Textbook of Medical Physiology

1. Physiology Explains Life: From Cells to Systems

The goal of physiology is to explain the physical and chemical factors that are responsible for the origin, development, and progression of life.

Physiology's Scope. Physiology seeks to understand the physical and chemical processes underlying life, from simple viruses to complex human beings. This vast field is divided into specialized areas like viral, bacterial, cellular, plant, and human physiology, each focusing on the unique functional characteristics of different life forms.

Human Physiology Focus. Human physiology specifically examines the mechanisms that enable the human body to function as a living organism. It explores how we maintain life through automatic responses to hunger, fear, cold, and the drive to reproduce, highlighting the body's intricate self-regulation.

Homeostasis. The human body operates as a complex automaton, with sensations and feelings playing a crucial role in its automatic life sequence. These attributes allow us to adapt and survive in diverse conditions. The concept of homeostasis, maintaining stable internal conditions, is central to understanding how our bodies function and thrive.

2. Cells: The Body's Foundational Building Blocks

The basic living unit of the body is the cell, and each organ is an aggregate of many different cells held together by intercellular supporting structures.

Cells as Units. The human body is composed of approximately 100 trillion cells, each a living unit with specialized functions. Organs are formed by aggregates of these cells, held together by intercellular structures.

Cellular Specialization. Different cell types are adapted to perform specific tasks. Red blood cells transport oxygen, while other cells have unique roles. Despite their differences, all cells share basic characteristics, such as the need for oxygen and nutrients to function.

Internal Environment. Cells exist within the extracellular fluid, the body's internal environment. This fluid provides the necessary ions and nutrients for cellular life. Cells can thrive as long as the internal environment maintains the proper concentrations of essential constituents.

3. DNA: The Blueprint of Life, Controlling Cells and Heredity

Each gene, which is a nucleic acid called deoxyribonucleic acid (DNA), automatically controls the formation of another nucleic acid, ribonucleic acid (RNA); this RNA then spreads throughout the cell to control the formation of a specific protein.

Genes as Directors. Genes, composed of DNA, are the control centers of the cell, dictating the synthesis of proteins. These proteins include structural components and enzymes that catalyze chemical reactions, influencing everything from energy production to cell structure.

Transcription and Translation. DNA's instructions are first transcribed into RNA, which then carries the genetic code from the nucleus to the cytoplasm. Here, the RNA directs the assembly of amino acids into specific proteins, a process called translation.

Genetic Regulation. The activity of genes is tightly regulated by feedback mechanisms, ensuring that cellular processes remain balanced. This regulation involves operons, repressor proteins, and activator proteins, all working to maintain cellular homeostasis.

4. Membrane Transport: Diffusion vs. Active Transport

Most substances pass through the cell membrane by diffusion and active transport.

Diffusion. Diffusion, or passive transport, relies on the random motion of molecules to move substances across the cell membrane. Lipid-soluble substances can diffuse directly through the lipid bilayer, while water-soluble substances use protein channels.

Active Transport. Active transport requires carrier proteins and energy to move substances against their concentration gradients. This process is essential for maintaining the differences in ion concentrations between the intracellular and extracellular fluids.

Endocytosis. Large particles enter the cell through endocytosis, including pinocytosis (ingestion of small globules of extracellular fluid) and phagocytosis (ingestion of large particles). Lysosomes then digest these substances within the cell.

5. Nerve Signals: Electrical Potentials and Action Potentials

The term homeostasis is used by physiologists to mean maintenance of static or constant conditions in the internal environment.

Resting Membrane Potential. All cells, especially nerve and muscle cells, maintain an electrical potential difference across their membranes. This resting membrane potential is typically negative inside the cell, due to the distribution of ions like sodium, potassium, and chloride.

Action Potentials. Nerve signals are transmitted through action potentials, rapid changes in the membrane potential that travel along the nerve fiber. These action potentials are initiated by depolarization, a reduction in the membrane's negativity, and terminated by repolarization, the restoration of the negative potential.

Voltage-Gated Channels. Voltage-gated sodium and potassium channels play a crucial role in action potentials. Sodium channels open rapidly to cause depolarization, while potassium channels open more slowly to facilitate repolarization.

6. Muscle Contraction: From Molecular Mechanisms to Movement

The most important type of movement that occurs in the body is that of the muscle cells in skeletal, cardiac, and smooth muscle, which constitute almost 50 per cent of the entire body mass.

Actin and Myosin. Muscle contraction relies on the interaction of actin and myosin filaments within muscle cells. These filaments slide past each other, shortening the muscle fiber and generating force.

