In chopping the term hypovolemia, its meaning can then be derived. Consisting of the words hypo, vol, and emia, the term literally means low blood volume. However, this is just the tip of the iceberg.
Hypovolemia is not simply just a condition characterized by low blood volume, but a state of diminished plasma volume. Making up around 55% of the total blood volume, blood plasma makes up the intravascular portion of the extracellular fluid and is composed of water, proteins, electrolytes, glucose, hormones, clotting factors and carbon dioxide. [2, 3, 4, 5]
Thus, characterized as a deficit in blood plasma, hypovolemia leads to volume deficit of extracellular fluid compartment, which consequently, are grounds for isotonic loss of body fluids and equal losses of water and electrolytes, specifically sodium. 
Hypovolemia: Its Pathologic Process
- Early volume loss leads to a decreased venous return, further causing a decrease in the end diastolic volume and subsequent reduction of cardiac output. As a result, there is diminished baroreceptor stretching in the carotid sinus and aortic arch.
- When these happen, subsequent events would occur: decrease in the parasympathetic activity and an increase in the sympathetic ones, as conducted by the vasomotor center. These would then lead to increased constriction of blood vessels, causing concomitant increase in the heart rate, contractility and cardiac output. Due to autoregulatory reflexes, blood is shunted to the essential organs: the brain, heart and kidneys. 
- Hypovolemia also acts as an isotonic disorder. As fluid volume diminishes, a simultaneous decrease in fluid transport and hydrostatic pressure ensue. The cellular components are then depressed of their nutrients for the manufacture of energy and its metabolism. Renal blood flow is decreased and this is recognized by the mesangial cells of the juxtaglomerular complex of kidneys.
- The renin-angiotensin system, triggered by a diminished renal blood flow, amplifies water and sodium reabsorption. Compensative mechanisms then occur, with the tachycardia, increased cardiac contraction, improved constriction of blood vessels and an elevated resistance of the systemic vascular components, all leading to an augmented mean arterial pressure (MAP) and increased cardiac output. [6, 8]
- Thirst responses are also triggered, liberating surges of both antidiuretic hormone and aldosterone. Production of which is influenced by angiotensin II, by the osmotic receptors found at the hypothalamus, and by trauma. Vasopressin, or ADH, is a potent agent for vasoconstriction, assisting reabsorption of water from the collecting system of nephrons. [6, 7]
- If there is continued volume depletion, the body finally gives in and the compensatory mechanisms exerted out. The blood pressure eventually drops, the heart unable to pump, failing to provide for the vital organs’ needs. At this point, hypovolemic shock may then ensue. [8, 9, 10]
An image of the human body, with almost the entire system affected by hypovolemia.
What Can Cause Hypovolemia?
- Hypovolemia can be caused by conditions that often present as excessive vasodilatation, reduced fluid intake, loss of blood, plasma, and sodium, and fluid shift towards the third space. [6, 11]
- Excessive vasodilatation and subsequent expansion of blood vessels can lead to inability of the nerves to function and inhibition of the vasomotor center of the brain. This often leads to hypovolemia. It is usually caused by either trauma, or vasodilator medications used to manage hypertension. 
- Reduced fluid intake can be rooted from coma, dysphagia, psychiatric illnesses, and environmental conditions preventing fluid intake. 
- Blood loss can also lead to decreased blood volume. This can result from internal bleeding, external injuries, or some complicated obstetric emergencies. It is usually associated with trauma, which is one of the most frequent causes of hypovolemia. [2, 8, 12]
- Losses of both plasma and sodium can be brought about by a concomitant fluid loss. Reasons of which include loose bowel movement, vomiting, nasogastric losses, diabetes, renal dysfunction associated with polyuria, excessive laxative or diuretic use, fistulas, and episodes of increased metabolic rates, such as fever and infections. [6, 8, 12]
- Third spacing may also cause eventual hypovolemia. These fluid shifts may occur in effusions, initial phases of burns, edema, peritonitis, intestinal obstructions, and even pancreatitis. [6, 13]
Signs and Symptoms of Hypovolemia
Various manifestations of hypovolemia include the following:
- Decreased blood pressure
- Poor skin turgor
- Prolonged capillary refill time
- Rise in the Body Temperature
- Fainting or syncope
- Body malaise
- Cold and clammy extremities
- Decreased urine output
- Lack of coordination in movement
- Alteration of mental status and coma, if severe. [ 2, 14, 15, 16]
In assessing for the presence of hypovolemia, it is critical to obtain the patient’s history to establish the reason for fluid disturbance. Strict fluid input and output monitoring must be done to evaluate fluid status in relation to dietary intake. Vital signs, along with blood pressure, should be monitored.
Active fluid losses from loose bowel movement, bleeding, wound drainage, and vomiting must be noted. Signs of circulatory overload such as headache, tachycardia, flushed skin, venous distention, difficulty of breathing, increased blood pressure, and cough must be watched out for.
In a hospital setting, measuring the hemodynamic status can be done with the use of the pulmonary artery pressure (PAP), central venous pressure (CVP), and pulmonary capillary wedge pressure (PCWP). These can determine hypovolemia, prevent its complications, and can also serve as guides for therapy. 
Blood tests such as glucose, electrolytes, blood chemistry, and arterial blood gases can also be done to confirm hypovolemia and prevent sequela of hypovolemic shock. 
Measurement of Central Venous Pressure (CVP) required to monitor hypovolemia and determine presence of its complications.
Treatment options for hypovolemia depend on the quantity of blood plasma lost.
- Mild cases of hypovolemia may only require intravenous fluids to replenish losses of the body. Correction of which may reverse the fluid and electrolyte disturbance and improve tissue perfusion. 
- For more severe losses or those due to trauma, blood transfusions may be the definitive management.
- Medications such as epinephrine, dobutamine and norepinephrine may also be given to compensate for decreased blood pressure. 
- Surgical repair of sites of bleeding must be done to prevent further blood loss.
- Oxygen inhalation must be provided to increase the efficacy of the remaining blood blow. 
- If the hypovolemic episode is due to an anaphylactic reaction of a medication, antidote administration should be given to avoid further progress to shock. 
Intravenous fluids are required for mild cases of hypovolemia.
For more severe cases of blood loss and hypovolemia, transfusions of blood products are warranted.
Prognosis of Hypovolemia
Hypovolemia does not always have a fatal sequela. However,the more damages there are to body and its internal organs, the more plasma is lost. Hence, the presence of shock and its fatal complications are increased. Thus, immediate medical intervention is vital. 
Also see about : Hypervolemia
- O’Neil, D. Blood Components. Palomar College. 1999
- Pathophysiology: A 2 in 1 Reference for Nurses. Lippincott Williams and Wilkins.
- Capillary Fluid Dynamic and Pathophysiology of Hypovolemia, Formulary. 2001 September; 36 (1): 7
- Sircar, S. Principles of Medical Physiology. Thieme Medical Publishing.
- Kreimer, U. Pathophysiology of Fluid Imbalance. Critical Care 2000. 2000 August. 4(2).
- Yildiz, F.Fluid Replacement in Treatment of Hypovolemia and Shock:; Crystalloids and Colloids., Archives Medical Review Journal. 2013; 22(3): 347-361.
- Takasu, A, et al. Effects of Increased Oxygen Breathing in a Volume Controlled Hemorrhagic Shock Outcome Model in Rats. 2000.