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BBC Bitesize. (2021). The three main functions of the cardiovascular system. https://www.bbc.co.uk/bitesize/guides/z9n6sg8/revision/3

Cleveland Clinic. (2020, January 24). Respiratory system: Functions, facts, organs & anatomy. https://my.clevelandclinic.org/health/articles/21205-respiratory-system

Elmore, L., PharmD, & BCPS, B. C. (2020, May 1). Immune system structure and function. Medium. https://medium.com/@lindsey_58509/immune-system-structure-and-function-249a7ecd8ed7

Encyclopedia Britannica. (2021, September 21). Respiratory system summary. https://www.britannica.com/summary/human-respiratory-system

File:Circulatory system en.svg. (n.d.). Wikimedia Commons. Retrieved December 11, 2021, from https://commons.wikimedia.org/wiki/File:Circulatory_System_en.svg

Harrison, R. J. (2020, February 4). Human reproductive system. Encyclopedia Britannica. https://www.britannica.com/science/human-reproductive-system

Island Health Works. (2019). Circulatory system. https://www.islandhealthworks.com/10-body-systems/circulatory-system

Myers, & DeWall. (2017). The endocrine system. macmillanhighered.com. https://www.macmillanhighered.com/BrainHoney/Resource/22292/digital_first_content/trunk/test/pel4e/pel4e_ch2_5.html

NIH. (2008, September 20). Introduction to the reproductive system. Welcome to SEER Training | SEER Training. https://training.seer.cancer.gov/anatomy/reproductive/

Shukla, R. (2017, June 7). Musculoskeletal system : An overview. Passionate in Knowledge. https://passionateinknowledge.com/2017/06/07/musculoskeletal-system-an-overview/

Szymik., B. (2011, May 3). Nervous system functions and parts. Ask a biologist. https://askabiologist.asu.edu/parts-of-nervous-system

Taylor, T. (2020, November 23). Respiratory system. Innerbody. https://www.innerbody.com/anatomy/respiratory

Wakim, S., & Grewal, M. (2021, September 4). Introduction to the digestive system. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Human_Biology/Book%3A_Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.2%3A_Introduction_to_the_Digestive_System#

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The Male & Female Reproductive System (Harrison, 2020)

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The reproductive system has four functions: To produce egg and sperm cells, transport and protect cells, nurture developing offspring, and produce hormones (NIH, 2008).

The reproductive system ensures the species' survival (NIH, 2008). The reproductive system works with other systems in the body, like the endocrine and urinary systems, to continually maintain homeostasis, which is the self-regulating process used by an organism that maintains stability while adjusting to conditions best for its survival (NIH, 2008).

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The main functions of the immune system are:
To fight disease-causing pathogens, like bacteria, viruses, parasites, and fungi.
To recognize and neutralize harmful substances encountered in the environment. And to fight disease-causing changes in the body, mainly cancer cells (Elmore et al., 2020).

The immune system is intricately-designed to play a vital role in the body's health and wellness (Elmore et al., 2020). Understanding the immune system structure and function can help people make the right choices to improve their health (Elmore et al., 2020). The immune system works with the circulatory system to produce blood cells through bone marrow (Elmore et al., 2020).

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Parts of the Nervous System (Szymik., 2011)

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The nervous system's function is:
To monitor changes, process and interpret information, deal with causes, effects, and illicit a response (Szymik., 2011).

The nervous system is a complicated highway divided into the Central Nervous System or CNS and the Peripheral Nervous System or PNS (Szymik., 2011). The nervous system works in unison with all of the body's organs, bones, and tissues.

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The Musculoskeletal System (Shukla, 2017)

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The primary roles of the musculoskeletal system are:
To provide support, movement, and protection. Moreover, the musculoskeletal system also gives the body's form, adds stability, stores salts, and forms and supplies new blood cells (Shukla, 2017).

The musculoskeletal system comprises two systems; the muscular and skeletal systems. The functions of the systems are movement and support (Shukla, 2017).

The skeletal system is composed of bones and cartilage; it performs crucial functions like supporting the body, facilitating movement, protecting internal organs, producing blood cells, storage, and releasing minerals and fat (Shukla, 2017).

The muscular system comprises flesh and is the muscle tissue throughout the body, used for the movement of the skeleton, maintaining posture through a steady contraction, and generating heat through cellular metabolism (Shukla, 2017). Humans have three types of muscle; skeletal, cardiac, and smooth muscle (Shukla, 2017).

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The Endocrine System (Myers & DeWall, 2017)

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The primary function of the endocrine system is:
To regulate and control blood sugar (insulin), control differentiation, growth, and function of the reproductive organs (testosterone (T) and estradiol), and aid in the development (growth) of the body and its energy production by supplying growth and thyroid hormones (Myers & DeWall, 2017).

The endocrine system comprises glands throughout the body that secrete hormones (Myers & DeWall, 2017). The endocrine system also acts as a chemical messenger (Myers & DeWall, 2017). Hormones travel through the blood, influencing mood, growth, reproduction, and metabolism (Myers & DeWall, 2017). Some hormone chemicals are identical to neurotransmitters; therefore, the endocrine and nervous systems are closely related (Myers & DeWall, 2017).

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The three primary functions of the respiratory system are:
To deliver oxygen to cells, remove waste and gases such as carbon dioxide through exhaling, and protect the airways from harm by substances or irritants (Taylor, 2020).

The respiratory system provides oxygen to the body’s cells while removing carbon dioxide, a potentially lethal waste product that may accumulate (Taylor, 2020). The airway, lungs, and muscles of respiration are the three major parts of the respiratory system (Cleveland Clinic, 2020).

The muscles and bones help push air inhaled in and out of the lungs (Cleveland Clinic, 2020). Bones and muscles in the respiratory system include ribs and diaphragm (Cleveland Clinic, 2020).

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Circulatory System ("File: Circulatory system en. svg," n.d.)

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The cardiovascular system's three primary functions are:
To transport oxygen, carbon dioxide, and nutrients throughout the body, cause the clotting of open wounds, and regulate the body's temperature (BBC Bitesize, 2021).

The circulatory system consists of:
The vascular system is a system of tubes, or vessels, that blood or lymph flow through, heart, which pumps blood through blood vessels, blood, and lymph (Island Health Works, 2019).

The circulatory system starts operating when the small intestine passes nutrients into the blood (Island Health Works, 2019). Oxygen begins with breathing; oxygen enters through the nostrils or mouth into the lungs (Island Health Works, 2019). Blood from the heart enters the lungs for oxygenation; then distributed to the cells and tissues (Island Health Works, 2019).

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The Digestive System (Wakim & Grewal, 2021)

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The digestive system consists of three primary functions related to food:
The digestion of food, absorption of nutrients, and waste elimination (Wakim & Grewal, 2021).

Digestion involves the process of breaking food down into smaller pieces that can absorb into the body (Wakim & Grewal, 2021). There is two processes mechanical and chemical digestion (Wakim & Grewal, 2021).
Mechanical digestion is a physical breakdown of large pieces into smaller pieces of food; it mainly occurs in the mouth and stomach (Wakim & Grewal, 2021).
Chemical digestion is a chemical process where food bonds get broken down from larger complex molecules into smaller, simplified nutrient molecules that are easier for the blood and lymph to absorb (Wakim & Grewal, 2021). The digestion happens from the mouth and stomach to the small intestines (Wakim & Grewal, 2021).