Content Summary:
The urinary system maintains homeostasis of the body fluids. It's organs include the kidneys, ureters, urinary bladder and urethra. The kidneys regulate the volume of plasma, the concentration of waste products and electrolytes within the plasma and the pH of the plasma. The ureters are tubes in which urine travels from the kidneys to the urinary bladder, and then through the urethra during micturition. The kidneys are the focal point of the urinary system and it is there that all the filtering and re-absorption occurs.

Summary #1:
Ultrafiltrate is the fluid that enters the glomerular capsule under hydrostatic pressure of the blood. A net filtration pressure of 10 mmHg occurs because the protein concentration of the tubular fluid is 2 to 5 mg per 100 ml compared to plasma which has 6 to 8 g per 100 ml. This encourages the osmotic return of filtered water. Because of the permeability of glomerular capillaries and large surface area, the net filtration produces a large volume of filtrate. The glomerular filtration rate (GFR) is calculated by measuring the volume of filtrate produced by both kidneys in 60 seconds. Women have a GFR average of 115 ml per minute and men have a GFR average of 125 ml per minute. This is approximately 180 L. per day, or about 45 gallons. The total blood volume is filtered within the urinary tubules every 40 minutes. Most of the water filtered from this process must be returned to the vascular system or our bodies would urinate continuously and die within minutes. The vasoconstriction or dilation of afferent aterioles affects blood flow which in turn affects the GFR.

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Summary #2
Urea is a waste product of amino acid metabolism and lends a significant contribution to the total osmolality of the interstitial fluid. Urea diffuses out of the inner collecting duct into the interstitial fluid, passes into the ascending limb of the loop of Henle which allows it to recirculate in the interstitial fluid, reacts with NaCl to make a hypertonic environment and water leaves the collecting duct by osmosis. Other waste products the kidneys remove are uric acid which comes from nucleic acid metabolism, and creatinine which comes from muscle metabolism.

Summary #3
Acute renal failure occurs when the kidneys lose their ability to excrete wastes and regulate homeostasis of blood volume, pH and the electrolytes during a period of hours to days. Blood creatinine concentration increases and the renal plasma clearance of creatinine decreases, possibly because of atherosclerosis or inflammation of the renal tubules that may cause reduced blood flow through the kidneys. Compromised kidney function may be caused by ischemia or excessive use of nonsteroidal anti-inflammatory drugs (NSAIDs).


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Application:
It is important for a health practitioner to understand the workings of the urinary system as many of the patients served may be dealing with issues concerning the kidneys, urinary bladder and the urethra. So many women suffer from UTI's so it is important to understand what may cause these and what suggestions I can give to improve their situation. After reading and studying this chapter, it truly becomes apparent as to the importance of increased water intake and limited salt intake. Many of my patients may have hypertension or diabetes and it will be important for me to understand how this may affect their ADL. On a personal note, I need to increase my intake of water on a daily basis. This has been very hard for me to stick with as I have attempted this many times. As each year passes, I see an imminent need for me to increase my water intake to keep my urinary system running smoothly.

Essential Questions:
Filtration occurs when filtrate is filtered from the blood plasma before reaching the interior of the glomerular capsule. Blood plasma fluid must go through three barriers: 1) the capillary fenestrae, 2) the glomerular basement membrane and 3) the slit diaphragm. Dissolved plasma solutes pass easily through these three barriers but plasma proteins do not because of their size and net negative charges. An example of filtration gone bad is when a defect occurs in the slit diaphragm and allows massive leakage of proteins into the filtrate causing proteinuria which may lead to diabetes mellitus, hypertension, kidney stones, multiple myeloma, PKD and/or renal artery stenosis.

Reabsorption is the return of filtered molecules from the tubules to the blood. This occurs no matter what the body's state of hydration is because blood volume and pressure must be maintained within the vascular system. Approximately 85% of the daily 1volume of filtrate produced (180 L ) is reabsorbed by the proximal tubules and descending LH. This reabsorption and the regulated reabsorption of the remaining volume of filtrate occurs by osmosis. Reabsorption by osmosis occus when the solute concentrations of plasma in the peritubular capillaries and the filtrate interact with the active transport of Na+ from the filtrate to the peritubular blood.

Secretion involves the movement of molecules and ions out of the peritubular capillaries in the interstitial fluid and are transported across the basolateral membrane of the tubular epithelial cells and into the lumen of the nephron tubule. Many foreign molecules called xenobiotics are secreted by membrane carriers into the filtrate of the proximal tubule. Drugs, toxins and even nicotine are eliminated this way.

Excretion is the elimination of excess ions and waste products from the blood. Clearing the blood of these products occurs through their excretion into the urine. Renal clearance is the ability of the kidneys to remove molecules from the blood and excrete them by way of urine flow. Concentrations of these waste products in the blood leaving the kidneys is lower than those concentrations entering the kidneys from the blood.

An electrolyte is an ion or molecule that is able to ionize and carry an electrical current. Common electrolytes in the plasma are Na+, HCO2 and K+. Plasma electrolyte control is important to maintain homeostasis. The control of Na+ allows for regulation of blood volume and pressure. The control of K+ allows for proper function of cardiac and skeletal muscles.

References: Human Physiology by Stuart Ira Fox, Anatomy and Physiology by Stanley E Gunstream, andTaber's Cyclopedic Medical Dictionary