What Is the Relationship Between Fluid and Electrolytes?
Water and electrolytes balance refers to whether the body's daily intake and discharge of water and sodium (main electrolytes in extracellular fluids) maintain balance and how to maintain balance.
Water and electrolyte balance
The water in the body and the substances dissolved in it are called body fluids. Body fluid content varies with age and gender (Table 1), and body fluid content decreases with age. The body fluid volume of an adult man is 60% of body weight and 50% of an adult woman. Women's body fluid content is about 6% to 10% less than men's, which is because women have more fat content. Adipose tissue
Normal human body fluid volume is quite stable, and daily water intake and discharge are in a dynamic balance (Table 3). Children's water requirements are about 2 to 4 times larger than adults. If there is insufficient water supply, the body fluid volume will rapidly decrease, leading to dehydration. This phenomenon is more likely to occur in children.
Table 3 Daily water in and out volume for adults (ml / 24 hours)
intake | Excretion |
Food 1000 | Respiration and Skin Evaporation 850 |
Drink 1200 | Renal excretion 1500 |
Oxidized water 300 | Feces excretion 150 |
Total 2500 | Total 2500 |
Body fluids are widely distributed in various parts of the body, and are divided into extracellular fluid (including plasma and interstitial fluid) and intracellular fluid (Table 1) according to the area of distribution. Extracellular fluid accounts for approximately 20% of body weight, of which plasma accounts for approximately 5%, and interstitial fluid accounts for approximately 15% (including lymphatic and cerebrospinal fluid). Cells live directly in extracellular fluid. The supply of nutrients and oxygen and the removal of end-products of metabolism depend on extracellular fluid. Therefore, extracellular fluid is called
The main component of body fluids is water, followed by electrolytes. There is a large difference in electrolyte concentration between extracellular fluid and intracellular fluid. The cations in extracellular fluid are mainly sodium ions.
Although the content of body fluids in various parts of the body is quite stable, it is by no means constant.
Although the daily intake of water and sodium salts varies greatly, the body's water and salt contents are relatively stable, which is due to the dynamic balance between intake and discharge. Adults consume 10.5 grams of table salt per day, and the total excretion is 10.5 grams (Table 4). The maintenance of this dynamic balance depends on the regulation of nerves and body fluids. The content of water in the body is closely related to the content of sodium chloride. When the content of sodium chloride in the body increases, the content of water also increases. Conversely, when the body is deficient in sodium chloride, the amount of water will decrease, and in severe cases, it can lead to circulatory failure. This is due to the decrease in blood volume and lower arterial blood pressure.
Table 4 Daily sodium chloride balance for normal adults
Intake | G / day | discharge | G / day |
food | 10.5 | sweat manure Pee | 0.25 0.25 10.0 |
total | 10.5 | total | 10.5 |
Regulation of water content in the body The content of water in the body depends on the balance between water intake and excretion. The control of water intake is mainly based on thirst, and the control of excretion is mainly determined by the concentration of antidiuretic hormone in plasma. That is, the amount of urine is adjusted by antidiuretic hormone. The thirst center and antidiuretic hormone-secreting cells are located in the hypothalamus. When the osmotic pressure of plasma crystals increases, the antidiuretic hormone secretion increases, and the thirst center can be stimulated to cause drinking water. In addition, afferent impulses from cardiovascular baroreceptors and volume sensors (mainly located in the right atrium) inhibit the secretion of antidiuretic hormones. When the blood pressure decreases and the blood volume decreases, the secretion of antidiuretic hormone increases, and the secretion of aldosterone also increases. In this way, sodium water retention is caused, and blood volume is increased. The regulation of extracellular fluid volume mainly includes the regulation of osmotic pressure and volume regulation.
Regulation of osmotic pressure: The osmotic pressure of extracellular fluid is generally maintained at a certain level. This is necessary to maintain the normal shape and function of cells. When the body is dehydrated, the osmotic pressure of plasma crystals increases, stimulating the supraoptic nucleus osmolarity receptor and thirst center (located in the lateral area of the hypothalamus) in the hypothalamus, causing increased secretion and release of antidiuretic hormone (ADH). The transport reaches the basal membrane of the renal tubules and collecting duct epithelial cells, and binds to the receptor on the membrane to produce cyclic adenylate in the epithelial cells. The permeability of the luminal membrane of the epithelial cells to water is increased, thereby increasing the reabsorption of water and reducing the amount of urine. On the other hand, thirst caused excitement, causing thirst, and actively drinking water. In this way, the water content in the body increases, so that the increased osmotic pressure of the crystals returns to normal levels. On the contrary, when the water content in the body increases, the osmotic pressure of the plasma crystals increases, and the secretion of antidiuretic hormones decreases. Therefore, the permeability of the renal tubules and collecting ducts to water decreases, water reabsorption decreases, urine volume increases, and excretion Excess water in the body, so that the osmotic pressure of the plasma returns to normal levels.
