An Introduction to Kidneys & Body Fluids

• What are the main fluid compartments of the body?

  1. Intracellular fluid (ICF) – inside cells
  2. Extracellular fluid (ECF) – outside cells
• What percentage of total body water is intracellular fluid (ICF)?

  ICF accounts for ~60% of total body water (~25L in a 70kg person).
• What percentage of total body water is extracellular fluid (ECF)?

  ECF accounts for ~40% of total body water (~15L in a 70kg person).
• What are the two major subdivisions of extracellular fluid (ECF)?

  1. Interstitial fluid (ISF) – surrounds cells (~75% of ECF)
  2. Plasma – fluid portion of blood (~25% of ECF)
• What percentage of extracellular fluid (ECF) is interstitial fluid (ISF)?

  ISF makes up ~75% of ECF (~12L in a 70kg person).
• What percentage of extracellular fluid (ECF) is plasma?

  Plasma makes up ~25% of ECF (~3L in a 70kg person).
• What is transcellular fluid, and how much does it contribute to total body water?

  Transcellular fluid includes specialised fluids (e.g., cerebrospinal, synovial, pleural, peritoneal) and accounts for ~1-2L of total body water.
• How is total body water (TBW) distributed across compartments?

  • ICF: ~60% of TBW (~25L)
  • ECF: ~40% of TBW (~15L)
  • Interstitial fluid: ~12L
  • Plasma: ~3L
•  What percentage of body weight is total body water (TBW)?

  TBW is ~60% of body weight in a healthy adult male (~42L in a 70kg person).
• What factors influence total body water percentage?

  • Higher TBW: Infants, males, lean individuals
  • Lower TBW: Elderly, females, obese individuals (fat contains little water)
• What is osmoregulation?

  Process of maintaining the balance of water and solutes in the body to ensure stable osmotic pressure. This prevents excessive water gain or loss in cells and maintains homeostasis.
• What is volume regulation?

  Volume regulation refers to the control of extracellular fluid (ECF) volume by regulating sodium and water balance. It ensures adequate blood pressure and organ perfusion.
• What is the primary organ responsible for osmoregulation and volume regulation?

  The kidneys play a key role in osmoregulation and volume regulation by filtering blood, reabsorbing water and electrolytes, and excreting excess fluids as urine.
• How does osmoregulation prevent excessive cell swelling or shrinking?

  By regulating the concentration of extracellular solutes, osmoregulation prevents:

  • Swelling (lysis): If ECF is too dilute (hypotonic).
  • Shrinking (crenation): If ECF is too concentrated (hypertonic).
• What is the role of antidiuretic hormone (ADH) in osmoregulation?

  ADH (vasopressin) increases water reabsorption in the kidneys by inserting aquaporins in the collecting ducts, reducing urine output and preventing dehydration.
• What triggers ADH release?

  • High plasma osmolarity (detected by osmoreceptors in the hypothalamus).
  • Low blood volume (detected by baroreceptors in blood vessels).
• How does aldosterone contribute to volume regulation?

  Aldosterone increases sodium (Na⁺) reabsorption in the distal tubule and collecting duct, which indirectly increases water retention and maintains blood volume.
• What triggers aldosterone release?

  • Low blood pressure (via the renin-angiotensin-aldosterone system, RAAS).
  • High potassium (K⁺) levels in the blood.
• What is the role of the renin-angiotensin-aldosterone system (RAAS) in volume regulation?

  RAAS increases blood volume and pressure by:

  1. Renin release (by kidneys in response to low BP).
  2. Renin converts angiotensinogen → angiotensin I.
  3. ACE (angiotensin-converting enzyme) converts it to angiotensin II.
  4. Angiotensin II stimulates aldosterone release, promoting Na⁺ and water retention.
• How do natriuretic peptides (ANP & BNP) regulate volume?

  Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) oppose RAAS, reducing blood volume by:

  • Inhibiting Na⁺ reabsorption, increasing sodium excretion (natriuresis).
  • Dilating blood vessels, lowering blood pressure.
  • Reducing ADH and aldosterone secretion.
• What are the main structural components of the kidney?

