13.1 - Homeostasis

Homeostasis is the maintenance of a relatively constant internal environment in living organisms. The physical and chemical factors of the internal environment of the body need to be maintained for the cells to function at optimum level.

• Examples of the physical factors are body temperature, blood pressure and osmotic pressure
• The chemical factors include salt and sugar levels, partial pressures of oxygen and carbon dioxide.

A negative feedback mechanism is a biological occurrence that initiates a corrective mechanism which reverses the original change and brings the system back to normal. When any factor becomes higher or lower than it should be, it will be adjusted to fall within the normal range again. Various organs in the body are involved in such regulation.

1. The integumentary, nervous, blood circulatory, muscle and endocrine systems are involved in controlling normal body temperature. Hypothalamus and thermoreceptors in the skin are responsible in detecting changes in body temperature.

Figure 1 - When the body temperature rises above normal set point, the erector muscles in the skin relax. The hair falls low towards the skin. However, during colder condition, the muscles are stimulated to contract , raising the hair which helps to trap warm air near the skin.

Figure 2 - When the internal temperature rises, vasodilation occurs. The arterioles in the skin relax, increasing the amount of blood flowing to the skin surface. More heat is radiated and lost to the outer environment. The sweat glands will also secrete sweat onto the surface of the skin. Heat absorbed is used to evaporate the sweat and the body feels cool. Meanwhile, when the internal temperature drops, vasoconstriction happens. The arterioles contract, decreasing the amount of blood flowing to the skin. Therefore, less heat is lost to the environment. The sweat glands are not stimulated. Sweating does not occur and the body heat is conserved.

Figure 3 - When the internal temperature rises, the skeletal muscles are not stimulated. Shivering does not occur. On the other hand, when the internal temperature drops, the muscles are stimulated to contract and relax more. Shivering occurs and heat is generated to keep us warm.

Figure 4 - When the internal temperature rises, the adrenal glands are less stimulated to secrete adrenaline. When the internal temperature drops, the glands will secrete more adrenaline. This increases the metabolism rate in the target tissues and more heat is generated.

Figure 5 - When the internal temperature increases, the thyroid gland is not stimulated to produce thyroxine. Metabolic rate is low and extra heat is generated. When the internal temperature drops, the gland is stimulated to produce more thyroxine. The metabolic rate increases and more heat is generated to keep the body warm.

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2. The endocrine gland, blood circulatory and digestive systems help in blood glucose level regulation.

Figure 6 - When the blood sugar level increases after meal, beta (β) cells of the islet of Langerhans will be stimulated to release insulin into the blood. Then, insulin will stimulate glucose uptake by muscle cells for cellular respiration. Insulin will also trigger the conversion of glucose to glycogen to be stored in the liver and muscle cells. Excess glucose is stored in the adipose cells. On the contrary, when the blood glucose level drops during fasting, alpha (α) cells of the islet of Langerhans will be stimulated to secrete glucagon into the bloodstream. Subsequently, glucagon will stimulate the conversion of glycogen back to glucose by the liver cells. Glucagon will also promote adipose tissues to break down and release fatty acids that can be metabolized to release energy. Failure of secretion or ability to utilize the insulin will lead diabetes mellitus.

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3. The respiratory, blood circulatory and nervous systems play major role in regulating the partial pressure of carbon dioxide. The respiratory control centre in the medulla oblongata.

• During vigorous exercise, cellular respiration causes the partial pressure of carbon dioxide to be higher. This causes the pH of the blood in the brain to decrease due to increased hydrogen ions.

  • Carbon dioxide + water <--> Carbonic acid <--> hydrogen ions + bicarbonate ions

• The changes in the blood pH are detected by the chemoreceptor in the medulla oblongata, carotid body and aortic body at the neck and heart. The impulses are then sent to the respiration and cardiovascular control centres in the medulla oblongata.

• The intercostal muscles, diaphragm and cardiac muscles are stimulated to contract and relax at a faster rate. As a result, the breathing rate, heart rate and ventilation rate increase. More carbon dioxide is exhaled from the lungs. The partial pressure of carbon dioxide drops and the blood pH goes back to normal level.

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4. Blood pressure is regulated by the blood circulatory and nervous systems. The sensory receptors that detect changes in the blood pressure is the baroreceptors.

• When the blood pressure increases , for instance during vigorous exercise, the baroreceptors in the carotid and aortic artery are stimulated. The cardiovascular control centre in the medulla oblongata is also stimulated** Vasodilation occurs at the arteries and the blood flow becomes smooth. Less contraction** of the smooth muscles. As a result, the blood pressure drops to normal level.

• When the blood pressure drops , for example during excessive bleeding, the baroreceptors in the carotid and aortic artery are less stimulated. The cardiovascular control centre in the medulla oblongata is less stimulated as well. Vasoconstriction occurs at the arteries, causing the blood flow to be restricted. Contraction of the smooth muscles helps the blood pressure increases to normal level.