The Fantastic Voyage Part II

Cynthia Gomez

Hello everyone, we are back for another exciting adventure, this one a little more appetizing than the last! Today’s Fantastic Voyage will follow the path of one tasty hamburger, some fatty french fries, and a cold and sweet root beer through a fifty-five year old man’s digestive, circulatory and urinary systems. I will be narrating all structures and functions as we explore; some imagery may be a little graphic, so hold on tight if you have a weak stomach!

Here we commence our journey straight into the mouth of our hungry friend, making our descent into the first part of this nine meter alimentary canal. Make sure to hold on tight, for this hollow chamber lined with mucous membranes like the rest of the digestive tract is where mechanical digestion takes place almost instantly. The teeth begin to chew the bites of food, breaking them down into smaller pieces while the salivary glands begin to secrete enzymes called salivary amylase to begin the chemical process of breaking down carbohydrates (Cleveland Clinic, 2014). Now that the carbohydrates from the hamburger, fries and root beer have begun to be digested, they have taken the shape of a moist bolus allowing it to pass through the rest of the tract with less friction. Now we are about to enter the pharynx, a tube-like structure made of muscle behind the nasal cavities and mouth. Together with the bolus, we quickly cross the upper esophageal sphincter into the esophagus. Notice the muscular, mucus lined walls of this twenty-five centimeter that connects the pharynx with the stomach (“The Human Digestion,” 2014). Quickly dodging the lower esophageal sphincter we approach the stomach. This strong walled and muscular organ is located behind the ribs, on the left side of the body. It is separated in three main parts, the fundus, the body, and the pylorus. You can feel the strong contraction of the walls as the stomach continues to break down the heavy meal while the glands begin to release a highly acidic concoction of enzymes, hydrochloric acid and mucus to digest proteins and fats into their respective building blocks, amino acids and fatty acids (Cleveland Clinic, 2014). This creates a soupy consistency called chime, which once partial digestion is complete, the smooth muscle fibers of the pyloric sphincter relax and allow us to continue to the first part of the small intestine.

We are now entering the duodenum, which continues the digestion process. The mucus lining of the intestines contains thousands of microscopic glands that secrete intestinal digestive juices such as amylase, bile, and a combination of intestinal and pancreatic enzymes (“Digestive System,” 2014). The amylase is secreted from the neighboring pancreas and it’s alkalinity helps further digest carbohydrates. The bile you see around us comes from the liver and gallbladder and it works to emulsify the fat and allow it to mix with water (“The Human Digestion,” 2014). The pancreatic and intestinal enzymes are working hard to finish breaking down proteins into amino acids. Get comfy and have a seat-we will be here for a while considering the small intestines measure approximately seven meters (Thibodeau & Patton, 2008). Now that the nutrients have been reduced to their basic building blocks, we continue down the jejunum and ileum, where more nutrient absorption takes place. If you look very closely at the lining of the walls around us, you will notice it is not really as smooth at it looks. The walls are sorted into many tiny circular folds called plicae, which then each of these folds can be further inspected to find millions of villi, tiny hair-like fingers filled with a complex net of capillaries (“Digestive System,” 2014). This complex structure allows for an increased surface area where the capillaries aid in the absorption of digested nutrients. Now that we are at the distal ileum, we will be crossing the mucosal membrane along with all of the nutrients and here we find our way into the superior mesenteric vein which as you would have guessed, drains blood from the small intestine.

The superior mesenteric vein lies just behind the pancreas and combines with the splenic vein to form the hepatic portal vein which is how we will be entering the liver. The liver takes up the entire upper right quadrant of the abdominal cavity, well into the left side. It is the largest exocrine gland, meaning the cells in the liver secrete bile into ducts that drain outside of the liver (Thibodeau & Patton, 2008). As we continue our venous pathway to the heart, we are now entering the hepatic veins which drain into the inferior vena cava. Notice that now we are following the flow of hepatic portal circulation, which means before returning to the heart we will have to travel through a second capillary bed in the liver where the high concentration of glucose from the meal just digested will be removed by liver cells and stored away as glycogen. Next, we follow the inferior vena cava into the right atrium of the heart. Feel how the ventricles contract and propel us past the atrioventricular valve into the right ventricle (“Basic venous,” 2010). Don’t forget to look up and notice the chordae tendineae, the string-like structures that attach the atrioventricular valves to the walls of the heart. As both ventricles contract together again, we now gain passage through the pulmonary semilunar valve, right at the entrance of the pulmonary artery ("Anatomy of the," 2014). At this point, there is no going back as the semilunar valves allow blood to leave the right ventricle into the lungs but prevents it from flowing back. From here we gain passage into the lungs- an organ large enough to fill almost the entire chest cavity, aside for the middle space which is occupied by the heart which we will be revisiting momentarily. As part of the pulmonary circulation, we are entering a confining and tightening corridor as we make our way through pulmonary arterioles and capillaries where the exchange of gases takes place between the lungs and the bloodstream. Here you will notice the blood changing colors from a deep purplish red to a more oxygenated bright red that is more common in arteries (“Basic venous,” 2010). Now let’s follow this rich blood through the lung venules into one of the four pulmonary veins that lead us back to the left atrium of the heart. Here we cross through the mitral valve and into the left ventricle which will bring us to a stop in front of the aortic semilunar valve. As it allows blood to leave the ventricle, out we go into the ascending aorta, taking us full circle into systemic circulation (“Basic venous,” 2010). As we follow this short curve through the arch of the aorta, we pass the descending aorta, through the thoracic part of the aorta, down the abdominal aorta and through a small tunnel on the side we sneak into the left renal artery which supplies the kidneys with blood.

