Body Fluids, Electrolytes and Balance of Acid and Base
Body Fluids and Electrolytes Overview
bodily fluids and Electrolytes
- Water is the most important component of the body.
- Homeostasis requires the body fluids to have a relatively constant volume and a stable composition.
- Anomalies in the control systems that maintain the constancy of body fluids cause some of the most common and serious clinical problems.
- An adult’s total body water volume is between 40 and 45 liters. It accounts for roughly two-thirds of our total body weight.
- Water surrounds almost all body cells and forms the majority of cellular protoplasm.
- Many electrolytes and other substances dissolve in this fluid.
- Fluids and electrolytes are constantly exchanged between the intravascular (blood) space and the body tissues.
- Several mechanisms regulate the flow of fluids between these two spaces, resulting in internal equilibrium.
- The two forces that ensure this balance are hydrostatic and osmotic pressures.
- Subsequent topics will cover the detailed pathophysiology of fluid imbalance.
Compartments for Body Fluids and Electrolytes
Body Fluid Compartments The body’s water is divided into two, which are as follows:
o The intracellular fluid compartment
o The extracellular fluid compartment
Intracellular fluid compartment
- This compartment is present inside of cells, and a plasma membrane divides it from the extracellular compartment.
- A typical adult consumes roughly 25 liters of ICF in total. Because the cell membrane contains mechanisms for selective uptake and discharge
- The composition of ICF is essentially within the control of the cell.
- The two compartments are separated by a plasma membrane that is extremely permeable to water but not to the majority of the electrolytes.
Extracellular Fluid Compartment
• The extracellular compartment exists outside the cell boundary.
• There are intravascular and extravascular spaces.
- Plasma is the intravascular compartment fluid.
- Extravascular compartment fluid is the fluid found in the interstitial space.
- Thus, extracellular fluid includes plasma, lymph, and interstitial fluid.
- Fluid is constantly exchanged between the extracellular and intracellular compartments.
- An average adult extracellular fluid volume is about 15 liters, with 3 liters being plasma and the remaining 12 liters being interstitial fluid.
- The composition of plasma and interstitial fluid is nearly identical, with the exception of a higher concentration of protein in the plasma.
• The electrolyte compositions of intracellular and extracellular fluids differ markedly.
• The most abundant electrolytes in intracellular fluid are potassium (K) and magnesium (Mg), both of which are positively (+) charged (cations), as well as phosphates (PO) and proteins, which are negatively (-) charged (anions).
• Extracellular fluids are positively charged with sodium (Na+) and negatively charged with chloride (Cl-) and bicarbonate (HCO3).
Normal Fluid Balance in the Body
Normal Water Balance
- Our bodies are constantly gaining and losing water or fluids.
- The amount gained must equal/balance the amount lost.
- If this does not occur, internal equilibrium is lost and systems cease to function normally.
Important factors Controlling Body Fluid Balance
Three important factors carry out this crucial role of regulating fluid balance in the body.
1. Plasma sodium concentration
- Anything that causes the body to retain sodium will also cause it to retain water.
2. The kidneys’ capacity to remove water from the body
- Before water is absorbed from the gut, but at the moment when the amount to be absorbed is roughly equal to what the body needs, thirst is sated.
- All of the above factors are dependent on the hormone anti-diuretic hormone (ADH) or arginine vasopressin, which is produced by the posterior pituitary gland.
- This hormone conserves water by increasing reabsorption from distal tubules and collecting tubules of the nephrone in the kidneys.
- As a result, its excess causes water retention and decreased urine production.while its absence will result in polyuria (production of very large volume of urine)
The daily fluid intake and demand are affected by a variety of factors, including:
- Outside temperature
- Physical exercise
- The type of food consumed
Water and Electrolytes Imbalance
Disruptions in Water and Sodium Balance
pure water deficiency
- This is direct water deprivation or loss without corresponding electrolyte depletion.
- Although rare, the condition may occur in a few clinical cases following water deprivation, particularly in people who are unable to swallow due to esophageal obstruction, patients in coma, or starvation.
The impact of a lack of pure water
- Increased extracellular compartment osmolarity.
When the extracellular compartment’s
- Osmolarity rises, Intracellular fluid is transferred to the extracellular compartment, causing cellular dehydration.
- Urine water excretion is reduced to a bare minimum, resulting in oliguria and very concentrated urine.
- Plasma changes include an increase in sodium, proteins, and urea concentrations.
Sodium and water deficiency combined
- In this case, sodium and chlorides are lost in equal amounts.
- However, chloride loss exceeds sodium loss in vomiting caused by gastric disease and diuresis caused by the use of some diuretics. As chloride is replaced by carbonate, this has no discernible effect on the volume of extracellular fluid.
Water and salt depletion effects
- Extracellular fluid osmotic pressure decreases.
- The osmolarity of intracellular fluid rises in comparison to extracellular fluid.