  • Cortex: Outer region containing glomeruli and convoluted tubules.
  • Medulla: Inner region with renal pyramids, loops of Henle, and collecting ducts.
  • Renal pelvis: Funnel-shaped cavity leading to the ureter.
  • Ureter: Transports urine to the bladder.
  • Renal artery: Supplies oxygenated blood.
  • Renal vein: Drains deoxygenated blood.
• What is the functional unit of the kidney?

  The nephron is the functional unit of the kidney, responsible for filtration, reabsorption, and excretion. Each kidney contains ~1 million nephrons.
• What are the main parts of the nephron?

  • Renal corpuscle (Filtration site)
  • Glomerulus: A capillary network for filtration.
  • Bowman’s capsule: Surrounds the glomerulus, collecting filtrate.
  • Renal tubule (Modification of filtrate)
  • Proximal convoluted tubule (PCT): Reabsorbs nutrients, water, and ions.
  • Loop of Henle: Maintains concentration gradient; descending limb reabsorbs water, ascending limb reabsorbs ions.
  • Distal convoluted tubule (DCT): Regulates ion balance and pH.
  • Collecting duct: Final site for water reabsorption; carries urine to the renal pelvis.
• What is the renal corpuscle, and what is its function?

  • The renal corpuscle consists of the glomerulus and Bowman’s capsule.
  • Function: Filtration of blood, allowing small molecules (e.g., water, ions, glucose, urea) to pass while retaining large molecules (e.g., proteins, cells).
• What is the role of the proximal convoluted tube (PCT)?

  • Reabsorbs ~65% of filtrate, including glucose, amino acids, sodium, and water.
  • Uses active transport for essential molecules.
  • Microvilli increase surface area for absorption.
• How does the Loop of Henle contribute to urine concentration?

  • Descending limb: Permeable to water → water moves out → filtrate becomes concentrated.
  • Ascending limb: Impermeable to water → actively pumps out Na⁺, Cl⁻ → filtrate becomes dilute.
  • Establishes a countercurrent multiplier to maintain medullary concentration gradient.
• What is the role of the distal convoluted tubule (DCT)?

  • Fine-tunes ion balance (Na⁺, K⁺, Ca²⁺, H⁺).
  • Aldosterone increases Na⁺ reabsorption and K⁺ secretion.
  • Regulates pH by H⁺ and HCO₃⁻ exchange.
• What is the function of the collecting duct?

  • Final site of water reabsorption (regulated by ADH).
  • Urine concentration adjusts based on body hydration levels.
  • Drains urine into the renal pelvis → ureter → bladder.
• Where does filtration occur in the kidney?

  Filtration occurs in the glomerulus within the renal corpuscle.
• What drives filtration in the glomerulus?

  Hydrostatic pressure of the blood forces water and small solutes out of the glomerular capillaries into Bowman’s capsule.
• What is the composition of the filtrate?

  Plasma-like fluid containing water, ions (Na+, K+, Cl-), glucose, amino acids, and waste products (urea, creatinine) but lacking proteins and blood cells.
• What is the glomerular filtration rate (GFR), and why is it important?

  GFR is the volume of filtrate formed per minute (~125 mL/min in healthy adults); it reflects kidney function.
• What is the primary purpose of tubular reabsorption?

  To reclaim essential substances (e.g., water, glucose, ions) back into the blood from the filtrate.
• Where does most reabsorption occur?

  Proximal convoluted tubule (PCT) – ~65-70% of filtrate is reabsorbed here.
• How is sodium (Na+) reabsorbed?

  Active transport via Na+/K+ ATPase pumps in the renal tubules.
• How is water reabsorbed?

  Water follows sodium via osmosis, mainly through aquaporins in the proximal tubule and collecting duct (if ADH is present).
• What is the renal threshold for glucose reabsorption?

  ~10 mmol/L (180 mg/dL); above this, glucose appears in the urine (glycosuria).
• What is tubular secretion?

  The active transport of substances from the blood into the filtrate for elimination.
• Where does most secretion occur?

  Distal convoluted tubule (DCT) and collecting duct.
• What substances are commonly secreted?

  H+ ions (for acid-base balance), K+ ions (regulated by aldosterone), creatinine, and drugs (e.g., penicillin, toxins).