Body Water Compartments and Balance

The amount of water in your body, which varies based on age, body fat, and muscle mass.

The fluid found inside cells, making up about two-thirds of your body's total water.

The fluid found outside of cells, making up about one-third of your body's total water.

Substances that dissolve in water and create electrically charged particles called ions.

The concentration of dissolved particles in a fluid, influencing water movement.

The difference in osmolality between two fluids, causing water to move from a lower concentration to a higher concentration.

The primary electrolyte in your extracellular fluid.

The primary electrolyte in your intracellular fluid.

A state of fluid loss from the body, leading to a decrease in total body water.

A condition where you have too much water in your cells, causing them to swell.

Edema is the accumulation of interstitial fluid (IF) in tissues, causing swelling. This happens when the volume of IF increases, not the volume of intracellular fluid (ICF).

Edema is caused by anything that increases fluid flow out of blood or decreases the return of fluid to the blood.

Electrolytes are minerals (ions) that conduct electricity in solution. They are essential for various bodily functions, including fluid balance, nerve impulses, and muscle contraction.

Sodium is the most abundant cation in the extracellular fluid (ECF) and plays a crucial role in regulating ECF volume and water distribution. It controls osmotic pressure, influencing water movement between compartments.

Sodium balance is regulated through a complex interplay of hormones and mechanisms. Aldosterone, a hormone produced by the adrenal cortex, plays a key role in sodium reabsorption by the kidneys. The renin-angiotensin-aldosterone system (RAAS) is activated in response to decreased blood pressure, stimulating aldosterone release.

ANP is a hormone released from the heart atria in response to high blood pressure. It acts to decrease blood pressure and volume by inhibiting the production of aldosterone and other hormones that promote sodium retention, and by promoting sodium and water excretion.

Potassium is the most abundant cation in the intracellular fluid (ICF) and is crucial for muscle contraction, nerve impulse transmission, and maintaining cell membrane potential.

Potassium balance is regulated primarily by the kidneys, which control potassium excretion. Aldosterone, a hormone released by the adrenal cortex, stimulates potassium secretion by the kidneys.

Calcium is essential for strong bones and teeth, muscle contraction, nerve impulse transmission, and blood clotting.

Calcium balance is regulated by a complex interplay of hormones, including parathyroid hormone (PTH) and calcitonin. PTH increases calcium levels in the blood by promoting its release from bones and absorption from the intestines. Calcitonin decreases calcium levels in the blood by inhibiting its release from bones.

Calcium in the extracellular fluid plays a crucial role in various bodily functions. It is essential for blood clotting, maintaining cell membrane permeability, regulating secretory activities, and most importantly, neuromuscular excitability.

PTH is a hormone secreted by the parathyroid glands that regulates calcium levels in the extracellular fluid. It increases calcium levels by promoting bone reabsorption, enhancing calcium reabsorption in the kidneys, and indirectly boosting calcium absorption in the small intestine.

Hypocalcemia is a condition characterized by low calcium levels in the extracellular fluid. It increases neuromuscular excitability, making muscles overly sensitive to stimulation. This can lead to muscle tetany, causing involuntary muscle contractions and spasms.

Hypercalcemia is a condition characterized by high calcium levels in the extracellular fluid. It inhibits neuronal and muscle cell activity, which can lead to fatal heart arrhythmias.

Chloride is the major anion accompanying sodium in the extracellular fluid. It contributes to maintaining the normal osmotic pressure of the blood. Chloride reabsorption is primarily passive, following sodium in the proximal convoluted tubule.

Maintaining a stable pH in body fluids is crucial for proper functioning. The normal pH of arterial blood is 7.4, while venous blood and interstitial fluid are slightly less acidic. Internal cellular fluid has a lower pH.

Chemical buffer systems consist of weak acids and their corresponding weak bases. These systems resist pH changes in body fluids by donating or accepting protons (H+). The main three types are bicarbonate, phosphate, and protein buffers.

The respiratory system eliminates carbon dioxide (CO2) from the blood, which is important for regulating H+ levels. Increased ventilation removes more CO2, reducing blood H+ concentration (increasing pH), while decreased ventilation allows CO2 to accumulate, raising blood H+ concentration (lowering pH).

The kidneys are the most potent mechanism for maintaining acid-base balance. They regulate blood bicarbonate levels by either conserving or generating new bicarbonate, or by excreting bicarbonate. The kidneys also regulate the excretion or retention of protons (H+), which is essential for bicarbonate maintenance.

The process of reabsorbing filtered bicarbonate ions from the renal tubules back into the bloodstream. This occurs in the proximal convoluted tubule (PCT) and the type A intercalated cells of the collecting duct.

Two mechanisms in the PCT and type A intercalated cells, involving the excretion of buffered H+ in the form of H2PO4 and NH4+, generate new bicarbonate ions to maintain pH balance.

The most important urine buffer is the phosphate buffer system. Secreted H+ binds to hydrogen phosphate (HPO4-), forming dihydrogen phosphate (H2PO4-), which is then excreted in the urine, generating new bicarbonate.

A major mechanism for excreting acid, involving the metabolism of glutamine in PCT cells. Each glutamine produces 2 ammonium ions (NH4+), which are excreted in the urine, and 2 'new' bicarbonate ions which are transported into the blood.

Type B intercalated cells in the collecting duct secrete bicarbonate while reabsorbing and generating hydrogen ions when plasma pH needs to become more acidic.

A condition where blood pH falls due to insufficient ventilation, leading to an increase in arterial partial pressure of carbon dioxide (above 45 mm Hg).

A condition where blood pH rises due to hyperventilation, leading to a decrease in arterial partial pressure of carbon dioxide (below 35 mm Hg).

A condition with low blood pH and bicarbonate levels caused by factors like alcohol ingestion, excessive bicarbonate loss, or lactate accumulation.

A condition with high blood pH and bicarbonate levels, often caused by vomiting acidic contents of the stomach or intake of excess base.

The lungs attempt to compensate for metabolic pH problems by adjusting ventilation to bring the arterial partial pressure of carbon dioxide closer to normal.