Source: APK2105 Textbook, University of Florida
Difficulty: Introductory Prerequisites: None. This is foundational material for the entire course.
This chapter lays the groundwork for everything else in APK2105. It introduces the levels of organisation in the body (cells, tissues, organs, organ systems), explains how the body separates its internal environment from the outside world, and introduces homeostasis, the central concept in physiology. If you understand how the body maintains stable internal conditions through feedback loops, every later chapter will make more sense. If you are joining the course late, start here.
The body is organised from cells up to organ systems, all working together. Epithelial tissue acts as the barrier between external and internal environments, while body fluids are divided into intracellular and extracellular compartments. Homeostasis keeps internal conditions stable through negative and positive feedback mechanisms.
Homeostasis
The body's process of maintaining constant internal conditions (composition, temperature, volume of ECF) despite changes in the external environment. Think of it as the body's autopilot, always correcting back to baseline.
Extracellular Fluid (ECF)
All fluid outside of cells. This is the body's internal environment because it surrounds most cells. It is high in sodium and includes both plasma and interstitial fluid.
Intracellular Fluid (ICF)
All fluid inside cells. Makes up roughly two-thirds of total body water. High in potassium. In simple terms, this is the fluid trapped within every cell membrane in the body.
Total Body Water (TBW)
The total fluid within the outer epithelial layer. Composed of approximately 2/3 ICF and 1/3 ECF.
Plasma
The liquid, noncellular portion of blood. Rich in proteins, unlike interstitial fluid.
Interstitial Fluid (ISF)
Fluid present outside of blood vessels but still within the ECF. Bathes most cells directly. Not rich in proteins (unlike plasma).
Negative Feedback
A regulatory mechanism where the body responds to a change by producing the opposite effect, pushing the variable back towards its set point. Think of a thermostat: temperature rises, the system cools it down.
Positive Feedback
A regulatory mechanism where the body responds to a change by amplifying it in the same direction. Rarer than negative feedback. Continues until a limiting trigger stops the loop.
Set Point
The normal or desired value of a regulated variable. The target that negative feedback loops aim for.
Error Signal
The difference between the desired value (set point) and the actual value of a regulated variable. This is what sensors detect and respond to.
Regulated Variable
A physiological parameter kept within strict limits, such as body temperature or blood glucose concentration.
Sensors
Specialised cells that detect changes in specific variables. For example, chemoreceptors in the blood detect oxygen and CO2 levels.
Integration Centre
The processing area that receives sensory input and generates an appropriate response, then sends signals to an effector.
Effector
The cell or organ that carries out the response directed by the integration centre.
Absorption
The transport of nutrients and other wanted substances from the lumen of the GI tract into the bloodstream.
Secretion
The transport of materials from the blood into the lumen, producing substances like acid and proteins.
Filtration
In the kidneys, the process by which fluid from the bloodstream enters the renal tubules.
Reabsorption
The selective transport of water, salts, and nutrients from the kidney tubules back into the bloodstream.
Inspiration / Expiration
Inspiration is the intake of oxygen; expiration is the release of carbon dioxide. In simple terms, breathing in and breathing out.
Neurons use electrical impulses to transmit sensory information from outside and inside the body, and to process that information
Muscle fibres (muscle cells) are further divided into three types:
Skeletal (voluntary)
Cardiac
Smooth (involuntary)
Epithelial cells are classified by:
Number of layers: simple (one layer) or stratified (multiple layers)
Cell shape: squamous, cuboidal, or columnar
They sit on a basement membrane of non-cellular material
Can form glands: exocrine glands (secrete via ducts to outside the body) or endocrine glands (secrete hormones into the body)
Connective tissue cells include blood, bone, and fat cells
Rule of thumb: small amounts of cells surrounded by lots of extracellular matrix (ECM)
ECM supports or binds structures together (bones, ligaments, tendons)
Contains fibres: elastic, collagen, and reticular
The body's major organ systems: endocrine, nervous, musculoskeletal, cardiovascular, respiratory, urinary, gastrointestinal, reproductive, immune, integumentary
Each organ system performs a distinct set of functions, but all depend on one another
Epithelial tissue forms a continuous barrier separating the external environment from the internal environment
This barrier lines both the outside (skin) and the inside (lungs, stomach, intestines)
The external environment provides oxygen and nutrients; these are passed through blood cells to the rest of the body
ECF is called the body's "internal environment" because it surrounds most of the body's cells
Respiration: O2 enters during inspiration, CO2 leaves during expiration
GI tract: nutrients, water, and salts move from the lumen into the bloodstream (absorption); blood products move into the lumen (secretion)
Kidneys: blood fluid enters tubules (filtration); useful substances are selectively returned to the blood (reabsorption)
Fluid compartments are separated by epithelial tissues and cell membranes, which are selectively permeable
TBW = ICF (≈2/3) + ECF (≈1/3)
ECF subdivides into plasma and interstitial fluid
ICF is high in potassium; ECF is high in sodium
Plasma is protein-rich; ISF is not
The body maintains stable internal composition, temperature, and volume of ECF
Normal ECF temperature: 37 °C (98.6 °F)
When disrupted (e.g. haemorrhage), compensatory mechanisms kick in. In haemorrhage, ISF rushes into plasma to maintain volume, though cells need days to replenish
Most homeostatic regulation uses negative feedback
The loop: sensor detects a deviation from the set point → generates an error signal → integration centre processes and sends a response → effector acts to reverse the change
The loop continues until the set point is met or nearly met
Example: body temperature rises → sensors detect it → the body activates cooling mechanisms → temperature returns to 37 °C
Amplifies a change rather than reversing it
Example: rising plasma oestrogen triggers increased LH release, which further drives the process, leading to ovulation
Allows rapid change in response to a stimulus
Always limited by a trigger that stops the loop, preventing indefinite escalation
Blood sugar is controlled by negative feedback
A rise in blood glucose triggers insulin release, which lowers blood sugar back to its set point
Without insulin, blood glucose remains elevated above the set point (relevant to understanding diabetes)
Negative feedback is the same principle behind a home thermostat: the system detects a deviation and corrects it. Your body does this constantly with temperature, blood pressure, and blood glucose.
