Homeostasis is the process by which living organisms maintain a stable internal environment despite

changes in the external environment. This stability is crucial for the proper functioning of cells and the
survival of the organism.

In the context of IGCSE biology, homeostasis involves the regulation of several factors within the body,
including:

1. **Temperature**: The body needs to maintain a relatively constant temperature (around 37°C in
humans) for enzymes and metabolic processes to work efficiently. If the temperature becomes too high
or low, it can interfere with these processes.

2. **Blood Glucose Levels**: The concentration of glucose in the blood needs to be kept within a
narrow range. Too high or too low levels of glucose can cause problems for the body, affecting energy
production and organ function.

3. **Water and Salt Balance**: The body needs to control the levels of water and salts, especially in the
blood and tissues, to prevent dehydration or overhydration, both of which can disrupt cellular function.

4. **pH Levels**: The pH of body fluids, particularly blood, must remain within a narrow range (around
7.4) to ensure enzymes and other biochemical processes work properly.

### Mechanisms of Homeostasis

Homeostasis is maintained through feedback systems, which involve sensors that detect changes in the
internal environment, control centers (usually the brain), and effectors that make adjustments. For
example:

- **Negative Feedback**: This is the most common mechanism of homeostasis, where a change is
detected and a response is triggered to counteract that change. For instance, if blood glucose levels rise
after eating, the pancreas releases insulin to lower the glucose level back to normal.

- **Positive Feedback**: This is less common but occurs when a change is amplified. An example is the
process of childbirth, where the release of the hormone oxytocin increases the intensity of uterine
contractions, leading to more oxytocin release until the baby is born.
### Example in Humans: Thermoregulation

The body regulates its temperature through processes like sweating (to cool down) or shivering (to
generate heat). When the temperature rises, sensors in the skin and brain detect the change, and the
brain sends signals to sweat glands to release sweat, which cools the body as it evaporates. When the
body is too cold, the brain sends signals to muscles to contract and generate heat (shivering).

In summary, homeostasis is essential for maintaining the optimal conditions needed for cells, tissues,
and organs to function effectively, ensuring the survival of the organism.

Insulin decreases blood glucose concentration.

When blood glucose levels are high, such as after eating, the pancreas releases insulin. Insulin helps cells
absorb glucose from the bloodstream, either to use as energy or to store as glycogen in the liver and
muscles. This process lowers the blood glucose concentration back to a normal level.

Electrical impulses travel along neurones.

Neurones (nerve cells) transmit electrical signals in the form of impulses. These impulses travel along
the axon of the neurone to transmit information between different parts of the body. When the impulse
reaches the end of the neurone, it can trigger the release of chemicals (neurotransmitters) to pass the
signal to the next neurone across a synapse.

The mammalian nervous system is made up of two main components: the **central nervous system
(CNS)** and the **peripheral nervous system (PNS)**. These two systems work together to coordinate
and control the body's activities.

### (a) **Central Nervous System (CNS)**

The **central nervous system** consists of the **brain** and the **spinal cord**.

- **Brain**: The brain is the control center of the body, responsible for processing information,
interpreting sensory input, and sending out commands to various body parts. It also plays a key role in
higher functions like thinking, memory, emotions, and decision-making.

- **Spinal Cord**: The spinal cord connects the brain to the rest of the body. It is responsible for
transmitting electrical impulses between the brain and peripheral nerves. The spinal cord also controls
reflex actions, which are quick, automatic responses to stimuli that do not involve the brain.
The CNS is protected by bones (the skull and vertebral column), membranes (meninges), and
cerebrospinal fluid.

### (b) **Peripheral Nervous System (PNS)**

The **peripheral nervous system** consists of all the nerves **outside** of the brain and spinal cord. It
connects the central nervous system to the rest of the body.

- **Sensory Neurones**: These carry information from sensory receptors (e.g., in the skin, eyes, ears) to
the CNS. For example, they transmit signals about temperature, light, and pain.

- **Motor Neurones**: These carry commands from the CNS to muscles and glands, enabling
movement and other actions. Motor neurones are responsible for voluntary actions (e.g., moving an
arm) and involuntary actions (e.g., controlling heart rate).

- **Autonomic Nervous System**: A part of the PNS that controls involuntary functions such as heart
rate, digestion, and respiration. It has two main divisions:

- **Sympathetic Nervous System**: Prepares the body for "fight or flight" responses, increasing heart
rate and energy expenditure.

- **Parasympathetic Nervous System**: Controls "rest and digest" functions, slowing down heart rate
and promoting energy conservation.

The PNS includes **cranial nerves** (connected to the brain) and **spinal nerves** (connected to the
spinal cord), which relay information to and from different parts of the body.

### Summary

- The **CNS** (brain and spinal cord) processes information and controls body functions.

- The **PNS** (nerves outside the CNS) connects the CNS to the body and is involved in transmitting
sensory information and carrying out motor commands.

The **pupil reflex** is the response of the pupil to changes in light intensity, allowing the eye to
regulate the amount of light entering the eye to protect the retina and optimize vision. This reflex
involves the **antagonistic action** of two types of muscles in the **iris**: the **circular muscles**
and the **radial muscles**.
### 1. **Circular Muscles**

- These muscles are arranged in a circular pattern around the pupil.

- When the circular muscles contract, the pupil **constricts** (becomes smaller).

- This action reduces the amount of light entering the eye, which is helpful in bright light conditions to
prevent damage to the retina.

### 2. **Radial Muscles**

- These muscles are arranged in a radial pattern (like spokes of a wheel) around the pupil.

- When the radial muscles contract, the pupil **dilates** (becomes larger).

- This allows more light to enter the eye, which is useful in low light conditions to improve vision.

### Antagonistic Action

- The circular and radial muscles act **antagonistically**—meaning they have opposite effects on the
size of the pupil.

- When one set of muscles contracts, the other set relaxes. For example:

- In bright light, the **circular muscles** contract to constrict the pupil, while the **radial muscles**
relax.

- In dim light, the **radial muscles** contract to dilate the pupil, while the **circular muscles** relax.

### The Pupil Reflex

This process is controlled by the **autonomic nervous system**:

- When light intensity increases, receptors in the retina detect the change and send signals to the brain.
The brain then sends a signal to the iris muscles to contract the circular muscles and dilate the radial
muscles.

- When light intensity decreases, the opposite happens: the circular muscles relax and the radial muscles
contract, causing the pupil to dilate.

### Summary:

- The **circular muscles** and **radial muscles** of the iris work **antagonistically** to regulate the
size of the pupil in response to light levels.

- In bright light, the pupil **constricts** due to the contraction of circular muscles and relaxation of
radial muscles.
- In dim light, the pupil **dilates** due to the contraction of radial muscles and relaxation of circular
muscles.

This reflex helps to control the amount of light entering the eye and protects the retina from damage.