Biology For Dummies

Part IV Systems Galore! Animal Structure and Function

In this part . . .

Has anyone ever told you that you’re an animal? Well you are — literally — because you’re a member of the animal kingdom. The body plans of animals are very diverse, but they all have certain things in common, including the need for oxygen and food. Some animals, such as humans, are made up of many complex organ systems that coordinate the structure and function of the body.

In this part, we introduce the fundamentals of the many organ systems in the human body. We also take a peek at some of the different ways that other animals do things, including how they obtain the nutrients they need from food and how they send those nutrients throughout their bodies.

Chapter 13

Pondering the Principles of Physiology

In This Chapter

Connecting structure with function

Understanding important physiological concepts such as evolution and homeostasis

Physiology is the study of the function of all living things in their normal state. The function of living things is closely tied to their structure and begins at the cellular level. In order to survive, living things must be able to regulate their functions and respond to changes in the environment. In this chapter, we help you see how the study of physiology is applicable at all levels of life and introduce you to some of the fundamental principles of physiology that apply to all the organ systems of the human body.

Studying Function at All Levels of Life

To truly be able to understand a living being, you need to have a good mental picture of its structure and the function of its body and cells. Enter anatomy and physiology. Anatomy is the study of the structure of living things, and physiology is the study of how these structures function. These two branches of biology go hand in hand because the function of an organism is dependent upon its structure.

For example, the function of the heart is to pump blood around the body. The heart muscle contracts, putting pressure on the blood and squeezing it out of the heart and into the arteries. In order for the heart to function properly, flaps of tissue within the heart, called valves, must close off chambers within the heart so that the blood doesn’t flow back into it. However, some people are born with a defect in their valves that prevents the valves from closing completely. In these people, the heart pumps the blood inefficiently because some blood flows back into the heart instead of going out into the arteries. A person with this type of heart defect may have poor circulation and tire easily. Heart valve defects are just one example of how differences in the structure of an organism can affect its function.

You can study anatomy and physiology at all levels of the organization of living things — from the smallest units of life (cells) to organisms. Some scientists even study ecological physiology by looking at how the physiology of organisms is interrelated with their environment. The complexity of anatomy and physiology grows as you move up the levels of the organization of living things, as you can see from the following:

Tissues are made of cells, which are made of molecules. The foundation of physiology rests upon the function of cells, but to understand the function of cells, or the details of a physiological process, you need to be able to follow the interactions of molecules within the cell. (For more on cells, see Chapter 4; for more on molecules, see Chapter 3.)

Organisms are made of organ systems, which are made of organs, which are made of tissues. In order to understand the function of an organism, you need to understand the functions of the organ systems and organs that make up the organism. And the functions of the organs depend upon the function of their tissues, which are groups of similar cells.

Organisms multiply to form populations, which interact with other populations to form communities, which interact with their environment to form ecosystems. An organism’s interaction with the living and nonliving things in its environment can influence its physiology. For example, environmental toxins such as polychlorinated biphenyls (PCBs) have estrogen-like properties and can affect the reproductive physiology of organisms. Likewise, interactions with pathogens can cause disease, which has a negative impact on an organism’s physiology. (See Chapter 11 for the scoop on ecosystems and Chapter 17 for details on pathogens.)

To understand the function of a structure at any level in the organization of living things, you often need to know something about one of the lower levels. However, knowing everything about the lower levels doesn’t necessarily tell you how the higher levels function. Sometimes the sum of all the parts is greater than what you expect. For instance, you likely wouldn’t predict the intellectual and emotional properties of the human brain just from studying how individual neurons function.

 The properties of an entire system that are greater than the functions of the individual parts have a special name — emergent properties.

Wrapping Your Head around the Big Physiological Ideas

In the human body alone, ten different organ systems interact with each other to regulate physiology (we introduce you to these systems in the rest of Part IV). Each system has its own parts and processes and makes unique contributions to the whole.

In the following sections, we present a few physiological concepts that are central to the functioning of all of your organ systems. With these big ideas in mind, you can more easily see the similarities in the different systems and understand some of the fundamental processes that regulate their functions.

Evolving the perfect form

Biological evolution, the study of how populations change over time, explains the relationship between structure and function that’s at the core of physiology. Scientists can look at the structures and functions of different kinds of organisms and compare them to reveal how biological evolution creates variations on a theme to improve the functioning of a part of an animal (be it tissue, organ, or an entire organ system) so that the animal can better cope in its environment.

For example, today’s scientists know that the function of the kidney is to reabsorb water into the body (we cover the kidney in more detail in Chapter 16). Within the kidney, a special tube called the loop of Henle helps set up conditions that allow mammals to reabsorb water from the fluid that enters the kidney, concentrating the urine and conserving water for the organism. Mammals that live in the desert are under strong selection pressure to conserve water (see Chapter 12 for more on biological evolution and selection pressures). Many desert mammals have an extralong loop of Henle in their kidneys that allows them to reabsorb most of their water. These mammals produce very concentrated urine, conserving water in a way that helps them survive in their desert environment. By comparing these desert mammals to their non-desert-dwelling relatives, scientists can discover evidence of the evolution of the loop of Henle’s function.

