Renal Physiology: Fluid Balance, Filtration and Reabsorption

Body Fluid Compartments

Liquids:

• Intracellular fluid (65% of body weight; 30-40% of body weight, 50-70% of total body water)
• Extracellular fluid (35% of body weight; 20% of body weight, 35% of total body water). It includes plasma (4%), interstitial fluid (11-15%) and transcellular fluid (1-3%).

Osmolarity and Tonicity

Osmolarity: total solute concentration of body fluids. Normal value: 290 ± 10 mOsm/L.

Tonicity: effective osmotic pressure of a solution compared with plasma. Isotonic solutions include 0.9% NaCl and 5% glucose.

Edema

• Intracellular edema: predisposed by hyponatremia, depletion of the metabolic system and lack of cellular nutrition. Caused by increased intracellular volume (decreased perfusion, reduced ionic pump function, inflammation).
• Extracellular edema: more frequent. Caused by fluid leaking from plasma into the interstitium, since capillaries and lymphatic vessels do not return fluid to the plasma. Mechanisms include:
  1. Increased capillary pressure.
  2. Decreased plasma protein.
  3. Increased capillary permeability (lack of vitamin C).
  4. Blockage of lymphatic drainage.

Hydration and Electrolyte Disorders

• Hyperhydration: due to renal, cardiac or hepatic failure.
• Hyponatremia: Na⁺ below 136 mEq/L.
• Hypernatremia: Na⁺ above 145 mEq/L; hypotonic solution is given.
• Hypokalemia: K⁺ below 3.5 mEq/L; flat T waves and U waves on ECG.
• Hyperkalemia: K⁺ above 5.5 mEq/L; peaked T waves. Treated with dialysis and calcium gluconate.

Renal System

1) Functions

The kidney regulates the osmolarity and volume of body fluids, blood pressure, acid-base balance, erythropoiesis, vitamin D activation, excretion of waste, and gluconeogenesis.

2) Juxtaglomerular Apparatus

• Juxtaglomerular cells store and secrete renin.
• The macula densa regulates renin secretion.
• Mesangial cells provide support, phagocytosis and contraction of glomerular vessels.

3) Formation of Urine

3.1) Glomerular Filtration

Glomerular filtration produces plasma without cells or proteins. About 180 L per day of plasma is filtered. The filtration fraction is the portion of renal plasma flow that passes into the filtrate, approximately 19%.

A) Filtration barrier:

• a. Glomerular capillary endothelium: pores prevent passage of cells.
• b. Basement membrane: negatively charged, repels proteins.
• c. Bowman's capsule epithelial cells (podocytes): negatively charged; pores block particles larger than 100,000 daltons.

B) Hydrostatic pressure: pressure inside the glomerular capillary.

C) Opposing forces: osmotic pressure in the capillary and in Bowman's capsule.

D) Net filtration pressure: 10 mmHg.

E) Factors: The filtration coefficient (Kf) depends on filtration surface area (total capillary area) and permeability. The effective filtration pressure (EFP) depends on glomerular capillary pressure (Pgc), which in turn depends on the degree of afferent/efferent arteriolar constriction.

3.2) Reabsorption

A) Driving force: osmotic pressure in plasma and interstitium.

B) Opposing force: capillary and interstitial pressure.

C) Net reabsorption pressure: 10 mmHg.

Regulation of renal blood flow: renal blood flow (RBF) depends on arterial pressure and the degree of smooth muscle contraction. Changes in RBF and GFR alter Na⁺ and H₂O excretion. The kidney keeps RBF and GFR constant at an arterial pressure of 80-180 mmHg through two mechanisms:

1. Myogenic mechanism: smooth muscle of the vessel contracts when stretched by increased arterial pressure.
2. Tubuloglomerular feedback: increased arterial pressure leads to increased glomerular pressure, which increases H₂O and NaCl delivery to the distal tubule. This stimulates the macula densa, causing vasoconstriction of the afferent arteriole.