Organism level systems Study Guide

The human eye is a complex sensory organ responsible for vision. Each main structure within the eye is specifically adapted to perform a particular function, working together to enable us to perceive light and form images. Light enters the eye through the cornea, a transparent, curved outer layer that initiates the focusing process. Behind the cornea is the iris, a pigmented diaphragm that controls the amount of light entering the eye by adjusting the size of the pupil. The pupil is the central opening in the iris. The lens, located behind the iris, further focuses light onto the retina. This focusing is achieved by changing its shape, a process called accommodation, controlled by ciliary muscles and suspensory ligaments. The retina, at the back of the eye, contains photoreceptor cells (rods and cones) that detect light and convert it into electrical signals. These signals are then transmitted to the brain via the optic nerve, which carries them for interpretation as visual images. The tough, outer sclera protects the delicate inner structures of the eye, while the choroid, rich in blood vessels, provides nutrients and oxygen to the retina and absorbs stray light. The vitreous humour fills the main cavity of the eye, maintaining its shape and holding the retina in place, while the aqueous humour in the front chamber provides nutrients to the cornea and lens.

The human body maintains a constant internal temperature, usually around 37°C, through a process called thermoregulation. The skin plays a crucial role in this process by enabling the body to lose or conserve heat. When the body temperature rises above normal, certain mechanisms in the skin are activated to cool it down. Sweat glands in the skin produce sweat, which is released onto the skin's surface. As this sweat evaporates, it takes heat energy from the body, leading to a cooling effect. Additionally, blood vessels near the skin surface, called arterioles, dilate (widen). This vasodilation increases blood flow to the skin, allowing more heat to radiate away from the body's surface into the surroundings. Conversely, when body temperature falls below normal, the skin helps to conserve heat. Arterioles constrict (narrow), reducing blood flow to the skin's surface (vasoconstriction). This minimises heat loss through radiation. Hair erector muscles contract, making hairs stand on end; this traps a layer of air close to the skin, which acts as an insulating layer, reducing heat loss by convection. These coordinated actions of the skin's structures ensure that the body's core temperature remains within a narrow, healthy range, critical for enzyme function and overall physiological processes.