Introduction to Human Physiology

What human physiology is – and why it matters.

Processes and functions of living organisms
Negative feedback loop
Sagittal plane
Dorsal cavity
Approximately 60%
Produce energy for the cell through cellular respiration
Striated muscle tissue
Transporting blood, oxygen, and nutrients throughout the body

Overview of Human Physiology


Human physiology is the study of the processes and functions that make us tick, and it is a critical component of understanding our bodies and how they work. Physiology, as defined by the Oxford Dictionary, is “the branch of biology that deals with the normal functions of living organisms and their parts.” This field of study is distinct from anatomy, which focuses on the structure of the body, as physiology delves into the mechanisms that drive these structures.

The history of human physiology can be traced back to at least 420 BCE. The works of Aristotle, Hippocrates, and Galen, laid the foundation for Western physiology. Important discoveries by Jean Fernel, William Harvey, and Claude Bernard further advanced our understanding of the human body.

Fernel introduced the term “physiology”, Harvey was the first to describe system circulation, and Bernard laid the groundwork for homeostasis. The study of physiology is essential for understanding disease, sports science, physical therapy, and occupational therapy, as it provides insights into the underlying processes that govern our bodies.


Building on the foundation of human physiology, homeostasis is a key concept that helps maintain the body’s internal environment. Homeostasis, as defined by Claude Bernard, a French physiologist, is the process by which the body maintains a stable internal environment despite external changes. Positive and negative feedback systems are integral to homeostasis,

Negative feedback loops are the most common type. They are characterized by a response that counteracts a change in the internal environment, restoring it to its original state. For example, if your body temperature starts to rise, your body will activate mechanisms to reduce your temperature, such as sweating and dilation of blood vessels near the skin’s surface. This reduces heat retention and promotes heat loss, bringing the temperature back to its normal range.


Positive feedback loops, on the other hand, amplify a change in the internal environment. Instead of reversing the change, they reinforce it, leading to an increase in the response. For example, when a baby suckles at the breast, nerve signals are sent to the mother’s brain, which stimulates the release of the hormone prolactin from the pituitary gland.

Prolactin stimulates milk production in the mammary glands, which increases the amount of milk available for the baby to suckle. As the baby suckles more, this further stimulates the release of prolactin, leading to increased milk production and so on. Homeostasis is critical for effective body function, as it ensures that our bodies can adapt to changes in the environment and maintain optimal conditions for survival.

Anatomical Terminology

Anatomical terminology is essential for accurately describing the human body and its structures. Regional and directional terms, such as anterior, posterior, medial, lateral, superior, inferior, proximal, distal, superficial, and deep, provide a universal language for discussing anatomy.

The three planes of the body – sagittal, frontal, and transverse – further aid in the precise description of body structures. The sagittal plane divides the body into left and right halves, separating it into two symmetrical sections. The frontal plane, also known as the coronal plane, divides the body into front and back sections.

The transverse plane, also known as the horizontal plane, divides the body into top and bottom sections. The dorsal body cavity is located on the posterior (back) side and houses the cranial and spinal cavitities. The ventral body cavity is located on the anterior (front) side and houses the thoracic and abdominopelvic cavities.

Body Cavities

Body cavities are essential for understanding human anatomy, as they house and protect our vital organs. Anatomy, as a field of study, focuses on the structure of the body. The major body cavities include the dorsal, ventral, spinal, cranial, thoracic, and abdominal cavities. Each cavity serves a specific function and is located in a particular region of the body.


The dorsal cavity, for example, contains the brain and spinal cord, while the ventral cavity houses organs such as the heart, lungs, and digestive system. The ventral cavity allows for the change in shape and size of organs based on their function. For example, the lungs, stomach, and uterus can all expand. Understanding the location and function of each body cavity is crucial for comprehending the organization of the human body and how its various systems interact.

Body Fluid Compartments

The distribution of water in the body is critical for maintaining fluid balance and overall health. The human body is composed of approximately 60% water, which is unevenly distributed across various organs and tissues. Organs such as the brain and kidneys require a higher water content to function optimally.


Intracellular (the fluid winthin cells) and extracellular (the fluid outside of cells) fluids play a vital role in body function, with imbalances leading to issues such as dehydration or overhydration. Dehydration occurs when the body loses more fluids than it takes in, leading to decreased fluid volume and increased concentrations of electrolytes in the ECF.

Overhydration, on the other hand, occurs when the body takes in more fluids than it can eliminate, leading to increased fluid volume and decreased concentrations of electrolytes in the ECF. Electrolytes, can also be affected by these imbalances. Electrolytes are substances that dissociate into ions (charged particles) when dissolved in a solution, such as water. They are essential for various physiological processes in the body, including regulating fluid balance, transmitting nerve impulses, and contracting muscles.