We enter the kidneys by traversing the glomerular membrane into Bowman’s capsule, giving us access into one of the millions of microscopic nephrons inside. The first part of the nephron is the renal corpuscle, which is made of the Bowman’s capsule we are in now and the glomerulus covered by Bowman’s capsule. The glomerulus consists of a complex network of capillaries that are delivered blood by the afferent arteriole and drained by the efferent arteriole (“Components of the,” 2014). If you take a good look, you will notice that the afferent arteriole is much larger in diameter and in length than the efferent arteriole. That is why as we trace the path of the glomerular capillaries, we will experience an increase in blood pressure. This is the body’s way of filtering out water and other solutes. Now we get to the second part of the nephron called the renal tubule. This long tube is divided into the proximal convoluted tubule, the descending Loop of Henle, then the ascending Loop of Henle into the distal convoluted tubule, and into the collecting tubules where urine is formed and drained into (“Inner Body,” 2014). This long twisting tube uses its form to achieve its function of reabsorbing water, solutes, sodium and chloride ions (Thibodeau & Patton, 2008). From here, the urine then moves through the minor and major calyx and is collected in the renal pelvis. Mucosal membranes line the ureters and pelvis, and the ureter is made up of a very thick and muscular wall that contracts to move urine along the long path into the bladder. The membrane is also covered with sensory nerve endings, which signal the walls when to contract and move the urine along (“Components of the,” 2014). As we enter the bladder, notice the elastic fibers and involuntary muscles that help the bladder expand to hold more urine or contract and expel it. An epithelial membrane lines the bladder in a way that most of the lining is loosely attached to the deeper muscle layer, forming rugae when not filled with urine and causing the lining to smooth out as it fills up and expands. As we swim down to the urethra, pay close attention to the trigone area, the only area where the lining is attached closely to the deeper muscles, giving it a smooth texture (“Inner Body,” 2014). We are approaching our destination as we get ready to fit through the narrow eight inch long urethra in this man and out of the urinary meatus, the external opening of the urethra, and the end of our amazing journey.

As we can very well see through the different journeys we have undertaken through what is the systematic structure of the human body, all of the different body systems work systematically to maintain homeostasis. They each have a different specialty and way of transporting, altering, cleansing, or balancing out nutrients, chemicals, and waste products. For example, the urinary system is probably one of the body systems most centered around homeostasis. It works to rid the body of excess wastes after digestion, assimilation and catabolism. It also works to maintain fluid and electrolyte balance. (Thibodeau & Patton, 2008). It does this by maintaining the relative consistency of body fluid volumes and the levels of many chemicals required for normal metabolic activity. It produces and excretes urine in direct response to the needs of the body to maintain stability. The circulatory system works as a transportation highway for hormones and nutrients between the heart and every organ of the body. The digestive system alters the physical and chemical composition of food so that it can be digested and absorbed by cells and later transported to the rest of the body via the circulatory system. It also works to eliminate the body of many waste products in the form of feces.

References:

Cleveland Clinic - The Structure and Function of the Digestive System. (n.d.). Cleveland Clinic. Retrieved April 30, 2014, from http://my.clevelandclinic.org/anatomy/digestive_system/hic_the_structure_and_function_of_the_digestive_system.aspx

Thibodeau, G., Patton, K. (2008). Structure and Function of the Body (13th ed.). St. Louis, MO: Mosby Elsevier Publishing.

Rinzler, C., & DeVault, K. (n.d.). The Human Digestion Process (or, What Happens after You Eat Food). - For Dummies. Retrieved April 30, 2014, from http://www.dummies.com/how-to/content/the-human-digestion-process.html

Urinary System. (n.d.). InnerBody. Retrieved April 30, 2014, from http://www.innerbody.com/image/urinov.html

Components of the Human Urinary System. (n.d.). . Retrieved January 1, 2014, from http://www.ivy-rose.co.uk/HumanBody/Urinary/Urinary_System.php

Digestive System. (n.d.). Digestive System. Retrieved April 30, 2014, from http://www.saddleback.edu/faculty/charrison/digestive.html

U.S. Department of Health & Human Services, National Heart Lung and Blood Institute. (2014). Anatomy of the heart. Retrieved from website: http://www.nhlbi.nih.gov/health//dci/Diseases/hhw/hhw_anatomy.html

Basic venous anatomy. (2012). Retrieved from http://vascular-web.com/asp/samples/sample104.asp