Introduction
The kidneys refer to the organs that are involved with several important regulatory roles in vertebrate animals. Kidneys play a crucial role in the urinary system as well as in the homeostatic functions such as electrolyte regulation, maintaining a balance between the acids and bases, and in regulating blood pressure. Kidneys filter the blood removing wastes, which are then taken to the urinary bladder as urine. During urine production, wastes such as ammonia and urea are excreted while substances such as glucose, water and amino acids reabsorbed. The kidney is also responsible for the production of hormones such as erythropoietin and calcitriol, and rennin enzyme (Seeley, Stephens and Tate).
Urine formation begins during blood filtering in the renal corpuscles with about 180 l of filtrate passing through every day. The filtrate has the same osmolarity as the blood except that it lacks protein and blood cell components. Osmolarity is measured in milliosmoles with the body fluid being kept at about 300mOsm/l. Reabsorption and secretion process take place as the filtrates flow through the proximal convoluted tubules, loop of Henle, distal convoluted tubules and the collecting ducts. Substances such as glucose, electrolytes and amino acids, which are useful in the body, are reabsorbed into the peritubular capillaries that surround the tubules. The 180 liters of filtrate produced in the renal corpuscles undergo reabsorption to produce approximately 1 liter of concentrated urine per day.
The produced urine usually has an osmolarity that ranges from 50-1200mOsm/l depending on the requirements of the body. Some of the factors that affect urine osmolarity include food and drinks taken, over hydration, dehydration among others. In the proximal convoluted tubules and loop of Henle, movement of filtrate occurs without any regulatory mechanisms, and urine volume and concentration are adjusted through secretion and reabsorption in the distal convoluted tubules and the collecting ducts.
Some of the mechanisms that regulate osmolarity of the body fluid are through the use of the hormone known as antidiuretic hormone. The hormone is released by the posterior pituitary gland when the body fluid osmolarity is high. The hormone reduces the osmolarity by stimulating the cells of distal convoluted tubules and the collecting ducts to increase water reabsorption through osmosis.
In this experiment, three groups ingested solutions with varying salt concentration to alter body fluid osmolarity for a short time. Urine samples were then collected every 30 minutes to obtain data for urine volume, osmolarity, and NaCl concentration. The aim of the experiment was to determine the effect of different salt solution concentrations on urine osmolarity.
Methods
Three test groups were formed each consisting of three members. Each group ingested one kind of solution hypertonic solution (50 ml of 8.0% salt solution) for the hypertonic group, hypotonic solution (500 ml water) for the hypotonic group, and isotonic solution (500ml of 0.8% salt solution. The isotonic solution was taken as 50 ml of 8.0% salt solution quickly followed by 450 ml of water.
The test subjects consumed their assigned fluid immediately after emptying their blabbers. Some of the urine was collected. The time at which the fluid was consumed was recorded as time 0. Osmolarity and NaCl determinations were performed on the samples, and the results recorded in their respective columns in the table provided. Since the test subjects started the lab with an empty blabber, urine volume was recorded as 0 ml. The bladder was emptied every 30 minutes for 2 hours and the volume recorded and a sample taken to the lab for osmolarity and NacL concentration analysis.
Results
The average measurements for the three groups were recorded in Table 1 below. Volume of the urine produced was highest in the group that ingested hypotonic solution followed by the group that ingested isotonic solution. The group that ingested hypertonic solution had the lowest amount of urine. Osmolarity and NaCl concentration were highest in the group that ingested hypertonic solution followed by the one that took isotonic solution and finally the one that ingested a hypotonic solution. This trend is clearly shown in Figure 1 for urine production, Figure 2 for urine osmolarity and Figure 3 for NaCl concentration.
Figure 1: Average rate of urine production (ml) at each time interval for the 3 test groups
Figure 2: Average urine osmolarity (mOsm) at each time interval of the 3 groups.
Figure 3: Average NaCI concentration (mg/ml) at each time interval for the three groups.
Discussion
Aldosterone production occurs in the outer section of the adrenal cortex and plays a crucial role in blood pressure regulation. The hormone acts on the collecting ducts and distal tubules of the nephrons where it increases ions and water reabsorption. When blood osmolarity is high, the juxtaglomerular cells found in the kidneys secrete rennin, which is then released into circulation. Renin in the plasma converts angiotensinogen released by the liver into its active form called angiotensin I, which is then converted to angiotensin II. It is angiotensin II that stimulates the secretion of aldosterone hormone by the adrenal cortex. Aldosterone acts on the distal tubules, as well as the collecting ducts causing an increase in water and sodium reabsorption back to the blood. Increased water content in the blood reduces blood osmolarity back to normal. Aldosterone also stimulates the cells of the distal tubules and the collecting duct to deploy more channels to reabsorb sodium in exchange for potassium ions (Seeley, Stephens and Tate).
The test group that ingested a hypertonic solution reflects the action of an increase in the secretion of aldosterone hormone. Increased levels of aldosterone increased blood osmolarity and this resulted in more water being reabsorbed and thus reduction in the amount of urine produced. Increased salt content increased the amount of salt in the body and thus reabsorption of NaCl was not necessary. This explains why NaCl concentration was the highest in the hypertonic solution test group.
Antidiuretic hormone (ADH) also known as vasopressin is a hormone that is produced by the posterior pituitary gland and responds to situations such as plasma volume reduction, increased blood osmolarity and cholecystokinin released by the small intestine. Increase in blood plasma osmolarity, in hypertonic test group, caused the release of ADH. The ADH target cells are located in the distal tubule and collecting ducts of the kidney. The hormone exerts its effect by inserting water channels also known as aquaporin-2 into the apical membrane of the collecting duct and distal tubule epithelial cells. The water channels allow water to be reabsorbed from the filtrate. The reabsorbed water reduces the plasma osmolarity back to normal (Seeley, Stephens and Tate).
The release of ADH in the hypertonic group caused less urine to be produced compared to the other groups. The hormone also caused the urine osmolarity to be higher compared with the other groups.
Work Cited
Seeley, R., T. D. Stephens and P. Tate. Anatomy and Physiology. 6th. New York: The McGraw Hill Companies, 2004. Print.
