Blood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and

oxygen — and transports waste products away from those same cells.

In vertebrates, it is composed of blood cells suspended in a liquid called blood plasma. Plasma, which comprises 55%
of blood fluid, is mostly water (90% by volume), and contains dissolved proteins, glucose, mineral ions, hormones,
carbon dioxide (plasma being the main medium for excretory product transportation), platelets and blood cells
themselves. The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white
blood cells, including leukocytes and platelets. The most abundant cells in vertebrate blood are red blood cells. These
contain hemoglobin, an iron-containing protein, which facilitates transportation of oxygen by reversibly binding to
this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely
transported extracellularly dissolved in plasma as bicarbonate ion.

Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to
resist infections and parasites. Platelets are important in the clotting of blood. Arthropods, using hemolymph, have
hemocytes as part of their immune system.

Blood performs many important functions within the body including:

 Supply (abastecimiento) of oxygen to tissues (tejidos) (bound to hemoglobin, which is carried in red cells)
 Supply of nutrients such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma
proteins (e.g., blood lipids)
 Removal of waste such as carbon dioxide, urea, and lactic acid
 Immunological functions, including circulation of white blood cells, and detection of foreign material by
antibodies
 Coagulation, which is one part of the body's self-repair mechanism
 Messenger functions, including the transport of hormones and the signaling of tissue damage
 Regulation of body pH (the normal pH of blood is in the range of 7.35 - 7.45).
 Regulation of core body temperature
 Hydraulic functions

Blood accounts for 7% of the human body weight, with an average density of approximately 1060 kg/m 3, very close to
pure water's density of 1000 kg/m3. The average adult has a blood volume of roughly 5 liters, composed of plasma
and several kinds of cells; these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white
blood cells), and thrombocytes (platelets). By volume, the red blood cells constitute about 45% of whole blood, the
plasma constitutes about 54.3%, white cells constitute 0.7%.

CELLS

One microliter of blood contains:

 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes: They contain the blood's hemoglobin and
distribute oxygen. The red blood cells (together with endothelial vessel cells and other cells) are also marked
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 by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is
referred to as the hematocrit, and is normally about 45%. The combined surface area of all red blood cells of
the human body would be roughly 2,000 times as great as the body's exterior surface.
 4,000-11,000 leukocytes: White blood cells are part of the immune system; they destroy and remove old or
aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. The
cancer of leukocytes is called leukemia.
 200,000-500,000 thrombocytes: thrombocytes, also called platelets, are responsible for blood clotting
(coagulation). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect
and clot, which then stops more blood from leaving the body and also helps to prevent bacteria from
entering the body.

PLASMA

About 55% of whole blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in
color. The blood plasma volume totals of 2.7 – 3.0 litres in an average human. It is essentially an aqueous solution
containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Plasma circulates dissolved
nutrients, such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins), and
removes waste products, such as carbon dioxide, urea, and lactic acid.

Other important components include:

 Serum albumin
 Blood-clotting factors (to facilitate coagulation)
 Immunoglobulins (antibodies)
 lipoprotein particles
 Various other proteins
 Various electrolytes (mainly sodium and chloride)

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

The normal pH of human arterial blood is approximately 7.40 (normal range is 7.35 – 7.45), a weakly alkaline solution.
Blood that has a pH below 7.35 is too acidic, whereas blood pH above 7.45 is too alkaline. Blood pH, partial pressure
of oxygen (pO2), partial pressure of carbon dioxide (pCO 2), and HCO3 are carefully regulated by a number of
homeostatic mechanisms, which exert their influence principally through the respiratory system and the urinary
system in order to control the acid-base balance and respiration. Plasma also circulates hormones transmitting their
messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

TRANSPORTE DE OXÍGENO

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is
chemically combined with the Hgb. About 1.5% is physically dissolved in the other blood liquids and not connected to
Hgb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species.

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With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated
blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed;
afterwards, venules, and veins carry deoxygenated blood back to the heart.

Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98-99% saturated with
oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated.
Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can
reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen
saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered
dangerous in an individual at rest (for instance, during surgery under anesthesia. Sustained hypoxia (oxygenation of
less than 90%), is dangerous to health, and severe hypoxia (saturations of less than 30%) may be rapidly fatal.

A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found
in an adult's lungs), and, so, fetuses produce another form of hemoglobin with a much higher affinity for oxygen
(hemoglobin F) in order to function under these conditions.

Carbon dioxide transport

When blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is
dissolved in the blood. Some carbon dioxide reacts with hemoglobin and other proteins to form carbamino
compounds. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions through the action of RBC
carbonic anhydrase. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Carbon dioxide (CO2), the main cellular waste product is carried in blood mainly dissolved in plasma, in equilibrium
with bicarbonate (HCO3-) and carbonic acid (H2CO3). 86%-90% of CO2 in the body is converted into carbonic acid,
which can quickly turn into bicarbonate, the chemical equilibrium being important in the pH buffering of plasma.
Blood pH is kept in a narrow range (pH between 7.35-7.45).

HEMOGLOBIN

Hemoglobin is the principal determinant of the color of blood in vertebrates. Each molecule has four heme groups,
and their interaction with various molecules alters the exact color. In vertebrates and other hemoglobin-using
creatures, arterial blood and capillary blood are bright-red, as oxygen imparts a strong red color to the heme group.
Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen during blood donation and
when venous blood samples are taken. Blood in carbon monoxide poisoning is bright-red, because carbon monoxide
causes the formation of carboxyhemoglobin. In cyanide poisoning, the body cannot utilize oxygen, so the venous
blood remains oxygenated, increasing the redness. While hemoglobin-containing blood is never blue, there are
several conditions and diseases wherein the color of the heme groups make the skin appear blue. If the heme is
oxidized, methaemoglobin, which is more brownish and cannot transport oxygen, is formed. In the rare condition
sulfhemoglobinemia, arterial hemoglobin is partially oxygenated, and appears dark-red with a bluish hue (cyanosis).
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