Fluid balance disorder

Introduction The main components of body fluids are water and electrolytes. It is divided into two parts, cell and extracellular fluid, the amount varies with sex, age and fatness. Adult men's body fluids are generally 60% of body weight; adult women's body fluids account for about 55% of body weight. Children's fat is less, so the proportion of body fluids is higher, in newborns, up to 80% of body weight. The amount of body fat increases with age. After 14 years of age, the proportion of body fluid in children is similar to that of adults. Disorders in the fluid balance can cause disturbances in the balance of the body.

Cause

The cause of fluid imbalance:

The body mainly maintains the balance of body fluids through the kidneys, keeping the internal environment stable. Renal regulation is affected by neurological and endocrine responses. The normal osmotic pressure of the body fluid is generally restored and maintained by the hypothalamic-posterior pituitary-antidiuretic hormone system, and then the blood volume is restored and maintained by the renin-aldosterone system. However, when the blood volume is sharply reduced, the body will maintain and restore the blood volume at the expense of the maintenance of the osmotic pressure of the body fluid, so that the perfusion of vital vital organs can be guaranteed and life-sustained.

When the body loses water, the extracellular fluid osmotic pressure increases, stimulating the hypothalamic-pituitary-antidiuretic hormone system, producing thirst, increasing drinking water, and promoting increased secretion of vasopressin. The far-curved renal tubules and collecting duct epithelial cells strengthen the reabsorption of water under the action of vasopressin, so the amount of urine is reduced, and the water is retained in the body, so that the extracellular fluid osmotic pressure is lowered. Conversely, when the body's water is increased, the extracellular fluid osmotic pressure is reduced, the thirst reaction is inhibited, and the secretion of antidiuretic hormone is reduced. The re-absorption of water from the distal convoluted tubules and collecting duct epithelial cells is reduced, and excess water is removed from the body. The external liquid osmotic pressure is increased. This response to vasopressin secretion is very sensitive. When the plasma osmotic pressure is less than 2% lower than normal, there is a change in the secretion of vasopressin, which keeps the body's water dynamic and stable.

On the other hand, when the extracellular fluid is reduced, especially when the blood volume is reduced, the intravascular pressure is decreased, and the blood pressure of the renal arterioles is also decreased accordingly. The baroreceptors located on the wall of the vessel are stimulated by the pressure drop, so that the glomerulus The cells increase the secretion of renin; at the same time, as the blood volume decreases and the blood pressure decreases, the glomerular filtration rate also decreases, so that the amount of Na+ flowing through the distal convoluted tubules is significantly reduced. The reduction in sodium stimulates the sodium receptor located in the dense plaque of the distal convoluted tubule, causing the peripheral cells to increase the secretion of renin. In addition, the decline in systemic blood pressure can also excite the sympathetic nerves and stimulate the secretion of renin by the cells in the pararenal cells. Renin catalyzes the presence of angiotensinogen in plasma, which causes it to transform into angiotensin I, which is then converted to angiotensin II, causing arteriolar contraction and stimulation of the adrenocortical globular zone, increasing the secretion of aldosterone and promoting the far-curved kidney. The small tube re-absorbs Na+ and promotes the excretion of K+ and H+. As sodium reabsorption increases, the reabsorption of CI- also increases, and the reabsorbed water increases. The result is an increase in extracellular fluid volume. After the circulating blood volume rises and the blood pressure gradually rises, which in turn inhibits the release of renin, the production of aldosterone decreases, so the reabsorption of Na+ decreases, so that the extracellular fluid volume no longer increases and remains stable.

an examination

Related inspection

Blood pH (pH) Determination of pH and pH of sweat electrolytes

Check and diagnose the imbalance of liquid balance:

The normal human body fluid maintains a certain H+ concentration, that is, maintains a certain pH value (the pH of the arterial plasma is 7.40 +-0.05). To maintain normal physiological and metabolic functions. In the metabolic process, the human body produces both acid and alkali, so the H+ concentration in body fluids often changes. However, the human body can pass the buffer system of the body fluid, the lungs and the regulation of the kidneys, so that the H+ concentration in the blood changes only in a small range, and the blood pH is maintained between 7.35 and 7.45.

