Structure and Function of Body Systems
Section A: Digestive System
Digestion is the process of mechanical and chemical breakdown of the large food particles by mastication, into small particles that can be easily absorbed by the body, in the small intestine, for conversion into nutrients needed for the well-being and well-functioning of the body. (Fox, 2015,50) This process is made possible by the digestive system, which comprises majorly of six parts, namely, the mouth, Pharynx, Esophagus, Stomach, Small intestines, and Large intestines. All of these organs end up forming a long continuous tube, that is open on both sides, known as the alimentary canal or the gastrointestinal tract, and are located majorly in the abdominopelvic cavity that is cushioned by the serous membrane, that holds the organs intact and in place.
The Gastrointestinal Tract (GT) has four layers that include serosa, muscularis, submucosa and mucosa, from the outermost layer to the innermost layer respectively. All these four layers are specialized differently, with some having well developed muscular layers, i.e. the serosa layer is highly fibrous others have modified linings for absorption, others have well developed mucous linings for the digestive juices secretion. The mouth, the first organ of the digestive system, also known as the buccal or oral cavity, is majorly for ingestion of the food particles to be masticated by the teeth. It comprises of bordering hard and soft palates above it, that distinguishes it from the nasal passages, and a uvula that blocks the nasal passages when there is swallowing taking place. The opening into the throat from the mouth is termed as fauces.
The tongue, a skeletal muscle, is also an accessory in the mouth, that lines the hind border of the buccal cavity. It is covered by a mucoid membrane that has rough and raised projections called papillae, and taste buds. (Silverthorn,2010,409) The tongue also has a thin fold of mucous membrane on its front surface that aids it stabilize at the base of the mouth. In addition, the mouth has three pairs of salivary glands; parotid, submandibular and sublingual. Parotid glands are the largest of all them, and are located just below the ears. The salivary glands secrete saliva, that has salivary amylase, an enzyme essential for the breakdown of carbohydrates and mucous that softens the masticated food to make boluses, for easy swallowing. The Pharynx is located at the throat, and on swallowing, the uvula closes the airway passages at the Pharynx to prevent choking.
The esophagus is a long tube, approximately 10 inches long, that connects the Pharynx to the stomach. It is located posterior to the trachea and heart but goes through the diaphragm. It passes the food boluses from the mouth to the stomach through peristalsis. The stomach is a sac-like organ that appears to be an expansion of the gastrointestinal tract and is located below the diaphragm and the liver. It has three regions, the fundus, the body, and the pylori. The stomach has well-developed muscles, namely, circular, longitudinal, and oblique, that is developed for the process of the churning of the food boluses, resulting in a mixture called chyme. (Fox,2015,70) Other muscles, cardioesphageal sphincter, and pyloric sphincter are located at the entrance and exit of the stomach openings that close and open when letting in the food boluses and letting out the chyme. The stomach also produces hydrochloric acid through the parietal cells that maintain the stomach’s acidity, fit for the digestion of proteins by the gastric juice. There are mucosal linings around the stomach secreted by the epithelial cells, for protection against the acidic hydrochloric acid.
The small intestine, also Ileum, is the longest organ of the alimentary canal, approximately an inch in diameter, and ten inches long for the living, that doubles on death. It is held in place by the mesenteries. The duodenum, the first part, is approximately ten inches long, and is the site for most absorptions. (Silverthorn,2010,412) The jejunum, the second part, is approximately eight inches and the Ileum, the third part and last part, is twelve inches long. The small intestine also has projections called villi, that increase the surface area of its function of absorption. The Ileum connects to the large intestine, through a valve called ileocecal valve. The large intestine is approximately two and a half inches in diameter and six inches long and is a major site for water and minerals absorption. It is subdivided into three regions, the cecum, which extends from the small intestine. It collects the indigestible material, the colon, and the rectum, which holds the undigested waste until it is released via the anus.
