Kidney Essay

Homeostasis is the maintenance of a steady state. With changes within and around living cells, conditions are maintained at a constant level. The ‘two major control systems, nerves and hormones, are mainly responsible for co-ordinating homeostatic mechanisms’ (Human Body) whilst using feedback. If a change in condition is detected a corrective mechanism is activated, conditions return to set point and the corrective mechanism is then switched off. The conditions are then at constant level. Some of the factors controlled by homeostatic mechanism are: body temperature, blood glucose level, water content of the body, respiration and urea being carried by the blood.

The changes within the homeostatic system is often referred to as positive or negative feedback. The reverse in change and the extent of the correction is closely monitored by the negative feedback system. The opposite to the negative feedback is the positive feedback which in a situation a change is made and is amplified rather than returned to normal. The picture shows below an example of a homeostasis mechanism, showing the changes the body would make in keeping the blood glucose at set point.

Picture 1

( http://www.bio.miami.edu/~cmallery/150/physiol/c45x10glu-homeostasis.jpg)

In the homeostatic system many organs have specific roles. The kidney as several roles as a homeostatic organ, one of the roles is the regulation of blood PH ‘the kidney excrete a variable amount of hydrogen ions into the urine and conserve bicarbonate ions’ (Principles of human anatomy) as these two activities help regulate the blood PH level.

The urogenital system ‘is a combination of two systems of the body: the reproductive system and the urinary system’ (http://www.wisegeek.com). The two systems share tissues in order to carry out their functions. The urinary system in males and females are very similar. The urinary system ‘consists of two kidneys, two ureters, one urinary bladder, and one urethra’ (Principles of human anatomy). The kidney is the main organ in the urogenital system.

The kidneys are located near the vertebral column on either side. The kidney ‘receives the largest blood supply of any organ, per gram of tissue’ (Human Biology). This ‘is supplied with oxygenated blood via the renal artery and drained of deoxygenated blood by the renal vein’ (generalmedicine). The kidney can be divided in to two main areas the cortex and the medulla. The cortex is the outer region which ‘contains the renal capsules (also called the bowman’s capsules) and the first convoluted and second convoluted tubules (also called the proximal and distal tubules)’ (Human Biology). The medulla consists of ‘the loop of henle and the collecting ducts’ (Human Biology) these ducts enable the delivery of urine into the open space of the kidney called the pelvis.

The nephron is the main functional unit of the kidney. A ‘human kidney contains about a million nephrons’ (Human Biology). The nephrons roles are filtering the blood of small molecules such as sodium, glucose and water but leaving the large molecules untouched. Also reabsorbing the required useful quantities of solutes in which the body still requires.

Picture 2

(http://www.standardofcare.com/mwiki/images/a/ae/Kidney_cIvyRose.JPG)

At ‘one end of the renal tubule is a cup-shaped membrane’ (The human body book) called the Bowman’s capsule that surrounds a knot of capillaries called the glomerulus. The glomerulus is the first step in filtrating blood into urine. Blood flows in at high pressure to the glomerulus, which comes from the afferent arteriole and leaves through the efferent arteriole. The blood plasma travelling in the afferent arteriole ‘of the kidney that becomes glomerular filtrate is the filtration fraction’. (Principles of anatomy and physiology page 1030). Although the filtration fraction will differ depending on your health ‘around (16-20%) is typical’ (Principles of anatomy and physiology) the rest of the blood would leave via the efferent arteriole.

The glomerulus acts as filtrate only small molecules such as sodium, glucose or amino acids enter. Certain molecules would not filtrate through EG: red blood cells or large molecule such as proteins. At this stage there is a combination of molecules that have filtered through some waste products and some molecules that are useful which can be still used by the body.

The fluid then passes along the first convoluted tubule (also called the proximal) which is a portion of the duct system of the nephron where the reabsorption starts. Certain molecules in the filtrate that the body does not want to lose such as glucose, too much sodium or amino acids, usually all the glucose and amino acids are reabsorbed. The removal of these molecules from the tubule occurs by active transport. These molecules that are reabsorb ‘show all the classic adaptations to active transport: a large surface area,

provided by microvilli, and many mitochondria to provide ATP to power the process’ (Page 236 Human biology).

