Topic E: Human Physiology: Regulation and Control (Elective)

This elective explores how the body senses change and responds to keep internal conditions within narrow limits, a process called homeostasis. You study the nervous system for fast responses, the endocrine system for slower, longer-lasting control, the kidney's role in water balance, the control of body temperature, and human reproduction. DSE questions frequently use the negative feedback idea and ask you to trace a control pathway from stimulus to response, so understanding mechanisms in sequence is key.

Homeostasis and negative feedback

Homeostasis is the maintenance of a stable internal environment despite changes outside. Conditions such as body temperature, blood glucose and water content are kept within narrow limits because cells, especially enzymes, work best under steady conditions. Control works mainly by negative feedback: a receptor detects a change from the set level, a coordination centre processes the information, and an effector brings about a response that returns the condition towards normal, after which the response is switched off. For example, a rise in blood glucose triggers actions that lower it, and a fall triggers actions that raise it. You should be able to describe any homeostatic system using the stimulus, receptor, coordinator, effector and response framework and explain why feedback is described as negative.

The nervous system and reflexes

The nervous system gives rapid, short-lived responses. It consists of the central nervous system, the brain and spinal cord, and the peripheral nerves. Information travels as electrical impulses along neurones: sensory neurones carry impulses from receptors to the central nervous system, and motor neurones carry impulses to effectors such as muscles and glands. A reflex action is a fast, automatic, protective response that does not require conscious thought; in a reflex arc the impulse passes from receptor, along a sensory neurone, through a relay neurone in the spinal cord, to a motor neurone and then the effector. You should be able to label a reflex arc, trace the pathway, and explain why reflexes are rapid and protective.

Synapses and the sense organs

Where two neurones meet there is a tiny gap called a synapse. The impulse cannot cross the gap directly; instead the arrival of an impulse releases a chemical transmitter that diffuses across and triggers a new impulse in the next neurone, ensuring impulses travel in one direction only. Sense organs contain receptors that detect specific stimuli: the eye detects light and the ear detects sound and balance. In the eye, light is refracted by the cornea and lens to form an image on the retina, and the lens changes shape during accommodation to focus on near or distant objects, while the iris controls how much light enters. You should be able to relate the parts of the eye to their functions and explain accommodation and the pupil reflex.

Hormonal coordination

The endocrine system provides slower but longer-lasting control using hormones, chemical messengers made by glands and carried in the blood to target organs. Unlike nervous control, hormonal effects are usually widespread and persist for some time. Insulin and glucagon from the pancreas regulate blood glucose: after a meal insulin lowers blood glucose by promoting its uptake and storage as glycogen, while glucagon raises it when levels fall; failure of this control causes diabetes. Adrenaline prepares the body for action, and reproductive hormones control sexual development and the menstrual cycle. You should be able to compare nervous and hormonal coordination in terms of speed, duration and how the message travels, and explain blood-glucose control as negative feedback.

Osmoregulation and the kidney

The kidneys regulate the water and salt content of the blood and remove the nitrogenous waste urea, formed in the liver from excess amino acids. Each kidney contains many tubules where blood is filtered under pressure, allowing water, glucose, salts and urea to pass into the tubule while large molecules and cells stay in the blood. Useful substances such as all the glucose and much of the water and salts are then reabsorbed, and the remaining urea, excess water and salts form urine. Water balance is controlled by a hormone that adjusts how much water the kidney reabsorbs: when the body is short of water more is reabsorbed and concentrated urine is produced. You should be able to outline filtration and reabsorption and explain osmoregulation as negative feedback.

Thermoregulation

Mammals keep their body temperature roughly constant so that enzymes work efficiently. Temperature receptors in the skin and brain detect change, and the skin acts as the main effector. When the body is too hot, blood vessels supplying the skin widen so more blood flows near the surface and loses heat, sweating increases so evaporation cools the skin, and hairs lie flat. When the body is too cold, these vessels narrow to conserve heat, sweating stops, hairs stand up to trap an insulating layer of air, and shivering generates heat by muscle contraction. You should be able to describe these responses as negative feedback, use the correct terms vasodilation and vasoconstriction, and explain why evaporation of sweat causes cooling.

Human reproduction

Human reproduction is sexual and involves the male and female reproductive systems. The testes produce sperm and the male hormone testosterone, while the ovaries produce eggs and the female hormones oestrogen and progesterone. The menstrual cycle, controlled by hormones, prepares the uterus lining each month for a possible pregnancy and releases an egg at ovulation. Fertilisation, the fusion of sperm and egg, normally occurs in the oviduct, and the resulting embryo implants in the uterus lining, where the placenta allows exchange of food, oxygen and waste between mother and fetus. You should be able to label the reproductive systems, outline the roles of the main hormones in the menstrual cycle, and explain the function of the placenta.