HOMEOSTASIS

NCS 2011 Laura Kannemeyer

Maintaining a constant internal environment (tissue fluid)

TISSUE FLUID Cells in body is surrounded by Extracellular fluid

How is homeostasis maintained

Keeps body in steady state. Detects any change from the norm and

then corrects this deviation. If too MUCH of a particular factor

process set in motion to reduce that factor If too LITTLE of a particular factor

process set in motion to increase that factor

How is homeostasis brought about?

Correction is brought about by

Negative feedback control system

o Receptor: detects change from normo Control centre: processes the

information/decides appropriate response.o Effector: responds and corrects the

deviation

Homeostatic control mechanism

receptor

effector

Control: brain

Control: brain

Feedback systems

1. Negative feedback: cancels or counteracts original stimulus(opposite present conditions), e.g., blood sugar balance, blood pH balance, blood pressure balance, body temperature "thermostat“

Definition: Negative feedback: (A principle governing most

control systems) a mechanism of response in which a stimulus initiates action that reverse or reduce the stimulus.

2. Positive feedback: increases original stimulusin reproductive processes: child birth (labor), sexual reproduction (orgasm)

Definition:Positive feedback: a mechanism of response in

which a stimulus initiates action that enhances the

stimulus.

Thermoregulation

Maintenance of a constant body temperature

by balancing

Heat production: thermogenesis with

Heat loss: thermolysis

ENDOTHERMS

Mammals that obtain heat from Cellular respiration.

Endo: insidetherm: heat

Human body temperature is 37ºC Optimum temperature for action of enzymes.

ie high temperatures denature enzymes low temperatures sluggishness

Homeothermic means that they can maintain a constant (high) body temperature.

Endothermic means that they can maintain their (high) body temperature by creating heat through metabolic means.

ECTOTHERMS

Gain heat from the environment

Heat gained by the body

Main source of heat is cellular respiration.

Behavioural means by absorbing from environs.

Increased secretion of thyroxin and adrenalin raises metabolic rate speeds up respiration releases more heat.

Heat lost from the body

Radiation: transfer of heat energy from a warm body to a colder environment.ie body losing heat

Conduction: transfer of heat energy from a hotter to a cooler of two objects in contact

Convection: occurs when a current of air / water flows over the surface of the body.ie Air warmed by the body rises and is replaced

by cooler air.

Evaporation: Change of liquid to a vapour / gas

SKIN OF THE MAMMAL

EPIDERMIS

Thick Layers of dead cells at the surface.(cornified) Contain keratin - “water-proof” the skin.  Forms horny pads on the soles of the feet and toes of many mammalsFunction: * effective in protecting the

underlying tissues from water loss, * invasion by microorganisms and * mechanical abrasion. 

DERMIS

Composed of dense fibrous connective tissue Contains blood vessels, nerve fibres and

sensory endings, erector muscles, glands and hair follicles. 

Sebaceous glands - produce oily and waxy secretions and

Sweat glands that are usually secreted into hair follicles 

HAIR FOLICLE

Hair forms a dense protective and insulating fur over the body. 

Eyelashes are modified hairs. 

Subcutaneous fat layer

Insulation waterproofing

Skin Receptors

How does body react in COLD?

Cold

End bulbs of Krause

Hypothalamus

Vasomotor center of medulla oblongata

Effectors

THERMOREGULATION ON A COLD DAY

Temperature of the environment is much lower than that of the body.

Heat is lost from the body through radiation, evaporation, conduction and convection.

Body temperature drops below normal.- Body’s thermostat – hypothalamus- Stimuli (a) changes in temperature of the blood (b) impulses from sense organs in the skin

ie end bulbs of Krause

1. To reduce heat loss:

The hypothalamus sends impulses to the:(a) Erector muscles in the skin Muscles contract Hair follicle rises / stand erect Goosebumps are formed Hair trap a thicker layer of air (air is a poor

conductor of heat) acts as an insulator Heat loss through radiation is ↓

(b) Involuntary muscles around the blood vessels in the skin

Circular muscles contract ↓ diameter of blood vessels – they constrict called vasoconstriction Less blood to the surface of the skin Thus less heat to the surface of the skin Thus ↓ loss of heat through radiation

(c) Sweat glands

Vasoconstriction cause less blood to flow to sweat glands

Become less active Less sweat is produced ↓ loss of heat through evaporation

(c) Subcutaneous layer of fat

Poor conductor of heat Serves to insulate body against heat loss.

When the body is cold

1. The small capillaries near the surface of the skin constrict (vasoconstriction) so less blood flows nearer to the surface, and less heat is lost through he blood.

2. The hairs on the skin stand erect by the hair erector muscles tightening. The hairs trap a layer of insulating air therefore less heat is lost to the surroundings.

3. Sweat Glands produce less sweat so less heat is lost through evaporation.

The muscles also vibrate very quickly when it is cold, so heat is produced to keep the body temperature at the correct level. This is also known as shivering!

Factors that warm up the body Low body temperatures initiate

shivering (contraction of voluntary muscles)

Behavioural factors eg wearing clothes / moving to warmer climate.

Increased production of thyroxin

Behavioural factors

The hypothalamus sends impulses to the cerebrum

The person becomes aware of the cold and goes over to voluntary actions:

(a) ↑ intake of warm food and drink (e.g. coffee)

(b) seeks shelter (indoors)(c) warmer clothes (d) seeks heat from the sun or heater

Increased production of thyroxin

Hypophysiso It is stimulated to secrete more TSHo this stimulates the thyroid glando to produce more thyroxin: ↑ in the general body metabolism ↑ in cellular respiration ↑ heat production

Hypothermia

Drop in body temperature to below 35ºCo Prolonged exposure to coldo Enzymes become sluggish: Irregular heart beat Low respiratory rate Drop in blood pressure Brain activity impaired Uncorrected cardiac arrest Coma death

How does body react in HOT?

Heat

Ruffinian bodies

Hypothalamus

Vasomotor center of medulla oblongata

Effectors

Body temperature rises above normal.- Body’s thermostat – hypothalamus- Stimuli (a) changes in temperature of the blood (b) impulses from sense organs in the

skin ie Ruffini’s corpuscles

THERMOREGULATION ON A HOT DAY

2. To increase heat production

The hypothalamus sends impulses to the:(a) Blood vessels to the skin Inhibits impulses to circular muscles relax Allows large volume of blood to flow to capillary loops vasodilation Heat in blood is lost through radiation, conduction, convection

(b) Sweat glands

Vasodilation more blood flow to seat glandsie ↑ sweating

Evaporation of sweat cools the body

Factors affecting rate of evaporation:• environmental temperature - ↑ temp, quicker evap.• air movement - ↑ wind, quicker evap.• humidity - ↓ dry air, rapid evap.

When the body is hot

1. The small capillaries near the surface of the skin dilate (vasodilation) so more blood flows nearer to the surface, and more heat is lost through he blood. The body therefore cools down.

2. The hairs on the skin lie flat because the hair erector muscles slacken. No layer of insulating air is trapped therefore heat can be lost more easily.

3. Sweat Glands produce more sweat so more heat can be lost through evaporation.

(c) Erector hair muscles

Muscles not stimulated. Hair follicle continue to lie flat.

Overheating of the body: Heat stroke / sunstroke Prolonged exposure to high temperatures Symptoms headache / dizziness / general

weakness sweating ↓ body appears flushed / pulse rate ↑ collapse of circulatory system death

Hyperthermia