Physiology is essentially defined as the study of how biological systems work in order to bring about the development and progression of life. This vast field of science covers all living organisms including viruses, bacteria, plants, humans and much more. Human physiology, in particular, deals with the ways by which the human body carries out its fundamental characteristics, making it a living system. As complex as it may be, an understanding of the basic concepts involved in the study of human functioning can be of help.
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Image: “Dissection of the shoulder” by University of Liverpool Faculty of Health & Life Sciences. License: CC BY-SA 2.0

Cells: The Basic Unit of the Human Body

Human Cell graphic

Image: “Prototypical Human Cell” by philschatz. License: CC BY 4.0

All living organisms, including human beings, start out with only a few number of cells. Cells are basically the fundamental unit of the human body. When clumped together using supporting structures, these cells make up a tissue. Different tissues combined for a purpose, in turn, become an organ.

There are basically a lot of types of cells. They are characterised according to their morphology, how they work, and their placement in the body, among the many qualities of different cells.


A neuron, which composes the majority of the nervous system, is different in terms of its ability to transmit neuronal signals, as compared to a white blood cell which is a part of the immune system. Each cell in the body develops certain characteristics which allow it to perform specific functions required of the organ system where it comes from.

Although it is given that cells in the body are very diverse, all of them share some characteristics. An example of which is the fact that all cells require nutrients and other raw materials for it to function well. Moreover, cells are also able to create copies of themselves which is exemplified during normal growth and in damage repair. However, there are some exceptions to this (neurons).

Fluid Compartments

In another article, you will learn that water comprises a significant portion of the structure of a cell. With this in mind, one would think that a human body is composed of a large percentage of fluids. True enough; fluids comprise a larger percentage, as compared to other compositions, such as minerals, fat, protein, etc. This percentage changes with age, giving neonates and infants the largest percentage of fluid in the body, while giving the elderly the lowest. Generally, an adult human body is made up of 60% fluid. Despite its relatively larger amount in the body, fluids are actually organised into compartments.

Intracellular fluid (ICF)

The water found inside the cell composes this compartment. This comprises the majority of the overall fluids in the human body. It contains large amounts of the so-called intracellular ions such as magnesium, phosphates and potassium.

Extracellular fluid (ECF)

This compartment is defined simply as the fluid that surrounds the cell. A French physiologist by the name of Claude Bernard coined the term milieu interieur, describing the ECF’s ability to dissolve and carry the requirements needed for all types of cells to thrive. Unlike the ICF, the ECF contains sodium, chloride and bicarbonate ions. This is further divided into two:

  • Intravascular space – the fluid that composes blood; confined within the lumen of the heart and blood vessels
  • Extravascular space – surrounds the tissues; it can either be interstitial (space directly surrounding cells; the microenvironment of the cells) or transcellular (areas in the body where only small amounts of fluid can be accommodated, e.g. peritoneal cavity, joints and inside the eyeball)

Overview on the Major Body Systems

In order to bring about harmony within the human body, otherwise known as homeostasis, different physiologic systems preserve a fairly constant milieu interieur. This is done by the different systems’ ability to maintain interdependent functions with each other.

Circulatory system

blood circulation

Image: “The human circulatory system. Red indicates oxygenated blood, blue indicates deoxygenated.” by Sansculotte. License: CC BY-SA 2.5

As defined earlier, extracellular fluid, particularly those found inside blood vessels, is the one responsible for the transport of oxygen, nutrients and metabolic wastes to and from the different areas of the body (see also blood circulation).

Respiratory system

The respiratory system answers the cell’s need for oxygen by supplying it through alveolar respiration. With gas transport occurring at the level of the alveoli, not only does the blood get oxygen for the cells, it also unloads carbon dioxide. The respiratory system also plays a big role in pH balance.

Gastrointestinal system (GIS)

As mentioned earlier, blood passes through most organs in the body, and that includes the gastrointestinal system. Just like the respiratory system, the GIS supplies the other nutrients needed for cell metabolism such as protein, fats and carbohydrates coming from the diet.

Accessory glands along the GIS

Organs such as the liver and the pancreas provide enzymes that participate in making food stuff easily available for absorption. Moreover, the liver also metabolises nutrients coming from the GIS, making it usable for the rest of the body.

