Homeostasis

Learning Objectives define homeostasis as the maintenance of a constant internal environment explain the basic principles of homeostasis in terms of stimulus resulting from a change in the internal environment, a corrective mechanism and a negative feedback identify on a diagram of the skin: hairs, sweat glands, temperature receptors, blood vessels and fatty tissue describe the maintenance of a constant body temperature in humans in terms of insulation and the role of: temperature receptors in the skin, sweating, shivering, blood vessels near the skin surface and the co-ordinating role of the brain

Definition Homeostasis is the maintenance of a constant internal environment.

The internal environment is made up of blood and tissue fluid.

What factors within the internal environment must be kept constant? The concentration of glucose The concentration of ions, e.g. sodium and potassium The concentration of carbon dioxide The osmotic pressure, determined by the relative concentrations of water and solutes (osmoregulation) Temperature (thermoregulation) The pH (acid-base balance) Nitrogenous waste products and other toxic substances, which are either eliminated or at least kept to a minimum

Importance of Homeostasis Homeostasis keeps the body environment under control and keeps the conditions right for cells to live and function. For eg. Enzymes in our body can only work at a narrow range of pH and temperature. Changes in these conditions can lead to enzyme denaturation or inactivation that can subsequently affect the workings of our body.

Negative Feedback Homeostasis involves an important principle called negative feedback which is the reverse effect of a stimulus. An increase in temperature will induce a feedback to decrease the temperature. A decrease in temperature will induce a feedback to increase the temperature.

NEGATIVE FEEDBACK CONTROL LOOP A norm or set point to be maintained A stimulus which is a change in the internal environment. A receptor which can detect the stimulus. A control centre A corrective mechanism to bring about the reverse effect of the stimulus. A feedback to the receptor when the set-point is reached. This causes corrective mechanism to stop.

Examples of homeostasis in Man: Regulation of blood glucose concentration Body cells need glucose for tissue respiration to provide energy. The concentration in blood plasma remains relatively constant. (70-90mg per cubic cm) Glucose level may rise after a meal. It falls during exercise or starvation. How is the concentration of glucose in blood plasma regulated? ?? If BGL rises – can lead to cells shrinking…

Regulation of blood water potential. Recall the functions of kidney to regulate water potential.

Regulation of carbon dioxide level. Changes in carbon dioxide concentration of blood automatically affects the rate of breathing. Exercising increase the concentration of carbon dioxide in blood hence also the rate of breathing.

Regulation of blood temperature

Functions of the Mammalian Skin Regulates body temperature Protects the body against damages Prevents water loss Excretory organ Sensory organ Produces vitamin D

Structure of the Human Skin

Structure of the Mammalian Skin: Largest organ of the body Made up of 3 parts: epidermis, dermis and subcutaneous(hypodermis) tissue. The hypodermis is a few layers of adipose cells (adipose tissue) where fat is stored. The fat serves as an insulation layer as well as a food storage organ.

The Epidermis Outer cornified layer Granular layer Innermost Malpighian layer

Parts of the Mammalian Skin Epidermis Outer cornified layer has: Dead, dry, flat cells filled with keratin which makes the skin water resistant and impenetrable. Protects body from mechanical injury Prevents water loss through evaporation Granular layer Living cells synthesizing keratin Flatter and ceases division Give rise towards cornified layer Innermost Malpighian layer Pigmented living cells(melanin- protects against harmful UV rays) Undergoes cell division- cells are pushed towards granular layer-change shape and structure and pushed towards granular layer.

Parts of the Mammalian Skin Dermis Blood capillaries Arterioles that supply blood to the capillaries are controlled by nerves Vasodilation: arterioles dilate; more blood sent to skin; heat lost Vasoconstriction: arterioles constrict; less blood sent to skin; heat retained Constriction and dilation helps to increase or decrease amount of blood brought to the surface hence amount of heat lost to environment

Parts of the Mammalian Skin Dermis Hair Formed in hair follicle (hollow tube-like structure) Base of follicle is the hair papilla where hair develops; cells in papilla are nourished by surrounding blood capillaries Hair erector muscle attached to hair follicle When contract, it cause the hair to stand forming “goosebumps” Sweat gland Coiled tubular gland producing sweat (water, salts NaCl and urea) Surrounded by blood capillaries Sweat evaporates from skin for thermoregulation

Parts of the Mammalian Skin Dermis Sebaceous gland At least 2 per follicle Secrete sebum into hair follicle Sebum lubricates hair maintain softness of skin prevents dehydration Antiseptic ( prevents growth of bacteria) Sense receptors Nerve endings (found in epidermis too) Detect changes in external environment e.g. temp, touch(pressure), pain

Parts of the Mammalian Skin Fatty tissues (adipose tissues) Insulating layer – reduce heat loss

Temperature Receptors in Skin Thermoregulation Ext. Temp Ext. Temp Body Temp Body Temp Temperature Receptors in Skin Blood Temp Blood Temp Hypothalamus Heat Loss Centre Heat Gain Centre Vaso-constriction Vasodilation Sweat Pdtn Rapid Breathing Sweat Pdtn Shivering Negative Feedback Normal

Heat is lost from the body to prevent overheating by: Through the skin by convection, radiation and a little conduction. Evaporation of sweat from the surface of the skin. In the faeces and urine. Expired air from lungs.

Advantages of constant body temperature: Can remain active throughout the day and year unchanged with temperature variation. Enzymes work best at a constant optimum temperature. Need not hibernate hence able to feed throughout the seasons. Exploit and colonize areas with different climate conditions.

To Summarise

What else? Glycemic index ( measures how quickly BGL increases after a meal. Test yourself 12.1, 12.2 (discuss) and review qs Frost bite and heat stroke, melanoma( find out). Why do you shiver when you’re having fever? Test yourself 12.3 for homework in your practical notebook.