• The study of the chemical and physical operations that make life possible is called physiology.
  • The study of the biophysical structures that make life possible is called anatomy. The branch of anatomy that studies tissues of the human body is called histology.
  • The study of chemicals and chemical structures that make up the cell and tissues of the human body is called biochemistry.
  • The branch of health sciences that studies the physical structures of the cell along with its chemical structures and physiology is called cytology.

Man against Nature

  • A human being survives in a world whose natural forces are inimical to human existence. Man must seek food, warmth, and a partner to procreate, as well as avoid dangers that can end his life. To achieve this at the unconscious level, the body has evolved a set of complex control systems that interact with each other to regulate the physiological operations of the body, as well as achieve physiological synergy. At the conscious level, man is expected to gain knowledge about his body and environment so as to develop a lifestyle that best suits his existing living conditions.
  • Physiological synergy means that the physiological operations of cells that are connected together and are jointly regulated by control systems work cooperatively to produce a greater outcome than the sum of the physiological outputs of individual cells. Basically, it is the synergy of cells working within an organ system (which is explained later).


Erythrocytes are the main cellular constituent of human blood. PHOTO CREDIT.
  • The basic functional and anatomical unit of life is the cell.
  • There are different types of cells.
  • Each cell has evolved to best perform a single function or a set of closely-related functions. In fact, it is this idea that each cell performs a single function that inspired the creators of programming languages to develop modular programming whereby the program is made up of small subunits called functions with each function performing a specific task or a set of related tasks. Basically, the function is to the program what the cell is to the body.
  • The healthy adult human body has about 40 trillion cells, with 62-71% of these cells being red blood cells (also called erythrocytes). The main function of the erythrocyte is to transport oxygen to tissues, where the oxygen is used in aerobic respiration (which will be discussed in another topic) to generate energy that powers the body. This reveals that the human body is principally preoccupied with generating energy to maintain life, and thus it can be concluded that energy is mandatory for life to exist. This is confirmed by another strange phenomenon where the number of microorganisms that live on the human skin and gut epithelium outnumbers the human cell population. These microorganisms are collectively designated as the microbiota.
  • Microbiota is made up of different species of microorganisms. They are named according to where they are normally found e.g microbiota that resides in the alimentary canal (also called the gastrointestinal tract [GIT] or alimentary canal) is called the gut microbiota. The population of the gut microbiota exceeds the total number of human cells. Most of the microbiota are single-celled (or unicellular) micro-organisms.
  • The human cells do share common characteristics that allow for their identification as a human cell (and not a unicellular microbiota). This is analogous to the concept that human beings share common characteristics that allow for their identification as humans, and not primates or plants.

