The digestive system has three main functions that relates to food. Digestion of food, absorption of nutrients from food and elimination of solid food waste.

The Digestive System works with a few other systems in the body to digest foods and liquids. Bacteria in the GI tract, called the Gut Flora helps with Digestion. Parts of the Nervous System and Circulatory Systems help as well. In fact the Digestive System works very close with Circulatory System to get nutrients absorbed and distributed throughout the body.

Griffin P. (2017) Your Digestive System and How it Works

https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works

The Circulatory System contains 4 main organs that have specific roles and functions. The vital circulatory system organs include Heart, Blood (blood is considered a tissue and not an organ), Blood Vessels, Lymphatic system.

Blood vessels have a complicated network that surrounds the small intestine absorbs the end products of digestion. The pituitary gland situated deep within the brain releases hormones that influence the musculoskeletal, integumentary, and reproductive systems. These hormones are carried to their target and through the Circulatory System.

Gabe B. (2021) Circulatory System

https://biologydictionary.net/circulatory-system/

Two subsidiary processes involve Respiration by the respiratory system. One process is breathing which is the physical of conducting air to and from the lungs. The other process is gas exchange. This is the biochemical process in which oxygen diffuses out of the air and into the blood while carbon dioxide and other waste gases diffuse out of the blood and into the air. All of the organs of the respiratory system are involved in breathing, but only the lungs are involved in gas exchange.

Cells cannot survive long without oxygen, and if there is too much carbon dioxide in the blood the blood becomes acidic (pH is too low). And if there is too little carbon dioxide in the blood the blood becomes too basic (pH is too high). The respiratory system works hand-in-hand with the nervous and cardiovascular systems to maintain homeostasis in blood gases and pH.

Suzanne W., Mandeep G. (2006) Structure and Function of the Respiratory System

https://bio.libretexts.org/Bookshelves/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.2%3A_Structure_and_Function_of_the_Respiratory_System

For example, the long-term secretion of growth hormone in the body influences the development of bones and muscles to increase height and also induces the growth of every internal organ. This happens over the course of many years. Hormones like cortisol, produced during times of stress, can change appetite, and metabolic pathways in skeletal and smooth muscle for hours or weeks.

The endocrine system is a collection of ductless glands that produce hormones and secrete them into the circulatory system. Endocrine glands work without ducts for carrying secretions towards target organs. Instead, hormones can act as chemical messengers for a large number of cells and tissues simultaneously.

Gabe B. (2021) Endocrine System

https://biologydictionary.net/endocrine-system/

The body contains three types of muscle tissue, Skeletal Muscle, Smooth Muscle, and Cardiac Muscle. Skeletal Muscle is voluntary and striated. These are the muscles that attach to bones and control conscious movement. Smooth Muscle is involuntary and non-striated. It is found in the hollow organs of the body (stomach, intestines, and around blood vessels.) Cardiac Muscle is involuntary and striated. It is found only in the heart and is specialized to help pump blood throughout the body.

When a muscle fiber receives a signal from the nervous system, myosin filaments are stimulated, pulling actin filaments closer together. This shortens sarcomeres within a fiber, causing it to contract.

Alexandra V. (2022) Introduction to the Biology of the Musculoskeletal System

https://www.merckmanuals.com/home/bone,-joint,-and-muscle-disorders/biology-of-the-musculoskeletal-system/introduction-to-the-biology-of-the-musculoskeletal-system

The Nervous System has two divisions; 

Central Nervous System (CNS) is the integration and command center of the body.

Peripheral Nervous System (PNS) represents the canal between the CNS and the body. It is further subdivided into the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS).

Jana V. (2023) Nervous System

https://www.kenhub.com/en/library/anatomy/the-nervous-system

The Innate Immune System is a rapid response system. It is the first to respond when it finds an invader. "It is made up of the skin, the eye's cornea, and the mucous membrane that lines the respiratory, gastrointestinal, and genitourinary tracts. These all create physical barriers to help protect the body. When this system recognizes an invader, it goes into action right away. The cells of this immune system surround and cover the invader. The invader is killed inside the immune system cells (called phagocytes)." (P.4)

The Acquired Immune System, makes special proteins (antibodies) to protect the body from invaders, with help from the Innate System of course. These antibodies are made by cells called B Lymphocytes after the invader has been exposed. It takes several days for antibodies to form, but after the first exposure, the Immune System will recognize the invader and defend against it. The Acquired Immune System changes all through out life. Immunizations train the Immune System.

Johns H. (2024) The Immune System

https://www.hopkinsmedicine.org/health/conditions-and-diseases/the-immune-system

Both sexes Gonads produce Gametes and Sex Hormones. Sex Hormones are Endocrine Hormones that control the development of Sex organs before birth, sexual maturation at puberty and reproduction once sexual maturation has happened.

The Reproductive System has various other functions like maturing gametes, delivering gametes to the site of fertilization and providing an environment for the development and growth of offspring.

Suzanne W., Mandeep G (2006) Introduction to the Reproductive System

https://bio.libretexts.org/Bookshelves/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.02%3A_Introduction_to_the_Reproductive_System