Composition of the Blood: Cells, Plasma, Values ​​and Function

It is composed of cells, fragments of cells and an aqueous solution (plasma).

Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to cells and transports metabolic waste products away from those same cells.

Composition of blood

It contains erythrocytes, leukocytes , thrombocytes (platelets) and plasma. The percentage of volume of all the blood cells in the whole blood is around 45% in adults. The rest consists of liquid plasma (for example, water, plasma proteins, electrolytes, etc.).

Blood represents 8% of human body weight, with an average density of around 1060 kg / m3, very close to the density of pure water of 1000 kg / m3.

The average adult has a blood volume of approximately 5 liters, which is composed of plasma and various cell types.

These blood cells that are also called corpuscles or “shaped elements” consist of erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets).

In volume, red blood cells constitute about 45% of the total blood, plasma about 54.3% and white blood cells about 0.7%.

Whole blood (plasma and cells) exhibits non-Newtonian fluid dynamics. If all human hemoglobin was free in the plasma instead of being contained in red blood cells, the circulatory fluid would be too viscous for the cardiovascular system to function effectively.


One microliter of blood contains:

Erythrocytes:  There are 4.7 to 6.1 million (in men) and 4.2 to 5.4 million (in women) of erythrocytes in the human body.

Red blood cells contain hemoglobin in the blood and distribute oxygen. Ripe red blood cells lack a nucleus and organelles in mammals. Red blood cells (along with endothelial vessel cells and other cells) are also marked by glycoproteins that define different types of blood.

The proportion of blood occupied by red blood cells is known as hematocrit , and is normally around 45%. The combined surface area of ​​all red blood cells in the human body would be approximately 2,000 times larger than the outer surface of the body.

Leukocytes:  In the human body there are 4,000 to 11,000 leukocytes. White blood cells are part of the body’s immune system, destroy and eliminate old and aberrant cells and cell debris, and attack infectious agents (pathogens) and foreign substances. Leukocyte cancer is called leukemia.

Thrombocytes:  The average human body hosts 200,000 to 500,000 thrombocytes. Also called platelets, they take part in the coagulation of the blood. The fibrin in the coagulation cascade creates a mesh over the platelet plug.


About 55% of the blood is blood plasma. Plasma is a fluid that is the liquid medium of the blood, which in itself is yellow. The total blood plasma volume of 2.7-3.0 liters in an average human.

It is essentially an aqueous solution that contains 92% water, 8% blood plasma proteins and traces of other materials.

Plasma circulates dissolved nutrients, such as glucose, amino acids and fatty acids (dissolved in the blood or attached to plasma proteins), and eliminates waste products, such as carbon dioxide, urea and lactic acid.

Other important components include:

  • Serum albumin
  • Blood coagulation factors (to facilitate coagulation).
  • Immunoglobulins (antibodies).
  • Lipoprotein particles.
  • Several other proteins.
  • Several electrolytes (mainly sodium and chloride).

The term serum refers to the plasma from which the coagulation proteins have been removed. Most of the remaining proteins are albumin and immunoglobulins.

PH values:

The pH of the blood is regulated to stay within the narrow range of 7.35 to 7.45, so it is slightly basic. Blood that has a pH below 7.35 is too acidic, while the blood pH above 7.45 is too basic.

The blood pH, the partial pressure of oxygen (pO2), the partial pressure of carbon dioxide (pCO2) and the bicarbonate (HCO3-) are carefully regulated by several homeostatic mechanisms, which exert their influence mainly through the respiratory system and the urinary system.

A gas test in the arterial blood measures them. The plasma also circulates hormones that transmit their messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

What is the function of the blood?

Blood is the most important means of transport in the human body. It transports gases (oxygen, carbon dioxide, nitrogen, etc.), as well as nutrients (metabolism) and final products of cellular metabolism. Therefore, the blood has the task of ensuring the exchange of substances.

The blood provides the tissues with blood gases and nutrients and, in turn, transports the final products (for example, carbon dioxide, urea, uric acid, creatinine, etc.) to the elimination organs (lung, liver, kidney). . In addition, it carries chemical messengers (hormones) to its target organs.