The first convoluted tubule is located ‘very close to blood vessels that carry blood away from the glomerulus’ (Page 236 Human biology) the reabsorbed molecules will move back into the bloodstream from the tubule. This process of reabsorption is very important. Only when excessive amounts of glucose in the blood or a person who suffers from diabetes will their reabsorption mechanism fail.

Picture 3

(http://www.colorado.edu/intphys/Class/IPHY3430-200/image/19-1j.jpg)

The kidney plays the role of salt balance. The re-absorption of salt occurs in the loop of Henle a region of the nephron. The loop of henle leads the proximal tubule to the distal tubule. The loop of henle seen above ‘is a long U-shaped region of the nephron that descends into the medulla and then returns to the cortex’. (Human Biology second edition) In the descending part of the loop of henle, water diffuses out by osmosis as the medulla has highly solute concentration therefore low water potential. The descending limb is permeable to water. Some sodium may diffuse as well. Due to the loss of water this increases the sodium concentration and in the ascending limb is when the sodium is actively pumped out. The ascending limb is also impermeable to water. This is where the important role of salt balance occurs.

As the fluid ascends up the loop of henle reaching the second convoluted tubule, this part of the nephron which fine tunes the remaining fluid. Playing an important role in homeostatic mechanisms including the regulation of pH, salt and water balance. The filtrate is now finely tuned and the fluid is a lot more concentrated than at the beginning as mostly waste products and excess water are present at this point.

At the end of the nephron is a collecting duct, by the time the filtrate as reached this location does the filtrate become urine. The collecting ducts consist of a series of tubules attached. The urine from other nephrons are collected here. At the collecting duct fluid balance occurs, reabsorption or excretion of the fluid depending of the bodies’ internal environment will happen in this region of the kidney.

The kidneys plays the role of water balance in the body. If the level of water in the plasma of the blood is low EG: due to exercise then ‘more water is reabsorbed back into the blood’. (BBC Bitesize)) However if the level of water in the plasma of the blood is high EG: due to excessive fluid intake ‘less water is reabsorbed back into the blood’.

Water balance is maintained by the release of a hormone called anti diuretic hormone (ADH). A region of the brain called the hypothalamus which contains osmoreceptor cells are sensitive to changes in the concentration of the blood. When the water in the blood plasma becomes low (the blood becomes too concentrated) the hypothalamus ‘responds in two ways –stimulating the thirst centre in the brain and it stimulates the pituarity gland to release anti-diuretic hormone (ADH)’ (Page 238 Human Biology) which then travels through the blood to the kidney.

This makes the distal convoluted tubule and the collecting duct more permeable to water, so more water is reabsorbed to re-enter the blood to be conserved. The urine is in small amount and very concentrated. When the water in the blood plasma is high (blood less concentrated) the hypothalamus detects this, responding with less ADH being released therefore making the distal convoluted tubule and the collecting duct less permeable to water so less water is conserved and just passed straight through, producing dilute urine.

Bibliography

* Principles of human anatomy by Gerard J Tortora / Mark T Nielsen
11th edition Printed USA copyright 2009

* (http://generalmedicine.suite101.com/article.cfm/the_human_kidney_structure_and_function) Date obtained 26/05/10

* Wise Geek-http://www.wisegeek.com/what-is-the-urogenital-system.htm). Date obtained 26/05/10

* Human Biology- Second edition Published 2002 By Harper Collins Limited. Authors Mike Boyle and Kathryn Senior. Pages 234-239

* Human Body- Published 2007 By Dorling Kindersley limited. Author Steve Parker forwarded by Professor Robert Winston. Page 197

* Picture 1- ( http://www.bio.miami.edu/~cmallery/150/physiol/c45x10glu-homeostasis.jpg) Date obtained 28/05/10

* Picture 2-(http://www.standardofcare.com/mwiki/images/a/ae/Kidney_cIvyRose.JPG) Date obtained 28/05/10

* Picture 3-(http://www.colorado.edu/intphys/Class/IPHY3430-200/image/19-1j.jpg) Date obtained 28/05/10 * (Principles of anatomy and physiology page 1030). * BBC Bitesize (http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr/homeostasis/waterbalrev2.shtml) Date obtained 28/05/10

Bibliography: * Picture 3-(http://www.colorado.edu/intphys/Class/IPHY3430-200/image/19-1j.jpg) Date obtained 28/05/10 * (Principles of anatomy and physiology page 1030).