The circulatory or cardiovascular system is responsible for moving nutrients, wastes and gases between body cells, transporting blood across the whole body and battling disease (Circulatory System). Its principal elements are the heart, numerous blood vessels, and blood.The heart forms the circulatory system's core. This 2-sided, 4-chambered pump which distributes blood to various arteries comprises of the right and left ventricles, and right and left atria. The ventricles, situated within the heart's lower half, are responsible for pumping blood to the whole body (away from our heart), whilst the atria, situated within the heart's upper half are in charge of receiving blood from different parts of the human body. The right and left ventricles pump de-oxygenated and oxygenated blood, respectively; de-oxygenated blood is pumped to lungs while oxygenated blood is pumped to the remainder of the human body (smith, 2013). These 4 chambers are connected to one another by means of valves which control blood flow and make sure its movement is unidirectional.

Blood vessels denote a sequence of elastic tubes which transport blood from and to the heart. Blood infused with oxygen exits the heart, supplying nutrients and oxygen by means of arteries throughout the body. After traversing the capillaries, the veins take wastes and deoxygenated blood back to the heart via the two vena cavae. Blood that exits the right ventricle via the pulmonary artery is oxygenated within the lungs. Carbon dioxide in the blood is disposed, and the blood goes back through the pulmonary veins to the left atrium and ventricle (smith, 2013). The process repeats in the same way.

The partly-viscous fluid, blood comprises of white (WBC) and red (RBC) blood cells, plasma, and platelets. Plasma refers to watery matter containing sugars, proteins, minerals and fats. In one cardiac cycle, ten pints of blood, on an average, travel across an adult's body. The RBCs contain hemoglobin, a substance whose function is transportation of oxygen to cells and return of carbon dioxide to the heart.

Digestive system

Figure 2. Human digestive system (adopted from Astudillo, 2016)

The human digestive system constitutes a tube via which food consumed via the mouth passes through the body and is finally egested from the anus. It comprises of a number of organs having their own respective functions and structures. In the course of its transportation through the digestive system, food breaks down into matter that can be readily absorbed into our bloodstream.

The digestive system's foremost structures are the oral structures and the mouth. But minimal food digestion occurs here. By means of chewing or mastication, food breaks down enough to get transported via the upper part of the digestive tract to the small intestine and stomach, which form the key sites of digestion. Chewing constitutes the foremost mechanical process in the digestion of food. Mastication muscles, namely, the buccinator, masseter, medial, lateral and temporal pterygoids aid the lower jaw's movements in the chewing of food (Keeton, Dworken, Hightower & Sircus, 2015).

Ingested liquids and solid food passes through the esophagus and reaches the stomach, which retains food, grinds and mixes it with gastric juices, and makes it more soluble and smaller. The stomach has the following chief functions: commencement of protein and carbohydrate digestion, conversion of food into the fluid 'chyme', and periodic discharge of the chyme into small intestine; these functions render the mixture's chemical and physical composition appropriate for the subsequent digestive stage (Keeton et al., 2015).

The body's small intestine forms the digestive system's main organ. Its basic operation is blending and transporting intraluminal matter, producing key digestive constituents such as enzymes, and nutrient absorption. A majority of processes solubilizing fats, carbohydrates, and proteins, and reducing them into fairly simple organic matter takes place in this part of the intestines (Keeton et al., 2015).

Endocrine system

Figure 3. The Human Endocrine System (adopted from Inner body, 2015)

The human endocrine system offers a mechanism to regulate, integrate, and coordinate every organ, system and cell in the body. It is chiefly responsible for regulating growth, reproduction, maturation, and metabolism. The following glands form part of the human endocrine system: pituitary, parathyroid, thyroid, pineal, and adrenal. Furthermore, numerous organs which do not wholly make up endocrine glands comprise of cells which secrete hormones; of these, four organs are: thymus, hypothalamus, gonads, and pancreas.

