Blood plays the role of a transportation system in our body. When pumped by the heart, blood delivers oxygen from the air we breathe, and all the nutrients from the food we eat, to all the cells of the body.
The blood also keeps the cells clean and healthy because it carries away waste products from the cells that are produced after using oxygen and nutrients. To regulate various processes in our body, glands produce hormones, and it is the blood that carries these hormones throughout the body. Blood also carries heat throughout the body.
A watery liquid like plasma– makes up more than half of the blood in the body. Plasma contains metabolic products, nutrients, and also substances and chemical compounds, which are so necessary for blood clotting.
Tiny cells make up the rest of the blood. Red blood cells such as red blood cells carry oxygen throughout the body and remove carbon dioxide from the lungs. red blood cells. White blood cells - leukocytes, are the remaining elements of blood. Leukocytes destroy pathogens that enter our body, thereby protecting us from all kinds of infections. 
Although red blood cells are the smallest cells in our body, a drop of blood contains approximately 5 million red blood cells, 10 thousand white blood cells, and 250 thousand platelets. Platelets are responsible for the formation of a blood clot in the place where the blood vessel is damaged.
There are only four blood groups: 0, A, B, AB. Every person's blood belongs to one of these groups.
The protein found in the blood is called hemoglobin. 
Hemoglobin is found in red blood cells and contains iron, and because of this, our blood is red. Sometimes our blood is dark red, and sometimes it is bright red. Changing the amount of oxygen in our blood explains the difference in color.
Types of blood vessels called arteries carry blood from the heart and lungs to the rest of the organs. Such blood is saturated with oxygen, which, when combined with hemoglobin, gives the blood its bright red color.
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 colour; 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 exactly arterial blood rich in oxygen and brighter in color, and venous 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 shaped 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.
Therefore, anyone can distinguish arterial bleeding from venous bleeding: 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
Science knows that different living organisms on the planet have different blood colors.
However, in humans it is red. Why is blood red? This question is asked by both children and adults.
The answer is quite simple: the red color is due to hemoglobin, which contains iron atoms in its structure.
What makes blood red is hemoglobin, which consists of:
- From a protein called globin;
- The non-protein element heme, which contains the ferrous ion.
It was possible to find out what gives the red color, but its elements turn out to be no less interesting. What elements give it this color is an equally interesting aspect.
Blood contains:
- Plasma. The liquid is light yellow in color, with its help the cells in its composition can move. It is composed of 90 percent water, with the remaining 10 percent made up of organic and inorganic components. Plasma also contains vitamins and microelements. The light yellow liquid contains many useful substances.
- The formed elements are blood cells. There are three types of cells: white blood cells, platelets and red blood cells. Each type of cell has certain functions and characteristics.
These are white cells that protect the human body. They protect him from internal diseases and foreign microorganisms penetrating from outside.
This is a white element in color. Its white hue is impossible not to notice during laboratory research, so such cells are determined quite simply.
White blood cells recognize foreign cells that can cause harm and destroy them.
These are very small colored plates whose main function- folding.
These cells are responsible for ensuring that the blood:
- It coagulated and did not flow out of the body;
- Coagulates quite quickly on the surface of the wound.
More than 90 percent of these cells are in the blood. It is also red because red blood cells have this hue.
They carry oxygen from the lungs to peripheral tissues and are continuously produced in bone marrow. They live for about four months, then are destroyed in the liver and spleen.
It is very important for red blood cells to carry oxygen to various tissues of the human body.
Few people know that immature red blood cells are blue, then acquire a gray tint and only then become red.
There are quite a lot of human red blood cells, which is why oxygen reaches peripheral tissues so quickly.
It is difficult to say which element is more significant. Each of them has an important function that affects human health.
Children often ask questions regarding the components of the human body. Blood is one of the most popular topics of discussion.
Explanations for children should be extremely simple, but at the same time informative. Blood contains many substances that differ in function.
Consists of plasma and special cells:
- Plasma is a liquid that contains useful substances. It has a light yellow tint.
- The formed elements are erythrocytes, leukocytes and platelets.
The presence of red cells - erythrocytes - explains its color. Red blood cells are red by nature, and their accumulation leads to the fact that a person’s blood is exactly this color.
There are about thirty-five billion red cells that move throughout the human body in the blood vessels.
Why are veins blue
The veins carry burgundy blood. They are red, like the color of the blood that flows through them, but not blue. The veins only appear blue.