Milieu Intérieur

circulatory system showing arteries and veins synowl
The human circulatory system showing arteries and veins. PHOTO CREDIT.
  • Up to 70% of the human body is liquid fluids that are collectively designated as the body fluid.
  • The body fluid is divided into two broad categories:
    • Intracellular Fluid (ICF): This is fluid found inside the cells. It forms about 2/3 of the total body fluid.
    • Extracellular Fluid (ECF): This is fluid located outside the cells. As expected, it forms 1/3 of the total body fluid. The ICF and ECF have different contents, and this allows for chemical analysis of body fluids in the medical sub-specialty known as clinical chemistry. The ECF is mainly composed of 2 different types of fluids:
      • Intravascular Fluid (IVF): This is the fluid found inside the circulatory system i.e fluid in the arteries, arteries, capillaries, venules, heart, and veins. It makes up about 21% of the ECF or 7% of the total body fluid. Because the IVF is principally blood, then one can calculate the total body fluid in a person, as well as his/her body weight e.g if the person has 5 liters of blood, then his/her total body fluid volume is about 71 liters, and the total weight of this person is expected to be about 101 kilograms (kg).
      • Extravascular Fluid (EVF): This is fluid found outside the circulatory system (and outside the cells). It makes up 69% of the ECF and about 27% of the total body fluid volume. Most of it is found in tissues, and this fluid is called interstitial fluid. The interstitial fluid makes up about 96% of EVF with the rest of the EVF being made up of transcellular fluid and cerebrospinal fluid (or CSF which is fluid found inside the central nervous system). Transcellular fluid is contained in synovial sacs and other cavities lined with serous membranes.
  • The constantly flowing ECF transports resources to cells as well as removes waste products and secretions released by cells. This results in ECF bathing the cells thus creating an environment in which cells can live in and thrive. The French Physiologist, Claude Benard, was the first to define this environment as the milieu intérieur or internal environment. This laid the foundation for the description of homeostasis and its complex control systems in 1929 by Walter Cannon, an American physiologist.
  • Homeostasis simply means maintenance of constant conditions in the ECF or internal environment. Usually, this involves maintaining the concentrations of soluble substances (called solutes) in the body fluid. This is achieved through complex control systems that regulate the concentrations of contents of the ECF and ICF.
  • These control systems operate to maintain the concentration of each solute in the ECF within a narrow range of limits that is defined as the normal range. If any of the solute concentrations exceeds its normal range, or falls below the normal range, then dysregulation is said to have occurred. Dysregulation is the initial phase towards the development of a disease. In physiology, disease is described as pathology, and this allows it to be defined as the pathophysiological state due to disrupted homeostasis. This means that disease and homeostatic dysregulation are linked.
  • The flow of ECF in the body can be divided into 2 stages. In the first stage, ECF flows in the circulatory system as IVF. The fluid that contains solutes in IVF is called plasma. In the second stage, plasma passes from the capillaries (whose walls are permeable) into the spaces between cells through a process called extravasation. The spaces between cells are called intercellular spaces and the fluid that is found in these spaces is aptly called intercellular fluid (which is also called interstitial [between tissues] fluid).
  • The walls of the capillaries have pores that allow fluid (and its dissolved substances) to flow in and out of the capillaries, and this is the basis of capillary permeability. Nonetheless, protein molecules cannot pass through these capillary pores and thus the intercellular fluid can be described as capillary plasma devoid of protein molecules. Because the fluid inside and outside the capillaries are constantly in motion (i.e kinetic motion), they allow for an exchange of fluid solutes between plasma and intercellular fluid. This exchange occurs through the process of diffusion whereby solutes move from the space where their concentration is high and into a space where their concentration is low. Diffusion occurs until the concentration of solutes in two fluid compartments, which are separated by a permeable wall, are the same.
  • The solutes in the ECF come from the following systems:
    • Respiratory System – The main function of this system is gaseous exchange. The blood that flows in the veins through the lungs move into smaller venules and then into capillaries which are in contact with alveolar sacs. The thin wall of the alveolar sac is separated from the wall of the pulmonary capillary by a very thin membrane called the alveolar membrane. This alveolar membrane allows dissolved gases to move between the alveolar sac and the pulmonary capillary. Carbon dioxide diffuses into the alveolar sac while oxygen diffuses from the alveolar sac into the capillary blood, and thus the blood that flows out of the alveolar sac is oxygenated. This oxygenated blood flows into arterioles and then into the arteries. The flow of blood within the lungs is called pulmonary circulation while the flow of blood through the rest of the body is called systemic circulation. As will be described later, pulmonary circulation involves the flow of blood from the right ventricle into the pulmonary artery that distributes the blood in the lungs for oxygenation and the return of the oxygenated blood back to the left atrium via the pulmonary vein. The left atrium then pumps this blood into the left ventricle. Systemic circulation involves the flow of oxygenated blood from the left ventricle via the aorta into arteries (distributed throughout the body) for deoxygenation with the deoxygenated blood returning to the right atrium via the venae cavae. The pulmonary and systemic circulations form the double circulatory system of human beings. Regarding the ECF, the respiratory system adds oxygen and removes carbon dioxide and other gaseous waste products produced by cells.
    • Gastrointestinal Tract (GIT): Oxygenated blood flows through the capillaries in the alimentary canal where it absorbs dissolved nutrients, including glucose (and other monosaccharides), fatty acids, oligosaccharides, and amino acids. This ensures that nutrients are added to the ECF. As will be discussed in a later topic, metabolites of some drugs (and other xenobiotics) are released from the bloodstream into the GIT for elimination from the body.
    • Liver: Oxygenated blood that flows into the liver absorbs dissolved nutrients, ingested drugs, and hormones; while at the same time passing harmful metabolites and waste products of cellular metabolism into the liver for detoxification. The detoxified materials are secreted into the bile fluid that flows into the GIT so that the bile can be excreted from the body in the form of feces.
    • Musculoskeletal System: This is the locomotor system that allows a person to move from place to place and perform any physical action, including the passing of ingested food through the GIT via muscle movements known as peristalsis. In the muscles, the nutrients and oxygen are metabolized through the process of aerobic respiration to generate energy in form of adenosine triphosphate (ATP) molecules. When oxygen is not available, or in short supply, energy production occurs through the process of anaerobic respiration which also produces lactic acid that is removed from the body via the respiratory system as carbon dioxide. Aerobic respiration generates 38 ATP molecules for each molecule of glucose that is metabolized into carbon dioxide and water, while anaerobic respiration produces only 2 ATP molecules for each molecule of glucose that is metabolized into 2 lactic acid molecules. The chemical process of oxidative phosphorylation is the reason why aerobic respiration produces 36 ATP molecules more than anaerobic respiration.
    • Kidneys: The flow of oxygenated blood through the kidney allows for the absorption of hormones collectively designated as renal hormones into the bloodstream for systemic circulation, while at the same time, cellular products of metabolism such as uric acid, urea, creatinine, and excess water (including water produced from cellular aerobic respiration) are removed from the bloodstream by the kidneys. This exchange occurs between the glomerulus and the Bowman capsule in the nephron. The functional unit of the kidney is known as the nephron.

