10 Chapter 10: Structure Determines Function

Learning Objectives

Learning objectives for the structure and function bioconcept.

  • You will be able to demonstrate and provide examples of the intimate relationship between structure (shape) and function in nature at several levels:
    •   molecular and cellular (proteins and cell types)
    •   individual (anatomy and physiology)
    •   population level and above (ecosystems)
  • Distinguish among the basic structures and functions of the four tissue types by:
    • providing or recognizing major examples of each tissue  type
    • summarizing how the tissues are organized into organs and systems
  • List the 11 organ systems, their components, and their functions.
  • Explain how and why organisms must maintain homeostasis within their internal environment.


10.1 Structure Determines Function

One of the overarching themes of biology is that structure determines function; how something is arranged allows it to perform a specific job. We see this at all levels in the hierarchy of biological organization from atoms up to the biosphere. Let’s take a look at some examples where structure determines function.

  • Molecular level – proteins. The shape (structure) of a protein determines its function. For example, there are two basic shapes for proteins: fibrous and globular (round). Fibrous proteins, such as collagen (Figure 18.1), are shaped like a rope and give strength to our skin to prevent it from tearing. Fibrous proteins are structural proteins because they help give shape to and support the skin. Globular proteins, such as hemoglobin (Figure 18.2), are used to transport oxygen in the blood. Other examples of globular proteins that have different functions are enzymes (catalyze or speed up chemical reactions in the body) and plasma membrane proteins (can transport substances across the cell membrane, play a role in cell communication, act as enzymes, or help identify the cell to the rest of the body).
Figure 10.1 Collagen, a fibrous protein found in the skin. CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=379964


Figure 10.2 Hemoglobin. A molecule of hemoglobin contains four globin proteins, each of which is bound to one molecule of the iron-containing pigment heme. (credit: modified from Openstax Anatomy and Physiology)
  • Cellular level – skeletal muscle cells. The structure of skeletal muscle cells allows them to have the function of contraction, which allows us to move. For example, skeletal muscle cells that make up your biceps brachii muscle are attached to both ends of the humerus bone by tendons and are packed full of contractile proteins (actin and myosin) (Figure 18.3). When the contractile proteins contract, they shorten the muscle cell, which then pulls on the ends of the humerus and allows you to flex your forearm (Figure 18.4).
Figure 10.3 Muscle Fiber (Cell) A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many fibrils, which give the cell its striated appearance. (credit: Openstax Anatomy and Physiology)


Figure 18.4 Biceps Brachii Muscle Contraction The large mass at the center of a muscle is called the belly. Tendons emerge from both ends of the belly and connect the muscle to the bones, allowing the skeleton to move. The tendons of the bicep connect to the upper arm and the forearm. (credit: Victoria Garcia)
  • Individual level (anatomy and physiology). In studying humans, anatomy is the study of the structure of the body (ex- where the quadriceps muscle is located) and physiology is the study of how the body functions (ex- how the quadriceps muscle contracts). Let’s take a look at the anatomy of the heart, which dictates the heart’s function. The heart consists of four hollow chambers (atria and ventricles) and is made of cardiac muscle cells (Figure 18.5). This structure allows the heart to have the function of pumping blood around the body. If the structure of the heart changes (ex- some of the heart chambers become stretched out or dilated), then the heart’s function decreases as the heart can no longer pump as much blood, which will eventually cause congestive heart failure.
Figure 10.5 Internal anatomy of the heart. This anterior view of the heart shows the four chambers, the major vessels and their early branches, as well as the valves. (credit: Openstax Human Biology)
  • Ecosystem level. An ecosystem consists of a community of all the different species living in a particular geographic area as well as all of the nonliving components (ex- water, sand, light, oxygen). If we look at the structure of a coral reef ecosystem, we see that the corals, which are the foundation species, provide protection and habitat for other species (Figure 18.6). The coral reef protects other species, such as fish, from ocean waves and currents and gives them a place to hide from predators.
Figure 10.6 By Fascinating Universe – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16657833

10.2 Human Tissue Types

The term tissue is used to describe a group of similar cells found together in the body that act together to perform specific functions. From the evolutionary perspective, tissues appear in more complex organisms.

Although there are many types of cells in the human body, they are organized into four categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall health and maintenance of the body. A disruption of the structure of a tissue is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function.

The Four Types of Tissues

Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities and passageways, and form certain glands. Examples of epithelial tissue include skin, mucous membranes, endocrine glands, and sweat glands. Connective tissue, as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body. Connective tissue is diverse and includes bone, tendons, ligaments, cartilage, fat, and blood. Muscle tissue is excitable, responding to stimulation and contracting to provide movement, and occurs as three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body (Figure 18.7).

The next level of organization is the organ, where two or more types of tissues come together to perform specific functions. Just as knowing the structure and function of cells helps you in your study of tissues, knowledge of tissues will help you understand how organs function.

