Integration of Systems

Discuss how different body systems interact with one another

As we've learned our bodies are complicated systems made up of cells, tissues, organs, and organ systems. In order for life to function properly, however, these systems must work together. Organs often perform roles in multiple systems, due to their unique functions. In this section, we'll learn how systems work together, and we'll learn about a few essential life functions that require work from multiple body systems.

Learning Objectives

  • Discuss how different body systems interact with one another
  • Explain how different organ systems relate to one another to maintain homeostasis
  • Explain how different organ systems work together to maintain blood solute levels


How Bodies Work

The organ level of organization in the body may be the most familiar to us from our everyday experiences. Many of the common ailments we hear about—an upset stomach, a broken bone, lung disease, skin cancer—are named for the organs they affect.

An organ is made up of tissues that work together to perform a specific function for the body as a whole. Groups of organs that perform related functions are organized into organ systems, which perform more general functions. Table 1 describes the structures and functions of some common organs.

Table 1. Structure and Function of Organs
Organ Primary function(s) Tissues it contains Organ system(s) it is a part of
brain control of body systems and behavior; cognition nervous, connective, epithelial nervous system; endocrine system
skin protection; support and containment; temperature and fluid regulation epithelial, nervous, connective, muscular integumentary system
stomach chemical and mechanical digestion of food epithelial, connective, muscular, nervous digestive system
sternum (breastbone) support; protection; blood cell production epithelial, connective, nervous skeletal system; immune system; cardiovascular system
kidney waste removal; fluid regulation epithelial, connective, nervous urinary system

Organ Systems, The Whole Body, and Populations

Organ systems are made up of organs that work together to perform a specific function for the body as a whole. Table 2 describes the organ systems and their primary organs and physiological functions that we will cover in subsequent pages.

Note that we have opted to organize the rest of this module into three basic groups: systems involved in "control," systems of "cell maintenance," and systems of "support." It is important to remember just as organs and systems work together that these categories are not mutually exclusive. For example, we have placed the reproductive system in the control category since it is involved in controlling the process and events of reproduction. However, the reproductive system is also a cell maintenance system, as it produces and maintains the actual cells used in reproduction. Just keep in mind these are groupings to help you mentally organize your learning more than they hard rules of anatomy and physiology.

Table 2. Organ Systems
Organ system Key Organ(s) Primary function(s)
nervous brain, spinal cord control of behavior and body systems; cognition
endocrine glands control of the body systems and development
reproductive penis, testes, prostate (male); uterus, ovaries, vagina (female) reproduction
sensory nerves and receptors associated with tongue, ears, skin, eyes, nose detect external stimuli and chemicals
cardiovascular heart, blood vessels transport of materials through the body; regulation of temperature
respiratory trachea, lungs gas exchange; regulation of temperature
immune thymus, tonsils, spleen defense agains infection
digestive tongue, esophagus, stomach, small intestine, large intestine, gallbladder, rectum digestion of food; waste removal
muscular muscles, tendons support; movement
skeletal bones, cartilage support; protection; movement; blood cell production
integumentary skin support; protection; regulation of fluid levels

The Whole Body

The organ systems of the body all work together to maintain proper physiological functions. Many times in the arena of anatomy and physiology, including in this course, we closely examine the molecules, cells, tissues and organs of the body to learn their forms and functions. However, it is important to consider that every molecule works as part of the entire system. Endocrine disorders such as diabetes affect glucose levels in the body. Altered blood glucose levels can affect many organ systems. For example, the immune system may not heal as well, the urinary system may experience kidney damage, and the cardiovascular system can experience vascular damage, even to the point of causing blindness. In the body, everything is interconnected.

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.

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 1. Click for a larger image. Organs that work together are grouped into organ systems.


Maintaining Homeostasis

Each organ system performs specific functions for the body, and each organ system is typically studied independently. However, the organ systems also work together to help the body maintain homeostasis.

Water Levels

For example, the cardiovascular, urinary, and lymphatic systems all help the body control water balance. The cardiovascular and lymphatic systems transport fluids throughout the body and help sense both solute and water levels and regulate pressure. If the water level gets too high, the urinary system produces more dilute urine (urine with a higher water content) to help eliminate the excess water. If the water level gets too low, more concentrated urine is produced so that water is conserved.

