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Function of the Cardiovascular, Respiratory and Gastrointestinal Systems

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Cardiovascular, Respiratory and Gastro- Intestinal System

Function of the Cardiovascular, Respiratory and Gastrointestinal Systems to Provide Oxygen and Glucose required for the Normal Function of Cells.

The human body is made up of numerous systems that aid in its function. These systems constitute specific organs that are responsible for their roles in the body. The three major systems that are common in the body include cardiovascular, the digestive and the respiratory systems. There exists an interrelationship between the cardiovascular, respiratory and gastrointestinal system to enable effective and efficient functioning of the body. The cardiovascular and the respiratory systems function together through performing gas exchange that entails passing oxygen from the alveoli into the bloodstream with carbon dioxide expelled out of the body. If the exchange of gases fails to take place, then the cells will automatically die. The gastrointestinal (GIT) system provides the cardiovascular and respiratory systems with the nutrients and minerals. The GIT takes nutrients and minerals from food eaten, breaks them down through digestion into a form utilized in the body. The cardiovascular, the digestive and the respiratory systems need to coordinate well to aid the body in carrying out its functions effectively, by fulfilling specific functions.

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The cardiovascular system primary function is pumping of the blood throughout the body. It constitutes the heart, which pumps the blood, pushing it through the vessels. The blood vessels supply the blood to different sections of the body before it passes again through the heart to undergo the same cycle. Blood transport O2 from the lungs to other parts of the body. It is responsible for the transportation of the waste products including CO2 eliminated from the different capabilities of relaxing and contracting, allowing the heart to pump blood. The valves allow the blood to flow in one direction, preventing backward flowage (Lin et al. 2012). Heartbeat is being controlled so that each chamber contracts only when there is full of blood. To attain this, the event of the cardiac cycle are being carefully co-ordinated in the contraction and relaxation of the atria and ventricles corresponding to pressure and volume changes in the heart and blood vessels. (figure 1). As the heart beats, it circulates blood through the pulmonary and systemic circuits of the body. There are two phases of the cardiac cycle called systole and diastole. In the diastole, the heart ventricles are relaxed and the heart is filled with blood. During atrial systole, the ventricles contract and the blood is pumped out of the heart to the arteries. Once the cardiac cycle is completed when the heart ers are filled with blood and then it is pumped out of the heart to the body.

There are three major blood vessels including arteries, capillaries, and veins. (Table.1). The arteries transport blood rich in oxygen from the heart to other parts of the body. The capillaries are responsible for the exchange of gases, nutrients, waste products and water to and from the bloodstream. Veins carry blood with low O2 back to the heart.

It also defends the body against infection, repairs body tissues, transports hormones, and controls the body’s pH. Blood is made of two main components; the plasma and the cells. The plasma is a watery pale-yellow liquid and it contains many dissolved substances like glucose and blood cells. The cellular component comprises the red blood cells, white blood cells and platelets. The red blood cells contain haemoglobin, a molecule which has a high affinity for oxygen (Anupam, 2012). Oxygen from inhaled air binds to the haemoglobin to form oxyhaemoglobin, which is transported to the tissues. The cardiovascular system, therefore, provides the cells with the substrate (glucose) and oxygen to enable them to produce ATP needed for their normal functioning(6).

Table.1. The structure of the arteries, veins and capillaries

There are three main types of blood vessels namely: arteries, veins, and capillaries. Arteriesare muscular in nature and they carry blood away from the heart. They have thick walls to enable them to withstand the pressure of blood being pumped by the heart. All arteries except the pulmonary artery carry oxygenated blood. Veinsare blood vessels that carry blood toward the heart. All veins except the pulmonary vein carry deoxygenated blood. Capillariesare the smallest type of blood vessels and have no valve. They connect very small arteries and veins. The exchange of gases and other substances between cells and the

blood takes place across the extremely thin walls of capillaries. Bloodis a fluid connective tissue. It circulates throughout the body through blood vessels by the pumping action of the heart. Blood in arteries carries oxygen and nutrients to all the body cells. Blood in veins carries carbon dioxide and other wastes away from the cells to be excreted.

