Internal Environment & Homeostasis: Salt, Water, and Renal System
The Internal Environment
It’s easy to identify external environmental factors: air, sunlight, temperature. But what about our internal environment? Claude Bernard coined the term “internal environment” to describe the fluids surrounding our body’s cells (interstitial fluid, blood, and lymph). These fluids contain vital substances like oxygen, nutrients, and minerals.
Water in the Body
In healthy adults, the internal environment’s water volume accounts for 37%, composed of:
- Interstitial fluid: Found between cells.
- Plasma: Part of the blood.
- Lymph: Formed from interstitial fluid.
- Transcellular fluid: Includes cerebrospinal fluid and synovial fluid.
The remaining 63% is intracellular fluid, residing within our cells.
Salt and Water Equilibrium
Maintaining the correct balance of water and salts is crucial for survival. Imbalances can lead to severe illness or even death. Our bodies must regulate these levels through intake and elimination. For example:
- High salt intake leads to decreased water elimination in urine and perspiration.
- Excessive salt intake can dehydrate cells.
- High water intake lowers salt levels in interstitial fluid, potentially leading to cell overhydration.
Homeostasis
Homeostasis refers to the state of equilibrium necessary for life. This includes maintaining water and salt levels, blood pH, body temperature, and blood sugar levels. Salt and water homeostasis is particularly important for maintaining isotonic conditions in blood and interstitial fluid. This balance is influenced by factors like physical activity, diet, and environmental temperature.
Renal System and Homeostasis
The kidneys play a vital role in homeostasis. They filter blood plasma, eliminating waste products and regulating blood pH, blood pressure, salt concentration, and water volume. They achieve this by:
- Excreting excess H+ and reabsorbing bicarbonate to regulate pH.
- Synthesizing renin, a protein involved in blood pressure regulation, salt concentration, and water volume.
Renal System and Water Balance
The kidneys control urine volume to regulate water loss. Increased water intake leads to increased plasma volume, which in turn increases urine production to maintain a constant plasma volume.
Renal System and Salt Balance
Sodium chloride is key in osmosis. Plasma sodium levels remain relatively constant, even with varying salt intake. The kidneys regulate salt elimination to maintain this balance.
Structure of the Renal System
Each kidney contains about 2.4 million nephrons, the structures responsible for urine production. Key components of the renal system include:
- Kidneys: Eliminate waste and regulate salt concentration, water volume, and blood pH.
- Ureters: Carry urine from the kidneys to the bladder.
- Urinary bladder: Stores urine.
- Urethra: Enables urination.
- Renal cortex: Outer region of the kidney.
- Renal medulla: Inner region containing renal pyramids.
- Renal pyramids: Conical structures containing the tubular system of the nephron.