Homeostasis

Homeostasis is the physiological control process by which the internal conditions of living organisms are maintained at equilibrium. Homeostasis is derived from the Greek words ‘homeo’ (which means ‘similar to’), and ‘stasis’ (which means standing still, or steady).

In reality, the internal conditions within the human body are never exactly static. Instead, they are always striving to reach the optimal equilibrium state. In other words, homeostasis is a state of dynamic equilibrium characterised by different responses to changes within the external and internal environments. These changes can be inside or outside the cell, tissue, organ, or organism.

The significance of homeostasis

Homeostasis is essential for the functioning and survival of organisms. Homeostasis is important for maintaining proteins' structures, water potential in the body, and successfully adapting the body's temperature to changing external conditions.

Maintaining the protein’s structure

Proteins are abundant macromolecules that are essential for cells to function effectively. However, proteins are very sensitive to changes in pH and temperature. Any change in these factors causes the proteins to denature and lose their native structure. When the native structure of a protein is lost, it will likely become inefficient or obsolete in its function.

Maintaining the water potential

Water potential is important for both plant and animal cells. As we know, water always moves from a system of high water potential to a system of low water potential.

In plants, the cells have a cellulose cell wall protecting them. Hence, the cells only become turgid when water diffuses in, and shrivel as water leaves them (Figure 1).

Fig. 1 - Plant cells in hypertonic, isotonic, and hypotonic solutions

In contrast, animal cells have no cell wall, so there is a risk of cellular damage when too much water diffuses in or out (Figure 2). Maintaining blood glucose levels at a dynamic equilibrium is essential to ensure a constant water potential for the cells. It also ensures that the cells receive a sufficient amount of glucose to use for respiration.

Plasmolysis and deplasmolysis is irreversible in animal cells, while plasmolysis in plant cells is reversible.

Fig. 2 - Red blood cells in hypertonic, isotonic, and hypotonic solutions

Adapting to a wider geographical range

The ability to maintain the body’s internal temperature at a constant level allows animals to be more independent of their external environment. Therefore, they will be able to survive in a wider variety of geographical ranges and in different climates. This feature has allowed mammals to inhabit most habitats, ranging from hot deserts to freezing polar regions.

Homeostasis’ control mechanisms

In order for any homeostatic mechanism to work effectively, five components are necessary (Figure 3):

1. An optimum point: the optimal condition where the system operates best.
2. A sensor: the receptor or group of receptors that would sense any changes or deviations from the desired value.
3. A coordinator: the control centre retains the value for the optimum point and has a way of comparing the current value, provided by the sensor, with the desired one.
4. An effector: the effector organ, often a muscle or a secretory gland, that has the ability to change the value of the variable to the value determined by the coordinator.
5. A feedback mechanism: the mechanism by which the receptor responds to the change in the variable, as it retunes to the optimum point due to the action of the effector. There are two types of feedback mechanisms; negative and positive.

Fig. 3 - Negative feedback loop and body temperature regulation in the body to return to homeostasis

Negative feedback mechanisms

Negative feedback is the most common type of feedback in living organisms. In negative feedback, the receptor detects the need for the re-establishment of the optimum point. It conveys the signal to the control centre which then turns off the effector.

Thermoregulation

An example of negative feedback is how body temperature is regulated in endotherms like mammals. They need to maintain their body temperature at a relatively constant level, despite fluctuations in the temperature of their environment.

The optimum temperature in the human body ranges between 36°C and 38°C. There are two different sensors in humans that detect changes in temperature:

1. Sensory cells in the skin detect external temperature changes.
2. Sensory cells in the hypothalamus that detect internal temperature changes.

These sensors are connected to the hypothalamus, which is the control centre for body temperature. When sensory cells detect a slight deviation of the body temperature from its optimum value, they send signals to the hypothalamus which then activates various mechanisms to restore the core body temperature. These mechanisms include:

In response to cold external environments:

1. Vasoconstriction of the arterioles near the skin. The arterioles ‘shrink’, reducing the diameter of the blood vessel and the amount of blood that circulates near the skin. As there is less heat loss to the environment, body heat is preserved.
2. Shivering of skeletal muscles. This produces metabolic heat, i.e., heat that is generated from ATP production.
3. Activation of hair erector muscles. This is the raising of hair on your skin, which creates an insulating layer of air and helps preserve body heat.
4. Increased metabolism and burning of fat. This helps generate more metabolic heat to raise the core body temperature.
5. Humans and animals also utilise behavioural mechanisms to avoid heat loss. These mechanisms include finding shelter, huddling together, or hugging their knees, which helps reduce heat loss by reducing the volume to surface ratio.

In response to hot external environments

1. Vasodilatation of the surface arterioles. This increases blood flow to the skin allowing more heat to be exchanged with the environment.
2. Increased secretion of sweat. This allows the body to lose heat as more water evaporates from the skin.
3. Relaxation of hair erector muscles. This lowers the hair on the skin. As a result, the insulating layer is removed allowing the skin to lose more heat.
4. Behavioural adaptations, such as avoiding the sun, staying in the shade, or jumping into the water!

Calcium regulation

Blood calcium levels are also regulated by a negative feedback mechanism which requires the action of the different hormones. One important hormone involved is the parathyroid hormone (PTH). This hormone is released from the parathyroid gland in response to low blood calcium levels.

PTH increases blood calcium levels by:

• Increasing bone resorption in the bones.
• Increasing calcium absorption in the intestine.
• Decreasing calcium excretion in the kidneys.

Osmoregulation

Another example of negative feedback is osmoregulation. ADH (antidiuretic hormone) is secreted in response to dehydration. ADH acts on kidneys and stimulates the retention of water. However, as the body hydrates, ADH release is inhibited (Figure 3).

Fig. 4 - Regulation of body water levels by negative feedback driven by the antidiuretic hormone (ADH)