The most important pair of buffer substances for HCO-3 and H2CO3 in the blood. The normal value of HCO-3 is 24 mmol/L on average, and H2CO3 is 1.2 mmol/L on average. The ratio of HCO-3 / H2CO3 = 24/1.2 = 20/1. The concentration of carbonic acid in the plasma is determined by the amount of CO2 dissolved in the physical state and the amount of carbonic acid formed by the water. Since CO2 in body fluids is mainly in the state of physical physical dissolution, the amount of H2CO3 is very small and can be ignored. Therefore, H2CO3 can be calculated by using carbon dioxide partial pressure (PCO2) and its solubility coefficient (0.03). The normal value of PCO2 is 40 mmHg, that is, H2CO3=0.03*40=1.2. Thus, HCO-3 / H2CO3 = HCO-3 / 0.03 * PCO2 = 24 / 1.2 = 20/1. As long as the ratio of HCO-3 / H2CO3 remains at 20/1, the pH of the plasma remains at 7.40. In terms of regulation of acid-base balance, lung respiration is the removal of CO2 and regulation of the respiratory component of the blood, PCO2, which regulates H2CO3 in the blood. Therefore, the body's respiratory function is abnormal, which can directly cause acid-base balance disorder, and can also affect the compensation of acid-base balance disorder. Renal regulation is the most important acid-base balance regulation system, which can discharge fixed acid and excessive alkaline substances to maintain the stability of plasma HCO-3 concentration. Abnormal kidney function can affect the normal regulation of acid-base balance and cause acid-base balance disorder. The mechanism of renal regulation of acid-base balance is: exchange of 1H+-Na+; reabsorption of 2HCO-3; 3 secretion of NH3 and H+ combined into NH+4; 4 acidification of urine and excretion of H+.

Differential diagnosis

Identification of symptoms that are confusing with fluid imbalance:

Isotonic water shortage: also known as acute water shortage or mixed water shortage. Surgical patients are most prone to this lack of water. Water and sodium are lost in proportion, serum sodium is still in the normal range, and the osmotic pressure of the extracellular fluid remains normal. It causes a rapid decrease in extracellular fluid volume, including circulating blood volume. The baroreceptor of the renal arterial wall is stimulated by the pressure drop in the tube, and the decrease of Na+ in the distal renal tubule fluid caused by the decrease in the filtration rate of the ball causes the excitability of the renin-aldosterone system and the secretion of aldosterone. Aldosterone promotes the reabsorption of sodium by the distal convoluted tubules, and the amount of water that is resorbed with sodium is also increased, causing the extracellular fluid to rise. Since the lost fluid is isotonic, the osmotic pressure of the extracellular fluid is not substantially changed, and initially the intracellular fluid is not transferred to the extracellular space to compensate for the lack of extracellular fluid. Therefore, the amount of intracellular fluid does not change. However, after the liquid loss lasts for a long time, the intracellular fluid will gradually move outward, and will be lost along with the extracellular fluid, causing the cells to be dehydrated.

Hypotonic water shortage: also known as chronic water shortage or secondary water shortage. Water and sodium are missing at the same time, but the lack of water is less than the loss of sodium, so the serum sodium is lower than the normal range, and the extracellular fluid is hypotonic. The body reduces the secretion of antidiuretic hormone, so that the reabsorption of water in the renal tubules is reduced, and the amount of urine is increased to increase the osmotic pressure of the extracellular fluid. However, the amount of extracellular fluid is reduced more, and the interstitial fluid enters the blood circulation. Although it can partially compensate the blood volume, the interstitial fluid is reduced more than the plasma. Faced with a significant reduction in circulating blood volume, the body will no longer consider osmotic pressure and try to maintain blood volume. Excited by the renin-aldosterone system, the kidneys are reduced in sodium, and CI- and water reabsorption increases. Therefore, the sodium chloride content in the urine is significantly reduced. Decreased blood volume will stimulate the posterior pituitary, which will increase the secretion of antidiuretic hormone and increase the reabsorption of water, leading to oliguria. If the blood volume continues to decrease and the above compensatory function is no longer able to maintain blood volume, shock will occur. This kind of shock caused by a large amount of sodium loss, also known as hyponatremia shock.

