Human body is an amazing mechanism, it is incredibly strong, yet incredibly vulnerable. To understand its function today, the body’s anatomical discoveries go back to third century B.C., when Greek scientist Herophilus performed dissection of human body for the first time, (Marco Sampaolo, 2017). The first step to modern human anatomy was made in 15-th century by Andreas Vesalius, who created the book ” De humani corporis fabrica” (On the Structure of the Human Body), which was first of its kind and scripted in an accurate manner by visualising the human body internally, (famousscientis, 2017).
In consequence of science and medicine we apprehend today the body’s anatomical design and within its unique structures performing a task, allowing the entire system to function as whole to maintain homeostasis. Moreover, major role lies in cooperation of circulatory, respiratory and digestive systems, which are crucial key systems for sustaining life.
The function of the circulatory system is to circulate the blood throughout the body via blood vessels by the pumping action of the heart. The heart is a muscular organ, within its cardiac muscle operates involuntarily without conscious control. The cardiac muscle within the myocardium (tissue layer in the heart wall) consists of intercalated discs (adjacent cells) between the muscle fibers, within electrical impulses occur for rapid and systematic communication amongst cells, thus allowing contraction and pumping action. The two other tissue layers within the heart wall are endocardium, which creates a smooth surface for effortless blood flow through the heart and epicardium, the vicseral layer within pericardium. (Cohen 2015)
The cardiovascular system is divided into two parts. The right side of the heart receive deoxygenated blood from all body tissues and is pumped into the lungs via right pulmonary artery, whereas the left side receives oxygenated blood from the lungs to be pumped into all tissues in the body via aorta. The atria (upper chambers of the heart) receive the blood, whereas the ventricles (lower chambers) pump the blood by involuntary action. (D. Shier, 2001). This circulation and distribution of oxygen is the physiological connection between the systems. Moreover, the blood vessels are direct tunnels for transportation of oxygen and nutrients to the tissues, thereby are also involved in exchange of gases and wastes. Blood flow is constant, its volume is regulated by the size of the blood vessel relevant to the size and the function of each organ.
The largest group of blood vessels are the arteries, where blood transportation occurs from the heart in to the tissues. Arterioles control the blood pressure. Within thick walls consist of smooth muscle allow to adjust in diameter, depending on the amount of the blood flow. Moreover, the blood flow is also determined by metabolites in arterial walls, causing the vessels to relax and dilute. Capillaries consist of single layer of endothelial cells, allowing exchanges of gases, the blood solutes and the tissue metabolic products to pass between the blood and the cells by diffusion action. Veins are flexible blood vessels and facilitate the changes in volume due to changes in the circulation (haemorrhage, transfusion) or in blood distribution. Blood flows is one-way transport system, within the valves in the veins prevent the back flow of the blood. (R. Hainsworth, 2001).

The cells within organs in the human body require oxygen in order to function effectively. Absence of oxygen for short period of time would have fatal consequences resulting in anoxia and death. Anoxia could be caused either internally, where heart or blood vessels fail to transport oxygen to the brain and externally as a result of lack of oxygen or by inhalation of environmental toxins. (T. Jewell, 2017).
Oxygen is transported to all body tissues by respiration, when fresh molecules are inhaled and carbon dioxide exhaled. The respiratory system begins with the nasal cavity, within nasal mucosa warms and moistens inhaled air, also traps particles and pathogens alongside with nasal hair. Behind nasal cavity locates pharynx, which connects with the larynx or the voice box. The rigid larynx consists of hyaline cartilage and vocal cords (folds of mucus membrane) varying in length, tension and distance in between, that regulates the pitch of sound. Moreover, epiglottis, the cartilage that covers the larynx during deglutition aids to keep food and liquids away from respiratory tract by moving downward as the larynx moves upward and forward. The trachea known as windpipe divides into bronchi, which enter the lungs at hilum, the connection point of blood vessels and nerves, making up the root of the lung.
The lungs are positioned in thoracic cavity with the heart in between and are subdivided into lobes alongside with the tubes of bronchi. The bronchi subdivides into lobar bronchi, which subdivides into segmental bronchi. Bronchioles are the smallest tubes within the bronchial tree, which end with the alveoli. The alveoli are tiny air sacs, within thin walls are made up from squamous epithelium, thus allowing external gas exchange via blood circulation between capillaries. Gases diffuse from higher concentration area to lower concentration area, based on cellular oxygen requirements and carbon dioxide production. Same process applies equally for internal gas exchange between blood and the tissues. (B.J Cohen 2000).