The hypophysis or pituitary gland comprises of the posterior and anterior lobes. The GH (growth hormone), somatotropin, or hGH (human growth hormone), is secreted by...
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The anterior lobes of the pituitary gland release them. The GH is stimulated through the GH-releasing hormone's secretion within the hypothalamus. The GH stimulates cell reproduction and growth within body tissues and aids in amino-acid transportation to tissue cells, followed by their conversion into essential body proteins. Further, the GH aids in releasing and utilizing fatty acids out of fat or adipose tissue as energy (Binnington and Obenchain, 2013). It also assists in regulating the level of nutrients in the blood after food consumption and while fasting. Lastly, it triggers the production of a growth factor similar to insulin within the liver. This growth factor's function is inhibition of additional GH release after sufficient quantities of thyroid-stimulating hormone and GH are secreted.
A second key endocrine gland is the adrenal gland. Its medulla produces two catechol amines: norepinephrine (NE) and epinephrine (E). The former is chiefly produced by the sympathetic nervous system's adrenergi neurons whereas the latter constitutes the chief secreted amine of the medulla (80%). Both are bound to alpha- and beta- adrenergic receptors. They alter cell activity (for instance, mobilization, quicker heart rate, adipose tissue fatty acids) by means of second messengers. Estrogen, andogren (or sex steroids), aldosterone, and cortisol are generated by the adrenal gland cortex. Aldosterone helps regulate the body's potassium and sodium balance. Rigorous exercise boosts aldosterone production. Cortisol reacts to numerous stressors such as exercise, ensuring the availability of fuel (i.e., free fatty acids and glucose) and amino acids to aid the tissue repair process (Crosta, 2015).

Pancreas makes up the third main endocrine gland. Its endocrine part (also called islets of Langerhans) comprises of numerous cells which generate hormones that flow directly into our bloodstream. One beta-cell-generated hormone is insulin, whose production occurs as a reaction to blood sugar increases. Insulin is also responsible for moving glucose into tissues and muscles from the bloodstream, for consumption as energy. Additionally, insulin aids the liver in absorbing glucose, which it stores in the form of glycogen to be utilized by the body in the course of exercise or stress, when it requires extra energy (Crosta, 2015). Another pancreas-generated hormone is glucagon, which originates from alpha cells in the pancreas when the body's blood sugar lowers. It is chiefly in charge of breaking down glycogen into glucose within the liver. Subsequently, this glucose goes into the bloodstream, restoring blood glucose levels to normal.

The respiratory system

Figure 4. The Human Respiratory System (adopted from Canadian Lung Association, 2015)

The respiratory system constitutes a set of organs whose function is breathing oxygen into the body and breathing carbon dioxide out. The respiratory system's main organs are the lungs, responsible for the aforementioned gas exchange in the course of the breathing process.

While breathing, oxygen comes into the mouth/nose, passes through the sinuses (hollows within the skull which are responsible for regulating the humidity and temperature of incoming air), and gets filtered by the windpipe or trachea (as maintained by the ALA (American Lung Association). The windpipe divides into two bronchi or tubes which transport air into the lungs. The bronchi contain a lining of small hairs, known as cilia, whose to-and-fro movement helps take the sticky mucus upwards and out. The mucus's function is collecting germs, dust, and other foreign harmful particles away from the lungs. It is expelled from the body while sneezing, spitting, swallowing or coughing (Zimmermann, 2016).

The bronchi give way to the two lungs' lobes. The ALA claims the lung on the left consists of 2 lobes while that on the right has three. The NHLBI (National Heart, Lung and Blood Institute) states that, for according the heart adequate space, of the two lungs, the left is relatively smaller. Lobes contain the alveoli -- tiny, spongy sacs that mark the site of carbon dioxide and oxygen exchange within the body. The walls of the alveoli are very thin (roughly 0.2 micrometers) and constitute a single tissue layer known as epithelial cells as well as miniature blood vessels known as pulmonary capillaries (Zimmermann, 2016).

The lungs are surrounded by a group of respiratory muscles capable of causing air inhalation or exhalation from the lungs. The diaphragm constitutes the chief respiratory muscle within the body. When this thin skeletal muscle sheet which makes up the thorax's bottom undergoes contraction, it moves to a very small degree (only some few inches) towards the abdominal cavity. The area in the thorax's cavity expands, enabling air to get pulled into the pair of lungs. Diaphragm relaxation…