Homeostasis. is: Homeostasis is: the of the in a relatively stable (state despite changes in the. the maintenance of the internal environment in a relatively.

Homeostasis

Homeostasis is: is:

the the maintenance of the of the internal environment in a relatively stable (in a relatively stable (steady) state despite changes in the state despite changes in the external environment..

• is the condition of a relatively stable internal environment, maintained within narrow limits

• when changes occur in the internal environment, homeostatic mechanisms act to restore it to the ‘normal’ state

• if the body deviates too far from the normal steady state (beyond its tolerance limits) of a variable, death can occur.

The Internal Environment• the body’s internal environment consists of the tissue fluid (surrounding

cells) and blood plasma (liquid part of the blood).

• both tissue fluid and plasma are located outside of cells. Together they are called extracellular fluid (extracellular fluid = internal environment).

• the composition of the extracellular fluid is regulated so that body cells can operate at their optimum

• fluids located outside cells are extracellular.

• fluids located inside the cells are intracellular.

Task

Quick Check 1-4

Internal environment = Extracellular fluid= Internal environment = Extracellular fluid= Interstitial fluidInterstitial fluid

Key Variables controlled in the internal environmentreading page 136 -138

• core body temperature• blood glucose concentration• water levels in body tissues• ph (hydrogen ion concentration)• ions, such as sodium, calcium and chloride ions• blood oxygen concentration• carbon dioxide concentration• blood volume• blood pressure

Key Body Systems contributing to homeostasisreading page 137

• nervous system

• endocrine system

• respiratory system

• circulatory system

• digestive system

• excretory system

• integumentary (skin)system

Stimulus-Response Model(**this is a key concept for Year 11/ 12)

• a change in the internal or external environment acts as a stimulus that is detected by receptors

• if the intensity of the stimulus is sufficient (threshold), messages are transferred to a control centre

• messages are then passed to effectors which produce a response

Transmission to nerves

StimulusStimulus

ResponseResponseTransmission to

nerves or hormones

Receptor

Control Centre

Effector

Stimulus-Response Model copy

Transmission to nerves

StimulusIncrease in blood carbon dioxideIncrease in blood carbon dioxide

ResponseDecreased carbon dioxideIn blood

Transmission to nerves or hormones(Negative feedback is normal, good!)

ReceptorIn arteries and brain

Control CentreRespiratory centre in brain

EffectorRespiratory muscles in

lungs (increased ventilations)

Negative Feedback in the Stimulus-Response Modelreading page 139

In the stimulus-response model, negative feedback occurs when the effector brings about a response that counteracts the original stimulus, so that the variable within

the internal environment is returned to its optimal level.

Nerves &

Nervous System

The Nervous System- Structural Classification reading pages 301 - 302

When classified according to structure, the nervous system has two subdivisions.

1. The central nervous system (CNS)consists of the brain and the spinal cordacts as the integrating and command center of the nervous systeminterprets incoming information and issues instructions based on past experience and current conditions

2. The peripheral nervous system (PNS)the part of the nervous system outside the CNSconsists mainly of nerves that extend from the brain and spinal cord

The Nervous Systemreading pages 170-172

SpinalCord

Brain

Nerves

Central Nervous System

Peripheral Nervous System

Neurons

Neurons (Nerve Cells)reading pages 172-175

• nerve cells or neurons are the basic structure of the nervous system

• a typical neuron has:• a nucleus within the cell body • dendrites: highly branched extensions of the cell body that receive and

then carry information towards the cell body• an axon: an extension that carries information away from the cell body

• three types of neurons exist including:• sensory (affector) neurons• connecting (inter) neurons• motor (effector) neurons

• the presence of the myelin sheath (on affector and effector neurons) increases the rate at which a nerve impulse is conducted along the axon.

Tasks

Biozone 255 - 256

Cells of the nervous system. (a) A typical sensory neuron (b) A typical motor neuron (c) Structure of a nerve (d) A typical connector or inter neuron

Relationship between different kinds of neurons.

Task:

copy and label

Reflex (Arc)Simplest of neural responses.

Involves as little as 3 neurons

Shortens link between stimulus and response.

