Revision:Homeostasis Basics

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H.1.1 State that homeostasis involves maintaining internal environment

State that homeostasis involves maintaining internal environment at a constant level or between narrow limits, including blood pH, water potential, oxygen and carbon dioxide concentrations, blood glucose and body temperature.

Things maintained by homeostasis:

• blood pH (controlled by both nervous and endocrine system)
• water potential (controlled by the endocrine system, kidney and ADH)
• Oxygen and Co2 concentrations (controlled by the nervous system, ventilation rate etc)
• blood glucose (controlled by the endocrine system, glucagon and insulin)
• body temperature (controlled by the nervous and endocrine systems)

H.1.2 Explain that homeostasis involves monitoring levels of variables and correcting changes in levels

Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.

H.1.3 Describe the control of body temperature

Describe the control of body temperature, including the roles of sweat glands, hairs, skin arterioles and shunt vessels, shivering, hormones (thyroxin, TRH, TSH), anterior pituitary gland, hypothalamus and thyroid.

Heat is sensed by the skin and the hypothalamus, as both contain thermo receptors. External environment temperature is sensed by the skin, and internal environment temperature is sensed by the hypothalamus.

When it is cold outside, messages are sent from the themoreceptors in the skin or from deep thermal receptors or via the blood to the cerebrum and the hypothalamus. The cerebrum makes the person aware of being cold, and can cause behavioral changes which are voluntary to, for example, put on a sweater. When the message has reached the hypothalamus, a series of reactions follow. TRH (thyroid releasing hormone) is released by the hypothalamus, its target organ being the anterior lobe of the pituitary gland. When TRH reaches the pituitary gland, it releases TSH (thyroid stimulating hormone) into the blood stream. The target organ of TSH is the thyroid gland. Upon receiving TSH, the thyroid then produces thyroxin. Thyroxin increases cellular metabolism to make heat.

Other things that happen:

• vasoconstriction (blood diverted from skin to keep heat)
• reduced sweating
• skin hairs raised (erector pilli muscle contracted)
• shivering
• increased metabolic rate

When it is too warm, messages are sent in the same way to the hypothalamus. This causes:

• increased sweating to release heat via water
• vasodilation (blood diverted to skin to lose heat)
• skin hairs lowered
• reduced metabolic rate

H.1.4 Explain the control of water potential

Explain the control of water potential including the roles of the hypothalamus, the posterior pituitary gland and feelings of thirst.

Osmoreceptors in the hypothalamus detect the water potential of the body. If the water potential is too low, then it will act in a way to keep more water in the body. If it is too high, then it will act in a way to excrete more water in the body. Once the hypothalamus has detected a change, a message is sent to the cerebrum, where feelings of thirst are generated if there is too little water in the body. The hypothalamus also sends a message to the posterior pituitary gland to induce ADH secretion. ADH increases the permeability of the collecting duct in the kidney, thereby increasing the amount of water reabsorbed by the body. also...(ADH being picked up by the receptors in the wall of the collecting duct causes an increase in cAMP level which leads to the insertion of aquaporins in the wall of the collecting duct.)

On the other hand, if there is too much water in the body, then no ADH is secreted, thus more water is excreted in the urine.

H.1.5 Explain the control of blood glucose concentration

Explain the control of blood glucose concentration, including the roles of glucagon and insulin secretion, a and B cells in the pancreatic islets, hypothalamus and feelings of hunger and satiety.

If there is too much glucose in the blood, then receptors in the pancreas detect this. They send a message to the cerebrum, inducing feelings of satiety (so that intake of food is decreased). They also send messages to the Islets of Langerhans (the B-cells) to produce insulin. This insulin is released into the bloodstream via capillaries, and has various effects. It increases the intake of glucose by all cells, and stimulates the conversion of glucose into glycogen (glycogenesis) in cells of the muscles and liver. This reduces the amount of glucose in the blood, so that they return to equilibrium.

If there is too little glucose in the blood, then the same receptors in the pancreas detect this. They send a message to the cerebrum, inducing feelings of hunger (so that intake of food is increased). They also send a message to the A-cells in the islets of langerhans to produce glucagon.

Glucagon is released into the bloodstream via capillaries and stimulates the conversion of glycogen into glucose (glycogenolysis) in liver. The liver is also stimulated to convert amino acids into glucose (gluconeogenesis). Thus, the levels of glucose in the blood increase so that equilibrium is reached.

A-cells make glucagon and affect mainly liver cells, to break down glycogen.

B-cells make insulin and affect all cells, to take in glucose.

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