Urinary System, Essay Example

Introduction

This paper addresses the anatomy and physiology of the kidneys, with particular reference to the functions of the urinary system.  Fig 1 provides a visual guide of  the kidneys within the context of the human anatomy.   Of particular significance being the renal arteries, veins, abdominal aorta, uretas, bladder and uretha.  The concept of urine creation is discussed later.

Kidney Anatomy

The kidneys are bean shaped and of a dark red colour.   One side is convex and the other side indented, hence the characteristic bean shape.   The cavity in the indented side is called the renal pelvis and this is attached to the ureter.   Each of the kidneys is enclosed within a membrane that protects them from potential diseases, infections or trauma.   The darker inner component of the kidney is terms the renal medulla that contains eight conical items termed renal pyramids.  The most common element that exists inside the kidney is termed  nephrons.  There are literally millions of these microscopic structure that perform the task of filtering the blood and removing waste product through the passing of urine.

It is the renal artery that is responsible for delivering blood to the kidneys each day ” Over 180 litres (50 gallons) of blood pass through the kidneys every day. When this blood enters the kidneys it is filtered and returned to the heart via the renal vein.”  (Anon, 2009)

Upon separating wastes from body fluids, the process termed excretion, there are four organs responsible for this.  The urinary system being one of the most important of these. It removes a large variety of metabolic wastes including, toxins, drugs, hormones, salts, hydrogen, iron and water.  The kidneys are the main organs of the urinary system.  Both of the kidney contain multitudes of blood vessels  The two kidneys are determined to hold about 160 kilometres of blood vessels. The kidneys are located in the abdominal cavit just above the waist. The kidney weighs approximately 150 gms and is the size of a conventional computer mouse.

Formation of Urine

The actual formation of urine occurs in that part of the kidneys called nephrons.  Each kidney contains over a million nephrons and in turn these occupy a network of capillaries.  ” Nephrons consist of a network of capillaries (called a glomerulus), a renal tubule, and a membrane that surrounds the glomerulus and functions as a filter (called Bowman’s capsule). The glomeruli are where urine production begins. Urine formation occurs in the renal tubules, which travel from the outer tissue of the kidney (called the cortex), to the inner tissue (called the medulla), and return to the cortex.”  (Healthcommunities.com, 2009)

The Cortex extends into the medulla and subsequently divides tissues into renal pyramids. These extend into funnel-like extensions (called calyces), it this location where the urine is collected.  Essentially the sections of the renal tube perform a different function. As the tube moves away from bowmans capsule it becomes highly coiled.  Here waste matter and toxic substances are removed from the blood and into a permeable membrane. At this stage useful substances like glucose, amino acids and vitamins are reabsorbed.

The urine then moves through the  loop of Henle This is a descending limb and an ascending limb. Certain sections of the loop are permeable to water and impermeable to substances in the urine (e.g., salt, ammonia.  This then moves to the  distal convoluted tubule. This section is usually water permeable. Here substances that remain in the urine are reabsorbed, which concentrates the strength of the  urine. Continuing through the distal convoluted tubule, the residue of the urine contains mostly waste products. The water and other useful substances have been reabsorbed. Finally the urine enters  the collecting tubule. Urine from several nephrons empty into each collecting tubule. Urine travels from the kidneys through the ureters to the bladder, where it is stored until it is eliminated from the body through the urethra.

Physiology of the Kidneys

The function and purpose of the kidneys is to regulate the fluid and electrolyte balance of the body by continually filtering the blood. A vital function in order to maintain constant extracellular fluid volume and composition. The Kidneys  excrete or conserve salt and water; control body pH, and also free the body of waste products of metabolism. The kidneys have three main filtration processes for cleansing the blood. Glomerular filtration, Tubule secretion and Tubule reabsorption.

The blood filtration process is one where the kidneys produce urine.  Here the blood enters the glomerulus under high pressure, this drives the substances across the leaky endothelial-capsular membrane into the nephron.”This membrane acts like a sieve allowing small substances to be filtered into the nephron, whilst large molecules such as plasma proteins remain within the blood. The filtered fluid is called ultra filtrate and passes from the Bowman’s Capsule into the proximal convoluted tubule. ”  (Rolfe, 2004).

The concept of tubile secretion also considers substances that are not filtered out by the bowmans capsule.  Instead these enter the nephron in a process called tuble secretion. Certain drug metabolites are deposited into the tubule from the blood stream. Also, metabolites produced by the cells forming the tubules themselves often enter the nephron in this way.” These processes are important in the regulation of the acid-base balance of the body. It is because of this that urine pH can dramatically vary from being either acidic or alkaline. ”  (Rolfe, 2004).

The concept of reabsorbtion occurs when  Substances that enter the nephron are not all excreted allowing potential  flow back into the blood. This often occurs with substances that are particularly beneficial to the body, including electrolytes (Na+, Cl-, K+, Ca++, HCO₃-, phosphate), amino acids, peptides, glucose and water.  This process occurs in the  proximal and distal convoluted tubules and also the loop of Henle. It can occur passively or alternatively  an active process resulting in energy in the form of ATP pushing the  substances from the nephron back into the blood stream.

