Organisation of the body and homeostasis

Levels of organisation

The body is built in layers of increasing complexity. Atoms combine into molecules, molecules form cells, similar cells form tissues, tissues combine into organs, and organs working toward a shared job form an organ system. The whole living individual is the organism. The kidneys, for example, are organs of the urinary system, and the heart belongs to the cardiovascular system.

The language of anatomy

Descriptions assume the anatomical position: standing upright, facing forward, feet together, arms at the sides with palms facing forward. Using one fixed reference lets everyone describe the same location in the same way, whatever position the person is really in.

Common directional terms come in opposite pairs:

• Superior / inferior: toward the head / toward the feet.
• Anterior (ventral) / posterior (dorsal): front / back.
• Medial / lateral: nearer the midline / farther from it.
• Proximal / distal: nearer the attachment of a limb to the trunk / farther from it.
• Superficial / deep: nearer the surface / farther inside.

Imaginary planes slice the body. A sagittal plane separates left from right, a frontal (coronal) plane separates front from back, and a transverse plane separates upper from lower. The trunk contains the thoracic cavity (heart and lungs) and the abdominopelvic cavity (digestive organs, kidneys, bladder), separated by the diaphragm.

Homeostasis

Homeostasis is the maintenance of a relatively stable internal environment, such as body temperature, blood glucose and fluid balance, even while outside conditions change. Most control uses a negative feedback loop, which has four parts:

1. A receptor detects a change in a variable.
2. A control centre compares the reading with a set point.
3. An effector carries out a response.
4. The response pushes the variable back toward the set point, which switches the loop off.

Positive feedback is rarer: the response amplifies the original change until an endpoint is reached. Labour contractions are the classic case, since stretching of the cervix triggers the hormone oxytocin, which strengthens contractions and causes more stretching.

Worked walk-through: a warm afternoon

Suppose a person walks into a hot room and core temperature creeps above its set point.

1. Receptors: temperature-sensitive neurons in the skin and in the hypothalamus detect the rise.
2. Control centre: the hypothalamus compares the reading with the set point, about 37 degrees Celsius, and finds it too high.
3. Effectors: sweat glands increase secretion, and blood vessels in the skin widen so more heat leaves through the surface.
4. Result: evaporating sweat and extra heat loss lower core temperature. Once it is back at the set point, the receptors stop signalling and the effectors relax.

Because the response opposes the change, this is negative feedback. In a cold room the same loop works in reverse: skin vessels narrow and skeletal muscles shiver to generate heat. Note that homeostasis is not a fixed number but a narrow range around a set point, with the variable constantly swinging slightly above and below it.