9.4 Body fluid; blood pressure regulation, Electrolyte Balance & Renin-Angiotensin-Aldosterone System

Body fluid

  • Intracellular fluid (ICF)

    • Located within cells

    • Major component of total body water (2/3)

  • Extracellular fluid (ECF)

    • Located outside the cell

      • Interstitial fluid: Between cells and tissue

      • Plasma: Fluid component of blood

  • Components: Water, protein, electrolytes, others

  • Functions: Transport/carriage, colloid osmotic pressure, membrane excitability, buffering

  • Compartments are separated by barriers

  • Capillary wall

    • Separates plasma and interstitial fluid

    • Movement passive via pores

  • Plasma (cell) membrane

    • Separates interstitial fluid and intracellular fluid

    • Movement regulated by passive and active mechanisms, such as channels, pumps

Fluid gain and loss

  • Fluid gain:

    • Ingested liquids, food, and metabolic production (e.g. from cellular respiration)

  • Fluid loss:

    • Urinary loss, sweating, respiratory evaporation, and gastrointestinal losses

  • Regulation of fluid balance:

    • Thirst mechanism: Activated by changes in serum osmolarity, detected by receptors in the hypothalamus

    • Water loss: regulated by kidneys, skin and lungs

Acidosis and alkalosis

  • Acid-base balance describes concentration of free hydrogen ions

    • A substance is an acid if it gives up H+ in solution

    • A substance is a base if it binds H+ in solution

  • a Major source of H+ is metabolically produced as a result of cellular respiration, particularly during the breakdown of glucose and fatty acids.

  • Acidosis

    • Blood pH falls below 7.35

    • Can cause CNS depression, coma

    • Results from an excess of H+ or insufficient bicarbonate (HCO3-)

  • Alkalosis

    • Blood pH rises above 7.45

    • Can cause CNS excitation, leading to seizures and spasms

    • Results from exceess bicarbonate or a loss of H+

  • Regulation

    • Respiratory system: adjusts CO2 levels to regulate H+ concentrations. Increased breathing reduces CO2 (raising pH), and decreased breathing increases CO2 (lowering pH)

Juxtaglomerular apparatus

  • It is a specialised collection of cells in the nephron, responsive to changes in blood pressure and sodium levels

  • Components

    • Granular cells (Juxtaglomerular cells): Release renin in response to low blood pressure or sodium levels

    • Macula densa: Senses changes in sodium concentration in the distal convoluted tubules and signals granular cells

  • Function: regulates blood pressure and electrolyte balance by controlling the release of renin, which triggers the Renin-angiotensin-aldosterone system pathway

Renin-Angiotensis-aldosterone system

  • Renin release

    • Triggered by low blood pressure, low sodium levls, or sympathetic nervous system activation.

    • Renin converts angiotensinogen (from the liver) into angiotensis I, which is then converted to angiotensin II by the enzyme ACE (angiotensin-converting enzyme) primarily in the lungs

  • Angiotensin II effects

    • Vasoconstriction: increases systemic vascular resistance, raising blood pressure

    • Aldosterone release: Stimulates the adrenal glands to release aldosterone, which acts on the distal convoluted tubule and collecting duct of the nephron to

      • Increase sodium reabsorption

      • Promoted potassium secretion

  • Aldosterone

    • Acts on the principal cells of the distal convoluted tubule and collecting ducts, promoting sodium retention and potassium excretion

    • Chloride (Cl-) follows sodium passively, contributing to fluid balance.

  • End result: The RAAS pathway enhances sodium and water retention, elevating blood volume and blood pressure to maintain homeostasis.