: the excretory system is also called the urinary system

: as far as systems go, the anatomy is fairly simple and straightforward. There are only a handful of organs involved: the kidneys, ureters, urinary bladder and the urethra.

: the main function of the excretory system is to remove tissue wastes from the body before they build up to toxic levels. The lungs remove carbon dioxide from the blood stream and are sometimes considered part of the excretory system for that reason. Other tissue wastes, such as salts and nitrogenous wastes are removed by the kidneys.

: Another function of the excretory system is to regulate water and electrolyte concentrations, blood pH, and volume of body fluids; and to control red blood cell production and blood pressure.

: Kidneys : the bean is named after it, not vice versa

-- a reddish brown, bean-shaped organ. It has a smooth surface and it runs about five inches long and 2-3 inches in width, with a thickness of just over an inch. It has a tough capsule around it.

-- there are two kidneys, one of each side of the vertebral column. The left kidney is generally slightly higher than the right.

-- the kidneys are behind the parietal peritoneum and against the deep muscles of the back.

-- kidneys have a convex and a concave side. The medial, concave side includes the indentation called the renal sinus, or renal hilus. This is where the renal blood vessels, nerves and lymphatic vessels enter the kidney. It is also where the ureter leaves.

-- the renal pelvis is a superior expansion of the ureter into a funnel shape.

-- the inside of the kidney is split into an inner medulla and an outer cortex. The cortex is where the tiny tubules called nephrons are located.

: Ureters : kidney to bladder pathway

-- a ureter runs about 25cm (10 inches) long. It moves from behind the parietal peritoneum down and somewhat forward to join with the urinary bladder.

-- there are three distinct layers of a ureter: an inner mucous coat, a middle muscular coat and an outer fibrous coat. The muscular layer helps to move urine from the kidney to the bladder by way of peristalsis.

: Urinary Bladder : temporary storage

-- located within the pelvic cavity, behind the pubic symphysis, beneath the parietal peritoneum

-- when empty, the bladder’s inner wall collapses into many folds, but it stretches out and becomes smoother and urine is "injected" into the bladder by the ureters

-- the internal "floor" of the bladder has a triangular area called the trigone. Each of the corners of the trigone has an opening. The posterior angles open into the two ureters, and the anterior angle opens to the neck of the bladder and into the urethra.

-- there are four total layers to the bladder: the inner mucous coat, which is continuous with that of the ureter, consists of several thicknesses of transitional epithelial cells. Because transitional epithelium can change shape, the bladder can expand and contract.

-- superficial to the mucous coat, there is a submucous coat, consisting of connective tissue and elastic fibers. Outside of that is a muscular coat of smooth muscle fibers, the interlocked of which comprise the detrusor muscle. The part of this muscle that surrounds the bladder neck makes the internal urethral sphincter, which prevents the emptying of the bladder until a certain pressure level is reached. The micturition reflex is carried to this sphincter by parasympathetic fibers.

-- the outer layer of the bladder is the serous coat, consisting of parietal peritoneum.

:Urethra : home free

-- a tube that takes the urine from the bladder to the outside

-- it is lined with mucous membrane and has a relatively thick layer of smooth muscle tissue.

-- it also contains mucous glands (urethral glands) that secrete mucus into the urethra.

: Kidney

-- the renal pelvis is divided into 2-3 tubes, called major calyces, and they in turn are subdivided into several minor calyces. A series of small openings piercing the renal papillae lead to each minor calyx.

-- the renal medulla is composed of conical masses of renal pyramids

-- the renal cortex is more granular in appearance and forms a sort of "shell" around the medulla. Renal columns dip into the medulla between adjacent pyramid tissue. The granular appearance is due to the nephron units, arranged randomly in the renal cortex.

-- the renal artery -- a branch of the abdominal aorta -- supplies blood to the kidneys. The renal artery supplies a great deal of blood; at rest, 15-30% of total cardiac output will be directed to the kidneys.

-- just inside the hilus, the renal artery branches several times into the interlobar arteries which pass between the renal pyramids and, at the junction of medulla and cortex, branch to form the arciform arteries. The lateral branches of the interlobar arteries, called afferent arterioles, lead to the nephrons.

-- venous blood returns through arterial-corresponding pathway. Blood from a nephron enters the interlobar vein, then into the renal vein, which then joins with the inferior vena cava.

: Nephron : a functional unit of excretion

-- there are about a million nephrons in each kidney, each consisting primarily of a renal corpuscle and a renal tubule.

-- the renal corpuscle consists of a cluster of capillaries (a glomerulus) and the Bowman’s capsule around it. The capsule is the expansion at the closed end of a renal tubule. Further down the tubule is a twisted portion called the proxmial convoluted tubule, which dips towards the renal pelvis to become the loop of Henle. The ascending loop of Henle forms a highly twisted area in the renal corpuscle called the distal convoluted tubule, several of which merge into one collecting duct. The ducts pass through the medulla and eventually join up to empty into a minor calyx.

-- after blood passes through the glomerulus, instead of entering a venule, it enters the efferent arteriole, which has a diameter larger than the capillary but smaller than the afferent arteriole. The small diameter causes some resistance to blood flow, and therefore a backup of blood in the glomerulus and a relatively high blood pressure there.