Supracellular and Subcellular Regulations

synowl neuron
A unipolar myelinated neuron is the functional unit of the nervous system. PHOTO CREDIT.
  • The nervous and hormonal systems are the two primary regulation systems in the body.
  • The nervous system is made up of 3 parts:
    • Sensory Input Portion: This is made up of the sensory receptors of neurons that detect the surroundings of the body, as well as the physical condition of the body. The main receptors are the visual receptors of the eyes, auditory receptors in the ear, olfactory receptors in the nose, taste buds in the tongue, and tactile receptors in the hands and feet.
    • Integrative Portion: This is made up entirely of the central nervous system (CNS) which is composed of the brain and the spinal cord. The brain is the cognitive organ of the body that stores information, generates thoughts and ideas, and sends commands to the voluntary muscles on what actions to take. The spinal cord connects the brain with the peripheral nerves in the body thus ensuring that the brain can receive neuronal input from the entire body. The nerves outside the spinal cord form the peripheral nervous system (PNS). The smooth muscles of the GIT are controlled by the enteric nervous system (ENS) which operates as the functional brain of the gut, though it receives signals from the CNS.
    • Motor Output Portion: This is part of the nervous system that sends signals to the musculoskeletal system. It is divided into two systems:
      • Autonomic Nervous System (ANS): It controls the actions of the smooth muscles (including muscles that surround glands). It operates at the subconscious level i.e it is outside the voluntary control of a person. It can also operate when the person is unconscious, and for this reason, it is called the vegetative nervous system. This is one of the reasons why life exists even when a person is in a state of coma.
      • Somatic Nervous System (SNS): It controls the actions of skeletal muscles. A skeletal muscle is any muscle that is attached to skeletal bones. This system can be controlled by a person and hence it is called the voluntary nervous system and the skeletal muscles are called voluntary muscles.
  • The hormonal system is made up of endocrine glands and tissues that secrete hormones. A hormone is a chemical compound that sends signals (in chemical form) to cells, hence it is called a biological signaling molecule that is engaged in cellular communication. For this reason, cellular communication mediated by a hormone is described as cell signaling. Hormones are transported to cells through the ECF. Regarding the cell, a hormone serves to regulate cellular metabolism (e.g insulin and thyroid hormone) and intracellular physiochemical operations such as mitosis, meiosis, and secretion of chemical substances.
  • It can be deduced that the nervous system focuses on supracellular regulation i.e regulation of activities of tissues, organs, and locomotion; while the hormonal system focuses on subcellular regulation i.e regulation of activities inside the cell. Therefore, both the hormonal and nervous systems regulate all the body functions.


  • The body is protected by two systems:
    • Immune System: This is made up of white blood cells which have two main functions. The first function is to identify the cells of the person – which are called host cells – and distinguish them from foreign cells. This ensures that the immune system does not attack host cells. If the immune system fails in this function, then autoimmunity results. Autoimmunity is the pathological state where the immune system attacks host cells, which usually results in the death of host cells or irreparable and irreversible cell damage. Any disease attributed to autoimmunity is known as an autoimmune disease e.g Type I insulin-dependent diabetes mellitus, multiple sclerosis, celiac disease, and systemic lupus erythematosus. The other function of the immune system is to destroy foreign cells. This can be achieved by ingesting the foreign cell through the process of phagocytosis or creating holes – using the complement system – in the foreign cells so as to initiate cell death through lysis.
    • Integumentary System: This is made up of the skin and all the natural external coverings of the body including hair, nails, and the specialized cornea of the eyes. It serves as the boundary or separator between the internal environment of the body and the external environment. At the immunological level, it is the first line of defense against infections with the immune system serving as the second line of defense. Up to 15% of the entire body weight is made up of skin.

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