Kidney Function and Hydration

• The nephron is the functional unit of the kidney, containing the glomerulus and tubules.
• The Kidney regulates fluid balance during steady state.
• The tubule regulates urine concentration: proximal convoluted tubules, loop of Henle, distal convoluted tubules, and collecting duct.
• The interstitium holds the glomerulus and tubules together.
• Blood flows into the kidneys through the renal artery, gets filtered, and waste is excreted in the urine; cleaned blood returns to the body through the renal vein.

Terminology

• Euhydration is normal body water content within the homeostatic range achieving + 2% total body water content.
• Dehydration is dynamic body water loss such as transition from euhydration to hypohydration.
• Rehydration is dynamic body water gain via fluid intake transitioning from hypohydration to euhydration.
• Hypohydration indicates a body water deficit.
• Over- or Hyperhydration indicates body water excess/elevation.

Total Body Water

• Total body water(TBW)volume is the combination of sodium and water.
• Euvolemic state indicates normal body water and sodium content within homeostatic range
• Hypovolemic state indicates body water and sodium deficit.
• Hypervolemic state indicates body water and sodium excess
• Hydration is related to water, and volemia is related to sodium.
• Serum sodium measures both hydration and volume.
• With a constant amount of sodium but too much water present, sodium laboratory values will decrease; water replacement alone can cause hyponatremia and require ion replacement

Water Loss and Gain

• Hydration status is influenced by sweating, urination, insensible water, respiration, fuel oxidation, feces etc..
• Insensible water loss accounts for water that cannot be measured.
• Processes leading to body mass loss include sweating, urination, insensible water loss, respiration, and feces.
• Processes leading to body mass gain include drinking, eating, and IV fluids.
• Urine output can range from 0.5-20L per day.
• Urine output, the biggest contributor of water excretion, can reach 20L per day if sick or just drinks a lot of fluids, is also dependent on sodium and water intake.
• Wherever sodium goes, water follows*

Daily Intake and Output of Water

• Intake refers to the volume of fluids ingested and from metabolism.
• Output includes insensible water loss from skin and lungs, sweat, feces, and urine.

Fluid Compartments

• Total body water (TBW) accounts for 50-70% of body mass.

Values of TBW

• Men is 60% and women is 50%.
• TBW = ~0.73 multiplied by fat-free mass with newborns around 70-75%
• Extracellular fluid (ECF) accounts for 20% of TBW.
• Intracellular fluid (ICF) accounts for 40% of TBW.
• Transcellular Fluid accounts for 1-2 L.
• Body water is divided across compartments Given a 70kg man:
• ECF is 14 L
• Plasma is 3L
• Interstitial Fluid is 11 L
• 40% of TBW is 28 L

Membranes

• Water moves to the compartment with a greater concentration of solutes.
• Water (intake) normally first goes to the plasma, then moves through the capillary membrane into the interstitial fluid and then through the cell membrane to reach the intracellular fluid.

Ion Concentration

• Na+ is highest in ECF at 140 mEq/L.
• K+ is highest in the ICF at 140 mEq/L.

Anions and Cations

• Cations are positively charged ions.
• Anions are negatively charged ions.
• Plasma has ~2% higher concentration of cations than interstitial fluid, also, plasma proteins, which have a net negative charge, bind cations, retaining them in the plasma (Donnan Effect).
• Anions have a slightly higher concentration in interstitial fluid due to repulsion from plasma proteins.

Osmolarity and Osmolality

• Sodium (Na+) is the most abundant electrolyte in the extracellular space and controls water movement between fluid compartments.
• Water follows solute to maintain osmotic equilibrium.
• Osmolality measures osmoles per kilo of water, whereas osmolarity measures osmoles per liter of water.
• Osmoles are comprised of sodium, proteins, carbohydrates, and other ions and are independent of the cells.
• Normal Osmolarity for humans is 282 mOsm/L, and it dependent on sodium/2x sodium (surrogate)
• When a hyperosmolar solution is added to the extracellular compartment, free water will then shift out of the cells and into the ECF.