ATP and Calcium. The process is powered by ATP, which provides the energy for the myosin heads to bind to actin and pull the filaments. Calcium ions play a critical role in initiating contraction by binding to troponin, which then allows myosin to attach to actin.

Types of Muscle. Skeletal muscle is responsible for voluntary movement, cardiac muscle pumps blood, and smooth muscle controls involuntary functions like digestion and blood vessel constriction. Each type has unique structural and functional characteristics.

7. Heart as a Pump: Cardiac Cycle and Regulation

Extracellular fluid is transported through all parts of the body in two stages.

Cardiac Cycle. The heart functions as a dual pump, with the right side circulating blood through the lungs and the left side circulating blood through the body. The cardiac cycle consists of diastole (relaxation and filling) and systole (contraction and ejection).

Cardiac Output. Cardiac output, the amount of blood pumped per minute, is determined by heart rate and stroke volume. The heart's ability to adapt to changing demands is crucial for maintaining adequate blood flow.

Regulation. The heart's pumping ability is regulated by intrinsic mechanisms, such as the Frank-Starling law, and by the autonomic nervous system. Sympathetic stimulation increases heart rate and contractility, while parasympathetic stimulation slows the heart rate.

8. Respiration: Ventilation, Diffusion, and Gas Transport

Because oxygen is one of the major substances required for chemical reactions in the cells, it is fortunate that the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid.

Pulmonary Ventilation. Respiration involves pulmonary ventilation (air movement), diffusion of gases between alveoli and blood, transport of gases in the blood, and regulation of respiration. Ventilation is driven by pressure differences created by the diaphragm and rib cage muscles.

Gas Exchange. Oxygen diffuses from the alveoli into the pulmonary blood, while carbon dioxide diffuses in the opposite direction. The efficiency of this exchange depends on factors like membrane thickness, surface area, and pressure gradients.

Gas Transport. Oxygen is transported primarily by hemoglobin in red blood cells, while carbon dioxide is transported in several forms, including dissolved CO2, bicarbonate ions, and carbaminohemoglobin.

9. Kidneys: Regulating Fluids, Electrolytes, and Acid-Base Balance

Passage of the blood through the kidneys removes from the plasma most of the other substances besides carbon dioxide that are not needed by the cells.

Kidney Functions. The kidneys perform multiple essential functions, including excreting waste products, regulating water and electrolyte balance, controlling blood pressure, and maintaining acid-base balance.

Urine Formation. Urine formation involves glomerular filtration, tubular reabsorption, and tubular secretion. These processes are carefully regulated to maintain the proper composition of body fluids.

Hormonal Control. Hormones like ADH, aldosterone, and angiotensin II play critical roles in regulating kidney function, influencing sodium and water reabsorption, potassium secretion, and acid-base balance.

10. The Body's Defense: Immunity and Inflammation

The human body has literally thousands of control systems in it.

Innate Immunity. The body's first line of defense against infection is innate immunity, which includes physical barriers, phagocytosis by neutrophils and macrophages, and chemical compounds that destroy invaders.

Acquired Immunity. Acquired immunity develops after exposure to foreign substances (antigens) and involves the formation of antibodies (humoral immunity) and activated T cells (cell-mediated immunity).

Inflammation. Inflammation is a complex response to tissue injury, characterized by vasodilation, increased capillary permeability, and migration of immune cells to the affected area.

11. Blood: Groups, Coagulation, and Transfusion

The basic living unit of the body is the cell, and each organ is an aggregate of many different cells held together by intercellular supporting structures.

Blood Groups. Blood is classified into different groups based on the presence or absence of specific antigens on red blood cells. The O-A-B system and the Rh system are the most important for blood transfusions.

Blood Coagulation. Blood coagulation is a complex process that involves a cascade of clotting factors, platelets, and fibrin formation. This process is essential for preventing blood loss after injury.

Transfusion. Blood transfusions can be life-saving but must be carefully matched to avoid transfusion reactions. These reactions occur when antibodies in the recipient's blood attack the antigens on the donor's red blood cells.

12. Environmental Physiology: Adapting to Extremes

The human body has literally thousands of control systems in it.

High Altitude. At high altitudes, the body adapts to low oxygen levels through increased pulmonary ventilation, increased red blood cell production, and changes in tissue metabolism.

Deep-Sea Diving. Deep-sea diving exposes the body to high pressures, which can lead to nitrogen narcosis and decompression sickness. Proper decompression procedures are essential to prevent these complications.

Space Physiology. Weightlessness in space causes fluid shifts, muscle atrophy, and bone loss. Exercise programs and other countermeasures are used to mitigate these effects during prolonged space missions.

Last updated: February 23, 2025