-- the efferent arteriole branches into the peritubular capillary system which forms a network around the renal tubule. The blood in this system is under relatively low blood pressure.

-- the macula densa is a structure comprised of special epithelial cells found at the point of contact between the distal convoluted tubule and the afferent and efferent arterioles. Close by this structure are some enlarged smooth muscle cells than, with the macula densa, form the juxtaglomerular complex.

: Urine Formation -- the primary function of the nephrons

-- begins when water and certain dissolved substances are filtered out of the glomerular capillaries and into the Bowman’s capsules.

-- the main force responsible for moving things through the walls of the glomerulus is hydrostatic pressure. Osmotic pressure in the plasma of the glomerulus and the hydrostatic pressure inside the Bowman’s capsule play a role as well. The filtration pressure is the net pressure acting to remove substances from the glomerulus.

-- glomerular filtrate consists of substances that enter the Bowman’s capsule, and is relatively the same as tissue fluid found in other areas of the body, except it lacks larger protein molecules.

-- the rate of glomerular filtration is directly proportional to the filtration pressure. In the average adult, the glomerular filtration rate is about 125ml/minute or 180l/day. This is equivalent to about 45gal/day. Since all of this is obviously not secreted as urine, where does the excess go? Most is reabsorbed through the renal tubules and reenters the plasma.

-- regulation of the filtration rate involves the juxtaglomerular complex and two negative feedback systems, triggered by decreasing filtration rates. The decrease of chloride ions reaching the macula densa would be one feedback loop stimulating the release of renin is one loop.

-- tubular reabsorption occurs throughout the renal tubule, but most occurs in the proximal convoluted tubule area. The epithelium of this section has microvilli that enhance the reabsorption by increasing the surface area to reabsorb the material. Glucose is one thing that is reabsorbed, mostly in the promixal convoluted area, water too (portions of the distal convoluted area are all but impermeable to water).

: glucosuria, or glucose in the urine, may occur after administering glucose to a insulin-dependent diabetic patient. It is due to the lack of insulin production, so that all glucose is not utilized.

-- amino acids are reabsorbed, for the most part, in the proximal convoluted area by active transport. Albumin present in the filtrate may by taken out in the proximal by pinocytosis; other substances removed in this area include creatine, lactic acid, citric acid, uric acid, ascorbic acid, phosphate ions, sulfate ions, calcium ions, potassium ions, and sodium ions.

-- water is reabsorbed passively via osmosis, and is closely associated with sodium ion reabsorption (an active transport). As water leaves the tubule, the substances left in the filtrate become more and more concentrated. Additional water may be removed from the filtrate in the case of ADH or aldosterone secretion. Both of these hormones reduce urine output by enhancing water reabsorption in the tubules.

-- a by-product of amino-acid metabolism, urea concentrations in the blood stream will increase as the amounts of protein in the diet increase. About 50% of the urea that enters as filtrate is reabsorbed, and the remainder goes out in the urine.

-- uric acid is a result of metabolism of certain organic bases in nucleic acids. Only about 10% of the amount filtered is excreted into the urine, though the active transport reabsorption mechanism seems more than capable of removing all of it from the tubule. Gout may result from increased plasma levels of uric acid, where the substance -- which is relatively insoluble -- precipitates out and forms crystals in the joints and other tissues.

-- certain substances will be transported from the plasma of the peritubular capillaries into the renal tubule, rather than enter the tubule through the Bowman capsule via filtration. Some of these substances will be secreted by active transport -- penicillin, creatinine, histamine, hydrogen ions (regulates blood pH), and some potassium ions.

: Composition of Urine

-- the actual composition of urine will vary, but in general, it will contain :

: about 95% water

: urea & uric acid

: trace amounts of amino acids

: electrolytes (varying concentrations depending on diet)

-- the volume of urine will also vary, and can be from 0.6-2.5 liters per day. Variations will depend on fluid intake, environmental conditions, relative humidity and emtional condition of the person in question. 50-60cc of urine output per hour is normal. Less than 30cc is indicative of kidney failure.

-- kidney stones are composed of uric acid, calcium oxalate, calcium phosphate or magnesium phosphate, and will form due to increased concentrations of the substance in question. 60% of sufferers pass their stones without problem, but others must have the stones removed. They are quite painful if they move into the ureter and may also be accompanied by nausea and vomiting.

: Micturition : urination

-- the need to urinate is stimulated by distension of the bladder as it fills. The first such urge usual occurs when there is about 150ml of fluid in the bladder. The pain receptors in the bladder are activated when there is about 500ml of fluid. 300ml is about the limit of comfort.

-- like the defecation reflex, the internal urethral sphincter will relax, and fluid will move about 3cm down the urethra to the external urethral sphincter, which is made of skeletal muscles and is under conscious control. When a decision has been made, the external sphincter relaxes, and urination occurs.

-- an automatic bladder occurs when spinal cord damage prevents voluntary control over the external sphincter. When the bladder reaches about 150ml, the reflex is triggered and urine is released automatically.