Receptors: Light

Receptors: Smell - Olfactory Receptors in the Nose

Receptors: Touch

Receptors: Sound

Endocrine System &

Hormones

Role of the Endocrine Systemreading page 134

• acts with the nervous system to coordinate and regulate the activity of body cells and so maintain homeostasis

• endocrine glands are ductless glands that secrete hormones into the bloodstream

• hormones are signalling molecules (proteins) that:• Are produced by cells within an organism• Produced within ductless glands called endocrine glands• Transported around the body in the bloodstream• act on target cells by binding to a receptor either on a plasma membrane or within a target

cell

• can communicate signals to a cell only if the cell has receptors that recognise hormone

• the receptor- hormone complex brings about a change in the target cell

Hormonesreading page 134

Examples: Hormones – Chemical MessengersGland Hormone Action

Hypothalamus Many Many body activities

Pituitary Growth Hormone The master gland

Thyroid Thyroxine MetabolismGrowth

Adrenals CortisolAdrenaline

MetabolismResponds to stress

Pancreas InsulinGlucagon

Blood glucose concentration

Gonads TestosteroneOestrogen

Fertility and sex characteristics

Homeostasis in action

Detecting & Maintaining Body TemperatureStimulus: External Temperature Change

•The skin is the barrier between our body and the external environment and can be two or three degrees below core body temperature.• •Core body temperature is maintained at about 37°C but changes in the external temperature cause changes in the temperature of exposed skin.

Receptor•Such change is detected by two kinds of temperature receptors in the skin (see figure 10.10, page 306).

•One kind of receptor detects the (stimulus) cooling of the skin and the other detects (stimulus) warming.

Transmission of Message•A change of temperature detected results in an increase in electrical information along affector (sensory) neurons

Detecting & Maintaining Body TemperatureControl Centre:

•Affector (sensory neurons) transmit impulses from skin receptors to the hypothalamus in the brain

Detecting & Maintaining Body TemperatureTransmission of Message

•Messages are relayed to effectors (glands and muscles) by effector neurons and/ or hormones (i.e. adrenalin, thyroxin etc)

Effectors•A number of effectors assist in increasing or decreasing temperature at the skin and internally including: Smooth muscles;

• Skeletal muscles; • Sweet glands• arterioles

Response•A number of number of responses to high and low temperature are employed by humans including:

• Vasoconstriction• Shivering• Piloerection• Increased metabolism• Vasodilation• Sweeting

Nerve Action Endocrine system

is faster Is slower

shorter lived more sustained(longer acting)

Why?

• Nerve action is due to electrical impulses, which travel very quickly (up to 200metres per second)

• Transmitter substance is active at a synapse for a fraction of a second only and then is inactivated

Why?

• endocrine hormones travel from their production site via the bloodstream to their target cells

• Hormones must be metabolised before their actions stop and inactivation time can take hours or days.

Research TaskHomeostasisDue Date: Thursday 31st March

Details of Task:Choose one of the following variables and produce a handout or PowerPoint presentation aimed at Year 11 Biology students outlining (addressing all the dot points) how the organism maintains a stable internal environment despite the changes in the variable in the external environment:

• Blood glucose levels in humans (Insulin and Glucagon)_• Temperature• Osmotic Control (i.e. H2O concentration in the blood)• Light (plants)• Seasonal changes (plants)• Salt concentrations in freshwater fish• Maintenance of salt concentration in salt water based fish• Calcium levels (Para hormone and Calcitonin)• Level of Thyroxin in the body (Thyroxin and Thyroid Stimulating Hormone)

CounterCurrent Heat Exchange

• Whales and dolphins also maintain their body temperature by using a countercurrent exchange system (see figure 10.29).

• There is a fine network of vascular tissue within the fins, tail flukes and other appendages.

• An outgoing artery is paired with an incoming vein.

• Blood coming from the body core to the skin is warm.

• Blood flowing from the skin back to the body core has been cooled.