Balancing the body to maintain homeostasis

More than 100 years ago, the French physiologist Claude Bernard noted that two different environments are important to animals:

The external environment: The external environment includes the Sun and the atmosphere that surrounds the animal. It experiences fairly large changes, such as temperature changes as the Sun rises and sets.

The internal environment: The internal environment includes the fluids that surround the cells in an animal’s tissues. It’s mainly affected by the diet of the animal and the amount of water that the animal drinks.

 If the internal environment of an animal changes too much, the conditions may kill the animal’s cells. Animals therefore use control systems to respond to and counteract changes in their external environment in order to keep their internal environment within a certain range that allows them to survive. In other words, animals (including you) are constantly trying to maintain homeostasis (balance) within their bodies.

Many different homeostatic processes maintain the balance of variables such as pH level, glucose level, and body temperature in an animal’s body. In order for homeostasis of a particular variable to be maintained, the animal must be able to

Measure the change of the variable in the body

Respond by changing the behavior of components in the body that regulate that variable

Most homeostasis control relies upon negative feedback. In negative feedback, a change triggers a response that reverses the change. For instance, after a meal, the amount of glucose in the blood increases. The body responds by releasing insulin into the bloodstream, which signals the body to transfer glucose from the blood into the cells and lowers the level of glucose in the bloodstream.

 The range of values that an organism can tolerate for a particular variable act like a set point for maintaining homeostasis. If changes occur in the internal environment, the changes are measured and compared to the set point. The difference between the state of the variable and the desired set point is used to generate signals that trigger actions, like negative feedback, designed to return the body to the set point. For example, blood glucose must stay within a certain range or else a person will develop the disease diabetes. When blood glucose rises after a meal, insulin triggers negative feedback that lowers the levels back to the normal range.

 To help you understand how homeostasis works, think of the body like a heating and cooling system. You determine the desired set point by setting the thermostat, and the thermostat measures the temperature of the room. If the temperature of the room is higher than the desired set point, the thermostat sends signals to turn on the cooling system. When the temperature of the room reaches the set point, the thermostat sends signals to turn off the cooling system. Just like your body, the heating system has a mechanism for measuring the change in the variable (in this case, temperature) and then responding to that change (by turning on the heating and cooling systems).

As you study the human body’s organ systems, you can expect to encounter many examples of homeostatic control and negative feedback. Although the details of each control system are different, they all have the same three components:

A receptor: The receptor measures changes in the variable, such as blood pressure, body temperature, or heart rate, and sends information to the control center.

A control center: The control center can be a neuron or gland so long as it processes the information, initiates a response to keep the variable within its normal range, and sends the response to an effector.

An effector: Often a muscle or a gland, an effector carries out the body’s response.

 Homeostasis doesn’t keep conditions in the body exactly the same all the time. The set point for a variable can change depending on the situation the organism is in. Your body temperature, for example, changes throughout the day. It may drop low while you sleep, or it may be high when you exercise. So you see, homeostasis keeps your internal environment within a particular ideal range, but it doesn’t keep it rigidly fixed at one point.

Getting the message across plasma membranes

Cells communicate with other cells, with tissues, and with organs. This communication is vital to the integration of all the body systems and to the maintenance of homeostasis (covered in the preceding section). The plasma membranes of cells separate them from their environment, maintaining a delicate balance between the outside and the inside of the cell (for more on plasma membranes, see Chapter 4). In complex, multicellular organisms such as humans, each cell has a specialized function (head to Chapter 19 for details on how cells become specialized). The function of the entire organism depends upon the coordinated functioning of all the cells within the body.

Signals are received by cells at the plasma membrane and then passed inside the cell by a process called signal transduction. During signal transduction:

1. A signal arrives at the plasma membrane of a cell and binds to a receptor in the plasma membrane.

 Signaling molecules that bind to receptors are called ligands. Each ligand binds specifically to its unique receptor, so cellular responses to each signal are very specific.

2. The receptor in the plasma membrane changes in response to the signal.

For example, the receptor may change shape.

3. The receptor interacts with a messenger molecule inside the cell that receives the signal and changes in response.

4. The intracellular messenger interacts with a target protein that causes a change in the behavior of the cell.

The change in behavior is often the result of changes in gene expression (see Chapter 8 for more on this topic).

Recognizing that what comes in, must go out

Organisms must take in matter and energy from their environment in order to survive, but they can’t create (or destroy) either one. Instead, they must transform matter and energy from one form to another. The reactions that make this possible are the metabolism of an organism (see Chapter 5 for more on metabolism).

 The Law of Mass Balance says that if the amount of a substance in the body is to remain constant, any input must be offset by an equal output. Simply stated, ins must equal outs.

Mass balance is the fundamental principle underlying the regulation of several systems in the human body, including

The concentration of oxygen and carbon dioxide in the respiratory system

The flow of blood through the heart

The clearance of materials through the kidneys

Water and electrolyte balance in the blood

In any of these systems, control mechanisms return the body to homeostasis when mass balance is disrupted.