Internal Environment
The term "milieu intérieur," or internal environment, was first coined by the renowned 19th-century French physiologist Claude Bernard. This internal environment refers to the extracellular fluid that surrounds cells, serving as the immediate environment in which they live and exchange life-sustaining substances.
In a physiological man, typically considered to be a 70 kg individual, total body fluid constitutes approximately 60% of total body weight, equivalent to 42 kg. This fluid is distributed between two primary compartments:
- Extracellular Fluid (ECF): Comprises about 20% of total body weight (14 kg).
- Intracellular Fluid (ICF): Constitutes roughly 40% of total body weight (28 kg).
Components of Extracellular Fluid
ECF can be further subdivided into:
- Plasma: Represents approximately 25% of ECF (5% of total body weight, about 3.5 kg).
- Interstitial Fluid: Makes up around 75% of ECF (15% of total body weight, about 10.5 kg). This fluid fills the spaces between cells, constantly bathing and sustaining them.
- Transcellular Fluid: Includes specialized fluids such as digestive fluids, synovial fluid, pleural fluid, peritoneal fluid, and pericardial fluid.
Electrolyte Composition of ECF and ICF
The electrolyte composition of ECF and ICF differs significantly, playing a crucial role in maintaining cellular function. The table below illustrates the concentrations of key electrolytes:
| Electrolyte | ECF (mmol/L) | ICF (mmol/L) |
|---|---|---|
| Na+ | 142 | 14 |
| K+ | 4.2 | 140 |
| Mg+ | 0.8 | To be continued in Chunk 2 |
| Na+ | 142 | 14 |
| K + | 4.2 | 140 |
| Mg+ | 0.8 | 20 |
| Ca2+ | 1.2 | 0 |
| Cl | 108 | 4 |
| – | 24 | 10 |
| HCO3 | | |
| NUTRIENTS | O2, glucose, fatty acids & amino | High concentrations of proteins |
| acids | | |
| WASTES | CO2, urea, uric acid, excess | – |
| water & ions | | |
It helps in nourishment of cells with nutrients It helps in removal of metabolic wastes from cells It is a medium of transport from one tissue to another Physiological factors
- Temperature
- Osmotic pressure
- Concentration of O2 in blood stream
- Concentration of CO2 in blood stream Chemical factors
- Salt level
- Glucose level – pH
Homeostasis is the maintenance of nearly constant conditions in the internal environment The word "Homeostasis" was coined by Walter Cannon in 1929 The internal environment is regulated by a feedback system Feedback is a system in which the process being controlled produces effects which serves as the control for that process Feedback systems include Positive feedback Negative feedback In positive feedback the initiating stimulus causes more of the same Examples of positive feedback in the body are; Blood clotting Parturition Milk ejection reflex Generation of nerve signals in a nerve fibre In negative feedback the response produced tends to reverse the initial stimulus Examples of negative feedback in the body are; Regulation of blood sugar level Regulation of blood pressure Regulation of blood pH
Regulation of O2 in the blood Regulation of CO2 in the blood Another feedback system is the feed-forward system In feed forward system response is made in anticipation to change Examples of feed forward systems are; Shivering Salivating (even before eating)
