (or brick-red) color, closer to the color of venous blood, while oxygen-enriched hemoglobin is bright scarlet. Ferric oxide is also insoluble in water, and is also incapable of giving up oxygen just like that. And also, for it to form, heme must be irreversibly destroyed. And the transformation of heme iron into trivalent iron (occurs in some poisonings) leads to the loss of heme’s ability to carry oxygen. Let me emphasize that oxygen bound in a complex with hemoglobin retains its molecular form, without oxidizing anything in hemoglobin.
The fact is that blood contains red blood cells. They, in turn, carry oxygen throughout the body. And the fact is that red blood cells or hemoglobin contain, or rather contain, divalent iron, which attaches oxygen and, together with hemoglobin, is carried by the blood to nourish the cells. But the iron salts in hemoglobin are red in color. and it is arterial blood that is rich in oxygen and brighter in color, while venous blood is darker. Of course, this process is very complex to be explained only from a chemistry perspective. But everyone knows that those who have little hemoglobin in their blood need to consume foods rich in iron.
In order to understand why blood is red, you need to understand its composition.
Blood consists of plasma and formed elements: leukocytes, platelets and erythrocytes.
Leukocytes and platelets are colorless.
Red blood cells contain hemoglobin, a red pigment that gives blood its red color.
BestFriend explained everything correctly, all that remains is to add what he kept silent about.
Hemoglobin is contained in special blood cells - red blood cells. This is a necessary condition for the transfer of oxygen into the cells of the body and its release for the oxidation of nutrients (ultimately, obtaining energy for life). Outside of red blood cells, hemoglobin is able to bind oxygen, but gives it away very reluctantly, only under the influence of enzymes. But why reinvent the wheel if everything the necessary conditions already created in red blood cells?
It is red blood cells that give blood its red color. Especially the arterial one, which is enriched with oxygen (it is bright red and opaque). But venous blood, if you look at it in a test tube, looks like cherry jam diluted with water. The secret of the trick is simple: red blood cells, having given oxygen to the cells, lose color, and also somewhat decrease in size, and go through the veins to the second circle - for a new portion of oxygen from the lungs.
That's why arterial bleeding Any person can distinguish from venous: bright red blood- from an artery, dark red - from a vein.
The leaves could have been of other colors if not for an accident during their evolution. There are also non-green plants in the world, but it just so happens that it is the green ones that have spread.
And blood doesn’t have to be red either, blue also exists, due to the content
What color is bloody? For most, the color of blood is associated with red.Red blood- uh it’s familiar and obvious.
However, red is not the only possible color blood. Blood can be blue, green, purple, and even colorless - all due to specific chemical substances, which are part of the blood of different organisms.
Hemoglobin and red color of blood
Most people know that human blood, like most other vertebrates, is red due to hemoglobin, which contains iron atoms in its structure.
Hemoglobin is also known as the respiratory pigment, and it plays important role in the body, transporting oxygen throughout the body to our cells, and also helps to take carbon dioxide from the tissues and “throw” it back into the lungs.
The large protein hemoglobin is made up of four small blocks that contain small regions called hemes, each containing an iron atom.
Heme, which contains a divalent iron atom that can attach or donate an oxygen molecule. In this case, the valence of iron, to which oxygen is attached, does not change.
It is thanks to this divalent iron oxide (Fe2+) hemoglobin turns red.All vertebrates, some species of insects and mollusks have iron oxide in their blood protein, and therefore their blood is red.
Blood of a different color
Red is not the only blood color possible in nature. And this is due to the fact that some living beings’ red blood cells contain not hemoglobin, but other iron-containing proteins.
Purple Blood
This is observed in some species of invertebrates, in particular mollusks.
Their blood contains protein hemerythrin, which is a respiratory pigment in the blood and contains five times more iron than hemoglobin. Hemerythrin, saturated with oxygen, gives the blood a purple tint, and when it gives oxygen to the tissues, such blood becomes pink.
Green blood
Another iron-containing protein is chlorocruorin- gives blood and tissue fluid green color. This protein is dissolved in the blood plasma and is close in composition to hemoglobin, but the iron in it is not oxide, as in the blood of mammals, but ferrous. That's why the color turns out green.