Hypertonic water shortage: also known as primary water shortage. Although water and sodium are missing at the same time, the lack of water is more than the lack of sodium, so the serum sodium is higher than the normal range, and the extracellular fluid is hyperosmotic. The thirst center located in the lower part of the hypothalamus is stimulated by hyperosmosis. The patient feels thirsty and drinks water, which increases the body's water to reduce the osmotic pressure. On the other hand, hypertonicity of the extracellular fluid can cause an increase in the secretion of antidiuretic hormone, so that the reabsorption of water by the renal tubules is increased, the amount of urine is reduced, and the osmotic pressure of the extracellular fluid is lowered and the capacity is restored. If the water shortage continues, the aldosterone secretion is increased due to a significant decrease in circulating blood volume, and the reabsorption of sodium and water is enhanced to maintain blood volume. When the water shortage is severe, the extracellular fluid osmotic pressure is increased, and the intracellular fluid is moved to the extracellular space. As a result, the amount of both internal and external fluids is reduced. Finally, the degree of water shortage in the intracellular fluid exceeds the extent of water shortage in the extracellular fluid. Water shortage in brain cells will cause brain dysfunction.

Excessive water: also known as water poisoning or diluted low blood sodium. It means that the total amount of water in the body exceeds the displacement, so that water remains in the body, causing a decrease in blood osmotic pressure and an increase in circulating blood volume. Too much water is less likely to occur. Only in the case of excessive secretion of vasopressin or renal insufficiency, the body consumes too much water or receives excessive intravenous infusion, which causes water to accumulate in the body, resulting in water poisoning. At this time, the amount of extracellular fluid increases, the serum sodium concentration decreases, and the osmotic pressure decreases. Since the osmotic pressure of the intracellular fluid is relatively high, the water moves into the cells, and as a result, the osmotic pressure of the inner and outer fluids of the cells is decreased and the amount is increased. In addition, the increased amount of extracellular fluid can inhibit the secretion of aldosterone, so that the far-nose renal tubules reduce the reabsorption of Na+, and the Na+ is excreted from the urine, so the serum sodium concentration is further reduced.

The normal human body fluid maintains a certain H + concentration, that is, maintains a certain pH value (the pH of the arterial plasma is 7.40 +-0.05). To maintain normal physiological and metabolic functions. In the metabolic process, the human body produces both acid and alkali, so the H+ concentration in body fluids often changes. However, the human body can pass the buffer system of the body fluid, the lungs and the regulation of the kidneys, so that the H+ concentration in the blood changes only in a small range, and the blood pH is maintained between 7.35 and 7.45.

The most important pair of buffer substances for HCO-3 and H2CO3 in the blood. The normal value of HCO-3 is 24 mmol/L on average, and H2CO3 is 1.2 mmol/L on average. The ratio of HCO-3 / H2CO3 = 24/1.2 = 20/1. The concentration of carbonic acid in the plasma is determined by the amount of CO2 dissolved in the physical state and the amount of carbonic acid formed by the water. Since CO2 in body fluids is mainly in the state of physical physical dissolution, the amount of H2CO3 is very small and can be ignored. Therefore, H2CO3 can be calculated by using carbon dioxide partial pressure (PCO2) and its solubility coefficient (0.03). The normal value of PCO2 is 40 mmHg, that is, H2CO3=0.03*40=1.2. Thus, HCO-3 / H2CO3 = HCO-3 / 0.03 * PCO2 = 24 / 1.2 = 20/1. As long as the ratio of HCO-3 / H2CO3 remains at 20/1, the pH of the plasma remains at 7.40. In terms of regulation of acid-base balance, lung respiration is the removal of CO2 and regulation of the respiratory component of the blood, PCO2, which regulates H2CO3 in the blood. Therefore, the body's respiratory function is abnormal, which can directly cause acid-base balance disorder, and can also affect the compensation of acid-base balance disorder. Renal regulation is the most important acid-base balance regulation system, which can discharge fixed acid and excessive alkaline substances to maintain the stability of plasma HCO-3 concentration. Abnormal kidney function can affect the normal regulation of acid-base balance and cause acid-base balance disorder. The mechanism of renal regulation of acid-base balance is: exchange of 1H+-Na+; reabsorption of 2HCO-3; 3 secretion of NH3 and H+ combined into NH+4; 4 acidification of urine and excretion of H+.

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