Gas transport and exchange in the blood is the physiological connection between all systems, where haemoglobin protein in red blood cells binds four oxygen molecules forming oxyhaemaglobin, which is released in each cell within tissues. Carbon dioxide diffuses from the cells into the blood, enters the red blood cells, within it reacts with water forming hydrogen ions and hydrogen carbonate ions (reversible process). Moreover, the hydrogen carbonate ions move to the plasma and are carried to the lungs, were the chemical change of the hydrogen carbonate process is reversed, thus releasing carbon dioxide for diffusion into the alveoli, within it is finally exhaled. Hydrogen ions act as a buffer in the blood by maintaining normal pH within range of 7.4. (Royal Society of Chemistry, 2004)
Furthermore, the cells use oxygen for cellular respiration to catabolise glycose, fatty acids (nutrient oxidation) to produce ATP for storage, equally release energy with some being released as heat to maintain body temperature. (A. B. Hopkin
Negative feedback keeps body temperature constant within the body, homeostasis maintains the balance of normal temperature regardless to internal and external changes. (B.J Cohen 2000)

The digestive system is constructed on functions of digestion, absorption and elimination, within the cells use nutrients for metabolism, growth and repair purposes. The control over digestive system evolves the enteric nervous system, that receives sensory stimuli and information, furthermore it evolves hormones produced by digestive organs. The digestive system begins with oral cavity, within the teeth and the muscular tongue mechanically brake food into small portions by mastication. Watery saliva moistens food for easy deglutition (involuntary reflex action), equally initiates carbohydrate digestion with the aid of salivary amylase enzyme. The bolus of food is pushed by the tongue into the pharynx, where the soft palate and uvula are raised to prevent food entering the nasal cavity. The food bolus then travels into the oesophagus, within it lubricates with mucus and moved into the stomach through esophageal hiatus (an opening in the diaphragm) by peristalsis action.
Some cells in the stomach’s lining secrete mucus for protection of its lining, other cells produce hydrochloric acid and pepsin (gastric juices) for chemical brake down of proteins. The food, mucus and gastric juices are mixed to form semi liquid mass called churn, which is discharged through pyloric sphincter into the small intestine. The small intestine is physiological contact to systems due to its digestion function of macromolecules (carbohydrates, lipids, proteins). The pancreas produces enzymes that digest lipids, carbohydrates, proteins and nucleic acid for transportation to duodenum via hepatopancreatic sphincter. The small intestine is composed of duodenum, jejunum and ileum, within most digestion occurring in duodenum. Inactive enzymes produced in pancreas activate in small intestine, within each specific enzyme act on certain type of nutrient molecule. (B.J Cohen 2000). Same principle applies to hydrolysis reaction for splitting molecules, when each reaction with specific enzyme requires one molecule of water. (T. Phillips, 2018). After digestion of macromolecules, fatty acids, monosaccharides, amino acids and nucleic acids are absorbed into the blood through villi, within arterioles and venules tunnelled with capillaries. The liver receives these nutrients, processes, stores or distributes as needed. Within liver the bile is manufactured, blood plasma proteins are formed, a waste product urea is synthesised and harmful substances (alcohol, drugs) are detoxified. Moreover, the glycose in the form of glycogen is stored within the liver, thus it facilitates the function of controlling the blood glycose levels.
Undigested food is transported into the large intestine, within the colon absorbs water and minerals as the final part of the gastrointestinal tract. Various bacteria within assist with digestion and synthesis of folic acid and nutrients. (C. Ashby). Moreover, the bacteria ferments undigested polysaccharides, which results in gas (nitrogen and carbon dioxide) release. The mucosal layer within the large intestine secures the organ from bacterial infections and secretes bicarbonates to balance the pH. (A. Mandal, 2018). The stool formed after digestion accumulates in rectum, the organ’s muscular wall enables to hold feces temporarily until localised nerves signalling brain for bowel movement to eliminate the stool through anus by peristalsis action. (N. Steely, 2017)