question: How does oxygen reach the brain?
comment: The red cells in your blood pick up oxygen in your lungs, then the heart pumps the blood around your body, including to the brain.
passages: Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin. The oxygen is held on the hemoglobin by four ferrous iron-containing heme groups per hemoglobin molecule. When all the heme groups carry one O molecule each the blood is said to be “saturated” with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase the oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as bicarbonate ions (HCO) in the plasma. However the conversion of dissolved CO into HCO (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and through alveolar capillaries on the other. The reaction is therefore catalyzed by carbonic anhydrase, an enzyme inside the red blood cells. The reaction can go in both directions depending on the prevailing partial pressure of CO. A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as carbamino groups. The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO, and carbamino groups) in arterial blood (i.e. after it has equilibrated with the alveolar air) is about 26 mM (or 58 ml/100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml/100 ml blood).
   Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin. The oxygen is held on the hemoglobin by four ferrous iron-containing heme groups per hemoglobin molecule. When all the heme groups carry one O molecule each the blood is said to be “saturated” with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase the oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as HCO ions in the plasma. However the conversion of dissolved CO into HCO (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and alveolar capillaries on the other. The reaction is therefore catalyzed by carbonic anhydrase, an enzyme inside the red blood cells. The reaction can go in either direction depending on the prevailing partial pressure of carbon dioxide. A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as carbamino groups. The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO, and carbamino groups) in arterial blood (i.e. after it has equilibrated with the alveolar air) is about 26 mM (or 58 ml/100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml/100 ml blood).
   Section::::Biological role.:Human physiology.
  
  Section::::Biological role.:Human physiology.:Content.
  
  The body produces approximately of carbon dioxide per day per person, containing of carbon. In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs, resulting in lower concentrations in the arteries. The carbon dioxide content of the blood is often given as the partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume. In humans, the blood carbon dioxide contents is shown in the adjacent table:
  
  Section::::Biological role.:Human physiology.:Transport in the blood.
   BULLET::::- CO inhibits blood platelet aggregation and adhesion.
  
  BULLET::::- CO may play a role as potential therapeutic agent.
  
  In mammals, carbon monoxide is naturally produced by the action of heme oxygenase 1 and 2 on the heme from hemoglobin breakdown. This process produces a certain amount of carboxyhemoglobin in normal persons, even if they do not breathe any carbon monoxide.
   Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide (about 20–25% of the total) as carbaminohemoglobin, in which CO is bound to the heme protein. The molecule also carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen.
   Blood supply to the cerebral cortex is part of the cerebral circulation. Cerebral arteries supply the blood that perfuses the cerebrum. This arterial blood carries oxygen, glucose, and other nutrients to the cortex. Cerebral veins drain the deoxygenated blood, and metabolic wastes including carbon dioxide, back to the heart.
   The heart functions as a pump in the circulatory system to provide a continuous flow of blood throughout the body. This circulation consists of the systemic circulation to and from the body and the pulmonary circulation to and from the lungs. Blood in the pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the process of respiration. The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs.
   Cerebral circulation is the movement of blood through the network of cerebral arteries and veins supplying the brain. The rate of the cerebral blood flow in the adult is typically 750 milliliters per minute, representing 15% of the cardiac output. The arteries deliver oxygenated blood, glucose and other nutrients to the brain, and the veins carry deoxygenated blood back to the heart, removing carbon dioxide, lactic acid, and other metabolic products. Since the brain is very vulnerable to compromises in its blood supply, the cerebral circulatory system has many safeguards including autoregulation of the blood vessels and the failure of these safeguards can result in a stroke. The amount of blood that the cerebral circulation carries is known as cerebral blood flow. The presence of gravitational fields or accelerations also determine variations in the movement and distribution of blood in the brain, such as when suspended upside-down. 
   Section::::Development.:Veins.
  
  The human venous system develops mainly from the vitelline veins, the umbilical veins and the cardinal veins, all of which empty into the sinus venosus.
  
  Section::::Function.
  
  Section::::Function.:Cardiovascular system.
  
  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 hemoglobin molecules. 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.
  
  Section::::Clinical significance.
   Section::::Storage.
   Section::::Controls of variables.:Levels of blood gases.
  
  Changes in the levels of oxygen, carbon dioxide, and plasma pH are sent to the respiratory center, in the brainstem where they are regulated.
   In vertebrates, oxygen is taken into the body by the tissues of the lungs, and passed to the red blood cells in the bloodstream. Oxygen is then distributed to all of the tissues in the body and offloaded from the red blood cells to respiring cells. Hemoglobin then picks up carbon dioxide to be returned to the lungs. Thus, hemoglobin binds and offloads both oxygen and carbon dioxide at the appropriate tissues, serving to deliver the oxygen needed for cellular metabolism and removing the resulting waste product, CO.
  