Table (1) The structure of arteries, veins and capillaries (Biology-forums, 2019)  

Figure 1

Figure 1: Structure of the heart



Table 2

The function of the heart is to oxygenate blood and pump it to all parts of the body from the left ventricle via the aorta.

                                            The structure of the heart (Centralgaheart, 2019)

Superior vena cava: This receives deoxygenated blood from upper extremities of the body and drains them into the right atrium.

Pulmonary artery: A hollow muscular tube that transports deoxygenated blood from the right ventricle to the lungs where oxygen is added to blood.

Pulmonary vein: Transports oxygenated blood from the lungs to the left atrium of the heart.

Inferior vena cava: This brings deoxygenated blood from lower extremities of the body and drains them into the right atrium.

Aorta: This is the biggest artery in the body. It sends blood into systemic circulation from the left ventricles to the entire body.

Tricuspid valve: This is the valve that closes and opens to allow blood flow from the right atrium to the right ventricle.

Bicuspid valve: (Is also known as mitral valve) This valve allows blood flow from the left atrium to the left ventricle

Septum: This divides the heart into two equal halves which is the upper halves and lower halves. The septum that divides the heart into two lower portions is the inter-ventricular septum.

Atria: These are the upper chambers of the heart. There are two atria in the heart, which are right atrium and the left atrium. The right atrium contracts and sends deoxygenated blood to the right ventricle to enter pulmonary circulation via the pulmonary vein. The left atrium contracts to send oxygenated blood to the left ventricle to start systemic circulation via the aorta.

Ventricles: These are the lower chambers of the heart. The left ventricle contracts to send deoxygenated blood into the pulmonary artery for pulmonary circulation. Left ventricle on the other hand contracts to send oxygenated blood received from left atrium into the aorta to initiate systemic circulation.

Apex cordis: This is the caudal/ tail part of the heart found at the region of the 5th intercostal space to the left side of the thoracic cavity.

Descending aorta:

Apex: This is the caudal/tail part of the heart found at the region of the 5th intercostal space to the left side of the thoracic cavity.

The Respiratory System

The primary function of the respiratory system entails obtaining oxygen from the atmosphere, supplying it to the cell, getting rid of CO2 in the body, produced from cell metabolism. The respiratory system constitutes of the lungs, airways, a section of the central nervous system aiding in controlling the respiration muscles, and the chest wall. The chest walls have respiration muscles including the diaphragm, the abdominal, and intercostal muscles, and the rib cage. The respiratory system functions entail gas exchange, pulmonary protection and metabolism, and phonation. Breathing occurs through inhalation and exhalation. Inhalation involves air getting into the lungs through chest volume expansion. Conversely, exhalation lungs expelling air into the lungs.

Table 2

                      (The Respiratory System, 2019)

 During respiration, air enters the respiratory system through the nose. As the air passes through the nasal cavity, mucus and hairs trap any particles in the air. The air is also warmed and moistened so it won’t harm delicate tissues of the lungs(9). Next, the air passes through the pharynx, a long tube that is shared with the digestive system. The epiglottis closes when food is swallowed to prevent choking(10). From the pharynx, air next passes through the larynx, or voice box. After the larynx, air moves into the trachea. In the chest, the trachea divides as it enters the lungs to form the right and left bronchi. The bronchi contain cartilage, which prevents them from collapsing. Mucus in the bronchi traps any remaining particles in the air. Cilia line the bronchi and sweep the particles and mucus toward the throat so they can be expelled from the body. Finally, air passes from the bronchi into the bronchioles. The bronchioles end in tiny air sacs called alveoli. 

