Table of Contents
Homeostasis Definition
For animals to survive and stay alive, their internal environment must be continuously monitored and checked for any changes resulting from internal or external factors. This is called homeostasis, which is the maintenance of a relatively constant internal environment for the cells within the body.
Internal and External changes
Any internal changes in the tissue fluid can directly disrupt the internal environment of the body. This internal environment includes all conditions within the body where cells operate, and can result in changes in cellular activities.
There are many features of the tissue fluid that influence cellular activity due to external and internal changes. Here are some examples:
Internal Temperature
At low body temperature, metabolic reactions slow down as a result of a decrease in kinetic energy. However, high temperatures denature proteins and enzymes required for these reactions.
Water potential
It is important to maintain the water potential of the blood within a narrow range. Increasing water potential results in the swelling of tissues (edema), while decreasing it can cause the cells to shrink.
Blood glucose concentration
Keeping the blood glucose concentration within a narrow range is necessary for cells to function properly. Glucose is required for cellular respiration. If its concentration in the blood increases, it can damage blood vessels and nerves over time.
pH
The activity of enzymes is also influenced by pH. Cytoplasm pH usually ranges from 6.5 to 7.0.
External changes can also affect the internal environment in the body. For example, disruptions in cellular activity can be the result of a lack of food or water, an extreme weather condition, or a stressful situation.
To clarify the difference between internal and external changes, the following table provides some examples.
| Example | Change (Internal/External) |
| Cold weather | External (the body responds by increasing core temperature) |
| High blood glucose concentration | Internal (the body responds by releasing insulin into the bloodstream) |
| High altitude (low oxygen) | External (body responds by increasing breathing rate) |
| loss of water (from sweating) | Internal (kidneys respond by absorbing more water) |
Roles of Nervous & Endocrine Systems in Homeostasis
So far, we have discussed what homeostasis is and why it is important, but now, it is time to understand its mechanisms and how the body detects changes in the internal environment.
Both the nervous and endocrine systems play a big role in homeostasis. They are responsible for regulating and restoring balance to the internal environment. They act as the “linking bridge” between the external or internal changes and the body.
The nervous system sends electrical signals to counteract changes, while the endocrine system releases hormones into the bloodstream, which act more slowly than nerve signals. The following table summarises their differences.
| Feature | Nervous System | Endocrine System |
| Mode of communication | Electrical signals (nerve impulses) | Chemical messengers (hormones) |
| Speed | Very fast, almost immediate | Slow, takes seconds to hours |
| Duration of effect | Short-lived | Long-lasting |
| Target | Specific cells, tissues, organs | Widespread |
Homeostasis Mechanism
Both the endocrine and nervous systems follow the same mechanisms to regulate the conditions of the blood and the tissue fluid. The body regulates its internal environment using a feedback system. It’s a cycle where a condition is monitored, changed, and evaluated.
A feedback system has three main components:
–Receptor: A receptor monitors changes that occur to the body, and send signals to the control centre (brain/spinal cord)
–Control centre: It includes the brain and the spinal cord. It processes the signals received from the receptor, and accurately calculate and measure the perfect output signal to the effector. This output is usually in the form of nerve impulses or hormones.
–Effector: The effector is any tissue or organ that responds to a stimulus, and includes muscles or glands. The effector helps restore a physiological factor back to its set point (ideal range).
Negative feedback loop
A Negative feedback loop is a type of feedback mechanism that occurs when there are changes in body conditions. It maintains homeostasis by reversing these changes back to their set point. A setpoint is the ideal level of a body condition maintained by homeostasis.
Negative feedback reduces the gap between current values and their set points.
The negative feedback loop begins when a receptor detects a change that causes an internal condition to move away from its set point. The receptor sends this information to the control centre, often the brain or spinal cord, which processes the signal and sends instructions to the effector. The effector then reverses the change, helping return the condition to its normal range.
Negative feedback loop examples
- Temperature
Receptors in the skin detect changes when the body temperature moves away from its set point (37°C). Imagine your body temperature drops to 36°C, your skin receptors will detect this change, sending information to the hypothalamus, part of the brain responsible for regulating temperature. The hypothalamus will process this information and send impulses to the blood vessels, causing them to constrict, and impulses to skeletal muscles to shiver, producing heat and raising the body temperature. - Blood glucose regulation
When blood glucose levels rise above their normal range, β-cells in the pancreas respond by releasing insulin into the bloodstream. Insulin causes cells to absorb glucose and liver cells to store it as glycogen. This lowers blood glucose levels back to their set point.
Positive feedback
A positive feedback loop is another type of a feedback mechanism. Even though it happens less frequently than negative feedback, it still plays an important role in the body. Positive feedback strengthens and reinforces the initial change rather than reversing it.
Positive feedback example
Childbirth is a very common example of positive feedback. When the baby’s head pushes against the cervix, stretch receptors detect this pressure. They send signals to the hypothalamus of the brain, stimulating the pituitary glands to release the hormone oxytocin into the bloodstream. Oxytocin makes the uterus contract more strongly, pushing the baby’s head harder against the cervix.
Negative feedback vs positive feedback
The clear difference between negative and positive feedback is their results.
- Negative feedback stabilises/reverses changes to bring a condition back to its set point
- Positive feedback strengthens or reinforces changes, moving a condition away from its set point