Blood

Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells.
Blood is composed of blood cells suspended in plasma. Plasma, which constitutes 55% of blood fluid, is mostly water, and contains proteins, glucose, mineral ions, and hormones. The blood cells are mainly red blood cells, white blood cells, and platelets. The most abundant cells are red blood cells. These contain hemoglobin, which facilitates oxygen transport by reversibly binding to it, increasing its solubility. 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.
Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled. Blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated.
Medical terms related to blood often begin with hemo-, hemato-, haemo- or haemato- from the Greek word αἷμα for "blood". In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its composition of cells and cell fragments suspended in plasma.

Functions

Blood performs many important functions within the body, including:

Supply of oxygen to tissues
Supply of nutrients such as glucose, amino acids, and fatty acids
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, the response to a broken blood vessel, the conversion of blood from a liquid to a semisolid gel to stop bleeding
Messenger functions, including the transport of hormones and the signaling of tissue damage
Regulation of core body temperature
Hydraulic functions
Constituents

In mammals

Blood accounts for 7% of the human body weight, with an average density around 1060 kg/m³, very close to pure water's density of 1000 kg/m³. The average adult has a blood volume of roughly or 1.3 gallons, which is composed of plasma and formed elements. The formed elements are the two types of blood cell or corpuscle – the red blood cells, and white blood cells – and the cell fragments called platelets that are involved in clotting. By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.
Whole blood exhibits non-Newtonian fluid dynamics.

Cells

One microliter of blood contains:

4.7 to 6.1 million, 4.2 to 5.4 million erythrocytes: Red blood cells contain the blood's hemoglobin and distribute oxygen. Mature red blood cells lack a nucleus and organelles in mammals. The red blood cells are also marked 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 body's immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents and foreign substances. The cancer of leukocytes is called leukemia.
200,000–500,000 thrombocytes: Also called platelets, they take part in blood clotting. Fibrin from the coagulation cascade creates a mesh over the platelet plug.

Parameter	Value	Refs.
Hematocrit	45 ± 7 for males 42 ± 5 for females
pH	7.35–7.45
base excess	−3 to +3
PO₂	10–13 kPa
PCO₂	4.8–5.8 kPa
HCO₃⁻	21–27 mM
Oxygen saturation	Oxygenated: 98–99% Deoxygenated: 75%

Plasma

About 55% of blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. The total blood plasma volume in an average human is 2.7–3.0 liters. 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, and removes waste products, such as carbon dioxide, urea, and lactic acid.
Other important components include:

Serum albumin
Blood-clotting factors
Immunoglobulins
lipoprotein particles
Various other proteins
Various electrolytes

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

Acidity

is regulated to stay within the narrow range of 7.35 to 7.45, making it slightly basic. Extra-cellular fluid in blood that has a pH below 7.35 is too acidic, whereas blood pH above 7.45 is too basic. A pH below 6.9 or above 7.8 is usually lethal. Blood pH, partial pressure of oxygen, partial pressure of carbon dioxide, and bicarbonate are carefully regulated by a number of homeostatic mechanisms, which exert their influence principally through the respiratory system and the urinary system to control the acid–base balance and respiration, which is called compensation. An arterial blood gas test measures these. Plasma also circulates hormones transmitting their messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

In non-mammals

Human blood is typical of that of mammals, although the precise details concerning cell numbers, size, protein structure, and so on, vary somewhat between species. In non-mammalian vertebrates, however, there are some key differences:

Red blood cells of non-mammalian vertebrates are flattened and ovoid in form, and retain their cell nuclei.
There is considerable variation in the types and proportions of white blood cells; for example, acidophils are generally more common than in humans.
Platelets are unique to mammals; in other vertebrates, small nucleated, spindle cells called thrombocytes are responsible for blood clotting instead.
Physiology

Circulatory system

Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and venous blood carries carbon dioxide, a waste product of metabolism by cells, to the lungs to be exhaled. However, one exception includes pulmonary arteries, which contain the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.
Additional return flow may be generated by the movement of skeletal muscles, which can compress veins and push blood through the valves in veins toward the right atrium.
The blood circulation was described by William Harvey in 1628.

Cell production and degradation

In vertebrates, the various cells of blood are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone, the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of T lymphocytes.
The proteinaceous component of blood is produced predominantly by the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney.
Healthy erythrocytes have a plasma life of about 120 days before they are degraded by the spleen, and the Kupffer cells in the liver. The liver also clears some proteins, lipids, and amino acids. The kidney actively secretes waste products into the urine.

Oxygen transport

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 hemoglobin. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species. Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O₂ per gram hemoglobin, which increases the total blood oxygen capacity seventyfold, compared to if oxygen solely were carried by its solubility of 0.03 ml O₂ per liter blood per mm Hg partial pressure of oxygen.
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 adult humans at rest, hemoglobin in blood leaving the lungs is about 98–99% saturated with oxygen, achieving an oxygen delivery between 950 and 1150 ml/min to the body. In a healthy adult at rest, oxygen consumption is approximately 200–250 ml/min, and deoxygenated blood returning to the lungs is still roughly 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. Sustained hypoxia, is dangerous to health, and severe hypoxia may be rapidly fatal.
A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures, so fetuses produce another form of hemoglobin with a much higher affinity for oxygen to function under these conditions.