Renal Function and Urine Formation: Key Physiological Processes

Posted on Jan 22, 2025 in Biology

Daily Water Losses

  • Insensible water loss: Respiratory evaporation and diffusion into the skin.
  • Sweat (exercise and temperature).
  • Feces.
  • Extracellular urine (60%).
  • Intracellular (40%).

Renal Function

Excretion of metabolic waste products, foreign chemicals, drugs, and metabolites of the water balance and hormones. Regulation of electrolyte balance. To maintain homeostasis, excretion of water and electrolytes is essential.

Regulation of Arterial Pressure (Long Term)

By regulating excretion of varying amounts of sodium and water.

  • Short-term secretion of vasoactive substances (renin) that form vasoactive products (angiotensin II).

Regulation of Acid-Base Balance

Excretion of acids. Regulating reserve buffer substances in body fluids.

Regulation of Red Cell Production

Erythropoietin stimulates red cell production.

Formation of Urine

Urinary excretion = Filtration – Reabsorption + Secretion

Glomerular Filtration

Filtration of glomerular capillaries to Bowman’s capsule. Filtrate is devoid of practically all proteins. Sales and organic molecules are similar in concentration to the plasma filtrate.

Glomerular Filtration Rate

Greater than other capillaries (greater hydrostatic pressure and increased capillary filtration coefficient). 20% of plasma passing through the kidney is filtered in the glomerular capillaries.

Filtration Net Pressure

Sum of hydrostatic forces and colloid osmotic pressure that favor or oppose filtration through the glomerular capillaries.

Filtration Path

  1. Glomerular hydrostatic (inside capillary): Favors filtration.
  2. Bowman’s capsule hydrostatic (not capillary): Opposes filtration.
  3. Colloid Protein Plasma Capillaries: Opposes filtration.
  4. Colloid Proteins of Bowman’s Capsule: Favors filtration.

The Kidneys

The kidneys regulate many aspects: the osmolality and volume of the liquid electrolyte balance. The acid-base balance and excretion of metabolic products and foreign substances. The production and secretion of hormones. Abdominal organs are retroperitoneal, at the level of T11 and L3. The medulla is divided into the cortex and contains approximately 1.25 million nephrons. The medulla forms 8-15 pyramids.

Tubular Processes

Proximal Tubular Reabsorption

65% of the load of sodium and water, and a lower percentage of filtered chloride than in the proximal tubule, is reabsorbed before reaching the loop of Henle.

Secretion of Acids and Bases

The proximal tubule secretes organic substances such as bile salts, oxalate, urate, and catecholamines, as well as drugs and toxic chemicals.

Water and Solute Transport in Henle’s Loop

Thin Descending Limb

Highly permeable to water, moderately permeable to solutes (urea and sodium). About 20% of filtered water is reabsorbed in Henle’s loop (the highest percentage in this segment).

Thick Ascending Limb

Waterproof, actively reabsorbs sodium, chloride, and potassium. About 25% of the filtered load of sodium, chloride, and potassium are reabsorbed in this segment of Henle’s loop. Also, some calcium, magnesium, and bicarbonate.

Thin Ascending Limb

Reabsorption capacity is much less than before.

Distal and Collecting Tubules

Second half of the distal and collecting tubules.

Chief Cells: Sodium reabsorption and secretion of potassium (sodium-potassium pump).

Intercalated Cells: Secrete an intense percentage of hydrogen ions (hydrogen ATPase).

Functional Characteristics of These Final Segments

  • Impermeable to urea.
  • Sodium reabsorption rate controlled by aldosterone.
  • Cells intensely secrete hydrogen ions.
  • Water permeability in these segments is controlled by [ADH].

Medullary Collecting Duct

Regulated water permeability by ADH. It is permeable to urea and secretes hydrogen ions.

Circulatory Function

  1. Blood flow to all body tissues is almost always accurately controlled in relation to tissue needs.
  2. Cardiac output is principally controlled by the sum of all local tissue flows.
  3. Blood pressure is controlled independently by controlling local blood flow or cardiac control.

Cross-Sectional Area

The arterial system of man is bifurcating. The sum of the cross-sectional area of the branches will be greater than the area of the original trunk.

Blood Stream Velocity

In a tube that contains a wide and a narrow segment, the velocities of the fluid in the two segments are inversely proportional to their cross-sectional area.

Blood Flow

Amount of blood passing a given point in the circulation in a given period.