Section B: The Renal System (Kidneys)
Kidneys are bean-shaped organs located between the peritoneum and the ventral part of the abdomen, intestines, and the liver and above the waist. The right kidney is slightly lower than the left kidney because of the liver taking space at the right kidney. A kidney is approximately ten to twelve centimeters long,5-7 cm in width, 3cm in thickness, and 135-150g in weight. (Derrickson,2006,620) They are organs of the renal or urinary system, with functions of excretion of body wastes through the urine, hormonal regulation, hormone calcitriol and erythropoietin, and the regulation of blood components e.g., ions, osmolality volume, pressure, and glucose. The kidney has eight structural parts, namely the ureter, the renal Cortex, the renal medulla, the nephrons, the proximal convoluted tubules, the distal convoluted tubules, the ducts, calyces and renal pelvises, and the loop of Henle.
Renal Cortex is a smooth textured organ, located superficially in the kidney, and extending from the capsule to the floors of the renal pyramids; cone-shaped projections, and to their in-between spaces. The Renal Medulla has approximately 7-17 renal pyramids, whose floors face the renal Cortex and its apex (Renal papilla). The papilla then faces the renal hilum, through which the ureter, nerves, blood vessels and lymphatic vessels emerge from. The part of the renal Cortex that protrudes between the pyramids is referred to as renal columns. (Derrickson,2006,1000) The nephron is the basic unit of a kidney, performing both functional and filtration purposes. They are microscopic in size, and are located in the parenchyma. Each of the millions of the microscopic nephrons has a renal corpuscle and a renal tubule. The corpuscle has a capillary network, also called glomerulus and a bowman’s capsule, also called glomerular, that encloses the glomerulus. The formation creates the smallest and thinnest filtration membrane that only allows the penetration of water and other very small solutes that is then passed to the renal tubule, which is composed of the proximal convoluted tubule, distal convoluted tubule and the loop of Henle.
The proximal convoluted tubule is the first passage of the fluid and is attached to the bowman’s capsule. It is also a site of reabsorption of large amounts of water and solutes i.e., water-soluble vitamins, ions e.g., Magnesium and Calcium, glucose, etc. from the fluid that has been filtered. (El-Husseini,2007,200) The loop of Henle is a connecting tubule of the proximal and distal convoluted tubules. Its descending part moves into the renal medulla and makes a turn, ascending to the renal Cortex. The distal convoluted tubule is a site for the reabsorption of the remaining ions and water that were no reabsorbed in the proximal convoluted tubule. At its end, almost all the filtered components have been reabsorbed back into the bloodstream. The rest of the filtered substances that have not been reabsorbed at this stage are excreted through urine.
The collecting ducts in unison from the distal convoluted tubules converge into bigger ducts that drain off the urine into calyces. The urine is then transported through the renal pelvis to the ureter and into the urinary bladder. The ureters, one for each kidney, are long tubes, approximately 25-30cm, with thick walls, about1-10mm, transport the urine from the pelvis to the urinary bladder by gravity, pressure, and muscular contractions that forcefully move the urine into the bladder. They bend at their ends while passing through the bladder. The urinary bladder, a hollow, striated organ, stores urine. It is spherical when full with urine, but collapsed when emptied. It has a capacity of 700-800ml and smaller in females than males. The urethra carries out the urine from the bladder. It is shorter in females than in men.
References
Fox, S., 2015. Human Physiology. McGraw-Hill Education. Pp 1-100
Silverthorn, D. U., Ober, W.C., Garrison, C. W., Silverthorn, A.C, and Johnson, B.R., 2010. Human Physiology: an integrated approach (pp 300- 509). San Francisco: Pearson/ Benjamin Cummings.
Tortora GJ, Derrickson B. 2006The endocrine system. In: Principles of Anatomy and Physiology.11th Ed. Hoboken, NJ: John Wiley & Sons Inc.: 616–665
Zahra A, El-Husseini A, Shoker A, can cystain C replace creatinine to estimate glomerular filtrate rate? A literature review. Am J Nephrol. 2007(2): 197-205