Renal System Physiology and Nephron Function

Renal System Functions and Mechanisms

The renal system functions to regulate the balance of the internal environment, ensuring both the excretion of metabolites and the retention of necessary anabolites (ions). The kidney performs its functions through various mechanisms, which include:

• Glomerular filtration
• Tubular reabsorption
• Secretion
• Excretion via urine

The Nephron: Morphofunctional Unit

The morphofunctional unit of the kidney is the nephron, which consists of a glomerulus and a tubule system. This system includes the proximal tubule, the loop of Henle, and the distal tubule, which empties into collecting ducts. The fluid that exits these ducts is urine.

Nephron Types and Distribution

Filtration occurs in the glomerulus, while reabsorption and secretion occur in the tubules. There are two types of nephrons: cortical (located in the cortex) and juxtamedullary (located in the medulla). The capillaries are distributed in parallel to the glomerular system.

Body Fluid and Urinary Functions

Approximately 60% of body weight (40 liters) is water, of which 15 liters is extracellular fluid. The primary urinary functions are:

• Waste Removal: Eliminating products such as urea, creatinine, and uric acid.
• Filtration: The leakage of fluid from glomerular capillaries into the renal tubule.
• Reabsorption: The transport of substances from inside the tubule back into the blood.
• Secretion: The transport of substances from the blood into the tubule.
• Excretion: The elimination of foreign substances into the urine.

Kidney Function and Regulation

The kidney is responsible for the purification of extracellular fluid, electrolyte balance adjustment, and pH adjustment. Other vital functions include:

• Blood pressure regulation
• Hormone release
• Gluconeogenesis

The Glomerulus and Filtration Rate

The glomerulus arises from an afferent arteriole and ends in an efferent arteriole. It differs from the systemic circulatory system because one arteriole feeds into another. The glomerulus is contained within Bowman's capsule. The normal Glomerular Filtration Rate (GFR) is approximately 125 ml/min. The filtration fraction is the percentage of plasma that becomes urine.

Tubular Resorption and Segment Functions

The tubules filter approximately 125 ml/min, which totals 180 liters per day.

Proximal Convoluted Tubule (PCT) Functions

The PCT absorbs 100% of glucose and amino acids. It also absorbs 85-90% of bicarbonate (HCO3-), uric acid, urea, and albumin, as well as 40-60% of water, sodium, potassium, calcium, magnesium, and organic acids and bases. Additionally, it secretes endogenous and exogenous substances, activates Vitamin D, and handles erythropoietin synthesis.

The Loop of Henle

The loop of Henle creates a medullary osmotic gradient through the countercurrent multiplier mechanism.

• Descending branch: Reabsorbs 40% of filtered water.
• Ascending limb: Reabsorbs 30-50% of Na, K, Ca, and Mg, and 5% of filtered bicarbonate.

Distal Convoluted Tubule (DCT) Functions

The DCT reabsorbs 3-7% of filtered Na, water, Ca, bicarbonate, phosphate, and Mg. In the final portion, it produces the final regulation of K excretion and the acidification of urine by H+. This is regulated by aldosterone and part of the final control of water excretion is regulated by ADH.

Collecting Tubule (CT) Functions

The CT handles 5% resorption. It is regulated by aldosterone and controls the final excretion of water via ADH regulation.