- Water leaves extracellular space to enter cells in order to balance the osmotic pressure of the two compartments
- Causing cells to become overly hydrated and swell, resulting in extracellular dehydration.
Causes of Water and Sodium Balance Disturbances
• Excessive vomiting and diarrhea.
• Excessive sweating.
• Excessive urination.
• Conditions such as diabetes mellitus, thiazide diuretic administration, Addison’s disease.
• Extensive hemorrhage and severe burn.
Formation of Oedema
- Denotes an increase in fluid in the interstitial tissue spaces or body cavities.
- Fluid collections in different body cavities are labeled differently depending on where they accumulate. As an example:
- Excess fluid in the pleural space is referred to as hydrothorax.
- Excess fluid in the pericardial sac is referred to as hydropericardium.
- Ascites (excess fluid in the peritoneal cavity)
- Anasarca (severe and generalized oedema with profound subcutaneous tissue swelling)
The Figure below is Oedema Formation Mechanism in Relation to Hydrostatic and Oncotic Pressure
Source: Kumar, 2008.
Oedema Formation Mechanisms
- Increased hydrostatic pressure
- Lower plasma osmotic pressure
- Obstruction of the Lymphatics
- Water and sodium retention
Hydrostatic Pressure Has Increased
- Impaired venous return can cause localized increases in intravascular pressure.
- For example, lower extremity deep venous thrombosis can result in edema confined to the affected leg’s distal portion.
- Generalized venous pressure increases, with subsequent systemic oedema, are most common in congestive heart failure, affecting right ventricular cardiac function.
- While increased venous hydrostatic pressure plays a role, the pathogenesis of cardiac edema is more complicated.
Lower Plasma Osmotic Pressure
- Low plasma osmotic pressure can be caused by excessive loss or decreased synthesis of albumin, a serum protein that is primarily responsible for maintaining plasma colloidal osmotic pressure.
- The nephrotic syndrome, in which glomerular capillary walls become leaky, is a major cause of albumin loss; patients typically present with generalized oedema.
- Reduced albumin synthesis occurs due to diffuse liver diseases (for example, cirrhosis) or protein malnutrition.
- In each case, decreased plasma osmotic pressure causes a net movement of fluid into the interstitial tissues, resulting in a decrease in plasma volume.
- As expected, decreased intravascular volume causes renal hypoperfusion, which is followed by secondary aldosteronism.
Obstruction of the Lymphatics
- Impaired lymphatic drainage and resulting lymphoedema are usually localized, but they can be caused by inflammatory or neoplastic obstruction.
- For example, the parasitic infection filariasis can cause extensive inguinal lymphatic and lymph node fibrosis.
- The resulting oedema of the external genitalia and lower limbs can be so severe as to earn the moniker elephantiasis.
- Breast cancer can be treated by resection and/or irradiation of the associated axillary lymph nodes.
- Serious upper extremity problems might occur from the scarring and loss of lymphatic drainage that follow.
- • In breast carcinoma, infiltration and obstruction of superficial lymphatics can also cause edema of the overlying skin, resulting in the so-called peau d’orange (orange peel) appearance.
Water and sodium retention
- Salt retention is another common cause of oedema.
- Salt retention with obligate thirst causes both increased hydrostatic pressure (due to intravascular volume expansion) and decreased vascular osmotic pressure.
- Salt retention can occur with any compromise ofrenal function, such as post-streptococcal glomerulonephritis and acute renal failure.
Balance of Acid and Base
- Optimal pH is maintained by maintaining the balance of acids and bases produced by the body cells.
The buffer system, which ensures that excess acid or alkali is safely eliminated from the body, is built into the body.
The lungs and kidneys are the organs that are most active in this activity.
- The cells of the proximal convoluted tubules secrete hydrogen ions to maintain normal blood pH.
- In the filtrate, hydrogen ions combine with buffers:
- Bicarbonate produces carbonic acid.
- Ammonia produces ammonium ions and
- Hydrogen phosphate produces dihydrogen phosphate.
- Carbonic acid is converted to carbon dioxide (CO2) and water (H2O), and carbon dioxide is reabsorbed, preserving the blood’s buffering capacity.
- Ammonium salts and hydrogen sulphate are lost as hydrogen ions through urine; other buffer systems include body proteins and phosphates, which absorb excess hydrogen ions (H+).
- The lungs control blood pH by increasing or decreasing the amount of carbon dioxide expelled (increased or decreased respiration).
- Carbonic acid is formed when carbon dioxide and water combine to form carbonic acid, which eventually dissociates to bicarbonates and hydrogen ions, lowering the body’s pH. As a result, any condition that causes the body to retain carbon dioxide will cause the blood to become acidic, and vice versa.
- The kidneys control blood pH by regulating the amount of hydrogen ion lost through urine.
- The normal blood pH ranges between 7.35 and 7.45.