The Body Fluids and Kidneys

---

Fluid Balance

• Maintenance of constant volume and stable composition is crucial for homeostasis.
• Abnormalities in control systems cause many clinical symptoms.
• Continuous exchange of fluids and solutes occurs with the external environment and different body compartments.
• Fluid intake varies and must be balanced by equal output to prevent changes in body volumes.

Water Intake

• Ingestion: about 2000 ml/day
• End product of carbohydrate oxidation: 200 ml/day
• Total intake: about 2200 ml/day

Water Loss

• Evaporation through respiration and diffusion: about 700 ml/day (insensible water loss)
• Sweat: variable, about 100 ml/day
• Feces: about 100 ml/day
• Kidneys: most important regulator, loss varies from 0.5 l to 10 l daily.

Body Fluid Compartments

• Two compartments: extracellular (plasma and interstitial fluid) and intracellular.
• An adult human has about 42 liters of fluids, varying with age.

Intracellular Fluid

• Fluid inside cells, about 28 liters.
• Consistent concentrations of constituents across cells.

Blood Volume

• Contains both extracellular (plasma) and intracellular fluids (inside red cells).
• Average blood volume: 7% of body weight (~5 l), with 60% plasma and 40% cellular components.

Composition of Extracellular and Intracellular Fluids

• Ionic composition of plasma and interstitial fluid is similar, except for protein concentration.
• Major ions in extracellular fluid: sodium, calcium, chloride, bicarbonate.
• Major ions in intracellular fluid: potassium, magnesium, phosphates, proteins.

Movement of Body Fluids and Electrolytes

• Continuous intake and output within the body and between compartments.
• Stable composition and volume maintain dynamic equilibrium (homeostasis).

Movement of Solutes Between Body Fluid Compartments

1. Membrane permeability.
2. Concentration and electric gradients.
3. Pressure gradients and active transport systems (e.g., sodium-potassium pump).

Movement of Water Between Body Fluid Compartments

• Controlled by hydrostatic pressure and osmotic pressure.
• Osmosis: water moves from low solute concentration to high solute concentration.
• Edema: excess fluid in interstitial spaces due to imbalances.

Regulation of Fluid Exchange and Osmotic Equilibrium

• Balance between plasma and interstitial fluid is determined by hydrostatic and colloid osmotic pressure.
• Balance between extracellular and intracellular fluid is by osmotic pressure of solutes.

Osmosis and Osmotic Pressure

• Movement of water from high water (low solute) concentration to low water (high solute) concentration.
• Osmotic pressure depends on the number of solute particles.

Edema

• Presence of excess fluids in body tissues, mostly extracellular.
• Causes: increased capillary pressure, decreased plasma proteins, increased capillary permeability, blockage of lymph return.

Renal Functions

• Excretion of metabolic waste products and foreign chemicals.
• Regulation of water and electrolyte balances.
• Regulation of body fluid osmolality and electrolyte concentrations.
• Regulation of arterial pressure.
• Regulation of acid-base balance.
• Secretion, metabolism, and excretion of hormones.
• Gluconeogenesis.

Excretion

• Kidneys eliminate metabolic waste products, toxins, and foreign substances.

Regulation of Water and Electrolyte Balances

• Kidneys adjust excretion rates to match intake for homeostasis.

Regulation of Arterial Pressure

• Long-term regulation through sodium and water excretion.
• Short-term regulation by secreting vasoactive substances.

Regulation of Acid-Base Balance

• Kidneys excrete acids and regulate body fluid buffer stores.

Regulation of Erythrocyte Production

• Kidneys secrete erythropoietin, stimulating red blood cell production in response to hypoxia.

Regulation of Vitamin D Production

• Kidneys produce active vitamin D (calcitriol), essential for calcium metabolism.

Glucose Synthesis

• Kidneys perform gluconeogenesis during prolonged fasting.

Urinary System Structure

• Components: kidneys, ureters, bladder, urethra.
• Kidneys divided into cortex and medulla, with nephrons as functional units.

Renal Blood Supply

• Blood flow to kidneys is about 22% of cardiac output.
• Renal artery branches into arterioles and capillaries, regulating hydrostatic pressure.

Nephron Structure

• Nephrons consist of glomerular capillaries and a long tubule.
• Types: cortical (short loops of Henle) and juxtamedullary (long loops of Henle).

Juxtaglomerular Apparatus

• Regulates blood pressure through renin secretion and response to NaCl content.

Glomerular Filtration

• Passive process driven by hydrostatic pressures.
• Efficient filtration due to large surface area and high pressure.
• Filtration membrane includes endothelium, basement membrane, and podocytes.

Filtration Forces

• Blood hydrostatic pressure, colloid osmotic pressure, and capsular hydrostatic pressure determine filtration rate.
• Glomerular filtration rate (GFR) of 125 ml/min (180 L/day).

Pathophysiology of Edema

• Causes include increased capillary pressure, decreased plasma proteins, increased capillary permeability, and lymphatic blockage.