This diagram shows the silhouette of a female surrounded by four micrographs of tissue. Each micrograph has arrows pointing to the organs where that tissue is found. The upper left micrograph shows nervous tissue that is whitish with several large, purple, irregularly-shaped neurons embedded throughout. Nervous tissue is found in the brain, spinal cord and nerves. The upper right micrograph shows muscle tissue that is red with elongated cells and prominent, purple nuclei. Cardiac muscle is found in the heart. Smooth muscle is found in muscular internal organs, such as the stomach. Skeletal muscle is found in parts that are moved voluntarily, such as the arms. The lower left micrograph shows epithelial tissue. This tissue is purple with many round, purple cells with dark purple nuclei. Epithelial tissue is found in the lining of GI tract organs and other hollow organs such as the small intestine. Epithelial tissue also composes the outer layer of the skin, known as the epidermis. Finally, the lower right micrograph shows connective tissue, which is composed of very loosely packed purple cells and fibers. There are large open spaces between clumps of cells and fibers. Connective tissue is found in the leg within fat and other soft padding tissue as well as bones and tendons.
Figure 10.7 Four Types of Tissue: Body The four types of tissues are exemplified in nervous tissue, stratified squamous epithelial tissue, cardiac muscle tissue, and connective tissue in small intestine. Clockwise from nervous tissue, LM × 872, LM × 282, LM × 460, LM × 800. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

10.3 Human Organ Systems

An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. Figure 18.8 below shows the eleven distinct organ systems in the human body. Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system. In this course, we will discuss some, but not all of these organ systems.

Organ Systems of the Human Body

This illustration shows eight silhouettes of a human female, each showing the components of a different organ system. The integumentary system encloses internal body structures and is the site of many sensory receptors. The integumentary system includes the hair, skin, and nails. The skeletal system supports the body and, along with the muscular system, enables movement. The skeletal system includes cartilage, such as that at the tip of the nose, as well as the bones and joints. The muscular system enables movement, along with the skeletal system, but also helps to maintain body temperature. The muscular system includes skeletal muscles, as well as tendons that connect skeletal muscles to bones. The nervous system detects and processes sensory information and activates bodily responses. The nervous system includes the brain, spinal cord, and peripheral nerves, such as those located in the limbs. The endocrine system secretes hormones and regulates bodily processes. The endocrine system includes the pituitary gland in the brain, the thyroid gland in the throat, the pancreas in the abdomen, the adrenal glands on top of the kidneys, and the testes in the scrotum of males as well as the ovaries in the pelvic region of females. The cardiovascular system delivers oxygen and nutrients to the tissues as well as equalizes temperature in the body. The cardiovascular system includes the heart and blood vessels.

The lymphatic system returns fluid to the blood and defends against pathogens. The lymphatic system includes the thymus in the chest, the spleen in the abdomen, the lymphatic vessels that spread throughout the body, and the lymph nodes distributed along the lymphatic vessels. The respiratory system removes carbon dioxide from the body and delivers oxygen to the blood. The respiratory system includes the nasal passages, the trachea, and the lungs. The digestive system processes food for use by the body and removes wastes from undigested food. The digestive system includes the stomach, the liver, the gall bladder (connected to the liver), the large intestine, and the small intestine. The urinary system controls water balance in the body and removes and excretes waste from the blood. The urinary system includes the kidneys and the urinary bladder. The reproductive system of males and females produce sex hormones and gametes. The male reproductive system is specialized to deliver gametes to the female while the female reproductive system is specialized to support the embryo and fetus until birth and produce milk for the infant after birth. The male reproductive system includes the two testes within the scrotum as well as the epididymis which wraps around each testis. The female reproductive system includes the mammary glands within the breasts and the ovaries and uterus within the pelvic cavity.
Figure 10.8 Human Organ Systems. Organs that work together are grouped into organ systems. (credit: Openstax Human Biology)

Table 10.1 below lists the 11 organ systems, their components, and functions.

Organ System Major Organs Function
Skeletal Bones, ligaments, cartilage Support and protection
Muscular Skeletal muscles, tendons Voluntary movement
Circulatory Heart, blood vessels Transport substances
Respiratory Nasal cavity, pharynx, larynx, lungs Gas exchange and sound
Digestive Mouth, stomach, intestines, liver, pancreas Obtaining nutrients
Urinary Kidneys, bladder Filtering blood, water balance
Integumentary Skin, hair, nails Protection
Reproductive Ovaries/testes, glands, uterus, vagina/penis Reproduction
Lymphatic Tonsils, spleen, lymph nodes Immune protection
Nervous Brain, spinal cord, nerves Integration, communication, and control
Endocrine Hypothalamus, pituitary, thyroid, adrenal, gonads Integration, communication, and control

10.4 Homeostasis

Before moving on to discussing the individual organ systems, it is important to review the concept of homeostasis. Homeostasis refers to the maintenance of a relatively stable state inside the body. Human organs and organ systems constantly adjust to internal and external changes in order to maintain this steady state. Examples of internal conditions maintained in homeostasis are the level of blood glucose, body temperature, and blood calcium level. These conditions remain stable because of control by negative feedback. If the blood glucose or calcium rises, this sends a signal to organs responsible for lowering blood glucose or calcium. The signals that restore the variable to the normal range (also called the set point) are examples of negative feedback. When homeostatic mechanisms fail, the person gets sick and could die.

