Homeostasis (PDF)

**Homeostasis:** - **Homeostasis is the maintenance of a constant internal environment (despite changes to the external/internal environment).** - Homeostasis is important to the human body because it is vital that organisms can maintain a constant internal environment to allow for...

**Homeostasis:** - **Homeostasis is the maintenance of a constant internal environment (despite changes to the external/internal environment).** - Homeostasis is important to the human body because it is vital that organisms can maintain a constant internal environment to allow for regular metabolism to occur i.e. ideal conditions for enzyme action. - Being able to react to changes in the environment ensures that an organism has a better chance of surviving difficult changing conditions. - A cell's delicately balanced chemical reactions work best within narrow limits of  temperature, pH, solute concentration etc. - In multicellular organisms e.g. humans all cells need to be maintained in optimal working conditions so tissues/organs/organ systems work together to ensure this. - In unicellular organisms e.g. protista the ability to maintain the internal environment is completely controlled by direct interactions with the external environment. **Homeostasis in plants:** - Plants need to be able to respond to changing internal conditions to ensure survival e.g. water levels. Water levels: - Plants control their water levels by ensuring that their root cells have very high solute concentration to increase osmosis of water into the plant - Plants have a waxy cuticle to reduce water loss from the plant. - Plants have stomata on the underside of the leaf as it is in a shaded area to reduce transpiration by sunlight. - Plants can also open and close the stomata, this also allows them to control carbon dioxide levels. **Homeostasis in humans examples:** **Internal Condition** **How it's controlled** ------------------------ -------------------------------------------------------------- Temperature Through actions of the skin i.e. sweating and shivering etc. Ion Content Through secretion in the kidneys Blood Glucose Hormone action of insulin/glucagon Water content Osmoregulation through kidneys and ADH **Waste Products** **How they are removed** Carbon Dioxide Excreted through the lungs Urea Removed through urine **Negative feedback mechanisms:** - Most homeostatic mechanisms work by negative feedback, i.e. if there is a change away from the normal optimum value, action is automatically taken to reverse this change. - This change can be positive i.e. an increase in temperature or negative i.e. a decrease in temperature but it will result a reaction to bring the body back to homeostasis and optimum levels Diagram of a diagram of a heat and electricity Description automatically generated **Temperature Control:** Positive change (increase away from the optimum: - When **temperature begins to rise** the body reacts by **flattening hairs,**  this means that air isn't trapped close to the surface of the skin allowing for more radiation of heat. - Blood vessels experience **vasodilation** resulting in increased blood flow. This means more blood flows to the surface which leads to increased heat loss and can often be seen as we flush red. - The skin will begin **sweating** which leads to increased evaporation resulting in increased cooling. Negative change (decrease away from the optimum): - When **temperature drops** we **shiver** which is muscle contractions being engaged and released quickly to create heat through respiration. - Hairs will undergo **piloerection** whereby the hair erector muscles pull on hairs to stand them up and trap a layer of air. We see this as goosebumps on the surface of our skin. -  Blood vessels will experience **vasoconstriction** resulting in decreased blood flow. Less blood carrying vital heat flows to the surface and this leads to reduced heat loss.  **Ectotherms vs endotherms:** **Ectotherms:** - **Ectotherms** are often referred to as cold-blooded animals**.** - Ectotherms will **rely on their external environment** to control body temperatures. - Ectotherms are completely reliant on their external environment for body temperature meaning that they can only survive in certain places. - Lizards have to live in hot countries so that they can absorb the heat they require by basking in the sun. - This form of temperature control is beneficial in that **less energy** is required as heat isn't generated through metabolism. Therefore less food needs to be consumed and long periods of nutrition deficiency can be survived. **Endotherms:** - **Endotherms** are often referred to as warm-blooded animals. - Endotherms will **create their own body temperature** through their internal metabolism.  - Endotherms can control their internal environment despite changes in the external environment. - These organisms can live anywhere there is enough food to produce significant body heat through metabolism.  - Polar Bears can live in frigid conditions as they create their own heat through metabolism - This form of temperature control requires **very high energy** demands meaning that large quantities of food need to be consumed regularly.\.  **The Skin:** **Functions of the skin** - Protection against bacteria and viruses as a strong barrier is formed to stop their entry.  - The skin produces vitamin D which is needed to absorb the mineral calcium from the bloodstream. - The **skin** is the sense organ that allows us to sense **touch**. The skin contains special receptor cells that allow us to feel touch, feel pain and pressure and feel temperature. - Controls temperature by controlling things like sweating, shivering, vasodilation and vasoconstriction.   **Structure of the skin:** - The epidermis protects the body from damage and pathogens. It is made up of;  1. The **Cornified Laye**r contains cells full of keratin.  2. The **Granular Laye**r where keratin is made.  3. The **Malpighian Layer** where cells are constantly dividing by mitosis to replace cells lost from the cornified layer. Melanin is also produced in this layer.  - **Melanin** gives **colour to the skin, hair and iris** as well as **protecting the skin against UV radiation. **