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The human digestive system is dedicated to pulling in nutrients necessary for all the cells of the body to function(Fowler et al, 2013). Its job is to bring in food, break it down physically (via chewing) and chemically to extract macronutrients, such as proteins, fats and complex carbohydrates, and process those macronutrients into smaller particles that the cells can use, such as amino acids, lipids, nucleotides and simple sugars(Fowler et al, 2013).

The process of chemical breakdown begins in the mouth, and continues as the food moves through the digestive system(Fowler et al, 2013). In the stomach, powerful acids work together with enzymes to break food down into chyme, which is moved into the small intestine, where it is introduced to bicarbonate rich juices from the pancreas to make it safe for the rest of the digestive system, as well as intestinal flora in the large intestine, which help to break down cellulose, a material we otherwise would be unable to digest(Fowler et al, 2013).

Throughout the small intestine, villi, little tendrils, move the chyme along while pulling digested molecules into their cells to be carried through to the circulatory system, where they are dispersed through the body(Fowler et al, 2013).

Finally, the large intestine primarily serves to move the indigestible waste through to the anus for storage, and later expulsion, all while reabsorbing water and mineral salts used throughout the digestive process(Fowler et al, 2013).

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The respiratory system has to do with air transfer. The diaphram controls breathing by pulling the lungs open to draw in air, and pushing them shut to expel air (Fowler et al, 2013). The air passes through the nasal cavity, which filters particles diffused in the air, conditions it to prevent damage from cold and dry air, and assesses the air quality through olfactory nerves (Fowler et al, 2013). From the nasal cavity, air is funneled into the trachea, which is lined with mucus, which helps to "grab" the air particles and pull them into the complex, branching bronchial system of the lungs (Fowler et al, 2013). The splitting pathways of the bronchi lead to alveoli, which directly touch capillaries of the circulatory system, where the air in the respiratory system deposits oxygen into the blood, and receives carbon dioxide, wereupon it is exhaled(Fowler et al, 2013).

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Image generated by ChatGPT (I figure the image for the respiratory system gave a pretty good visual of the circulatory system)

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The circulatory system is actually a binary system; it is a system comprised of two subsystems which are interwoven together. The heart is actually two pumps, rather than one pump (Fowler et al, 2013). Since the pulmonary pump provides blood to the lungs for respiration, while the systemic pump provides blood to the rest of the body, the systemic pump is larger than the pulmonary pump (Fowler et al, 2013). Each pump is comprised of two chambers: the atrium, which draws in blood during the diatole phase (where the heart relaxes) of the cardiac cycle, and the ventricle, which pushes blood out during the systole phase (where the heart contracts) of the cardiac cycle (Fowler et al, 2013).

Arteries are the blood vessel which moves blood away from the heart (Fowler et al, 2013). As the arteries move away from the heart, they branch out into smaller and smaller vessels, eventually becoming capillaries, where blood cells travel in single file, so as to allow for nutrient and waste transfer for the muscles and organs of the body (Fowler et al, 2013). Also of note for arteries further from the heart, these smaller arteries have more muscle surrounding them to allow for constriction of blood flow.

Once the blood reaches the organs and muscles, the blood is sent back to the heart through veins (Fowler et al, 2013). Veins are fundamentally different from arteries in that they have thinner vessel walls, due to the lower blood pressure, and they have valves throughout their length to prevent blood from backflowing (Fowler et al, 2013).

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The endocrine system is centered around hormone production and dispersal, and is made up of endocrine glands throughout the body (Fowler et al, 2013). Hormones are the cells' way of communicating with each other, as each hormone causes specific behavior in the cells they interact with (Fowler et al, 2013). Each cell that a hormone interacts with has receptors dedicated to that hormone on their plasma membrane (Fowler et al, 2013). One example of an endocrine gland is the thyroid glands in the neck (Fowler et al, 2013). They produce thyroxine and triiodothyronine, which stimulates metabolic rates, increasing energy levels (Fowler et al, 2013). The thyroid glands also produce calcitonin, which decreases blood-calcium levels (Fowler et al, 2013). To counter this, parathyroid glands produce parathyroid hormones, increasing blood-calcium levels (Fowler et al, 2013). Another example of the role of the endocrine system is epinephrine and norepinephrine, which interacts with the respiratory, circulatory and musculoskeletal systems, stimulating them to increase their activity in response to fight-or-flight being triggered (Fowler et al, 2013). Epinephrine and norepinephrine are produced in the testes and the adrenal cortex (Fowler et al, 2013).