The Gastro-intestinal system (GIT) majorly helps with the digestion of food. It is through food that the body gets the energy to fuel its activities and enable the other systems top function effectively. The GIT begins from the mouth to the anus. It made up of numerous parts, which aid in its function. The mouth forms the entry point of the GIT, which allows for the mechanical breakdown of food. In the mouth, the food is chewed and crushed by the teeth, which are four types, the incisors, and canines for cutting and biting, and the premolar and molar for chewing. Also, there is the tongue, which secrets the salivary glands, capable of mixing the food and making it ready for digestion. Food leaves the mouth and passes through the oesophagus moving down to the small intestines and the stomach. The system constitutes of the liver, gallbladder, pancreases, with each producing specific enzymes that assist in the digestion of particular components of food.

Table 3


                                               The digestive system






This is the upper/superior opening of the digestive system. It contains the teeth, tongue and salivary glands.

In the mouth, entry of food, chewing and swallowing takes place; partial digestion of carbohydrates and starch food occur here by secretion of salivary amylase.

Salivary glands

Are located inside the mouth and are

They secrete an enzyme called salivary amylase that breaks down starch into maltose.


This is a muscular tube about 25cm long; that connects the mouth to the stomach.

Transport chewed food from the mouth to the stomach.


A sac like organ; muscular and hollow.

Stomach holds food and mixes it. It also secrete gastric acids (hydrochloric acids) and some enzymes in the process of breaking down food.


This is the largest gland in the body and weighs about 1.2kg in an adult. Consist of 4 lobes and is wedge shaped.

Stores the bile that is secreted by the liver, and releases it into the small intestines for fat digestion.


Is an organ that lies in the abdominal cavity, about 15 cm and weighs approximately 80g.

Produces enzymes which helps in the breakdown of protein foods, fatty foods and carbohydrate.

Oxygen transportation in the body is carried out by the blood. However, since oxygen is partially soluble in liquid, only 1.5% of the total amount dissolves in the blood and transported (Pittman, 2011). Most of the oxygen molecules are transported from the lungs to the tissues of the body by a specialized transport system that depends on the erythrocyte or the red blood cells (RBCs).  Erythrocytes constitute of a metalloprotein, haemoglobin, which binds the molecules of oxygen to them (Figure 3). Heme is a part of haemoglobin, which has iron, and it is often the one that binds with the oxygen. A single haemoglobin molecule has constitutes of heme with iron molecules, with each haemoglobin molecule able to contain a maximum of four molecules of oxygen (Pittman, 2011). Through diffusion, oxygen moves from the alveolus into the capillary through the respiratory membrane. Also, it diffuses into the RBCs and bound by the haemoglobin, forming oxy-haemoglobin (Hb-O2). Oxy-haemoglobin refers to bright red molecules that give the blood a bright red colour, signifying the presence of oxygen in the blood. The blood with oxygen leaves the lungs, transported down the body to aid in the function of the body organs.

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The transportation of Carbon dioxide (CO2) molecules occurs in the blood, transported from the body tissues to the lungs by any of the three methods including Co2 direct dissolution into the blood, haemoglobin binding, or in the form of bicarbonate ion (Arthurs and Sudhakar, 2005). Numerous characteristics of CO2 in the blood determine its transportation. CO2 is highly soluble in blood compared to oxygen, with about 5-7% of CO2 dissolving in the plasma. Also, most of the CO2 molecules, about 85%, are transported as a section of the bicarbonate buffer system. In the system, CO2 diffusion into the RBCs occurs. CO2 is swiftly converted into carbonic acid by the Carbonic anhydrase. Carbonic acid, due to its instability directly separates into bicarbonate and hydrogen ions. The fast conversion of CO2 into bicarbonate ions enable continuous CO2 uptake into the bloodstream down its concentration gradient. Also, it causes the production of hydrogen ions, which are cable of changing the pH of the blood in case of too much production (Arthurs and Sudhakar, 2005). Upon the blood reaching the lungs, there is backward transportation of the bicarbonate ion into the RBC, exchanging with a hydrogen ion, which separates for the haemoglobin and binding with bicarbonate ion. It results in the production of the carbonic acid intermediate, converted back into CO2 through carbonic anhydrase enzymatic action. The CO2 produced expels out of the lungs through the exhalation process. The buffer system is beneficial in that CO2 is getting soaked up into the blood with minimal change in the system’s pH. It is significant since it only a little change in the body’s pH is capable of causing severe injury or death. Also, the existence of the buffer system enables individual to live within high altitudes. The bicarbonate system is cable of adjusting to allow CO2 regulation while maintaining the appropriate PH of the body when there are partial changes in the O2 and CO2 pressures.


