Circulatory system Circulation of<unk>  through the human heart<unk>  is circulated around the body through<unk>  vessels by the pumping action of the heart. In humans,<unk>  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.<unk>  then enters the left ventricle to be circulated again. Arterial<unk>  carries oxygen from inhaled air to all of the cells of the body, and venous<unk>  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<unk>  in the body, while the pulmonary veins contain oxygenated<unk> . Additional return flow may be generated by the movement of skeletal muscles, which can compress veins and push<unk>  through the valves in veins toward the right atrium. The<unk>  circulation was famously described by William Harvey in 1628. Cell production and degradation In vertebrates, the various cells of<unk>  are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red<unk>  cells; and myelopoiesis, the production of white<unk>  cells and platelets. During childhood, almost every human bone produces red<unk>  cells; as adults, red<unk>  cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), 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<unk>  (including clotting proteins) 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 left About 98.5% of the oxygen in a sample of arterial<unk>  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<unk>  liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below). Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O2 per gram hemoglobin, which increases the total<unk>  oxygen capacity seventyfold, compared to if oxygen solely were carried by its solubility of 0.03 ml O2 per liter<unk>  per mm Hg partial pressure of oxygen (about 100 mm Hg in arteries). With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated<unk>  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<unk>  back to the heart. Under normal conditions in adult humans at rest, hemoglobin in<unk>  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<unk>  returning to the lungs is still roughly 75% (70 to 78%) saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous<unk> , which can reach less than 15% in a trained athlete; although breathing rate and<unk>  flow increase to compensate, oxygen saturation in arterial<unk>  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 less than 90%), is dangerous to health, and severe hypoxia (saturations 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), so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) to function under these conditions. Carbon dioxide transport CO2 is carried in<unk>  in three different ways. (The exact percentages vary depending whether it is arterial or venous<unk> . Most of it (about 70%) is converted to bicarbonate ions by the enzyme carbonic anhydrase in the red<unk>  cells by the reaction CO2 + H2O → H2CO3 → H+ + ; about 7% is dissolved in the plasma; and about 23% is bound to hemoglobin as carbamino compounds. Hemoglobin, the main oxygen-carrying molecule in red<unk>  cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the<unk>  due to increased oxygen levels is known as the Haldane effect, and is important in the transport of carbon dioxide from the tissues to the lungs. A rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect. Transport of hydrogen ions Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin. Lymphatic system In mammals,<unk>  is in equilibrium with lymph, which is continuously formed in tissues from<unk>  by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the thoracic duct, which drains into the left subclavian vein, where lymph rejoins the systemic<unk>  circulation. Thermoregulation<unk>  circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing<unk>  flow to the surface (e.g., during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss. In contrast, when the external temperature is low,<unk>  flow to the extremities and surface of the skin is reduced and to prevent heat loss and is circulated to the important organs of the body, preferentially. Rate of flow Rate of<unk>  flow varies greatly between different organs. Liver has the most abundant<unk>  supply with an approximate flow of 1350 ml/min. Kidney and brain are the second and the third most supplied organs, with 1100 ml/min and ~700 ml/min, respectively. Relative rates of<unk>  flow per 100 g of tissue are different, with kidney, adrenal gland and thyroid being the first, second and third most supplied tissues, respectively. Hydraulic functions The restriction of<unk>  flow can also be used in specialized tissues to cause engorgement, resulting in an erection of that tissue; examples are the erectile tissue in the penis and clitoris. Another example of a hydraulic function is the jumping spider, in which<unk>  forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs. Invertebrates In insects, the<unk>  (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called tracheae allow oxygen from the air to diffuse directly to the tissues.) Insect<unk>  moves nutrients to the tissues and removes waste products in an open system. Other invertebrates use respiratory proteins to increase the oxygen-carrying capacity. Hemoglobin is the most common respiratory protein found in nature. Hemocyanin (blue) contains copper and is found in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigment (bright-green, blue, or orange). In many invertebrates, these oxygen-carrying proteins are freely soluble in the<unk> ; in vertebrates they are contained in specialized red<unk>  cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging<unk>  filtering organs like the kidneys. Giant tube worms have unusual hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals.
Blood
Serum 
Body fluid
Venous blood
Urine
Cord blood
Feces
Arterial blood