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Bones and muscles. For such a seemingly simple combination, the musculoskeletal system is essential for everything we do. The skeletal system has five main functions, which are supplemented by skeletal muscles (Fowler et al, 2013). These are body support, storage for lipids and minerals absorbed through the digestive system, blood cell production, protection of the various internal organs, and allowance of movement (Fowler et al, 2013).

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The central nervous system shares control over all the body's other systems with the endocrine system, although its control of these systems is more direct and immediate, thanks to its use of both chemical and electrochemical methods (Fowler et al, 2013). In addition to controlling other organ systems, the nervous system takes in sensory information and communicates this data to the brain, also a part of the nervous system (Fowler et al, 2013). Barring a belief in the soul, the nervous system, more specifically within the brain, contains the purest essence of what could be considered "us;" all cognitive functions, including but in no way limited to, our thoughts, memory, ability to learn and personality, all of these things are simulated in our brains (Fowler et al, 2013).

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The immune system protects the body from infectious pathogens, and can be divided into two parts: innate immunity and adaptive immunity (Fowler et al, 2013). Innate immunity involves both internal defenses and external defenses (Fowler et al, 2013). External defenses can include the keratin in skin and hair, and extends to the acidity in the fluids of mucus membranes (Fowler et al, 2013). Internal defenses can include the highly acidic contents of the stomach, as well as the microbiome inside the rest of the digestive system, which outcompetes newcomers to the environment if they make it past other defenses (Fowler et al, 2013). White blood come in different types, and thus can be included in both innate immunity and adaptive immunity (Fowler et al, 2013). In adaptive immunity, B and T white blood cells cooperate to kill pathogens; T cells specialize in killing pathogens, but cannot recognize threats, and thus rely on B cells, which can mark targets for T cells, and can even specialize in recognizing previously dealt-with threats, remaining forever vigilant for a resurgence of former menaces (Fowler et al, 2013).

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During fetal development, the male and female development diverges at around 7 weeks, depending on whether testosterone is introduced to the fetus or not (Fowler et al, 2013). If testosterone is introduced, what would be a female development turns into a male development; the material for the ovaries is instead used to produce testes, what would become a labia is instead turned into a scrotum, the clitoris is repurposed as a penis, etc (Fowler et al, 2013).

Gamete production in males begins at puberty, and is called spermatogenesis; this is an ongoing process of gamete production (Fowler et al, 2013). A stem cell undergoes mitosis, producing one stem cell and one spermatocyte (Fowler et al, 2013). The two spermatocytes undergo meiosis 1 to produce two secondary spermatocytes, which divide after meiosis 2, each producing two spermatids; as the four spermatids reach the lumen of the tubule, they grow flagellum, at which point they are proper sperm (Fowler et al, 2013).

Gamete production in females differs, as stem cells called oogonia undergo mitosis repeatedly in order to produce several million of them (now called oocytes) before birth (Fowler et al, 2013). They all begin the process of meiosis (also before birth), but stall the process in prophase 1 until puberty, whereupon endocrine hormones start the process of several follicles to develop in the ovaries each month (Fowler et al, 2013). From this point, the oocytes in the follicles finish their first round of meiosis, producing unequal daughter cells, with one daughter generally not surviving (Fowler et al, 2013). The surviving daughter cell is called the secondary oocyte, which is moved into the uterus, where it begins meiosis again, this time freezing in metaphase 2 until it is either fertilized or shed (Fowler et al, 2013).

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Fowler S., Roush R., Wise, J. et al (2013, April 25 & updated 2025, Feb 10) Concepts of Biology, Openstax https://openstax.org/details/books/concepts-biology?Book%20details