Musculoskeletal system

The musculoskeletal system has a relatively straightforward function when it comes to the maintenance of homeostasis: it allows one to move around for the purpose of food ingestion, avoidance of danger and performance of vital activities and reflexes (e.g. breathing, sneezing and defecation).

Urinary system

Scheme of Renal Blood Flow

Image: Detail: “Nephrons and Vessels” by Phil Schatz. License: CC BY 4.0

The kidneys perform a somewhat similar job with the lungs, in the sense that they contribute to the disposal of some metabolic wastes and participate in the buffering of the blood’s pH. All of the blood passes through the kidneys in order to be filtered, resulting in the formation of urine.

Nervous system

Aside from the fact that organ systems have to co-ordinate with each other in order to maintain homeostasis, the human body also has to interact harmoniously with the environment in order for it to function well. Both of these can be accomplished with the help of the nervous system through its affective, integrative and effective portions. The autonomic system also helps out in regulating vital body activities, such as the respiratory rate, heart rate and gastrointestinal motility.

Endocrine system

The endocrine system also contributes to intersystem communications in a way that is similar to the nervous system. However, instead of using neuronal impulses, it makes use of chemical transmitters in the form of hormones in order to regulate organ functions.

Immune system

The cells composing this system work by monitoring which, among the cells, do not belong to the system. In cases where invaders do make their way inside the body, the components of the immune system either directly attack or set off efficient cascades that effectively deactivate and/or eradicate the foreign body.

Integumentary system

Without our skin, we would be continuous with the environment. The skin and its appendages serve as a protective structure, shielding our internal environment from the constantly changing environment outside our bodies. It also functions as a thermoregulatory and an excretory organ.

Reproductive system

Although reproduction does not offer a direct benefit to the homeostasis of a body system, it does serve the function of allowing the lineage of a certain being to continue in enjoying a harmonious internal environment.

Regulatory Systems of the Body

As mentioned earlier, there is a vast diversity among the cells found in a single human body. This also means that numerous tissues and organs exist with so many different functions. However, in order for homeostasis to occur, these diverse systems have to work harmoniously. This is where the regulation of the key functions of each system comes into play.

Regulatory systems control the intracellular and extracellular environment. These may also drive inherent functions of each organ, or maintain good relationships between organs within, or among, different systems.

These control mechanisms explain how the body maintains a physiologic level of oxygen, carbon dioxide, trace metals and other substances in the body. Another example of which is the nerve receptors found in various areas in the body that help in regulating key processes such as breathing, digestion and thermoregulation.

Negative feedback systems

Negative Feedback Loop

Image: “Negative Feedback Loop” by Phil Schatz. License: CC BY 4.0

The most common attribute among the many regulatory systems is that they work in a negative feedback fashion. This can be described as the body’s inherent ability to suppress a cellular activity once the amount of substrate for that particular cell (it may be the cell’s own products or their analogue) overwhelm its receptors. This also applies to other body processes.


Application is in blood pressure maintenance. If the receptors within blood vessels register that there is low pressure and not enough blood is getting through the tissues, the autonomic system drives the cardiovascular system to increase the heart rate and constrict the blood vessels in order to achieve a normal blood pressure.

Positive feedback systems

In contrast, a few processes in the human body make use of the positive feedback system. Instead of inhibition, certain stimuli actually promote the activity of a certain cell or organ.


A prominent example of which is during childbirth. In this process, the stimulus (cervical stretch when the head of the foetus is about to come out) actually brings about more oxytocin released from the posterior pituitary gland, thereby causing more uterine contractions. In the long run, this mechanism actually allows the baby to be delivered. Other examples of body processes that make use of positive feedback are blood clotting and nerve impulse transmission.

Review Questions

The correct answers can be found below the references.

1. Which, among the following compartments in the human body, contains the most amount of fluid?

  1. Extracellular compartment
  2. Plasma
  3. Intracellular compartment
  4. Interstitial fluid compartment

2. Most of the body processes are controlled by:

  1. A mechanism by which a feedback ultimately increases the activity of a certain organ.
  2. Means of a dampening control system in which the output, when applied to certain tissues and organs, inhibits their activity.
  3. A regulatory system that neatly compartmentalises each organ system in order for it to perform its specific function.
  4. There is no inherent control system.
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