2.4: Human Organs and Organ Systems

Heartbreak

Did you ever wonder why the heart is associated with love? The heart was once thought to be the center of all thought and the site of all emotions. This idea may have stemmed from very early anatomical dissections that revealed many nerves in the heart region, as well as the fact that the heart starts racing during excitement or high emotional states. As we now know, the brain controls thoughts and emotions, but the heart and the brain are connected, which is why thoughts and feelings in one may lead to changes in the other.

Figure \(\PageIndex{1}\): "L'Absinthe" by Edgar Degas (~1875) illustrating how the feeling of sadness can affect our whole being (Musée d'Orsay CC0)

Human Organs

An organ is a group of tissues serving a common function in the same structural unit. Organs exist in most multicellular organisms, including not only humans and other animals but also plants. In single-celled organisms such as bacteria, the functional equivalent of an organ is an organelle.

Vital Organs

The human body contains five organs considered vital to an individual's survival. They are the heart, brain, kidneys, liver, and lungs. The locations of these five organs and several other internal organs are shown in Figure \(\PageIndex{2}\). If any of the five vital organs stops functioning, the death of the organism is imminent without medical intervention.

1. The heart is located in the center of the chest and functions to keep blood flowing through the body. Blood carries substances that cells need and removes waste products from cells.
2. The brain is located in the head and functions as the body’s control center. It is the seat of all thoughts, memories, perceptions, and feelings.
3. The two kidneys are located in the back of the abdomen on either side of the body. Their function is to filter blood and form urine, which is excreted from the body.
4. The liver is located on the right side of the abdomen. It has many functions, including filtering blood, secreting bile that is needed for digestion, and producing proteins necessary for blood clotting.
5. The two lungs are located on either side of the upper chest. Their main function is to exchange oxygen and carbon dioxide with the blood.

Organ Tissues

Although organs consist of multiple tissue types, many organs are composed of a main tissue associated with the organ’s major function and other tissues that are supportive. For some organs, the main tissue may be unique. For example, cardiac muscle is found only in the heart, where it pumps blood. Supportive tissues include nervous tissues, which control the heart's beating, and connective tissues, which form heart valves that keep the heart's blood flowing in one direction.

Figure \(\PageIndex{2}\): Locate the five vital organs described above: heart, brain, kidneys, liver, and lungs (Mikael Häggström CC0)

Human Organ Systems

Organs that are functionally related often cooperate to form whole organ systems.

The video below describes the 11 human organ systems. Each system is also described in more detail in the text that follows. Most of these human organ systems are also covered in separate chapters in this text.

Video

 

Integumentary System

The integumentary system includes the skin (the body's largest organ), hair, and nails. The skin encloses and protects the body, is the site of many sensory receptors, serves as the body’s first line of defense against pathogens, and helps regulate body temperature and eliminate waste products through sweating.

Skeletal System

The skeletal system consists of bones, joints, and teeth. Bones are connected by tendons, ligaments, and cartilage. The skeletal system supports the body and provides a framework for its shape. The skeletal system enables the body to move in partnership with skeletal muscle tissue. Bones also protect internal organs, store calcium, and produce red and white blood cells.

Muscular System

The muscular system consists of three types of muscles:

• skeletal muscle tissue, which is attached to bones by tendons and allows voluntary movement of the body.
• smooth muscle tissue, which controls the involuntary movements of internal organs, such as those of the digestive system or blood vessels.
• cardiac muscle tissue, which controls the heart's involuntary beating.

Nervous System

The nervous system includes the brain and spinal cord, which make up the central nervous system, and nerves that run throughout the rest of the body, which make up the peripheral nervous system.

The nervous system controls both voluntary and involuntary responses of the human organism and also detects and processes sensory information.

Endocrine System

The endocrine system consists of glands that secrete hormones into the blood, which then circulate throughout the body. Endocrine hormones are chemical messengers that control many body functions, including metabolism, growth, and sexual development. The master gland of the endocrine system is the pituitary gland, which produces hormones that control other endocrine glands. Some of the other endocrine glands include the pancreas, thyroid gland, and adrenal glands.

Cardiovascular System

The cardiovascular system (also called the circulatory system) includes the heart, blood, and three types of blood vessels: arteries, veins, and capillaries. The heart pumps blood, which travels through the blood vessels. The main function of the cardiovascular system is transport. Oxygen from the lungs and nutrients from the digestive system are transported to cells throughout the body. Carbon dioxide and other waste products are removed from the cells and transported to organs such as the lungs and kidneys for elimination from the body. The cardiovascular system also equalizes body temperature and transports endocrine hormones to cells in the body where they are needed.

Urinary System

The urinary system includes the pair of kidneys, which filter excess water and a waste product called urea from the blood and form urine. Two tubes called ureters carry urine from the kidneys to the urinary bladder, which stores it until it is excreted from the body through another tube called the urethra. The kidneys also produce renin, an enzyme, and various hormones. These substances help regulate blood pressure, red blood cell production, and calcium and phosphorus balance in the body.

Respiratory System

The respiratory system includes the nasal passages, lungs, and a long tube called the trachea, which carries air between the nasal passages and lungs. The main function of the respiratory system is to deliver oxygen to the blood and remove carbon dioxide from the body. Gases are exchanged between the lungs and blood across the walls of capillaries lining tiny air sacs (alveoli) in the lungs.