Control of Homeostasis

When a change occurs in an person’s environment, an adjustment must be made. A receptor (often a neuron) senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector, which returns the regulated variable back to the normal range. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems.

Negative Feedback Mechanisms

Any homeostatic process that changes the direction of the stimulus back toward the normal range is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is the maintenance of blood glucose levels. After a person has eaten, blood glucose levels rise. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 18.9. However, if a person has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Negative feedback loops are the predominant mechanism used to maintain homeostasis.

Illustration shows the response to consuming a meal. When food is consumed and digested, blood glucose levels rise. In response to the higher concentration of glucose, the pancreas secretes insulin into the blood. In response to the higher insulin levels in the blood, glucose is transported into many body cells. Liver cells store glucose as glycogen. As a result, glucose levels drop. In response to the lower concentration of glucose, the pancreas stops secreting insulin.
Figure 10.9 Blood glucose levels are controlled by a negative feedback loop. (credit: modification of work by Jon Sullivan)


Another example of the use of negative feedback to maintain homeostasis is thermoregulation. Animals, such as humans, that maintain a constant body temperature in the face of differing environmental temperatures, are called endotherms. We are able to maintain this temperature by generating internal heat (a waste product of the cellular chemical reactions of metabolism) that keeps the cellular processes operating optimally even when the environment is cold.

Thermoreceptors (made of neurons) in the internal organs, spine, and brain send information about the body temperature to the control center in the hypothalamus in the brain. The hypothalamus acts as the body’s thermostat and can raise or lower the body temperature to keep it in the normal range (around 98.6 ºF or 37 ºC). If the body temperature is above the normal range, the hypothalamus will send signals to the sweat glands to cause sweating and to the smooth muscle around the blood vessels in the skin to cause vasodilation. Vasodilation, the opening up of arteries to the skin by relaxation of their smooth muscles, brings more blood and heat to the body surface, facilitating heat loss and cooling the body. Conversely, if the body temperature is below the normal range, the hypothalamus will tell the skeletal muscles to contract to cause shivering, which will generate body heat. Signals are also sent to the smooth muscle around the blood vessels in the skin to cause vasoconstriction. Vasoconstriction, the narrowing of blood vessels to the skin by contraction of their smooth muscles, reduces blood flow in peripheral blood vessels, forcing blood toward the core and vital organs, conserving heat.

The normal range (set point) for body temperature may be changed during an infection. Some of your immune system cells release chemicals called pyrogens, which cause the hypothalamus to reset the body temperature normal range to a higher value, resulting in a fever. The increase in body heat makes the body less optimal for bacterial growth and increases the activities of immune system cells so they are better able to fight the infection.

Flow chart shows how normal body temperature is maintained. If the body temperature rises, blood vessels dilate, resulting in loss of heat to the environment. Sweat glands secrete fluid. As this fluid evaporates, heat is lost from the body. As a result, the body temperature falls to normal body temperature. If body temperature falls, blood vessels constrict so that heat is conserved. Sweat glands do not secrete fluid. Shivering (involuntary contraction of muscles) releases heat which warms the body. Heat is retained, and body temperature increases to normal.
Figure 10.10 The body is able to regulate temperature in response to signals from the nervous system in a negative feedback loop.

Positive Feedback

A positive feedback loop pushes the regulated variable further away from the normal range. Positive feedback is not often used in the body, but it is used in blood clotting, sneezing, and generating nerve signals. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure 18.11. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This pushes the baby’s head toward the cervix, stretching it. The stretched cervix sends a signal to the pituitary gland in the brain to release more oxytocin. The increased oxytocin causes stronger uterine contractions, which push the baby further into the cervix, stretching it more. Increased release of oxytocin, stronger uterine contractions, and further stretching of the cervix continues until the baby is delivered and the positive feedback loop is turned off because the cervix is not longer being stretched as much.

Prior to birth, the baby pushes against the cervix, causing it to stretch. Stretching of the cervix causes nerve impulses to be sent to the brain. As a result, the brain stimulates the pituitary to release oxytocin. Oxytocin causes the uterus to contract. As a result, the baby pushes against the cervix in a positive feedback loop.
Figure 10.11 The birth of a human infant is the result of positive feedback. (credit: Openstax Biology 2e)
Adapted from Openstax Human Biology and Biology 2e

Media Attributions

  • Collagen
  • Hemoglobin
  • Muscle fiber
  • Biceps
  • Internal Anatomy of the Heart
  • Coral reef


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Human Biology Copyright © by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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