Internal Temperatures

Similarly, the cardiovascular, integumentary (skin and associated structures), respiratory, and muscular systems work together to help the body maintain a stable internal temperature. If body temperature rises, blood vessels in the skin dilate, allowing more blood to flow near the skin's surface. This allows heat to dissipate through the skin and into the surrounding air. The skin may also produce sweat if the body gets too hot; when the sweat evaporates, it helps to cool the body. Rapid breathing can also help the body eliminate excess heat. Together, these responses to increased body temperature explain why you sweat, pant, and become red in the face when you exercise hard. (Heavy breathing during exercise is also one way the body gets more oxygen to your muscles, and gets rid of the extra carbon dioxide produced by the muscles.)

Conversely, if your body is too cold, blood vessels in the skin contract, and blood flow to the extremities (arms and legs) slows. Muscles contract and relax rapidly, which generates heat to keep you warm. The hair on your skin rises, trapping more air, which is a good insulator, near your skin. These responses to decreased body temperature explain why you shiver, get "goose bumps," and have cold, pale extremities when you are cold.

Case Study: Fevers

So what happens when you have a fever? Does this mean your body is unable to maintain its homeostasis, in the same way your house will get too hot if your air conditioner is broken?

In extreme cases, a fever can be a medical emergency; but fever is an adaptive physiological response of our body to certain infectious agents. Certain chemicals called pyrogens will trigger your hypothalamus to shift the set point to a higher value. This is more like you programming the thermostat in your house to a higher temperature to save energy on a hot day when you are not going to be home during the day. These pyrogens can come from microorganisms that infect you, or they can be produced by your body cells in response to an infection of some sort.

Practice Questions

  1. As the level of pyrogens increases in your blood, and the set point resets higher, chemoreceptors now stimulating the hypothalamus are responding to ________ as the variable, rather than thermoreceptors responding to body temperature as the variable.

    1. temperature
    2. pyrogens
    3. heart rate
    4. blood pressure




  2. The control center is the _________.

    1. skeletal muscle
    2. sweat glands
    3. blood vessels
    4. hypothalamus




  3. Because the set point has been increased, you now feel cold even though you have what would normally be a body temperature within the healthy range. This produces the “chills” you feel when you get a fever. In response, the hypothalumus will work to increase body temperature. Which response will do this?

    1. The hypothalamus will stimulate sweat glands and dilating blood vessels as effectors to cool off the body.
    2. The hypothalamus will stimulate skeletal muscles to shiver and constricting blood vessels.






Although the evidence is only indirect, fever is believed to enhance the body’s immune response. The increased temperature may actually impair the replication of infecting bacteria and viruses that are adapted to survive best at your normal homeostatic body temperature range. This can give your immune cells a chance to destroy the microorganisms before they can rapidly multiply and spread in the body. There is also some indirect evidence that increased body temperature slightly modifies several metabolic reactions in ways that also allow the immune system to function more efficiently.

Practice Questions

  1. Once the new higher set point is reached, the thermoreceptors stimulate the _________ as the control center.

    1. skeletal muscle
    2. sweat glands
    3. blood vessels
    4. hypothalamus




  2. In response, the sweat glands and blood vessels (effectors) are stimulated to _________.

    1. secrete sweat for evaporation and dilate vessels for increased heat loss from blood near the surface of the skin.
    2. shiver to create heat and constrict vessels to conserve heat by keeping blood away from the surface of the skin.






Unfortunately during some infections, pyrogen levels come in "waves." This adjusts your temperature set point up and down. When pyrogen levels dip, you get the other part of the fever experience: “the sweats” and feeling flushed. As long as the pyrogen levels continue to increase and decrease you will feel like you are swinging back and forth.

Practice Question

  1. Once the pyrogen level is reduced because the infection is under control, the ________ (control center) will reset the higher set point to normal.

    1. thermoreceptors
    2. chemoreceptors
    3. hypothalamus






Your body will continue to swing back and forth between the body's normal upper and lower temperature limits, but because it is now within your “normal” temperature range, you probably won’t even notice that your body is still at work, maintaining the homoeostasis of this variable.

Practice Question

  1. Patients often get a fever after an operation. Which of the following would not be a reasonable cause of such a response?

    1. Tissue trauma from the operation has stimulated body cells to release pyrogens.
    2. Despite precautions, some bacteria have infected the person during the operation.
    3. The operation has damaged the thermoreceptors
    4. Post-operative medications have impacted the immune system, causing the release of pyrogens.






Homeostasis of Ions

Body functions such as regulation of the heartbeat, contraction of muscles, activation of enzymes, and cellular communication require tightly regulated calcium levels. Normally, we get a lot of calcium from our diet. The small intestine absorbs calcium from digested food.

