Themes in Human Biology

Everyone has a body and, by adulthood, a general understanding of how it works. But to truly understand the intricate functions of the human body—and the problems that occur when something goes wrong—you must approach the study of the body in an organized way. This course will help you understand the functions of the human body. The text will discuss the details of many complex functional systems but will also look at how all of these systems work in harmony to keep you healthy. As you move through this course, you should keep four main themes in mind: structure and function, homeostasis, levels of organization, and integration of systems.

1. Structure and Function

The first theme is the connection between structure and function. You will be studying anatomy, which focuses on the body’s structures, and physiology, which focuses on the body’s functions. In fact, it is virtually impossible to study one without the other because function relies so completely upon structure.

In biological systems, there is a close link between the structure of a cell or body part and its function. Two factors determine something’s structure:  its three-dimensional shape and the materials it is made of. The structure that something takes directly influences its possible functions. Consider a piece of wood. Depending on how it is shaped, someone could use it as a spear for hunting, a cup for drinking, a pipe for smoking, or even a flute for playing music. The wood also possesses unique properties that influence how it can be used. For example, a piece of wood could never function as a hot air balloon, no matter its shape. It would always be too heavy to lift off the ground.

At the smallest levels of biological organization, the structure of a molecule determines its function within a cell. In this course, you might investigate the structure of a molecule and then learn what it does. It will help you to remind yourself that this molecule’s structure and function are connected.

Enzymes are molecules found within a cell that speed up the rate of the chemical reactions necessary to support life. Enzymes break down food molecules, help build muscle proteins, can destroy toxins, and much more. An enzyme’s ability to function depends directly on its three-dimensional shape. If exposed to excessive heat or harsh chemical conditions, enzymes unravel and change shape. When this occurs, the enzymes stop working, and the chemical reactions of life slow down or cease.

At a larger scale, the structure of a cell is directly linked to its function within the body of a multicellular organism. The structure also determines function at higher levels of biological organization. Your body’s organs function as they do because of how tissues are put together, forming filters, pumps, levers, surfaces for gas exchange, and pipes for air and fluid flow. Consider the following examples. The structure of the bones in the skeletal system provides the support necessary for the function of walking upright. The vocal cords would not be able to fulfill their role—the production of sound—if their structure were disrupted. The large surface area of the small intestine allows it to perform its primary function: absorbing nutrients from food efficiently.

2. Homeostasis

The second theme will be homeostasis, the body’s natural tendency to maintain a relatively stable internal environment even though conditions around and within us constantly change. Homeostasis drives most of the body’s functions. Homeostasis occurs at all different levels. For example, body temperature is regulated around 98.6, a temperature that is optimal for cell function and organism function. To maintain this temperature, we sweat to cool down on a hot day and shiver to increase our temperature when we are cold. Other variables, like blood pressure, blood pH, and blood calcium concentrations, are similarly maintained within a narrow range that is optimal for human health. Many diseases occur because of disruptions in homeostasis.

3. Levels of Organization

The third theme will be the hierarchical organization of body parts. You can think of the body’s components as being organized into a hierarchy of levels. Like all things in the physical world, your body is built from chemical building blocks. The smallest of these building blocks are atoms of elements, which combine to form larger and more complicated structures called molecules. These molecules, such as water, proteins, carbohydrates, nucleic acids, and lipids, are used to build cells. Cells are the smallest unit of organization capable of carrying out all life processes. Groups of related cells that work together to perform specific functions make up tissues, and tissues that work together form organs. Organs do not work independently; they are organized into organ systems that complete more complex tasks. How can you identify hierarchical organization? This type of structure is characterized by the following:

• Each level is made of parts from the level below.
• Complex structures are built out of simpler parts.
• Simple structures can be combined in many different ways.

What are the results of hierarchical organization?

• Each new level of organization has characteristics that were not present in the level below. These are called emergent properties. An emergent property is a new feature that arises from combining structures from the level below. For example, Sodium is a silvery, soft, explosive metal. Chlorine is a poisonous green gas. When sodium and chloride combine, table salt is created. Table salt is neither silvery nor green, neither explosive nor poisonous. Table salt is made only of sodium and chloride but has emergent properties (such as making french fries taste delicious) that are not present if the two chemicals are separate.
• Within a level, when the same parts are organized in a different way, differences in function are created. Changing how the elements of a level are organized generates different functions. Consider the digestive system as an example. The function of the digestive system is to bring food into the body and break it down physically and chemically into nutrients that our bodies can use. The digestive system includes the mouth, stomach, intestines, among others. Imagine the result if the order of organs was different!

Understanding this hierarchy is important because disruptions might occur at any level. For example, the depletion of calcium atoms from the body can lead to weak bones. Or a single mutation in a DNA molecule can lead to organ dysfunction, such as the disturbed lung function found in individuals with cystic fibrosis. In fact, disturbed function at higher levels in the hierarchy can be traced to disordered structure at the cellular and molecular levels.