Tonicity

• The ability of a solution to modify the volume of cells by altering their water content.
• Solutions are either isotonic, hypertonic, or hypotonic:
  • Isotonic: water concentration in the intracellular and extracellular fluids is equal.
  • Hypotonic: Solution that has a lower concentration of impermeant solutes, and water will diffuse into the cell, causing it to swell.
  • Hypertonic: Solution that has a higher concentration of impermeant solutes, and water will flow out of the cell into the ECF, causing the cell to shrink.

Filtration

• The movement of water across a semipermeable membrane.
• Determinants of capillary filtration (Starling forces):Kf * (PC – PIF – ㅠC + πIF)
• Capillary filtration occurs because fluid and colloid osmotic pressure forces operate at the capillary membrane and move fluid outward or inward through the membrane pores.
• Lymphatic reabsorption: Filtered water and solutes are absorbed into the lymphatic system to prevent fluid accumulation in the interstitial space.
• At the venous end, because fluid has left the capillary, the concentration of plasma proteins increases - oncotic pressure is now dominant.

Clinical Correlates: Hypernatremia and Hyponatremia

• Plasma sodium is a reasonable indicator of plasma osmolarity under many conditions, with other osmoles including glucose and amino acids
• Normal sodium value is 140 meq/L
• Hyponatremia is excess water and loss of sodium.
• Hypernatremia is lack of water and high sodium.

Clinical Causes of Hyponatremia

• Gastrointestinal losses of K+, Cl, Na+, and water.
• Use of diuretics (increased Na+ excretion in urine)
• Loop diuretics inhibit Na+ reabsorption in the Loop of Henle, leading to significant Na+ and water loss.
• Thiazide diuretics act on the distal tubule, blocking Nat reabsorption.
• K+-sparing diuretics act on the collecting duct, reducing Nat reabsorption while conserving K+.

Dehydration and Overhydration

• Dehydration is seen in Diabetes insipidus and in adrenal insufficiency
• Overhydration is seen in Cushing disease, excessive ADH, and primary aldosteronism.

Effects on the Brain

• Normonatremia sees balanced levels of Na+, K+, organic solutes, and H2O
• Acute Hyponatremia: Rapid decrease in the sodium concentration in the blood, leading to water influx into brain cells causing swelling.
• Chronic Hyponatremia: Brain adapts by gradually expelling solutes to minimize swelling, which makes rapid correction dangerous.

Syndrome of Inappropriate ADH (SIADH)

• Inappropriate secretion (oversecretion) of ADH.

Edema

• Extracellular edema is caused when there's leakage from the plasma to interstitial space across capillaries and
• Intracellular edema is caused by hyponatremia, depression of metabolic systems in the tissues and lack of nutrition.
• The components of edema are the same as the factors that can increase the capillary filtration rate: increase in capillary filtration coefficient, increases in capillary hydrostatic pressure; decrease plasma colloid oncotic pressure.

Causes of Edema

• Heart Failure impairs normal blood pumping from veins to arteries; Arterial Pressure Changes; Increased Renin/ADH.

Decreased Plasma Proteins

• Decreases plasma colloid osmotic pressure from failure to produce normal protein amounts or leakage from plasma and can be caused by burns or Wounds; Liver disease or Serous protein malnutrition

Physiology of Hydration

• Hypohydration - body water deficit
• Hypovolemia - decreased plasma volume
• Hyperosmolality - increased plasma osmolality
• Plasma osmolality: concentration of dissolved solutes in the blood

Sweating

• Factors impacting the variability in sweating rate include exercise intensity, body size, environmental conditions, heat acclimatization, fitness, clothing or equipment worn and body composition
• Abnormality in the heart will affect the kidney by trying to absorb more fluid due to the activation of RAAS

Minimal change disease

• Patients that have a kidney disease normally undergo a renal biopsy
• The outermost layer of the glomerular membrane is the epithelial cells or podocytes.
• Urine is also observed to be frothy (with bubbles) indicating proteinuria.