Control of Plasma Volume and Proximal Tubule Composition

Control of Plasma Volume

• The kidneys handle sodium ions to regulate extracellular fluid (ECF) volume.
• Handling sodium ions in the proximal convoluted tubule (PCT) involves glomerulotubular balance and ECF volume effects.
• Sodium ion reabsorption in the loop of Henle is a key process, though not solely through the countercurrent system.
• Sodium ion uptake occurs in both the early and late distal tubules.
• Hormones, sympathetic nerves, dopamine, and Starling forces regulate sodium chloride reabsorption.
• The renin-angiotensin system controls sodium uptake in response to blood pressure changes.
• The body's water is either intracellular or extracellular (composed of interstitial fluid and plasma).
• Extracellular fluid composition influences intracellular fluid.
• Kidneys regulate extracellular fluid composition, influencing intracellular composition.

Proximal Tubule Composition

• 65% of water and 67% sodium are reabsorbed
• 100% of glucose and amino acids are reabsorbed
• 90% of bicarbonate is reabsorbed

Loop of Henle Composition

• 25% of sodium and 15% of water are reabsorbed

Distal and Collecting Ducts Composition

• 20% of water, 8% of sodium, and 10% of bicarbonate are reabsorbed
• These processes maintain sodium ion balance.

Sodium Ion Balance

• Kidneys balance sodium ion excretion and ingestion daily.
• Sodium is the primary cation of ECF.
• Sodium ion concentration dictates ECF volume; this influences plasma, blood volume, and blood pressure.
• An excess of sodium ion excretion leads to negative sodium ion balance, resulting in reduced ECF volume and blood pressure.

ECF Expansion/Contraction

• Increased ECF volume is detected by sensors in the vascular system (volume/baroreceptors).

• Low-pressure baroreceptors in the atria and pulmonary vasculature signal the brainstem via the vagus nerve.

• This signaling reduces sympathetic nerve outflow and increases antidiuretic hormone (ADH) secretion, thus promoting water reabsorption.

• Kidneys increase NaCl and water excretion to reduce ECF volume if it increases. They decrease NaCl and water excretion to increase the ECF volume if it decreases.

• Changes in sodium concentration in the extracellular fluid do not generally affect extracellular fluid osmolarity.

Fluid Compartments

• Fluid is located intracellularly and extracellularly (composed of interstitial fluid and plasma)
• Extracellular fluid composition affects intracellular fluid
• Kidneys control extracellular fluid composition, allowing for changes in intracellular composition

Reference

• Textbook of medical physiology by Guyton & Hall, 9th edition.