Now I am going to talk about examples of negative feedback. Negative feedback loops control the release of hormones from many endocrine glands so that homeostasis is maintained. Hypothalamus lies at the base of the brain to which it is attached by numerous nerves. This has many functions:
- It regulates activities such as thirst, sleep and temperature control.
- It monitors level of hormones and other chemicals in the blood passing through it.
- It controls the functioning of the anterior pituitary gland.
-
Produces ADH and oxytocin which are stored in the posterior pituitary gland. (diagram showing role of hypothalamus as the link between nervous and endocrine systems).
Hypothalamus is linked between the nervous and endocrine systems. By monitoring the level of hormones in the blood, the hypothalamus is able to exercise homeostatic control of them.
Thyroxine controls metabolic rate through a negative feedback loop. Metabolic rate is rate at which all cells in the body carry out their biochemical reactions. It is a vital whole body function that must be controlled within very strict limits. Hypothalamus in brain detects even small decrease in metabolic rate and response by releasing more thyrotropin releasing hormone. This acts on the pituitary gland, causing it to release more thyroid-stimulating hormone. This passes to thyroid gland, which responds by secreting thyroxine, the hormones which acts on individual cells to increase metabolic rate. When metabolic rate gets back to normal levels, the hypothalamus responds by releasing less thyrotropin releasing hormone and in a healthy person, homeostasis is maintained. (Diagram showing control of metabolic rate involving a negative feedback loop)
The control of blood glucose is an example of homeostasis. A negative feedback mechanism operates to detects and correct the level of blood glucose, maintaining it within ‘safe’ limits. The pancreas plays an important role in the control of blood glucose level. If the levels are too high, beta cells in the islets of Langerhans respond by releasing insulin. This travels to all parts of the body at the blood, but mainly affects cells in muscles, liver and adipose tissue. Insulin lowers the blood sugar level by making the cell membrane more permeable to glucose. It activates transport proteins, allowing glucose to pass into cells. If the levels of blood glucose get too low, alpha-cells in the islets of Langerhans secrete glucagons. This hormone fits into receptor sites on cell membranes, and activates the enzymes inside the cell, which convert glycogen into glucose. The glucose then passes out of the cells and into the blood, raising blood sugar levels. (Diagram)
In osmoregulation, the method of negative feedback is used. The amount of water reabsorbed is geared to the body’s needs, and it is upon this that the solute concentration of the blood and tissue fluids depends. In the brain there are groups of cells sensitive to a rise in the solute concentration of the blood, such as might occur if the person loses a lot of water or takes in an excessive amount of salt. These osmoreceptors are situated in the hypothalamus region of the brain at the base of pituitary gland. When receptors are stimulated, a hormone is released from the posterior lobe of the pituitary gland into the bloodstream. The hormone is carried to the kidneys where it speeds up the rate at which water is reabsorbed. A result of this is that less urine is produced per unit time and it has a higher solute concentration. Stimulation of osmoreceptors also makes the person feel thirsty, so that means the person would drink. This will cause the solute concentration of the blood to fall. If drinking in excess, this results in the solute concentration of the blood falling below its normal value. The osmoreceptors are now less stimulated thatn before. This causes less hormone to be produced, less water is reabsorbed by the kidney, and a more copious and dilute urine is produced. This causes the solute concentration of the blood to rise. The production of a large quantity of watery urine is known as diuresis, and clearly the hormone counteracts this condition. The hormone used is known as antidiuretic hormone, (ADH). If more water is needed in the blood, more ADH is produced, this hormone causes the lining of the collecting ducts more permeable to water, facilitating the osmotic movement of water into the surrounding tissues. The distal convoluted tubule is also affected, so this also reabsorbs water. (Diagram)
Control of ventilation is an example of homeostasis, in which a system acts to maintain a steady state. Ventilation of respiratory system is controlled by the breathing centre in a region of the hindbrain called medulla oblongata. The ventral portion of this centre controls inspiratory movements and is called the inspiratory centre. The remainder controls breathing out and is called the expiratory centre. Control also relies on chemoreceptors in the carotid and aortic bodies of the blood system. These are sensitive to minute changes in the concentration of carbon dioxide in the blood. When this level rises, increased ventilation of the respiratory surfaces is required. Nerve impulses from these chemoreceptors stimulate the inspiratory centre in the medulla. Nerve impulses pass along the phrenic and thoractic nerves to the diaphragm and intercostals muscles. Their increased rate of contraction causes faster inspiration. As lung expands, stretch receptors in their walls are stimulated and impulses passes along the vagus nerve to the expiratory centre in the medulla. This automatically switches off the inspiratory centre, the muscles relax and expiration takes place. Inspiration takes place again. (Diagram)
Control of body temperature is effected by the hypothalamus. Within the hypothalamus is the thermoregulatory centre which has two parts: a heat gain and a heat loss centre. The hypothalamus monitors the temperature of blood temperature of blood passing through it and in addition receives nervous information from receptors in the skin about external temperature changes. Any reduction in blood temperature will bring about changes, which conserve heat. A rise in blood temperature has the opposite effect. (Diagram) Nearly all structures of the skin play some part in temperature regulation. (diagram) The skin is divided into two main layers, the epidermis at the surface, and the dermis beneath. Below the dermis is another layer, not strictly part of the skin, called the hypodermis. In cold conditions heat energy is liable to be lost from the body, but this is counteracted by the following responses:
-
The hairs are raised into a more vertical position by contraction of erector pili muscles. The contraction of erector pili also leads goose pimples. (diagram)
-
The arteriole leading to the superficial capillaries constrict. As a result blood flow to the surface of the skin is reduced, thereby cutting down the loss of heat energy from the blood to the surroundings. This vasoconstriction is brought about by the sympathetic nervous system and is particularly powerful in exposed structures such as ears, which are particularly susceptible to cold. Blood largely passes beneath the insulating layer of subcutaneous fat and so loses little heat to the outside. (diagram)
- Shivering-At low temperatures, the skeleton muscle of the body may undergo rhythmic, involuntary contractions, which produce metabolic heat. Asynchronous twitching of groups of muscle may precede this shivering.