4. Integration of Systems

Have you ever played a game of Jenga, where you try to remove one block at a time from a tower structure without causing the entire tower to collapse? In this game, each block depends on the other blocks for stability, and if you’re not careful, removing a single block can destroy the entire structure. Life is much the same. The various parts of living systems are interdependent at levels from the individual to the biosphere. The Merriam-Webster Dictionary defines interdependence as 1: the state of being dependent upon one another and 2: a mutually dependent relationship.

Your trillions of cells are intimately dependent on each other for survival. Your cells are specialized, and you exist because of a massive team effort from all these different cells. If any one of your major organs were to fail suddenly, you might die very quickly. Moreover, your health also depends on trillions upon trillions of bacterial cells that live in and on your body. They provide your cells with vitamins, help keep out invaders, and may even influence your mood. So your body itself is a community of interdependent cells.

Each section of the course will discuss the integration of the body’s systems. Every organ system relies on the healthy functioning of other systems to carry out its functions. When these systems all work together, the organism thrives. A breakdown in one system can cause failures in other systems as well.

Vital Functions for Human Life

This section introduces you to the major organ systems of the body. To put these systems in context, we will first discuss the vital functions of life.

Within any organism, there is a multitude of functions taking place at any given time. Humans, for example, can breathe, talk, digest food, process visual images, and move their bodies simultaneously. While these activities are important, some are essential to the human body’s survival. They are vital functions– processes or actions of the body on which life is directly dependent. You will examine five vital functions in this course: control and regulation, structure, support, and movement; exchange with the environment; transport within the body; and protection from the environment.

Major Organ Systems of the Body Grouped by Primary Function

Next, we will describe each major category of function and the systems that execute the vital functions. An organ system is an integrated collection of organs in the body that work together to perform a vital function. This course will organize the organ systems of the body based on the vital functions defined earlier. Keep in mind, however, that most systems of the body could be placed in more than one category. The major organ systems of the body and their primary functions are shown in Table 1.1.

Control and Regulation

An organism must constantly gather information and react accordingly to maintain its internal equilibrium. In humans, the nervous system is made up of the brain, nerves, and spinal cord. Sensory organs react to environmental stimuli and signal other systems when actions are needed to bring the body back into balance. The endocrine system, which produces hormones and other regulatory substances, plays a crucial role in maintaining balance among chemical messengers within the body. Locally, most body cells can produce chemical messages that influence the metabolism of other cells. And some organs in other body systems produce chemicals that can travel through the body to regulate metabolic processes in other organs.

Structure, Support, and Movement

The skeletal and muscular systems support the body and allow it to move away from danger, toward food sources, etc. Certain parts of the skeletal system, such as the skull and ribcage, also help to protect the internal organs, such as the brain, heart, and lungs, from damage. The muscular system also allows for movement within the body.

Exchange with the Environment

An organism constantly interacts with its environment. Our bodies must obtain food, water, and oxygen from the world to survive. The human body must also eliminate waste before it reaches toxic levels. Two organ systems are primarily responsible for exchanging material with the environment. The digestive system brings food and water into the body and eliminates solid wastes. The respiratory system brings in oxygen and removes carbon dioxide.

The urinary system doesn’t take in anything from the environment but does eliminate waste products of metabolism from the body fluid. However, this is just one of several roles that the urinary system plays in maintaining homeostasis of body fluids, so we will explore it more extensively in the following vital function category involving body fluids.

Fluid Transport within the Body

Single-celled organisms can directly absorb nutrients and oxygen from the environment into themselves, which they use to support essential cell functions. Single-celled organisms excrete waste products in a similar fashion. In multi-celled organisms like humans, however, most cells are not exposed directly to the outside environment. Instead, body cells rely on organ systems to transport and regulate the composition of internal fluids throughout the body. The cardiovascular system provides the “pipes” that carry blood and a pump that pushes blood. The lymphatic system returns interstitial fluid to blood. This system also participates in immune function and so will be discussed in the next section. The urinary system filters blood to regulate body fluid homeostasis, including volume, pressure, and chemical composition.

Protection from the Environment

Our organs must be protected from potentially damaging environmental substances, including pathogens (disease-causing microorganisms). The integumentary system forms the immune system’s first line of defense. It is made up of skin, hair, and nails. Our skin, and mucous membranes that line body cavities in contact with the environment, prevent most pathogens from entering the body and destroying healthy body cells. Organs of the lymphatic system provide the many roaming cells of the immune system with a home base. These cells, and the molecules they produce, defend us from foreign organisms that manage to enter our body tissues or fluids.

As you can see, several organ systems work together to accomplish vital functions throughout the body. Since the organ systems are distributed throughout large regions of the human body, it is necessary to define orientation within the body and communicate the proper terminology as you study these integrated structures and functions.

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