Regulation of body water content
Volume regulation: When the blood volume changes, the blood pressure will affect the baroreceptors or volume sensors in the low pressure system (venous) and high pressure system (artery). Baroreceptors are mainly located in the carotid sinus and aortic arch, while volume sensors are mainly located in the right atrium and pulmonary vein. When the blood volume increases, the afferent impulses of the above-mentioned receptors increase, and the secretion of antidiuretic hormones and aldosterone is reflexively suppressed, thereby reducing the reabsorption of water and sodium by the renal tubules, increasing the urine volume, reducing the extracellular fluid volume, and blood. The amount returns to normal levels. On the contrary, when the blood volume is reduced, the afferent impulses of the above-mentioned receptors are reduced, and the secretion of antidiuretic hormones and aldosterone cannot be inhibited. Therefore, the secretion of these two hormones increases, causing the renal tubules to reabsorb water and sodium, causing sodium Water is retained, so blood volume is restored (Figure 2).
In general, the adjustment of osmotic pressure is much more sensitive than the volume adjustment. A 1.2% increase in plasma crystal osmotic pressure can cause a significant increase in antidiuretic hormone secretion, and a decrease in blood volume by more than 8% can cause a significant increase in antidiuretic hormone. . However, in terms of the maximum value of antidiuretic hormone secretion, the effect of plasma osmotic pressure is not as great as that of volume regulation. The maximum increase of plasma osmotic pressure can not make antidiuretic hormone secretion more than 10 times, but when the blood volume loss is more than 25%, the secretion of antidiuretic hormone can reach 20-50 times. That is to say, in the case of a large decrease in blood volume, the role of volume regulation far exceeds the regulation of osmotic pressure. At this time, even if the plasma osmotic pressure is lower than normal, the secretion of antidiuretic hormone is still significantly increased, indicating that In severe cases, ensuring the physiological significance of blood volume is much greater than ensuring osmotic pressure.
In addition, the renin-angiotensin-aldosterone system also plays an important role in the regulation of plasma volume. When the blood volume is reduced, renal blood flow is reduced, causing renin to be secreted by cells near the glomerulus. Renin changes the angiotensinogen in the plasma to angiotensin (A), and then to angiotensin (A), and A can increase the secretion of aldosterone and stimulate the increase of aldosterone secretion in the central nervous system. Renal tubules have increased sodium reabsorption; thirst centers are excited, which increases water intake. Therefore, sodium water retention increases the extracellular fluid volume and restores blood volume.
Regulation of sodium content in the body The regulation of water balance relies on regulating the amount of drinking water (by thirst) and renal drainage, which is closely related to the plasma sodium content. The regulation of sodium balance in humans mainly depends on regulating the amount of sodium excreted by the kidneys. The sodium retention capacity of the kidney is very strong. When the body is deficient in sodium, the sodium in the urine is significantly reduced or even disappeared. The amount of renal sodium excretion is regulated by the following factors.
Renin-angiotensin-aldosterone system: This is the main regulating system for sodium retention, because the sodium content in the glomerular filtrate decreases when the body is deficient in sodium, and the lumen fluid containing less sodium flows through the dense plaque It causes the excitement of dense plaques, so that the cells near the glomerulus secrete renin into the blood, resulting in an increase in the concentration of angiotensin . The adrenal cortex globular band is highly sensitive to angiotensin , which increases the secretion of aldosterone, thereby enhancing the renal tubular reabsorption of sodium.
Ventricular sodium-sensitive receptors: There are sodium-sensitive receptors in the ventricle, which play a certain role in regulating water-salt balance. Perfusion of hypertonic sodium chloride solution into the carotid artery or increase of sodium ion concentration in cerebrospinal fluid can increase the excretion of sodium in urine. Slowly instilling hypertonic sodium chloride solution into the third ventricle or lateral ventricle of the sheep causes drinking water and the effect of inhibiting water diuresis, while increasing the excretion of sodium and urine. If the infusion of hypertonic sucrose solution does not cause the above effects, it means that the sensor is sensing sodium, not osmotic pressure. Injecting hypertonic sodium chloride solution into the fourth ventricle can also cause an increase in urinary sodium excretion, but without the effects of drinking water and increased secretion of antidiuretic hormones. It can be seen that sodium-sensitive sensors that control urinary sodium excretion are widely distributed in the ventricular system, while sodium-sensitive sensors that control water balance are only distributed in the anterior wall of the third ventricle. After the anterior wall of the third ventricle of the sheep was damaged, the hypertonic sodium chloride solution was slowly injected into the ventricle, and the effects of drinking water and antidiuretic hormone secretion no longer appeared.
Sodium-releasing hormone: When the amount of extracellular fluid increases, a substance is produced in the kidney, which can inhibit the reabsorption of sodium by the proximal tubules and increase the amount of urine sodium excretion. Therefore, this substance is called sodium-releasing hormone . However, the chemical structure of the substance, the specific site and the mode of action are unclear, so some people are skeptical about the existence of this hormone. [1]
