What is Physiology?

If anatomy is the blueprint of the body — showing what is where — physiology is the instruction manual — explaining how everything actually works. Anatomy asks: *"What is it?"* Physiology asks: *"How does it work? Why does it work that way? What happens when it goes wrong?"* For example: - Anatomy tells you the heart has four chambers and four valves. - Physiology tells you WHY — the left ventricle needs thick walls because it pumps blood to the whole body under high pressure; the valves are there to prevent backflow. The two subjects are inseparable. You cannot understand how the lungs work without knowing their structure (anatomy), and knowing the structure without understanding the function gives you only half the picture. This is why in medical school, anatomy and physiology are almost always taught together. Every structure in the body exists for a reason — understanding that reason is what physiology is all about.

Physiology covers every function in the body, organised by system. Here is a quick overview of the major areas you will study: Cardiovascular physiology — how the heart pumps blood, how blood pressure is generated and regulated, how blood flow is distributed to different organs depending on what the body needs. Respiratory physiology — how breathing works, how oxygen gets from air into the blood, how carbon dioxide is removed, and how the body controls breathing rate. Renal physiology — how the kidneys filter the blood, how they regulate water and salt balance, and how they keep the blood's pH (acid-base balance) stable. Neurophysiology — how nerve cells generate and transmit electrical signals, how reflexes work, how the brain processes sensation and controls movement. Gastrointestinal physiology — how food is broken down and absorbed, how the gut moves food through, and how different organs (stomach, pancreas, liver) contribute to digestion. Endocrine physiology — how hormones are made, released, and act on target organs to regulate almost every process in the body. Muscle physiology — how muscles contract at the molecular level, how they fatigue, and how exercise changes them over time. The unifying theme across all of these areas is homeostasis — the body's constant effort to keep everything in balance. We will explore homeostasis in detail in the next lesson.

One of the most remarkable things about the human body is that it can regulate itself automatically — without you having to think about it. Your heart rate adjusts when you stand up. Your kidneys change how much urine they produce depending on how much water you drank. Your pupils widen in a dark room within seconds. All of this self-regulation happens through control systems — the same basic concept used in engineering. Every control system has three parts: 1. A sensor (receptor) Something that detects a change. In the body, sensors include chemoreceptors (detect changes in blood oxygen or CO₂), baroreceptors (detect changes in blood pressure), thermoreceptors (detect temperature changes), and osmoreceptors (detect changes in blood salt concentration). 2. A control centre Something that processes the information and decides what to do. In the body this is often the brain (specifically parts like the hypothalamus or brainstem), but it can also be local — cells in the pancreas directly sense blood glucose and respond without involving the brain. 3. An effector Something that carries out the response. Effectors are usually muscles or glands. For example, sweat glands (reduce temperature), the heart (increase or decrease heart rate), the adrenal glands (release adrenaline), or blood vessels (dilate or constrict). This sensor → control centre → effector loop runs continuously throughout your life, adjusting thousands of variables at once. When one part of the loop fails — due to disease, injury, or drug effects — the body loses the ability to regulate that variable, and illness follows.

The most important principle in physiology is this: the structure of something tells you exactly what it is designed to do. This principle works at every level — from molecules to whole organs. At the molecular level: Haemoglobin (the protein in red blood cells that carries oxygen) has a special shape that grabs oxygen tightly in the lungs (where oxygen is plentiful) and releases it in the tissues (where oxygen is scarce). Its shape is perfectly tuned for this job. Change the shape even slightly — as happens in sickle cell disease — and it no longer works properly. At the cellular level: Cells that are designed to absorb things (like gut lining cells and kidney tubule cells) have thousands of tiny finger-like projections called microvilli on their surface. These massively increase surface area — more surface area means more absorption. A smooth cell would absorb far less. At the organ level: The small intestine is 6 metres long and coiled — giving it a huge length for absorption. The alveoli of the lungs are tiny spheres (maximising surface-area-to-volume ratio) with walls just one cell thick (minimising the distance oxygen must travel). The heart has one-way valves because blood must flow in one direction only. Whenever you see an unfamiliar structure in medicine, ask: *"What does this shape tell me about what this thing does?"* The answer is almost always right there in the structure. A clinical example: A patient has atherosclerosis — fatty plaques building up inside the artery walls. The artery's inner diameter (lumen) narrows. Physiology tells you what happens next: narrower tube → higher resistance to flow → the heart must work harder to push blood through → over time the heart muscle thickens (hypertrophy) → eventually the heart may fail under the strain. Understanding structure-function relationships lets you predict the entire chain of events.

Every symptom a patient has, every sign a doctor finds on examination, every result on a blood test — all of it reflects physiology either working normally or breaking down. Symptoms explained by physiology: - Shortness of breath in heart failure — the failing heart cannot pump enough blood forward. Pressure backs up into the lungs. Fluid leaks into the alveoli. Gas exchange is impaired. The patient feels breathless because blood oxygen falls and CO₂ rises — both detected by chemoreceptors that drive the sensation of breathlessness. - Swelling (oedema) in the ankles — in heart failure or low albumin states, fluid leaks from capillaries into the tissues. Gravity pulls it to the ankles. - High blood pressure — could be from increased blood volume (kidneys retaining too much salt and water), increased heart output, or narrowed blood vessels increasing resistance. The treatment targets the specific mechanism. Blood tests explained by physiology: Every routine blood test measures a variable the body normally regulates tightly. When a result is abnormal, it means a control system has failed or been overwhelmed: - High blood glucose → insulin system failing (diabetes) - High creatinine → kidneys not filtering (kidney disease) - Low haemoglobin → not enough red blood cells (anaemia) - Abnormal sodium → water/salt regulation failing Physiology gives you the framework to understand *why* these things happen — and what to do about them.