7.11, 7.12 - Feedback, homeostasis and thermoregulation

Homeostasis and dynamic equilibrium

Body cells depend on their surroundings even when the outside environment or their own activity changes. Their immediate environment is the tissue fluid, whose conditions depend on exchanges with the blood.

Temperature, blood pH and the availability of substances such as glucose must be regulated. Enzyme-catalysed reactions depend on suitable conditions: cooling can slow reactions, while sufficiently high temperatures can disrupt protein structure and enzyme function. Large pH changes can also alter proteins and their activity. Homeostasis therefore supports the coordinated reactions on which cells depend; it does not make every variable identical everywhere in the body.

During exercise, faster muscle metabolism increases oxygen demand, carbon dioxide production and heat production. Increased cardiac output and ventilation help match gas transport to this demand, as in 7.9. Temperature control must also change its activity. A constant rate of heat loss that was adequate at rest may be inadequate during running.

For example, a runner can have a roughly stable core temperature while producing and losing heat much faster than at rest. When heat gain and heat loss balance, there is no continuing net increase in stored body heat. Small fluctuations and a modest exercise-related rise can still occur: homeostasis means regulation within a suitable range, not an absolutely fixed temperature of exactly 37°C.

Negative feedback and narrow limits

A control system needs information about the variable it regulates and a way to alter it. A receptor detects a change; a coordination centre processes information and directs a response; an effector, such as a muscle or gland, brings about the response. Information about the outcome then affects the system's further activity: this is feedback.

If temperature rises, negative feedback increases processes that remove heat. If temperature falls, it favours conserving or producing heat. Negative feedback can therefore cause either an increase or a decrease in a variable. The word negative describes opposition to the original change; it does not mean harmful, nor does it mean that every response reduces something.

As the deviation becomes smaller, the corrective response is reduced. This avoids maintaining the full correction after it is no longer needed. Detection and responses take time, so small fluctuations or temporary overshoots can occur. A regulated variable may oscillate around a reference value rather than drawing a perfectly horizontal line. Continuous detection and adjustment keep these deviations within narrow limits when the control system has sufficient capacity.

Consider a room-cooling system as an analogy. Warming increases cooling, and cooling decreases as the room approaches the reference temperature. The key biological principle is the relation between the original deviation and the response, rather than whether the equipment is a fan, a gland or a muscle.

Positive feedback amplifies change

Some biological events require a process to build rapidly towards completion rather than maintain a variable near a stable value.

During childbirth, stretching of the cervix sends sensory information to the brain. This promotes oxytocin release, which strengthens uterine muscle contractions. Stronger contractions push the baby against the cervix, producing more stretch and further promoting the response. The loop reinforces its starting change. Birth removes the stretching stimulus, bringing this particular loop to an end. Positive feedback is not intrinsically harmful; its usefulness depends on the biological context and a way for the process to terminate.

Compare the two types by tracing one complete loop. Negative feedback makes the initial deviation smaller; positive feedback makes it larger. Merely seeing an increasing hormone concentration or an increasing response does not identify the type: you must know its effect on the original variable. Positive feedback alone cannot hold a variable close to one reference value because it reinforces deviations rather than correcting them.

The hypothalamus and temperature sensing

The hypothalamus, a region of the brain, coordinates important thermoregulatory responses. Thermoreceptors are temperature-sensitive receptors or cells. Temperature-sensitive cells in the hypothalamus respond to the temperature of the blood supplying it, giving information about internal, or core, temperature. Core temperature refers to the temperature of deeper body tissues; the skin surface can be much cooler.

Thermoreceptors in the skin detect local skin temperature changes. Sensory nerve pathways carry this information to central thermoregulatory circuits, including the hypothalamus. Skin information helps the system respond to a change in the environment before a large core-temperature change develops. It is not a direct measurement of the temperature of every deep organ. Both skin and core information contribute; regulation is not a choice between using only one set of sensors.

The hypothalamus integrates this information and adjusts nerve signals to effectors. Sweat glands alter secretion, smooth muscle in the walls of skin arterioles alters their diameter, and skeletal muscles can produce shivering. The sensor detects, the coordination centre directs, and the effector acts: these are different roles, even though the hypothalamus contains temperature-sensitive cells as well as coordinating circuits.

The feedback diagram previews the two sets of responses that you will examine next. Each corrective pathway opposes its original temperature change, and its activity decreases as the deviation is corrected.