What Is Homeostasis?

Homeostasis is the maintenance of a stable internal environment through dynamic regulatory processes. Key components include:

• Set Points: Desired values for variables (e.g., plasma osmolality).
• Receptors: Sensors that detect deviations (e.g., osmoreceptors).
• Control Centers: Integrate signals (e.g., hypothalamus).
• Effectors: Organs or tissues that restore balance (e.g., kidneys).

Kidney Overview

The kidneys are paired retroperitoneal organs that filter blood, reabsorb essential substances, secrete wastes, and produce hormones. Each kidney contains about one million nephrons—the functional filtration units.

Fluid & Electrolyte Balance

Mechanisms: Glomerular filtration, tubular reabsorption and secretion adjust volume and electrolyte composition.

Key Ions: Sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), calcium (Ca²⁺), magnesium (Mg²⁺).

Acid–Base Regulation

Mechanisms: Secretion of H⁺ in distal tubule, reabsorption of bicarbonate (HCO₃⁻), generation of new HCO₃⁻.

Outcome: Maintains blood pH around 7.35–7.45 by excreting acid or base equivalents.

Blood Pressure Control (RAAS)

Renin–Angiotensin–Aldosterone System (RAAS): Juxtaglomerular cells release renin → angiotensin II formation → vasoconstriction and aldosterone secretion → Na⁺ and water retention.

Effect: Regulates systemic blood pressure and extracellular fluid volume.

Waste Excretion & Detoxification

Filtration: Removal of metabolic wastes (urea, creatinine) and xenobiotics.

Secretion: Active transport of organic acids, bases, and drugs into tubular lumen for elimination.

Endocrine Functions

• Erythropoietin (EPO): Stimulates red blood cell production in bone marrow.
• Calcitriol (Active Vitamin D): Enhances intestinal Ca²⁺ absorption.
• Renin: Initiator of RAAS.