Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the diastolic blood pressure (the bottom number), which are the maximum and minimum blood pressures, respectively. A systolic blood pressure of less than 90 millimeters of mercury (mmHg) or diastolic of less than 60 mmHg is generally considered to be hypotension. Different numbers apply to children. However, in practice, blood pressure is considered too low only if noticeable symptoms are present. Primary symptoms generally include dizziness and lightheadedness. Other symptoms include fatigue, shortness of breath, headaches, shakiness, increased thirst, irregular heartbeat, chest pain, confusion.Hypotension is the opposite of hypertension, which is high blood pressure. It is best understood as a physiological state rather than a disease. Severely low blood pressure can deprive the brain and other vital organs of oxygen and nutrients, leading to a life-threatening condition called shock. Shock is classified based on the underlying cause, including hypovolemic shock, cardiogenic shock, distributive shock, and obstructive shock.Hypotension can be caused by strenuous exercise, excessive heat, low blood volume (hypovolemia), hormonal changes, widening of blood vessels, anemia, vitamin B12 deficiency, anaphylaxis, heart problems, or endocrine problems. Some medications can also lead to hypotension.
Thus any cause that influences the rate or volume of air entering the lungs (ventilation) or any cause that influences the transfer of air from the lungs to the blood may cause hypoxemia. As well as these respiratory causes, cardiovascular causes such as shunts may also result in hypoxemia. Hypoxemia is caused by five categories of etiologies: hypoventilation, ventilation/perfusion mismatch, right-to-left shunt, diffusion impairment, and low PO2. Low PO2 and hypoventilation are associated with a normal alveolar–arterial gradient (A-a gradient) whereas the other categories are associated with an increased A-a gradient. : 229

Ventilation
If the alveolar ventilation is low, there will not be enough oxygen delivered to the alveoli for the bodys use. This can cause hypoxemia even if the lungs are normal, as the cause is in the brainstems control of ventilation or in the bodys inability to breathe effectively. Respiratory drive
Respiration is controlled by centers in the medulla, which influence the rate of breathing and the depth of each breath. This is influenced by the blood level of carbon dioxide, as determined by central and peripheral chemoreceptors located in the central nervous system and carotid and aortic bodies, respectively. Hypoxia occurs when the breathing center doesnt function correctly or when the signal is not appropriate:

Strokes, epilepsy and cervical neck fractures can all damage the medullary respiratory centres that generates rhythmic impulses and transmit them along the phrenic nerve to the diaphragm, the muscle that is responsible for breathing.