Understanding positive feedback helps explain why labour contractions intensify over time (oxytocin positive feedback loop) and why blood clotting cascades rapidly once started.
Students often think ECF and ICF have the same ionic composition. They do not. ICF is potassium-rich; ECF is sodium-rich.
Students frequently confuse absorption and reabsorption. Absorption happens in the GI tract (lumen to blood). Reabsorption happens in the kidneys (tubules back to blood).
Positive feedback is not "the body going wrong." It is a controlled, purposeful amplification with a built-in stopping mechanism.
The body's "internal environment" is the ECF, not the inside of cells. This trips students up because ICF sounds like it should be the internal environment, but cells are bathed by ECF.
⚠️ Be able to distinguish negative feedback from positive feedback with examples. This is tested repeatedly.
⚠️ Know the fluid compartment ratios: TBW = 2/3 ICF + 1/3 ECF. Know which ions dominate each compartment.
⚠️ Understand the difference between absorption, secretion, filtration, and reabsorption, and know which organ system each occurs in.
⚠️ The negative feedback loop components (sensor → integration centre → effector) are a staple exam question. Be able to label each part in a given scenario.
True or False: ECF is high in potassium and ICF is high in sodium.
Fill in the blank: The process by which fluid enters kidney tubules from the bloodstream is called __________.
True or False: Positive feedback always leads to a runaway, uncontrolled response.
Fill in the blank: The difference between the set point and the actual value of a variable is called the __________.
True or False: Plasma and interstitial fluid both contain high levels of protein.
Answers: 1. False (it is the reverse). 2. Filtration. 3. False (triggers limit the loop). 4. Error signal. 5. False (plasma is protein-rich; ISF is not).
Q: Define homeostasis and explain why it is important for normal body function.
A: Homeostasis is the body's process of maintaining constant internal conditions (composition, temperature, volume of ECF) despite changes in external conditions. It is essential because cells require a stable environment to function properly; significant deviations can impair cellular processes and lead to disease or death.
Q: A patient experiences significant blood loss (haemorrhage). Describe the immediate compensatory response involving body fluid compartments.
A: Interstitial fluid (ISF) rushes into the plasma to help maintain blood volume. This is a short-term compensation; the cells that lost fluid will need several days to replenish their volume.
Q: Explain the components of a negative feedback loop using blood glucose regulation as your example.
A: The regulated variable is blood glucose. When glucose rises above the set point, sensors (pancreatic beta cells) detect the change and generate an error signal. The integration centre triggers the release of insulin (the effector response). Insulin lowers blood glucose. The loop continues until blood glucose returns to or near its set point.
Q: How does positive feedback differ from negative feedback? Give one physiological example of each.
A: Negative feedback opposes a change, returning the variable to its set point (e.g. body temperature regulation). Positive feedback amplifies a change in the same direction (e.g. rising oestrogen triggers more LH release, driving ovulation). Positive feedback loops have built-in triggers that stop them.
Q: Distinguish between absorption and reabsorption.
A: Absorption is the transport of nutrients from the GI tract lumen into the bloodstream. Reabsorption is the selective return of water, salts, and nutrients from kidney tubules back into the bloodstream.
Homeostasis and feedback loops underpin virtually every later topic in APK2105, from thermoregulation to endocrine control to cardiovascular adjustments during exercise.
The fluid compartment concepts here connect directly to Chapter 2's discussion of membrane transport and selective permeability.
Understanding epithelial barriers prepares you for later material on gas exchange in the lungs and nutrient absorption in the GI tract.
homeostasis, negative feedback, positive feedback, set point, error signal, regulated variable, sensor, integration centre, effector, ECF, ICF, TBW, plasma, interstitial fluid, ISF, absorption, secretion, filtration, reabsorption, body fluid compartments, epithelial barrier, internal environment, external environment, inspiration, expiration, APK2105, anatomy and physiology, University of Florida, Chapter 1