This can be explained by the law of physics about the reflection of light and perception:
When a ray of light hits the body, the skin reflects some of the waves and looks light. However, it transmits the blue spectrum much worse.
The blood itself absorbs light of all wavelengths. The skin gives a blue color for visibility, and the vein is red.
Human brain compares color blood vessel against warm skin tone, resulting in blue.
Blood of a different color in various living creatures
Not all living organisms have red blood.
The protein that gives this color in humans is hemoglobin, contained in hemoglobin. Other living beings have other fat-containing proteins instead of hemoglobin.
The most common shades besides red are:
- Blue. Crustaceans, spiders, mollusks, octopuses and squids boast this color. AND blue blood It has great value for these creatures, as it is filled important elements. Instead of hemoglobin, it contains hemocyanin, which contains copper.
- Violet. This color is found in marine invertebrates and some mollusks. Typically, such blood is not only purple, but also slightly pink. Color pink blood in young invertebrate organisms. IN in this case protein - hemerythrin.
- Green. Found in annelids and leeches. The protein is chlorocruorin, close to hemoglobin. However, iron in this case is not oxide, but ferrous.
The color of blood varies depending on the protein it contains. Whatever color the blood is, it has a huge amount useful substances necessary for a living organism. Pigment is important for every organism, despite its diversity.
Video - Secrets and mysteries of our blood
Blood is a combination of many substances - plasma and formed elements. Each element has strictly defined functions and tasks; certain particles also have a pronounced pigment, which determines the blood color. Why is human blood red? The pigment is contained in red hemoglobin; it is part of the red blood cell. It is for this reason that there are organisms on Earth (scorpions, spiders, monkfish) whose blood color is blue or green. Their hemoglobin is dominated by copper or iron, which gives the characteristic color of the blood.
To understand all these elements, it is necessary to understand.
Compound
Plasma
As already indicated, one of them is plasma. It takes up about half of the blood composition. Blood plasma turns blood into a liquid state, has a light yellow color and is slightly denser in properties than water. The density of plasma is provided by the substances dissolved in it: salts, fats, carbohydrates and other elements.
Shaped elements
Another component of blood is the formed elements (cells). They are represented by red blood cells blood bodies, - white blood cells, platelets - blood platelets. It is red blood cells that answer the question why blood is red.
At the same time circulatory system about 35 billion red blood cells move around. Appearing in the bone marrow, hemoglobin is formed - this is a red pigment, saturated with protein and iron. The task of hemoglobin is to deliver oxygen to vital parts of the body and remove carbon dioxide. Red blood cells live on average 4 months, then they disintegrate in the spleen. The process of formation and breakdown of red blood cells is continuous.
Hemoglobin
The blood, enriched with oxygen in the lungs, disperses to the vital organs of the body. At this moment it has a bright scarlet color. This occurs due to bonding with oxygen, resulting in oxyhemoglobin. As it passes through the body, it distributes oxygen and becomes hemoglobin again. Next, hemoglobin absorbs carbon dioxide from tissues and is transformed into carbohemoglobin. At this moment, the color of the blood changes to dark red. Immature red blood cells also have a bluish tint; as they grow, they then become colored grey colour and then turn red.
The color of the blood may vary. Answers to the questions why blood is dark red or bright red. Different shade A person's blood receives depending on whether it moves towards the heart or away from it.
Very often people wonder why veins are blue and blood is red? The fact is that venous blood is the blood that flows through the veins to the heart. This blood is saturated with carbon dioxide and deprived of oxygen, has lower acidity, contains less glucose and significantly more final metabolic products. In addition to being dark red, venous blood also has a bluish, blue tint. However, not so strong as to “stain” the veins blue.
Why is blood red? It's all about the process of passing light rays and the ability of bodies to reflect or absorb solar rays. The beam must pass through the skin to reach venous blood, fat layer, the vein itself. Sunbeam consists of 7 colors, three of which blood reflects (red, blue, yellow), the remaining colors are absorbed. Reflected rays pass through tissues a second time to enter the eye. At this moment, red rays and low-frequency light will be absorbed by the body, and blue light will be transmitted. We hope that we have answered why a person has dark red and bright red blood.
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 the 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, crayfish it is blue because it contains the protein hemocyanin, which includes copper, which is responsible for the hue. In sea worms, the blood protein contains ferrous iron, which is why it is green.
