Human Eye Function and Endocrine System Regulation

The Pupil Reflex and Light Intensity

The pupil reflex occurs when the eye adjusts the light intensity (the amount of light) entering through to the retina. Pupil size alters according to the light intensity:

• Pupil widens (dilates): Occurs when there is less light available.
• Pupil constricts: Occurs when there is more light available.

Muscle Control of the Pupil

The pupil is controlled by two types of muscles:

• Circular muscles: These circle around the pupil. When they contract, the pupil constricts.
• Radial muscles: These run outwards from the edge of the pupil. When they contract, the pupil dilates.

Photoreceptors: Rods and Cones

The retina contains two main types of photoreceptors:

• Rods: Sensitive to dim light and used for night vision. A lack of Vitamin A can lead to night blindness. Rods only detect black and white light.
• Cones: These detect colors in bright light. They are fewer in number than rods, and most are found at the fovea.

Vision and Lens Accommodation

Rays of light are refracted (bent) first by the cornea and then by the lens to focus on the retina.

• For distant objects: The ciliary muscles relax and the suspensory ligaments pull tight, making the lens thin. Consequently, the light does not bend as much.
• For close objects: The ciliary muscles contract, allowing the lens to become thicker (fat), thus bending the light more.

Introduction to Hormones

Hormones are chemical messengers produced by particular groups of cells within an organism. They regulate the growth or activity of specific target cells capable of responding. Hormones are produced in endocrine glands, which contain many secretory cells, and are part of the endocrine system.

The Role of Adrenaline

Adrenaline is secreted by the adrenal glands. While small amounts are produced normally to help maintain normal blood pressure, larger amounts are secreted when you are nervous or excited. Known as the 'fight or flight' hormone, it provides extra energy and muscle power, though its effects do not last long.

Functions and Effects of Adrenaline

• Metabolic Activity: Increases the rate of metabolic activity.
• Heart Rate: Increases to provide more oxygen to the brain.
• Liver: Breaks down glycogen, which increases the metabolic rate.
• Blood Flow: Arterioles to the digestive system constrict, while arterioles to the muscle system dilate (causing 'butterflies in the stomach').
• Respiration: Bronchioles get wider, allowing more air to move in and out of the lungs.

ADH and Homeostasis

Antidiuretic Hormone (ADH) controls the water content of the blood. Maintaining a constant internal environment is called homeostasis, which is why this hormone is vital to the process.

Hormones in Food Production

Hormones are sometimes used to increase the weight and milk production of animals. This increases productivity and reduces costs simultaneously. However, there are concerns that these hormones may affect humans when they consume the resulting products.

Key Endocrine Glands and Their Functions

• Pancreas: Produces Insulin to lower blood sugar concentration.
• Testis: Produces Testosterone to control puberty in males.
• Ovary: Produces Progesterone and Oestrogen to control puberty in females, the menstrual cycle, and ovulation.
• Thyroid Gland: Produces Thyroxine to control the rate of metabolism (heat production by the liver) and body mass.
• Adrenal Glands: Produce Adrenaline to prepare the body for action.
• Pituitary Gland: Controls growth and produces Trophic hormones (which control other endocrine organs) and Antidiuretic hormone (which controls kidney function).