Lymphatic System

The lymphatic system is sometimes considered to be part of the immune system. It consists of a network of lymph vessels and ducts that collect excess fluid (lymph) from the extracellular spaces of tissues and transport it to the bloodstream. The lymphatic system also includes many small collections of tissue, called lymph nodes, and an organ called the spleen, both of which remove pathogens and cellular debris from the lymph or blood. In addition, the thymus gland in the lymphatic system produces some types of white blood cells (lymphocytes) that fight infections.

Digestive System

The digestive system consists of several main organs — including the mouth, esophagus, stomach, and small and large intestines — that form a long tube called the gastrointestinal (GI) tract. Food moves through this tract where it is digested, its nutrients absorbed, and its waste products excreted. The digestive system also includes accessory organs (such as the pancreas and liver) that produce enzymes and other substances needed for digestion, but through which food does not actually pass.

Male and Female Reproductive Systems

Contrary to the belief that biology exists only as a binary (male or female), there is a spectrum of neurobiology, genes, hormones, physical development, environmental factors, and reproductive organs that any individual may have.

There is also an important distinction between what an individual considers their sex versus their gender. 

These will all be covered in more detail in the reproductive and developmental chapters.

Communication among Organ Systems

Communication among organ systems is vital if they are to work together as a team. They must be able to respond to each other and change their responses as needed to keep the body in balance. Communication among organ systems is primarily controlled by the autonomic nervous system and the endocrine system.

• The autonomic nervous system is the part of the nervous system that controls involuntary functions such as heart rate, blood flow, and digestion. You do not consciously think about your heart to make it beat faster or actively squeeze muscles to push food through your intestines. In fact, you generally do not think about these functions at all. The autonomic nervous system orchestrates all the required signals, sending messages between different parts of the nervous system and between the nervous system and other organ systems via chemical signals called neurotransmitters.
• The endocrine system consists of glands that secrete hormones directly into the bloodstream, where they circulate to cells throughout the body. The hypothalamus, a part of the brain, initiates the signal by secreting hormones directly into the pituitary gland, which is located just beneath it. The pituitary gland is the master gland of the endocrine system: its hormones either activate or inhibit the activity of other endocrine glands. For example, if the pituitary gland secretes thyroid-stimulating hormone (TSH), the hormone stimulates the thyroid to secrete thyroid hormone. The thyroid hormone then travels to body cells, and their metabolism increases.

Examples of Organ System Interactions

Cellular Respiration

Cellular respiration is a good example of organ system interactions because it is a basic life process in all living cells. Cellular respiration is the process by which glucose (or other forms of chemical energy, such as protein or fat) is broken down in the presence of oxygen to produce energy (in the form of ATP molecules). In other words, it is the fundamental process by which cells obtain usable energy to power other cellular processes. Carbon dioxide is a waste product of cellular respiration and must be removed from the body.

Which organ systems are involved in cellular respiration? 

• The digestive system provides the glucose needed for cellular respiration from the food you eat.
• The respiratory system gathers the oxygen required from the air you breathe.
• The cardiovascular system delivers glucose from the digestive system and oxygen from the respiratory system to individual cells.
• The carbon dioxide produced during cellular respiration leaves the body via the opposite route. 

Fight-or-Flight Response

The fight-or-flight response is an example of the nervous and endocrine systems' control over the responses of other organ systems.

The fight-or-flight response begins when the nervous system perceives sudden danger, as shown in Figure \(\PageIndex{3}\). The brain sends a message to the endocrine system (via the pituitary gland) to stimulate the adrenal glands to secrete the hormones cortisol and adrenaline. These hormones flood the circulation and affect other organ systems throughout the body, including the cardiovascular, urinary, sensory, and digestive systems. Specific responses include increased heart rate, bladder relaxation, tunnel vision, and a shunting of blood away from the digestive system and toward the muscles, brain, and other vital organs needed to fight or flee.

 

Figure \(\PageIndex{3}\): A concept map of the fight-or-flight response. A threat (an attack, harmful event, or threat to survival) triggers brain signals (from the amygdala to the hypothalamus). ACTH (adrenocorticotropic hormone) is secreted by the pituitary gland, triggering the release of cortisol and adrenaline.

The physical effects of this cascade include increased heart rate, relaxed bladder, tunnel vision, shaking, dilated pupils, flushed face, dry mouth, slowed digestion, and hearing loss (Jvnkfood CC-BY 4.0).

Feature: Human Biology in the News

Organ transplantation has been performed by surgeons for more than six decades, and you’ve no doubt heard of people receiving heart, lung, and kidney transplants. However, you may have never heard of a penis transplant. The first U.S. penis transplant was performed in May of 2016 at Massachusetts General Hospital in Boston. The 15-hour procedure involved a team of more than 50 physicians, surgeons, and nurses. The patient was a 64-year-old man who had lost his penis to cancer in 2012. The surgical milestone involved grafting microscopic blood vessels and nerves of the donor organ to those of the recipient. As with most transplant patients, this patient will have to take immunosuppressing drugs for the rest of his life so his immune system will not reject the organ. The transplant team said that their success with this transplant “holds promise for patients with devastating genitourinary injuries and disease.” They also hope their experiences will be helpful for gender reassignment surgery.

 

Explore More

https://bio.libretexts.org/link?16777#Explore_More

Professor Anthony Atala is working to answer an important question: Can we grow new replacement organs rather than transplanting organs from other people? In his state-of-the-art lab, he and his associates are actually growing human organs, including blood vessels, bladders, and kidneys. Watch the fascinating TED talk below to see how they are doing it.