The respiratory system works hand in hand with the cardiovascular system to ensure that our cells receive the necessary oxygen to carry out aerobic respiration and keep our bodies alive and functioning.   The respiratory system and the cardiovascular system depend on each other to fulfill their duties and neither one can function without the other system.  Should one of these systems cease to be functional then the other system will follow soon after because our body will no longer be able to sustain life.  The respiratory system is made up of two functional portions, the conducting portion and the respiratory portion.  The conduction portion transports air, breaching the gap between the oxygen in the atmosphere and the oxygen in our bodies.  This includes the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles.  The respiratory portion is responsible for carrying out gas exchange and this is where the respiratory system begins to integrate with the cardiovascular system.  The respiratory portion includes respiratory bronchioles, alveolar ducts, and alveoli.The respiratory system can also be divided into two separate structural classifications, the upper respiratory tract and the lower respiratory tract. The upper respiratory tract begins with the nose where oxygen enters and continues on through the nasal cavity, paranasal sinuses, and the pharynx; all of which are part of the conducting portion of the respiratory system.  The lower respiratory tract begins with structures associated with the conduction portion of the system, the larynx, trachea, bronchi, and bronchioles.  This is where the respiratory portion begins with the respiratory bronchioles, alveolar ducts, and alveoli.  Gas exchange, when O2 crosses from the respiratory system and into the cardiovascular system, takes place in the alveoli.To get oxygen from the atmosphere around us to our cells involves three sequential steps: Ventilation (the movement of air into and out of the lungs), gas exchange (occurs between the oxygen and your blood in the lungs and then between your oxygenated blood and your body cells in the your tissue), and the final step is cellular respiration, when the cells use the oxygen delivered to them to keep your body running as it should.  During normal inspiration contraction of the parasternal, external intercostal, and diaphragm (via phrenic nerve inervation) muscles increase the volume of the thoracic cavity.  Boyle’s Law states that the pressure of a given quantity of a gas is inversely proportional to its volume, like so many types of concentrations in the body, oxygen flows from an area of greater pressure to lower pressure.  Inhalation of air is an active process utilizing muscles of inspiration to bring about a pressure difference, by increasing the volume of the thoracic cavity containing the lungs a pressure difference is created thereby  inducing oxygen to flow from an area of greater concentration (the air around us) and into our lungs (through all the upper and lower respiratory structures).  With each inhalation the surrounding air moves through the upper respiratory tract to the lower respiratory tract and into the alveoli where gas exchange takes place.  The alveoli are surrounded by capillaries filled with venous blood from the heart by way of the pulmonary artery, low in oxygen concentration and high in carbon dioxide concentration.  Once again utilizing the advantages of a pressure gradient the body carries out the transfer of oxygen and carbon dioxide by flowing from the areas of high concentration to the areas of low concentration.  The freshly inhaled air in the alveoli is high in oxygen and low in carbon dioxide so the oxygen easily crosses the thin epithelium of the alveoli and into the capillaries while the carbon dioxide easily flows from the capillaries and into the alveoli.  The now oxygen rich blood in the capillaries surrounding the alveoli will re-enter the cardiovascular system entering the left atrium of the heart by way of the pulmonary veins.  This oxygen rich blood will now be pumped from the right atrium to the left atrium of the heart and then distributed throughout the body where cellular respiration occurs in all the cells that our body is made from.The carbon dioxide that was transferred to the alveoli will be exhaled passively as the inspiratory muscles relax.  The relaxation of the inspiratory muscles causes the thoracic cavity volume to decrease resulting in an increase in pressure.  This pressure increase drives the “used up air”, full of carbon dioxide, out of the lungs because a pressure gradient has once again been created only this time the pressure was greater inside the thoracic cavity then the surrounding atmospheric pressure.  This air will now move back up the lower respiratory tract to the upper respiratory tract and out into the surrounding atmospheric air.The functionality of the respiratory system is crucial to maintaining homeostasis  and therefore the life of our bodies  There are so many variables involved in maintaining this and even a slight fluctuation in one of these numerous variables can set the entire respiratory system awry causing shortness of breath and a hypoxic state in our bodies.  The sensation of shortness of breath is due to the metabolic demands of our bodies not being met; there is not enough oxygen in circulation for sufficient cellular respiration.  The reason for the shortage of oxygen at the cellular level can be the result inadequate gas exchange between the alveoli and the capillaries due to poor perfusion (an inadequate amount of blood traveling through the pulmonary capillaries possibly relating to a cardiovascular issue) or do to a disturbance in ventilation.  A disturbance in ventilation would be due to an inadequate amount of oxygen crossing the alveolar membrane to the capillaries and can be a result of inadequate oxygen reaching the alveoli from the air we breathe.  Considering the number of structures involved in getting atmospheric air down to the alveoli the malfunction can occur in numerous possible locations.   Conditions such as asthma, pneumonia, and congestive heart failure stimulate receptors in the lungs causing shortness of breath.  Airway obstructions, chest muscle weakness, neuromuscular diseases, and lung collapse all cause mechanical disruption of the airway, lung, or chest wall.  The respiratory system is a very complex and vital element of our anatomical make-up and proper balance and functionality of our respiratory system is crucial to our health.