Figure 3: Transportation of CO2 and O2 through the blood. Source: https://www.ptdirect.com/training-design/anatomy-and-physiology/respiratory-gas-transport

The exchange of gas in the lungs is the function of the respiratory system. Through the pulmonary artery, deoxygenated blood goes from the heart into the lungs, where it branches becoming capillary network constitution of pulmonary capillaries. The pulmonary capillaries develop the respiratory membrane with alveoli. Figure 3; The layers of the cell underlining the alveoli and the surrounding capillaries are all one cell thick and in close contact with each other. Gas exchange occurs as the blood pumping occurs through the capillary network. Oxygen passes through the air-blood barrier quickly into the blood found in capillaries. In the same way, CO2 passes into the alveoli from the bold and exhaled. However, some of the CO2 returns on haemoglobin, and is dissolvable in plasma or exist in converted form. The process of gas exchange in the lungs takes place through three processes, including perfusion, ventilation, and diffusion (Wagner, 2015). Ventilation transfers oxygen from the atmosphere into the alveoli, and concurrently get rid of the CO2, transported from the blood into the air. Diffusion refers to the process through which O2 within the alveoli passes into the pulmonary capillary across the alveolar wall. Perfusion, on the other hand, transports the blood by pulmonary circulation, allowing for the intake of O2 by the constant flowing RBCs.

Ventilation and perfusions are the two essential aspects of gas exchange within the lungs. The volumes of the involved perfusion and ventilation need to be well-matched to allow for efficient transfer of gases. However, chances of imbalance could be possible due to factors such as impacts of regional gravity on blood, infections or blocked alveolar ducts (Johansson, 2014). Ventilation is an irregular, bidirectional inspiration and expiration processes, while perfusion is unidirectional, moving to the right atrium from the left ventricle. Oxygen enters alveoli at a faster rate when ventilation is enough, with the partial pressure within the alveoli remaining high. Conversely, the partial pressure of the alveoli decreases when the ventilation becomes sufficient. Diffusion of the oxygen does not occur effectively if there is no significant difference in partial pressure, expected between the blood and the alveoli. (figure  Inspiration is supported by respiratory muscles mostly including the diaphragm and exterior intercostal muscles. The muscles cause the relaxation of the thoracic cage, resulting in the minimization of the intrapleural pressure covering the lungs, causing passive expansion of the lungs. Expiration is commonly passive and takes place upon the relaxation of the elastic muscles and enables the lung to expel air by causing it to recoil elastically. Diffusion does not need energy expedition by the organism since it is passive. It only reflects the random motion of molecules, which happens to equalize the concentration of molecules in space over time.

Figure 3: Gaseous exchange in the alveoli

The production and secretion of enzymes occur in almost all parts forming the digestive system including the stomach, pancreas, intestinal mucosa, salivary glands, and stomach

The final stage of digestion always occurs within the villi of the enterocytes. The enzymes are mostly hydrolases. The digestion began in the mouth, with carbohydrates, through salivary amylase and continued in the small intestines with pancreatic amylase.  Ptyalin, a type of amylase, is the only significant enzyme found in saliva. The parotid glands produce it. It begins the digestion of carbohydrates including starch from plants and muscle glycogen (Dane and Hänninen, 2009).  Salivary amylase hydrolyzes starch into the disaccharides maltose and other small glucose polymers such as maltotriose. However, oral digestion is partially significant since ptyalin action food within a shorter duration.