Blue blood
However, the color range of the blood of living beings is not limited to red, purple and green. For example, octopuses, octopuses, spiders, crabs and scorpions - blue bloods in the most literal sense. The reason is that in these animals and insects the respiratory pigment of the blood is not hemoglobin, but
Blood forms the basis of the life of a living organism. Circulating through the system of blood vessels, veins and arteries, it transports oxygen and substances necessary for metabolism or resulting from it to various organs. metabolic processes.
But the functions of blood are not limited to transporting nutrients and metabolic products. Blood regulates body temperature and transports hormones responsible for vital processes; protects the body from infections and damage.
What is blood for: basic functions
Almost all processes in the body related to respiration and digestion are associated with blood supply. It is the blood that transports oxygen from the lungs to the tissues, and carbon dioxide from the tissues and organs to the lungs. The secretion products of endocrine glands - hormones - are transported throughout the body with the blood, and this ensures coordination between various organs.
Nutrients from small intestine Through capillaries, thanks to the blood, they travel from the digestive tract to the liver. This is where the modification happens fatty acids, glucose, amino acids and regulation of their quantity, depending on what the body needs for this moment to a greater extent. 
Further, the transported substances reach their “destination points” through tissue capillaries. End products enter the blood from tissues, which are then excreted from the body, for example, in urine.
In warm-blooded organisms, blood plays a primary role in the process of maintaining optimal body temperature, or thermoregulation. In different parts of the body, the absorption and release of heat must be balanced, and this balance is made possible precisely because the blood carries heat.
The main center of thermoregulatory processes is located in the brain - the hypothalamus, which is sensitive to changes in the temperature of the blood passing through it. The hypothalamus regulates the processes by which heat is released or absorbed.
For example, heat loss can be adjusted by changing the diameter blood vessels skin, which, in turn, changes the volume of blood flowing near the surface of the body (and this is where heat is lost most easily).
About the color of blood
Blood is a liquid, the fluidity of which is determined by its viscosity and the nature of the movement of its components. Blood viscosity depends on the number of red blood cells and proteins it contains, and affects the speed of blood movement and blood pressure.
Blood consists of pale yellow plasma, which contains three types of cellular elements: red blood cells (erythrocytes), white cells (leukocytes), and platelets (platelets). The total volume of blood in the body of an adult male is about five liters, with most of it being plasma and the rest mostly red blood cells. Red blood cells contain the pigment hemoglobin, which gives blood its red color.
Main function red blood cells transport oxygen, and hemoglobin plays a key role in this process. Hemoglobin is an organic pigment that contains a compound of porphyrin with iron (heme) and the protein globin.
It is known that the blood in the arteries and veins has different shades: venous blood is dark, arterial blood is bright scarlet. This happens because the arteries carry blood from the heart and lungs and are saturated with oxygen. And through the veins, blood from tissues and organs flows to the heart, the hemoglobin in this blood is almost devoid of oxygen, which is why it has dark color.
Can blood be a different color?
Of course it can. For example, the blood of octopuses, scorpions, crayfish, spiders are blue, because instead of hemoglobin it contains hemocyanin, and in it the metal is not iron, but copper. 
If iron colors human blood red, copper gives the blood of octopuses and other organisms a blue or blue tint. By the way, when octopus blood is saturated with oxygen, it darkens, and in the veins, on the contrary, it turns pale.
And in nature there are sea worms whose blood is green. It gets this color thanks to the ferrous iron it contains.
Surely every person has asked the question: “Why is blood red?” To get the answer, you need to consider what it consists of.
Compound
Blood is a rapidly renewing connective tissue, which circulates throughout the body and carries gases and substances necessary for metabolism. It consists of a liquid part, called plasma, and formed elements - blood cells. Normally, plasma makes up about 55% of the total volume, cells – about 45%.
Plasma
This pale yellow liquid performs very important functions. Thanks to plasma, cells suspended in it can move. It consists of 90% water, the remaining 10% are organic and inorganic components. Plasma contains microelements, vitamins, and intermediate metabolic elements.
