Generality
Volemia is synonymous with circulating blood volume, hence the overall amount of blood present within the circulatory system.
In the adult, the average volume is around 4.7 - 5 liters, with small individual variations:
- it is generally 10% higher in men than in women;
- it is greater in the child than in the adult and the elderly (as long as it refers to the body surface, therefore expressed in relative terms (L / m2) and not absolute).
What's this
Volemia is the total volume of blood, that is, of plasma and figurative elements (red blood cells, white blood cells and platelets).
The volume includes both the mass of blood circulating in the vessels, and that immobilized in some organs that act as deposits, such as liver or spleen.
The volume of circulating blood is controlled by the kidneys and varies according to body weight, age and sex.
Volume control
Volume control is entrusted to complex regulatory mechanisms, among which the kidney undoubtedly plays a leading role.
Maintaining blood volume within limits compatible with the optimal fulfillment of the body's vital functions, they mainly concur:
- The dietary intake of water and mineral salts, in particular sodium;
- The amount of water lost daily through urine, feces, sweat, perspiratio insensibilis and breathing.
If the blood volume increases ( hypervolemia ) the pressure also increases; this concept becomes easily understandable considering a balloon (blood vessel) filled with increasing amounts of water (blood); as the volume increases, the pressure exerted by the blood on the walls of the vessels also increases, to the point of causing them to burst (in extreme cases). The only solution to avoid the sad event is to remove excess water from the balloon, ie lower the volume by returning it to normal; all this is provided by the kidney, through a passive mechanism and an active mechanism influenced by the hormones aldosterone (decreases the excretion of sodium and water, increasing that of potassium and hydrogen ions) and vasopressin (favors the reabsorption of water at the renal level) .
Thanks to these hormones, the kidneys can make water losses with urine ridiculously small, but they certainly cannot restore the lost liquid, as if by magic. In these circumstances, a series of physiological mechanisms are then activated to maintain vital functions even in conditions of hypovolemia ; among these we mention vasoconstriction (also favored by vasopressin) and increased sympathetic heart stimulation, both aimed at supporting arterial pressure under hypovolemia conditions. The blood venous reserve also contributes to the same purpose, which in similar circumstances tends to reduce in favor of the arteries (the activation of the sympathetic nervous system favors the contraction of the veins).
Why do you measure
Volume measurement is indicated in the diagnosis and monitoring of patients suffering from congestive heart failure, chronic hypertension or kidney failure and of patients in intensive care.
Normal values
In the adult, the average volume is about 4.7 - 5 liters, with small individual variations.
The female sex generally has a lower blood volume than the male one. The children, on the other hand, have a greater volume than the adult and the elderly.
Under normal conditions, the volume remains constant and corresponds to about 7% of body weight. Out of the total blood mass, 55% is due to the plasma and the remaining 45% to the figurative elements, especially to the red blood cells which are the largest among the blood cells.
High Volemia - Causes
Increased volume (hypervolemia):
- Simultaneous ingestion of liquids and salty foods (in this case the plasma osmolarity also increases);
- Conspicuous fluid ingestion or intravenous infusion of the same;
- Extended stay in height;
- Pregnancy;
- Muscle exercise;
- Exposure to high temperatures;
- Primary or secondary polyglobulia (increase in erythrocyte quota).
Low Volemia - Causes
Volume drop (hypovolaemia):
- Dehydration;
- Severe diarrhea;
- He retched;
- Excessive sweating;
- Bleeding;
- Severe burns;
- Exposure to low temperatures;
- Standing up for several hours;
- Insufficient introduction of liquids;
- Administration of diuretics;
- Nephrotic syndrome;
- Blood donation;
- Alcohol consumption;
- Diabetic ketoacidosis.
Symptoms of severe hypovolemia (reduction greater than 15-20%):
- Dizziness;
- Nausea;
- Weakness;
- Extreme thirst;
- Pallor;
- Tachycardia;
- Hypotension;
- Anxiety;
- agitation;
- Increased respiratory rate;
- Sweating (for sympathetic stimulation);
- Cold skin.
How to measure it
For obvious reasons, volume in the living must be measured indirectly. To this end, the total plasma volume is first established, calculated by intravenously introducing known quantities of an indicator and evaluating the concentration in a plasma sample after a few minutes. In this way it is possible to trace the total plasma volume by means of simple mathematical formulas.
Once this data is obtained, it is related to the hematocrit, that is, to the quantity of cells present in the blood (obviously there are many more cells in the blood and the more it increases its volume). The data thus collected are entered in the following formula for the calculation of the volume:
Volemia = [Plasmatic volume / (100 - hematocrit)] x 100
Calculate your Volemia
The blood volume of an individual can be calculated indicatively even from the height and weight values:
COMPUTER
Taking advantage of Nadler's formula:
- Males = 0.3669 * Height (m) 3 + 0.03219 * Weight (kg) + 0.6041
- Females = 0.3561 * Height (m) 3 + 0.03308 x Weight (kg) + 0.1833
Preparation
The volume can be calculated starting from the hematocrit and the volume of the plasma. The patient must therefore undergo a blood test. If in addition to these parameters, you must perform some other blood test that includes fasting (such as determining blood sugar or cholesterol) you will need to refrain from food and drink. The doctor will still be able to provide useful information for the preparation of these analyzes.
Interpretation of Results
- Hypervolemia is the increase in the volume of circulating blood; this leads to an increase in body weight and consequences such as heart failure and hyponatremia. Hypervolemia is often caused by renal dysfunction (acute and chronic insufficiency and nephrotic syndrome). Other causes can be iatrogenic (due to excessive infusion of liquids, especially if rich in sodium) or secondary to heart and liver failure.
- Hypovolemia is the reduction in the volume of circulating blood. This is frequently due to conspicuous blood loss (haemorrhage), secondary to trauma, surgery or injury to internal organs. Hypovolemia can also result from the depletion of fluids via the skin (excessive sweating, burns, etc.), gastrointestinal (diseases that cause vomiting and profuse diarrhea) or kidney (diabetes mellitus or insipid, adrenal insufficiency and diuretic abuse). Furthermore, the decrease in the circulating blood mass can occur due to an increase in capillary permeability secondary to inflammation, sepsis and acute pancreatitis.
