14.4 Homeostasis

2026 Syllabus Objectives

  1. Describe homeostasis as the maintenance of a constant internal environment
  2. State that insulin decreases blood glucose concentration
  3. Explain the concept of homeostatic control by negative feedback with reference to a set point
  4. Describe the control of blood glucose concentration by the liver and the roles of insulin and glucagon
  5. Outline the treatment of Type 1 diabetes
  6. Identify in diagrams and images of the skin: hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue
  7. Describe the maintenance of a constant internal body temperature in mammals in terms of: insulation, sweating, shivering and the role of the brain
  8. Describe the maintenance of a constant internal body temperature in mammals in terms of vasodilation and vasoconstriction of arterioles supplying skin surface capillaries

What is Homeostasis? 🔑

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Homeostasis is the maintenance of a constant internal environment within the body.

Why is Homeostasis Important?

Homeostasis helps your cells work as efficiently as possible by maintaining optimal conditions for metabolic processes:

  • Constant temperature (around 37°C37°\text{C}) - Helps enzymes work at their optimum rate
  • Constant water content - Prevents cells from being damaged by absorbing or losing too much water by osmosis
  • Constant glucose concentration - Ensures there is always enough fuel for respiration

Key Point: Homeostasis involves both the nervous system and hormones working together to maintain internal stability.

Control of Blood Glucose Concentration 🩸

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Why is Blood Glucose Control Important?

The control of blood glucose concentration is a very important part of homeostasis:

Why cells need glucose:

  • Cells need a steady supply of glucose for respiration to release energy
  • Brain cells are especially dependent on glucose and die quite quickly if deprived of it

Problems with high blood glucose:

  • Can cause water to move out of cells into the blood by osmosis
  • Leaves cells with too little water to carry out normal metabolic processes

The Role of the Pancreas

The pancreas functions as two glands in one:

  1. Exocrine function - Most of the pancreas makes pancreatic juice, which flows along the pancreatic duct into the duodenum

  2. Endocrine function - Scattered throughout the pancreas are groups of cells called islets that produce two hormones:

    • Insulin - lowers blood glucose concentration
    • Glucagon - raises blood glucose concentration

How Insulin Works

Insulin is a hormone secreted by the pancreas that decreases blood glucose concentration.

When blood glucose rises (e.g., after eating a sugary meal):

  1. Islets in the pancreas detect the high glucose concentration
  2. Pancreas secretes insulin into the blood
  3. Insulin reaches the liver and causes liver cells to:
    • Absorb glucose from the blood
    • Use some glucose for respiration
    • Convert excess glucose into the insoluble polysaccharide glycogen
    • Store glycogen in the liver

How Glucagon Works

Glucagon is a hormone secreted by the pancreas that increases blood glucose concentration.

When blood glucose falls too low:

  1. Pancreas secretes glucagon
  2. Glucagon causes liver cells to:
    • Break down glycogen stores into glucose
    • Release glucose into the blood

The Complete Control Cycle

The diagram below shows how blood glucose concentration is regulated through a continuous cycle:

Pancreas→Insulin secreted (high glucose)→Liver stores glucose as glycogen\text{Pancreas} \rightarrow \text{Insulin secreted (high glucose)} \rightarrow \text{Liver stores glucose as glycogen} Blood glucose falls→Normal levels→Low levels\text{Blood glucose falls} \rightarrow \text{Normal levels} \rightarrow \text{Low levels} Glucagon secreted→Liver breaks down glycogen→Blood glucose rises\text{Glucagon secreted} \rightarrow \text{Liver breaks down glycogen} \rightarrow \text{Blood glucose rises}

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