Cages
There are three types of shaped elements:
- leukocytes are white cells that perform protective function, protecting the body from internal diseases and foreign agents penetrating from the outside;
- platelets - small colorless plates responsible for coagulation;
- Red blood cells are the same cells that make blood red.
Red blood cells give blood its red color
These cells, called red blood cells, make up most of the formed elements - more than 90%. Their main function is to transfer oxygen from the lungs to peripheral tissues and carbon dioxide from tissues to the lungs for further removal from the body. Red blood cells are continuously produced in bone marrow. Their lifespan is about four months, after which they are destroyed in the spleen and liver.
The red color of red blood cells is given by the hemoglobin protein found in them, which is capable of reversibly binding to oxygen molecules and transporting them to tissues.
The color of blood varies depending on whether it flows from the heart or to the heart. The blood that comes from the lungs and then travels through the arteries to the organs is saturated with oxygen and has a bright scarlet color. The fact is that hemoglobin in the lungs binds oxygen molecules and turns into oxyhemoglobin, which has a light red color. Upon entering the organs, oxyhemoglobin releases O₂ and turns back into hemoglobin. In peripheral tissues, it binds carbon dioxide, takes the form of carbohemoglobin and darkens. Therefore, the blood flowing through the veins from the tissues to the heart and lungs is dark, with a bluish tint.
An immature red blood cell contains little hemoglobin, so at first it is blue, then becomes gray, and only when ripe it becomes red.
Hemoglobin
This is a complex protein that includes a pigment group. One third of the red blood cell consists of hemoglobin, which makes the cell red.
Hemoglobin consists of a protein - globin, and a non-protein pigment - heme, containing ferrous ion. Each hemoglobin molecule includes four hemes, which account for 4% of the total mass of the molecule, while globin accounts for 96% of the mass. the main role in the activity of hemoglobin belongs to the iron ion. To transport oxygen, heme reversibly binds to the O₂ molecule. Ferrous oxide is what gives blood its red color.
Instead of a conclusion
The blood of humans and other vertebrates is red due to the iron-containing protein hemoglobin.. But there are living beings on Earth whose blood contains other types of protein, and therefore its color is different. In scorpions, spiders, octopuses, and crayfish, it is blue because it contains the protein hemocyanin, which includes copper, which is responsible for the shade. In sea worms, the blood protein contains ferrous iron, which is why it is green.
Although Valentine's Day has led us to believe completely different information, our heart actually has a dull Brown color. Why is blood red? Let's find out the real reason.
The most pressing question for many
There are many organs in our body that have the most different colors. For example, we have bright pink lungs, a brown liver and a gray brain. And by the way, red blood flows through your veins and arteries. Each of us has probably wondered more than once why blood is red. We have the answers for you.
What is blood really?
Human blood is not just a liquid. It contains a lot of different elements that spread nutrients throughout the body and fill our tissues with oxygen. Our blood mainly consists of plasma, in which blood cells are suspended ( shaped elements), and any substances that are transferred (besides oxygen) dissolve here. Plasma is the most important component of this important liquid and is very pale in color with a yellow tint. But as soon as the formed elements dissolve in it, it sharply changes its color and becomes slightly cloudy. The most common type of blood cell found in plasma is red blood cells, which contain a protein called hemoglobin.
What is the truth about the color of blood?
The generally accepted opinion is that it is iron, which can be found in hemoglobin, that gives our blood this red color, but everyone who believes so is very much mistaken. The red color is formed due to heme, a special pigment that is part of hemoglobin and contains iron ions. Oxygen, in turn, combines with iron, and it is this interaction that makes our blood red. The other components of a blood cell do not affect its color in any way.
Light or dark?
If hemoglobin contains high level oxygen, then it will reflect certain wave lines of light, absorbing all others, and thereby give the blood a bright red color. If it contains less oxygen, the reflected waves will be slightly different, the blood will become slightly darker.
What about blue blood?
As for people of aristocratic origin, the so-called blue-blooded individuals, they have almost the same red liquid as everyone else. But with hypoxia (dangerous low level oxygen in the blood) the wavelengths of reflected light reach the violet hue at the end of the spectrum. And then you can see blue veins through the skin.