TorporTorporTorpor:  a state of lowered body temperature and metabolic activity assumed by many animals in response to adverse environmental conditions, especially cold and heat. The torpid state may last overnight, as in temperate-zone hummingbirds and some insects and reptiles; or it may last for months, in the case of true hibernation and the winter torpor of many cold-blooded vertebrates

Antifreeze Substances"Groundsels also grow here [on Mount Kenya]. They are relatives of the dandelions and ragworts that flourish as small yelllow-flowered weeds in European gardens. On Mount Kenya, they have evolved into giants. One grows into a tree up to thirty feet tall. Each of its branches ends in a dense rosette of large robust leaves. As the branches grow, so each year the lower ring of leaves in the rosette turn yellow and die. But they are not shed. Instead, they remain attached and form a thick lagging around the trunk. This is of crucial importance to the groundsel. The living leaves in the rosette contain special substances that prevent frost damage to the tissues and even though they may become covered by hoar frost during the night, they thaw out rapidly in the powerful warmth of the morning sun.

Plant responses to temperature in a hot environment

• Text 322 – 325

• Quick Check 19 – 2

Green plants depend on radiant energy from the sun to carry out photosynthesis.

However, only a small fraction of energy absorbed is used.

To prevent overheating, a plant must lose much of the radiant energy it absorbs.

A plant does this in the following ways (see figure 10.30)(summarise)

Plants in a cold environment

Many plants survive in sub-zero temperatures without being damaged by these extremely low temperatures.

Unlike animals, plants do not produce an ‘antifreeze’.

They gradually become resistant to the potential danger of ice forming in their tissues as the temperature falls below 0°C.

How does this occur?

Water Balance in mammals• Read pages 330 – 335

• Quick check 25 – 30

Tonicity, page 28

Key Terms: •Tonicity•Isontonic•Hypertonic•Hypotonic

Homework

• PRAC Preparation• . Reading 362 - 364

Water Balance in fish

Water in living organisms must be maintained at a relatively stable level.

Water loss from and water gain by an organism occur in many ways.

Kidneys are essential organs for water balance.

Kidneys eliminate nitrogenous wastes from the body at the same time as maintaining water balance.

Vasopressin, renin and aldosterone are important compounds in the control of water balance and blood pressure in humans.

Water Balance in Plants

Water makes up about 90–95 per cent of the living tissues of plants.

Plants often grow in situations where they are continually losing water.

Plants cannot move around and search for water.

They have features that help them obtain and retain sufficient water for their cells to operate effectively.

Under conditions of water shortage, plants maximise their opportunity to obtain and conserve water and at the same time minimise loss.

They do these things in a number of special ways.

TranspirationThe movement of water from the roots, through the stem, and to the leaves where it may pass through the stomata as water vapour is called the transpiration stream.

The loss of water vapour from a plant is called transpiration and occurs mainly through the stomata with some loss through the cuticle.

Up to 98 per cent of water absorbed by a plant can be lost through transpiration.

Only 2–5 per cent is retained within the plant.

Leaf Structure and water loss

The waterproof outer layer of a leaf, the cuticle, reduces water loss.

The thickness of the cuticle is just one of the ways in which the structure of a leaf can vary depending on its particular environmental conditions (see figure 10.45).

The thinner the cuticle, the more transpiration occurs.

Plants that live in dry areas are known as xerophytes and show a variety of specialised features.

The shape of a leaf is important in relation to the amount of water it loses through transpiration.

The edges of some flat leaves curl over and form a protective layer above the stomata in order to reduce transpiration rate.

Factors affecting TranspirationFactors affecting TranspirationThe transpiration rate is much greater on a hot windy day than on a hot still day.

On a still day, a boundary of still air surrounds the leaf. Water vapour leaving stomatal pores tends to stay close to the leaf, keeping the humidity of the boundary layer similar to the humidity inside the leaf.

On a windy day, water vapour that diffuses out of stomatal pores is immediately blown away from the leaf; there will still be a significant difference in humidity inside and outside the leaf, and more water vapour will diffuse from the leaf.

The windier a day is, then the higher is the transpiration rate.

Air temperature is another factor that influences the transpiration rate of a plant.

The higher the temperature, the greater will be the amount of water lost from the plant. As water evaporates from the surfaces of a plant,particularly the leaves, heat is required so the evaporating process cools the surface of the leaf.

Stomatal pores close if excessive water loss occurs.

As long as there is sufficient water in the soil to replace the water that is being lost by a plant, stomata stay open.