The endocrine system is the control center for regulating blood calcium homeostasis. The parathyroid and thyroid glands contain receptors that respond to levels of calcium in the blood. In this feedback system, blood calcium level is the variable, because it changes in response to the environment. Changes in blood calcium level have the following effects:

  • When blood calcium is low, the parathyroid gland secretes parathyroid hormone. This hormone causes effector organs (the kidneys and bones) to respond to increase calcium levels. The kidneys prevent calcium from being excreted in the urine. Osteoclasts in bones reabsorb bone tissue and release calcium.
  • When blood calcium levels are high, the thyroid gland releases calcitonin. Calcitonin causes the kidneys to reabsorb less calcium from the filtrate, allowing excess calcium to be removed from the body in urine. Calcitonin also suppresses the formation of active vitamin D in the kidneys; without vitamin D the small intestines don't absorb as much dietary calcium. Osteoblasts, stimulated by calcitonin, use calcium in the blood to add to bone tissue.


Practice Questions

Based on the above description of calcium homeostasis, try to answer these questions:

  1. What is the variable?
  2. What is the receptor?
  3. What is the control center?
  4. What is the effector?


  1. urine
  2. endocrine system
  3. parathyroid hormone or calcitonin
  4. calcium levels










Calcium imbalance in the blood can lead to disease or even death. Hypocalcemia refers to low blood calcium levels. Signs of hypocalcemia include muscle spasms and heart malfunctions. Hypercalcemia occurs when blood calcium levels are higher than normal. Hypercalcemia can also cause heart malfunction as well as muscle weakness and kidney stones.

Practice Question

What problem(s) is/are associated with calcium homeostasis dysfunction?

  1. heart disease
  2. bone disease
  3. both
  4. neither






Watch this video for another discussion on homeostasis and organ systems:

Blood Calcium Levels

As you have learned, proper calcium levels are important to maintain whole body homeostasis. Calcium ions are used for the heartbeat, the contraction of muscles, the activation of enzymes, and cellular communication. The parathyroid and thyroid glands of the endocrine system detect changes in blood calcium levels. When the parathyroid glands detect low blood calcium levels, several organ systems alter their function to restore blood calcium levels back to normal. The skeletal, urinary, and digestive systems all act as effectors to achieve this goal through negative feedback.

The release of parathyroid hormone from the endocrine system triggers osteoclasts of the skeletal system to resorb bone and release calcium into the blood. Similarly, this hormone causes the kidneys of the urinary system to reabsorb calcium and return it to the blood instead of excreting calcium into the urine. Through altered function of the kidneys to form active vitamin D, the small intestine of the digestive system increases the absorption of calcium.

When the thyroid gland detects elevated blood calcium levels, the skeletal, urinary, and digestive systems contribute to lower blood calcium levels back to normal. Release of the hormone calcitonin from the thyroid gland of the endocrine system triggers a series of responses. The osteoblasts of the skeletal system use excess calcium in the blood to deposit new bone. The kidneys of the urinary system excrete excess calcium into the urine instead of reclaiming calcium through reabsorption. Lastly, the kidneys stop forming active vitamin D, which causes decreased intestinal absorption of calcium through the digestive system.

Practice Question

Graves' disease is an autoimmune disease in which the thyroid is overactive, producing an excessive amount of thyroid hormones. Some of the symptoms are heart palpitations and hand tremors.

Which system is impacted by the altered calcium levels in Graves' disease, according to the symptoms listed above?

  1. skeletal
  2. muscular
  3. urinary
  4. digestive






Blood Glucose Levels

The endocrine functions of the pancreas and liver coordinate efforts to maintain normal blood glucose levels. When pancreatic cells detect low blood glucose levels, the pancreas synthesizes and secretes the hormone glucagon. Glucagon causes the liver to convert the polymerized sugar glycogen into glucose through a process known as glycogenolysis. Glucose then travels through the blood to allow all cells of the body to use it.

If pancreatic cells detect high blood glucose levels, the pancreas synthesizes and release the hormone insulin. Insulin causes polymerization of glucose into glycogen, which is then stored in the liver through a process known as glycogenesis.

The nervous and digestive systems also play a role in maintaining blood glucose levels. When the stomach is empty and blood glucose levels are low, the digestive system and the brain respond by making you feel hungry—your stomach may "growl," and you may feel pain or discomfort in your midsection. These sensations prompt you to eat, which raises blood glucose levels.

PRactice Question

The liver and pancreas are part of both the endocrine system and the digestive system. What is the utility of having integrated digestion and regulation?







Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

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