When the food is ingested, it passes from the oral cavity through the esophagus to the stomach, small and than large intestine. The process of digestion starts in the mouth where the food is mechanically and chemically digested. We use our teeth to smash food into smaller parts, while saliva contains enzymes that can dissolve carbs. In the stomach and small intestine, the chemical digestion continuous while the epithelium of the small intestine is the place where all of the crucial molecules are absorbed. The large intestine has the function in water absorption and the elimination of waste.

Metabolic waste is excreted by the skin, urinary, digestive, and respiratory system. The metabolic waste is eliminated through the sweat, urine, feces, and expiration, which is the way of eliminating the carbon dioxide.

The oxygenated blood goes through the pulmonary veins to the left atrium and then the left ventricle. From the left ventricle arises aorta which branches into smaller arteries and arterioles. These small vessels bring blood through the capillary network where gas exchange happens between the erythrocytes and the tissue. Unoxygenated blood goes into the venules and veins. All veins in our organism merge into superior and inferior vena cava. These large vessels transport blood to the right atrium. From that point, blood goes to the right ventricle, pulmonary artery and lungs. In this organ, blood gets oxygenized and through the pulmonary veins goes into the left heart, which closes the circle.

The blood comes from the arterioles through the capillary network to the veins. However, some amount of plasma transfers through the capillary membrane to the tissues, which is now called the interstitial fluid. When it enters the lymphatic vessels it is called the lymph. The lymphatic system has a role in collecting this fluid and transport it to the large vein vessels where it enters the circulation.

The air is inhaled through the nose, where it is warmed and filtered. It travels through pharynx and larynx to the trachea, which is known as the windpipe. The trachea branches into two bronchi, one for each lung. They further divide into smaller tubes, until the level of bronchioles, which have the smallest diameter. These structures lead to the alveolar sacs, where the gas exchange is made.

Digestion can be divided into mechanical and chemical. The food is roughly divided into smaller parts through the process of mechanical digestion. However, in the process of dissolving fat, protein, and carbon molecules, enzyme activity is crucial. Therefore, we can conclude that without certain enzymes, food would be only partially digested and our body wouldn’t be able to absorb the nutrients.

The nitrogen is the final product of the protein metabolism. Substances that contain nitrogen are uric acid and urea, which are toxic when found in high blood concentration. Therefore, they are excreted by the urinary system. The blood is filtered in the kidneys and the excess of urea is passed from the blood vessels through the renal tubules to the ureter and bladder, where the urine is stored. When the process of miction begins, the urine is excreted through the urethra.

Skeletal muscles contribute to the blood circulation and the process of inspiration.
Blood circulates through the veins mainly opposed to the gravitation. Blood flow to the heart is accomplished by the pressure in the thorax and abdomen, muscular contractions near the veins and one-way valves that doesn’t allow the blood to go retrogradely.
Respiratory muscles must contract to decrease the pressure in the airway. We inhale air when the intrapulmonary pressure is lower than the atmospheric. Therefore, inspiration is active while expiration is the passive process.