  Section::::Cytochrome c oxidase.
   Neurons do not have internal reserves of energy in the form of sugar and oxygen, so their firing causes a need for more energy to be brought in quickly. Through a process called the hemodynamic response, blood releases oxygen to them at a greater rate than to inactive neurons. This causes a change of the relative levels of oxyhemoglobin and deoxyhemoglobin (oxygenated or deoxygenated blood) that can be detected on the basis of their differential magnetic susceptibility.
   Section::::Neural signal under normal oxygen uptake.
   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.
  
  Section::::Physiology.:Carbon dioxide transport.
   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.
   The function of the "right heart", is to collect de-oxygenated blood, in the right atrium, from the body via the superior vena cava, inferior vena cava and from the coronary sinus and pump it, through the tricuspid valve, via the right ventricle, through the semilunar pulmonary valve and into the pulmonary artery in the pulmonary circulation where carbon dioxide can be exchanged for oxygen in the lungs. This happens through the passive process of diffusion. In the "left heart" oxygenated blood is returned to the left atrium via the pulmonary vein. It is then pumped into the left ventricle through the bicuspid valve and into the aorta for systemic circulation. Eventually in the systemic capillaries exchange with the tissue fluid and cells of the body occurs; oxygen and nutrients are supplied to the cells for their metabolism and exchanged for carbon dioxide and waste products In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.
   The heart functions as a pump and acts as a double pump in the cardiovascular system to provide a continuous circulation of blood throughout the body. This circulation includes the systemic circulation and the pulmonary circulation. Both circuits transport blood but they can also be seen in terms of the gases they carry. The pulmonary circulation collects oxygen from the lungs and delivers carbon dioxide for exhalation. The systemic circuit transports oxygen to the body and returns relatively de-oxygenated blood and carbon dioxide to the pulmonary circuit.
   The alveoli are tiny air sacs in the lungs where gas exchange takes place. The mean number of alveoli in a human lung is 480 million. When the diaphragm contracts, a negative pressure is generated in the thorax and air rushes in to fill the cavity. When that happens, these sacs fill with air, making the lung expand. The alveoli are rich with capillaries, called alveolor capillaries. Here the red blood cells absorb oxygen from the air and then carry it back in the form of oxyhaemaglobin, to nourish the cells. The red blood cells also carry carbon dioxide () away from the cells in the form of carbaminohemoglobin and releases it into the alveoli through the alveolor capillaries. When the diaphragm relaxes, a positive pressure is generated in the thorax and air rushes out of the alveoli expelling the carbon dioxide.
   The minimum tissue and venous partial pressure of oxygen which will maintain consciousness is about . This is equivalent to approximately in the lungs. Approximately 46 ml/min oxygen is required for brain function. This equates to a minimum arterial ppO of at 868 ml/min cerebral flow.
   In fish, the hypoxic signal is carried up to the brain for processing by the glossopharyngeal (cranial nerve IX) and vagus (cranial nerve X) nerves. The first branchial arch is innervated by the glossopharyngeal nerve (cranial nerve IX); however all four arches are innervated by the vagus nerve (cranial nerve X). Both the glossopharyngeal and vagus nerves carry sensory nerve fibres into the brain and central nervous system.
  
  Section::::Oxygen sensing.:Locations of oxygen sensors.
   Since the blood arriving in the alveolar capillaries has a formula_3 of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 13 kPa (100 mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly. Similarly, since the blood arriving in the alveolar capillaries has a formula_3 of also about 6 kPa (45 mmHg), whereas that of the alveolar air is 5.3 kPa (40 mmHg), there is a net movement of carbon dioxide out of the capillaries into the alveoli. The changes brought about by these net flows of individual gases into and out of the functional residual capacity necessitate the replacement of about 15% of the alveolar air with ambient air every 5 seconds or so. This is very tightly controlled by the continuous monitoring of the arterial blood gas tensions (which accurately reflect partial pressures of the respiratory gases in the alveolar air) by the aortic bodies, the carotid bodies, and the blood gas and pH sensor on the anterior surface of the medulla oblongata in the brain. There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the bronchioles and pulmonary capillaries, and are therefore responsible for directing the flow of air and blood to different parts of the lungs.
   Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the 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 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.
   Endogenous nitrous oxide can possibly play a role in modulating endogenous opioid and NMDA systems.
  
  Section::::Carbon monoxide.
  
  Carbon monoxide is produced naturally by the human body as a signaling molecule. Thus, carbon monoxide may have a physiological role in the body, such as a neurotransmitter or a blood vessel relaxant. Because of carbon monoxide's role in the body, abnormalities in its metabolism have been linked to a variety of diseases, including neurodegenerations, hypertension, heart failure, and inflammation.
  
  Summary of function:
  
  BULLET::::- CO functions as an endogenous signaling molecule.
  
  BULLET::::- CO modulates functions of the cardiovascular system.
   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 (for exceptions, see below). Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O 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 O per liter blood per mm Hg partial pressure of oxygen (about 100 mm Hg in arteries).
  