Cellular Physiology

Cells are the basic building blocks of life,. Cells are responsible for various processes, including metabolism, reproduction, communication, homeostasis, and differentiation (the process by which unspecialized cells develop into specialized cells with specific functions). The basic structure of a cell includes the cell membrane, nucleus, cytoplasm, organelles, and mitochondria.


The cell membrane, also known as the plasma membrane, is a thin, flexible layer that encloses the cell and separates its internal environment from the external environment. The nucleus is the control center and contains the cell’s genetic material in the form of DNA.

An organelle is a specialized subunit within a cell that has a specific function. Mitochondria are organelles that produce energy for the cell through a process called cellular respiration. The cytoplasm is the gel-like substance that fills the cell and contains various organelles and structures.

There are over 200 different cell types in the human body, each with specific functions and locations. As cells age and die, they are replaced by new cells, with skin cells serving as a prime example of this process.


The human body is composed of four primary tissue types: epithelial, connective, muscle, and nervous. Each tissue type has a unique structure and function, and they work together to support the body’s overall function.

Epithelial tissue, for example, forms the outer layer of the skin and lines internal organs, while connective tissue (bone, blood, and lympg tissue) provides support and structure to various body parts. Striated muscle tissue is responsible for voluntary movement while smooth muscle tissue, like the muscles in your stomach, are involuntary. Nervous tissue is made up of nerve cells and transports “messages” around the body.


Understanding the relationship between different tissue types and their functions is essential for comprehending the organization of the human body. For example, think of your body healing a wound. The epithelial tissue covers the wound, the connective tissue provides support and structure, the muscle tissue contracts to stop bleeding and prevent infection, and the nervous tissue coordinates the healing process.

Organ Systems

The human body consists of 11 major organ systems. Each system serves a specific function and is composed of various organs that work together to maintain overall health.

The circulatory system is responsible for transporting blood, oxygen, and nutrients throughout the body, while the respiratory system facilitates gas exchange.

The integumentary system (your body’s outer layer) provides protection from external factors, regulates body temperature, and synthesizes vitamin D.

The lymphatic system helps maintain fluid balance, filters pathogens and other foreign substances from the blood, and produces immune cells.


The endocrine system regulates body functions through the secretion of hormones.

The gastrointestinal system breaks down food, absorbs nutrients, and eliminates waste.

The urinary system filters waste products from the blood, regulates electrolyte balance and blood pressure, and produces urine.

The muscoskeletal system provides support and structure, facilitates movement, and protects internal organs.

The nervous system coordinates and controls body functions, and processes and responds to sensory input.

The reproductive system produces and transports gametes, facilitates fertilization and development of the fetus, and produces sex hormones.

The immune system defends the body against pathogens and foreign substances, and plays a role in tissue repair and homeostasis.

Integration of Systems

The various organ systems in the human body work together to maintain homeostasis and ensure optimal function. For instance, the nervous and endocrine systems regulate the body’s internal environment, while the cardiovascular and respiratory systems deliver oxygen and nutrients to tissues and remove waste products.


For example, during exercise, the musculoskeletal system produces movement by contracting skeletal muscles, which requires oxygen and energy. The respiratory system responds by increasing the rate and depth of breathing to supply the body with more oxygen and remove carbon dioxide.

The circulatory system works in conjunction with the respiratory system to deliver oxygen and nutrients to the working muscles and remove waste products. The endocrine system releases hormones such as adrenaline to increase heart rate and mobilize energy stores. The urinary system regulates electrolyte balance and removes waste products generated during exercise. The nervous system coordinates the movement and adjusts the breathing and heart rate based on the body’s needs.

Physiological Adaptations

The human body is capable of adapting to various environments and stressors, such as altitude, exercise, temperature, and internal and external stressors like bacterial and viral infections, air and water pollution, and dietary imbalances. These adaptations can manifest as genetic changes, developmental adjustments, or acclimatization.


For example, individuals living at high altitudes may develop increased red blood cell production to compensate for lower oxygen levels, while those exposed to extreme temperatures may undergo changes in their metabolic rates to maintain body temperature. Understanding these physiological adaptations is essential for appreciating the resilience and adaptability of the human body in the face of diverse challenges.

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The Endocrine System;

The system that governs all hormonal activity.

The Cardiovascular System;

Your body's way of getting oxygen to where it's needed.

The Musculoskeletal System;

The interlinked system of muscles and skeletons that holds us all together.

The Respiratory System;

The system built around the exchange of gases.

The Urinary System;

How your body disposes of liquid waste.

The Nervous System;

The system governing sensation and neural communications.

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