Within the  distal convoluted tubule, certain changes  are made to the composition of urine. It is here that the hormone aldosterone influences the reabsorption of sodium, this is accomplished in order to keep the salt levels in the blood at a constant  ” For example. On a hot day the body may loose salt in the form of sweat. Low levels of salt in the plasma raises levels of aldosterone. The hormone acts on the tubule cells to increase the reabsorption of sodium from the ultra filtrate back into the blood stream. ”  (Rolfe, 2004)

The kidneys are also important for maintaining the water balance in the blood, termed water homeostasis.  Example :  Where water is lost by the body in hot weather by means of sweat glands. This triggers the release of ADH into the blood acting on the cells of the distal tubule which increases water reabsorption, thereby reducing the mount excreted. In hot weather, the tendency is to produce small volumes of urine as the body is fighting to conserve water.

The end product is the actual production of  urine itself : ” Urine is made mainly of water, and contains nitrogenous wastes such as urea and uric acid, salts and metabolites from endogenous (i.e. those produced by the body) and exogenous sources. A healthy adult will produce over a litre of urine every day. The pH will range from 4 to 8.5. These figures will dramatically vary depending on many factors such as fluid and salt intake, environmental temperature, and the presence of disease or use of medication. ”  (Rolfe, 2004)

The risk of kidney disease

There are a number of areas where you can put the kidneys at risk or obtain kidney disease.  The Medical Education Institute in the USA provides very informative statistics.

• Diabetes : Almost 40% of new dialysis patients have diabetes, making it the fastest growing risk factor for kidney disease. Type 2 diabetes is the number one cause of kidney failure, responsible for more than one of every three new cases.  (Beth, 2009)
• Hypertension :  Hypertension is the number two cause of kidney failure. Normal blood pressure is less than 130/85—and this is the target for people who have diabetes, heart disease, or CKD. Weight control, exercise, and medications can control blood pressure—and perhaps prevent or slow the progress from kidney disease to kidney failure.
• Blockages :  Scarring from infections or a malformed lower urinary tract system (birth defect) can force urine to back up into the kidney and damage it. Blood clots or plaques of cholesterol that block the kidney’s blood vessels can reduce blood flow to the kidney and cause damage. Repeated kidney stones can block the flow of urine from the kidney and are another kind of obstruction that can damage the kidneys.

Kidney Stone Formation

Kidney stones are formed by the composition of  different types of crystals. These are usually :

• calcium oxalate,
• calcium phosphate,
• a combination of calcium oxalate and calcium phosphate,
• magnesium ammonium phosphate
• uric acid,
• cystine
• miscellaneous types such as occur with drug metabolites.

Genetically inherited disorders are also responsible for differing types of kidney stone infections. ” High protein and salt intake increase the risk of calcium stone formation. High purine diets (meat, fish, chicken) lower urinary pH and cause increased excretion of uric acid. Vitamin B6 deficiency leads to increased formation and excretion of oxalate. Dehydration, excessive vitamin C intake, calcium supplementation, and calcium containing antacids may also lead to stone formation. Also, geography plays a part with more stones noted in the southeast, also known as the stone belt.”  (Anon, Anatomy and Stone Formation, 2005).

Kidney stones that obstruct are those that result in pain, the most common symptom indicating the presence of kidney stones. This pain often detected in the  flank with “radiation into the lower abdomen and inguinal areas as well as to the testicle in males and the labia in females. Stone pain or renal colic can vary from a mild ache to the worst pain imaginable. Women often describe it as worse than labour pain, and patients often say there is no position which gives relief of pain.”  (Anon, Anatomy and Stone Formation, 2005)

Blood in the urine, occurs in 95% of the cases. This may be visible to the human eye, or so small that it can only be detected by a urine dip stick or microscopic exam.

Those patients who are have a disposition towards Kidney Stone formation  are advised to drink plenty of  fluids in order to expel  at least 2 quarts of urine per day. This urine flow washes crystals from the system before they begin crystallising to make stones. Fluids that are high in citrate are recommended. Citrates act as an inhibitor to stone formation. Lemonade made with frozen concentrate, real lemon juice or real lemons is one of the fluids recommended for its citrate content.

Diets low in protein help to prevent stone formation, so patients are advised to cut back on meat, fish, and chicken. Stone formers are not good candidates for high protein, high fat, low carbohydrate diets.  (Anon, Anatomy and Stone Formation, 2005)

Calcium restriction has been somewhat controversial in medical terms.  It is useful to restrict intake under certain circumstances, however normally patients are instructed to eat a normal calcium diet without supplements. In most cases the  calcium binds with oxalate and is excreted, thus lowering the probability of stone disease. Finally it is useful to avoid certain stone provoking drugs i.e. as certain diuretics, calcitrol, or probenicid.

Works Cited

Anon. (2005). Anatomy and Stone Formation. Retrieved 11 19, 2009, from StoneCentre: http://www.urostonecenter.com/anatomy.asp

Anon. (2009). Comprehensive Kidney Facts. Retrieved 11 18, 2009, from Comprehensive Kidney Facts: http://www.comprehensive-kidney-facts.com/kidney-anatomy.html

Beth, W. (2009, 11 22). Risk Factors for CKD. Retrieved 11 22, 2009, from Life Options: http://www.lifeoptions.org/kidneyinfo/ckdinfo.php?page=3

Healthcommunities.com. (2009). Renal System Anatomy. Retrieved 11 18, 2009, from Nephrology Channel: http://www.nephrologychannel.com/anatomy.shtml

Rolfe, D. V. (2004). RLO Physiology of the Kidneys. Retrieved 11 18, 2009, from The University of Nottingham : http://www.nottingham.ac.uk/nmp/sonet/rlos/bioproc/kidneyphysiology/2.html