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An organ is a self-contained group of tissues that performs a specific function in the body. The heart, liver, and stomach are examples of organs in humans. The word organ comes from the Latin organum, which means “instrument”. This in turn comes from the Greek word ὄργανον (órganon), which refers to a musical instrument or “organ of the body”. Organs are found in most animals and plants. All animals except for less specialized ones like those in the phylum Porifera (sea sponges) have specialized tissues grouped into organs. The human body has 78 different organs. The largest organ is the skin, while the smallest organ is the pineal gland, which produces the hormone melatonin. Some organs are necessary for survival; these are called vital organs. Humans have five vital organs that they cannot live without: the brain, heart, liver, kidneys, and lungs. (A person can live with one kidney instead of two, or one lung, but they must have one working kidney or lung to survive.) Other organs perform important functions, but a person can live without them. For example, if a person has stomach cancer and undergoes a total gastrectomy, their entire stomach is removed, and their esophagus is surgically connected to their small intestine. They may suffer some hardships and may have to alter their diet by eating smaller meals and taking nutritional supplements, but they can still digest food in their intestines just fine, and can therefore live without a stomach. Other examples of non-essential organs include the bladder, spleen, and gallbladder. Some organs are vestigial, meaning that they perform little or no function in the body. Evolutionarily, over time, they have become unnecessary. The appendix, a small tube connected to the large intestine that can sometimes become inflamed and require removal, has been widely thought to be vestigial. It no longer has a significant role in digestion. However, it does still play a role in immune functioning and maintaining good gut bacteria. Although we may think of plants as being simpler than animals, plants also have organs. This includes reproductive organs such as stamens and pistils (contained in flowers), roots, stems, and leaves. Each of these organs performs specialized tasks such as reproduction, absorbing nutrients from the soil, and performing photosynthesis. There are four different levels of organization in multicellular organisms, and organs make up one of these levels. From simplest to complex, an organism is made up of cells, tissues, organs, and organ systems. Cells make up the most basic level of organization; the cell is the building block of a living organism. This is followed by tissues. Tissues are groups of cells that work together and have a similar structure and function. The four types of tissues in the human body are muscle, epithelial, connective, and nervous tissue. Organs, as stated before, are groups of tissues that work together to perform a certain function. Organ systems represent the highest level of an organism’s bodily organization. They are made up of groups of organs that work together in order to carry out a certain function. For example, the digestive system includes organs such as the esophagus, stomach, small intestine, and large intestine, and all of these organs play a role in the digestion of food. The body’s organs are grouped into organ systems based on the functions they perform. Humans have 11 different organ systems. Here are all of the organ systems with some examples of organs found in each system:
This image depicts parts of the circulatory system, including the heart, arteries, and veins: 
1. Organs are self-contained groups of what? B is correct. While organs are technically made up of cells (as is everything else in an organism), organs themselves are defined as groups of specialized tissues that work together to carry out a function in the body. In turn, groups of organs that work together make up organ systems. An entire organism is made up of different organ systems. 2. Which is NOT a vital organ in humans? A is correct. The bladder performs an important role in collecting urine that is to be excreted from the body, but it is not necessary for a person to have a bladder in order to survive. If the bladder is removed during a process called a cystectomy, which is sometimes performed on a patient with bladder cancer, a person can still survive. Instead, urine will be collected outside the body in a urostomy bag. The five vital organs that are needed for survival in humans are the brain, heart, liver, kidneys, and lungs. 3. Which living thing does not have organs? D is correct. Sea sponges are multicellular and classified as animals, but they are very simple; they do not have any organs or even true tissues. Instead, they are made up of groups of cells. Other animals and plants have specialized tissues and organs.
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
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.
Organ Systems, The Whole Body, and PopulationsOrgan 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.
The Whole BodyThe 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. Figure 1. Click for a larger image. Organs that work together are grouped into organ systems. Maintaining HomeostasisEach 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 LevelsFor 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 TemperaturesSimilarly, 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.
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
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
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
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
Homeostasis of IonsBody 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:
Based on the above description of calcium homeostasis, try to answer these questions:
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.
What problem(s) is/are associated with calcium homeostasis dysfunction?
Watch this video for another discussion on homeostasis and organ systems:
Blood Calcium LevelsAs 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.
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?
Blood Glucose LevelsThe 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.
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 UnderstandingAnswer 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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