Urinary System of Organisms - Essay Example

HOMEOSTASIS s Living organisms getrid of the metabolic wastes by the process of excretion since accumulation of the metabolic wastes in the body will lead to toxicity. In other words excretion maybe defined as the process of nitrogenous waste products such as ammonia, urea, uric acid etc., along with excess pigments, salt and water. The process helps keep internal chemical environment constant. This is called homeostasis. The organs which remove the waste products of metabolism from the animal body are termed excretory organs. In man and other vertebrates the primary organs of excretion are the kidneys which are mainly associated with the urinary system. The urinary system consists of kidneys, ureters, urinary bladder and urethra. The metanephric kidneys in humans filter the blood and forms urine which is carried by the ureters to the urinary bladder where it is stored till it is passed out through the urethra. The reflex of voiding urine is known as micturition. If the urine content of the urinary bladder reaches more than 300ml, the reflex of micturition is triggered. The urine that is primarily formed by the kidneys is highly dilute and contains a lot of salts and other substances which are beneficial and useful to the body. The urine needs to be concentrated and the salts must be reabsorbed else all of the water and important salts would be excreted leading to salt and water imbalance in the body. This is where the role of the kidneys comes in. The fact is that the structure, function and location of the kidneys help in maintaining homeostasis in the human body. It was in 1902 that Hober coined the term osmoregulation; Osmoregulation is a collective of a number of processes that regulate water content, volume and movement. Each kidney is made up of about 1.25 million nephrons which are the prime functional units of the kidneys. It is the structure of nephrons that play the most pivotal role in maintaining the homeostatic balance of the body. The structure of the nephrons is complicated and consists of three major parts- Renal corpuscle (Bowman’s capsule and Glomerulus), Nephric Tubules (PCT, Henle’s loop and DCT) and Collective tubule. Each of the parts of the nephrons play important role in formation of urine and maintaining internal chemical balance. Bowman’s capsules act as Ultrafiltration and filters out water and other substances from the plasma as the blood flows through the glomerulus except for the blood cells and plasma proteins. The glomerulus filtrate so formed passes through the tubules. During this course the composition, osmotic pressure and pH changes due to reabsorption of water and solutes. This reabsorption from the nephric filtrate in the blood capillaries is called selective reabsorption and is done by two mechanisms- active reabsorption and passive reabsorption. The urine thus produced is drained continuously by the nephrons into the renal pelvis from where it is carried to the urinary bladder by peristaltic movements and is stored till it is voided out. Urine composition sometimes differs owing to some disease because of failure of the kidneys to reabsorb certain substances. For example Glycosuria, is the disease resulting from presence of glucose in the urine. Ketosis is the presence of ketones or acetone bodies due to metabolism of fatty acids instead of glucose. Water and Salt Regulation (OSMOLALITY) When the water intake of a person is very high, the urine excreted is hypotonic so as to remove excess water while in case of excessive water loss from the body urine tends to be hypertonic. In this way the osmotic concentration of blood is maintained. Two counter current mechanisms are present in the kidneys to regulate the water. Henle’s Loop: The Loop of Henle is an important part of the nephrons. It is responsible for concentration of the urine. The glomerular filtrate flows in opposite directions in the two limbs f Henle’s loop. During the passage of fluid through the ascending limb of the Loop of Henle, sodium ions leave the tubular fluid and re-enter the interstitial (peritubular) fluid. However, the ascending limb is not permeable to water; the fluid in the tubule becomes hypotonic. The descending limb on the other hand is permeable to both water and passive diffusion of NaCl. Therefore the ascending and descending limb of the Loop of Henle forms counter-current mechanism. Both passive and active movements of NaCl establish an osmotic gradient in the interstitial fluid. Owing to the entry of NaCl ions in the Nephric filtrate reduces the difference in salt concentration between the Nephric filtrate and the interstitial fluid owing to which very little water is reabsorbed. Therefore, the contents in the descending limb become hypertonic. The Vasa Recta is another counter current mechanism that helps to regulate water concentration in the urine. Inside the renal medulla, the efferent arteriole forms a peritubular capillary network around the tubules. Some of these vessels are wide, thin walled and in loop form. Therefore, the blood flows in opposite direction in the two limbs of the vasa recta, the blood entering the descending limb comes in close proximity to the outgoing blood in the ascending limb. So the concentration of NaCl increases in the interstitial fluid of the medulla because of the counter current system of the vasa recta. The contents of the descending limb become hypertonic. One of the most important hormones associated with water regulation is ADH (antidiuretic hormone), the secretion of which depends on water intake. If water intake is low ADH production increases to help reduce urine volume while if excess water is taken ADH production is produced decreases so that only small amounts of water is reabsorbed and the excess water is excreted along with urine. Therefore ADH helps the kidneys to regulate water concentration and controls the tonicity or osmolality of body fluids. Regulation Of pH The kidneys maintain the body fluids at a constant pH by maintaining a balance between H+ ions ingested and H+ ions excreted. The kidneys help in increasing pH by excretion of hydrogen ions and reabsorption of HCO3- ions. Henderson-Hasselbalch equation shows a relation between CO2, HCO3- ions and pH pH ∝ HCO3- is reabsorbed in the proximal distal tubule. Free hydrogen ions reacts with bicarbonate which leads to formation of carbon dioxide and water. The products so formed is carried to the epithelium of the tubule where enzyme carbonic anhydrase gives rise to carbonic acid which again breaks down to form hydrogen ions and HCO3- which is then reabsorbed from the epithelium. Excretions of hydrogen ions take place with the help of two different mechanism namely- phosphate mechanism and ammonia mechanism. Body fluid volume and Na maintenance The nephrons have an important role in regulation of Na ions which automatically controls ECF volume in the body by the rennin-angiotensin mechanism. In case blood pressure falls rennin secreted by the juxtaglomerular cells play active role in reabsorption of sodium ions. K+ ion Homeostasis It is important to adjust the excretion of potassium ions according to the dietary intake. Chronic homeostatic regulation of these ions is done by the kidneys. During the movement of the filtrate most of the potassium ions are reabsorbed. 65% of the ions are reabsorbed before the filtrate reaches the proximal tubules while more 15% is reabsorbed in the Loop of Henle. Presence of minerralocorticoids such as aldosterone alters the composition of normal urine. Since the primary function of the hormones is to regulate potassium and sodium ions in the kidneys. In case of hyperaldosteronism, causes retention of sodium ions leading to high volume of ECF. It also causes K+ excretion and hypokalemic alkalosis. Sharp decreased in the levels of corticoids may cause diseases because of excessive sodium ions excretion and retention of potassium ion. Conclusion The urinary system as a whole is not only crucial for living organisms for the purpose of excretion of unwanted substances but the intricate mechanisms involved in the formation and concentration of the urine plays pivotal role in maintaining the internal chemicals balance of the body. REFERENCE Despopulous,A., Silbernagl,S (2003). Color Atlas of Physiology. 5th ed. New York: Thieme. 154-168. Mackenna,B.R. Callander,R. (1997). Illustrated physiology. london: Churchill Livingstone. 167-217. Read More