The primary enzymes involved in the digestion of the fat include the gastric, intestinal lipase, lingual, and pancreatic. However, the fundamental process of fat digestion takes place in the small intestines through pancreatic lipase and the bile acids. Lipase converts lipids into free fatty acids within the stomach and duodenum, ready for utilization across the gut boundary into the lacteal system. Also, lipase enzymes secret phospholipases, significant in breaking down phospholipids including phosphatidylcholine. Lingual lipase is secreted by the serous lingual glands found within the dorsal section of the tongue. According to Wang et al. (2013), about 30% of the triglyceride found in the diet is digested within the stomach through the action of a combination of gastric and lingual lipase, with the production of fatty acids. Lingual lipase seems to have minimal effect on the digestion of lipids. Bile aids in fat emulsification to enable it to bind to the lipase enzymes.

Protein digestion starts in the stomach with the help of pepsins, which is the active form of pepsinogen, secreted from the primary cells of the gastric glands. Its digestion continues with the help of proteolytic enzymes secreted by the pancreas including chymotrypsin, carboxypeptidase, and trypsin, secreted in inactive forms. Proteases found in the pancreatic juices aid in the breaking down of peptide bonds into smaller peptide units and specific amino acids. The end products for protein digestion include amino acids produced by the mucosal and intestinal dipeptidase. Proteases helping in amino acids breakdown from the ends of polypeptide protein chains are known as exopeptidases. However, the ones that split internal bonds in the peptides chain are called endopeptidase.

Furthermore, through the CVS, the cells get of the wastes resulting from cell metabolism especially in the form of CO2. On the other hand, the respiratory system brings in the fresh oxygen into the body for cells to uses, as it gets rid of the excess CO2, which if high may result to the death of most cells. The GIT is the source of energy for the cells. It converts food into a form that can be utilized by the cells, thus supplying the cells with the energy to carry out their functions. Also, it is through food that the dead cells replacement with the new ones occurs. A combination of the three systems is significant for the cells found in the body.

In conclusion, the cardiovascular, respiratory and gastrointestinal systems work in synergy to provide the cells with energy for their normal function. The cardiovascular system transports oxygen provided by the respiratory system through respiration, glucose provided by the gastrointestinal system through digestions, to the cells. In the cell, through cellular respiration, the cells break down glucose to form ATP using oxygen, ATP is the energy currency of the cell, this helps the cells in the body to perform their normal function.                                                Reference list

  • Arthurs, G. and Sudhakar, M. (2005). Carbon dioxide transport. Continuing Education in Anaesthesia Critical Care & Pain, 5(6), pp.207-210.
  • Anupam, S. (2012). Practical Pediatric Hematology. Jaypee Brothers Medical Publisher Pvt. Ltd.
  • Boyle, M and Senior, K. (2008). Human biology. 3th edition. London: Collins.
  • Centralgaheart.com. (2019). anatomy-of-the-heart – Central Georgia Heart Center. [online] Available at: http://centralgaheart.com/the-anatomy-of-your-heart/anatomy-of-the-heart-2/ [Accessed 1 May 2019].
  • Dane, S. and Hänninen, O., 2009. ENZYMES OF DIGESTION. Physiology and Maintenance-              Volume II: Enzymes: The Biological Catalysts of Life, Nutrition and Digestion, p.45.
  • Johansson, M.J., 2014. Gas Exchange in the Normal Lung: Experimental studies on the effects of  positive end-expiratory pressure and body position (Doctoral dissertation, Linköping University Electronic Press).
  • Lin, C.J., Lin, C.Y., Chen, C.H., Zhou, B. and Chang, C.P., 2012. Partitioning the heart:               mechanisms of cardiac septation and valve development. Development, 139(18), pp.3277-3299.
  • Pittman, R.N., 2011. The circulatory system and oxygen transport. In Regulation of Tissue Oxygenation. Morgan & Claypool Life Sciences.
  • Wagner, P.D., 2015. The physiological basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases. European Respiratory Journal, 45(1), pp.227-243. 
  • Wang, T.Y., Liu, M., Portincasa, P. and Wang, D.Q.H., 2013. New insights into the molecular mechanism of intestinal fatty acid absorption. European journal of clinical investigation, 43(11), pp.1203-1223.


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