Topic 23: The Excretory System

Excretion (kiválasztás) refers to the removal of metabolic wastes from the cells and the body.  It involves many organs:
- liver:  involved in detoxification, hemoglobin breakdown and amino acid breakdown
- skin: via sweating
- lungs: removal of carbon dioxide
- urinary/excretory system: produces urine (vizelet)


The structure of the excretory system

http://www.tutorvista.com/content/biology/biology-ii/excretion-and-osmoregulation/excretion-osmoregulation-man.php

The excretory system functions by filtering the blood, reabsorbing important compounds and secreting wastes in the form of urine, in order to maintain the body's fluid volume, pH and electrolyte concentrations.

http://www.medindia.net/patients/patientinfo/acuterenalfailure.htm

The kidney (vese) is about 10cm long and found at the back of the abdominal cavity.  It is surrounded by the capsule, with the cortex forming the periphery of the kidney (lighter colour) and the medulla forming the inner part (darker colour).  The nephron (nefron) is the functional unit of filtration.  There are about a million nephrons in each kidney, which run through the cortex and the medulla.  The Bowman's  capsule and the glomerulus, along with the proximal and distal convoluted tubules are found in the cortex, will the Loop of Henle runs into the medulla.

http://www.buzzle.com/articles/labeled-diagram-of-the-human-kidney.html

This image shows the part of the nephron found in the cortex (ligter pink) and the part in the medullar (darker reddish) of the kidney.  http://lyceum.algonquincollege.com/lts/onlineCourses/anatomy/content/module14-6.htm

Filtering process:

1.  Blood enters the glomerulus.  It is under high pressure.  Many of its contents pass across the capillary walls into the Bowman's capsule.  Water, glucose, vitamins, amino acids, carbamide (product of protein breakdown), uric acid (product of nucleotide breakdown), ammonia, salts and ions all make up the primary filtrate (szűrlet).  About 180L of primary filtrate is produced each day.  

http://faculty.pasadena.edu/dkwon/chapter%2019%20web%20page/chapter%2019%20urinary_files/frame.htm

2.  Reabsorption  occurs throughout the nephron, but varies in the different parts, depending on which transporter molecules are found there.  Most of the transport out of the nephron is active (requires ATP), but some is passive and movement of water occurs by osmosis.

In the promixmal convoluted tubule essentially 100% of the glucose and amino acids are reabsorbed.  Reabsorption of glucose requires transport proteins.  If a person has diabetes, and so very high levels of blood glucose, these transport proteins can't transport all of the glucose out of the filtrate before it has flowed to the next part, so glucose remains in the filtrate (and then the urine).  Thus glucose in the urine is a sign of diabetes.  About 2/3's of the ions such as sodium, potassium, calcium and chloride are reabsorbed, and so is about half the urea.  Meanwhile hydrogen ions, many medications and toxins are secreted into the nephron.  While all this is happening about 2/3's of the water is also reabsorbed by osmosis.

In the Loop of Henle, the decending loop allows for the reabsorption of water, while the ascending loop allows for passive and active transport of salts.  

In the distal convoluted tubule the reabsorption of the remaining water is dependent on the presence of the hormone vasopressin (also called anti-diuretic hormone, or ADH for short).  It affects the water permeability of the tubule wall and if it is present, then more water will be reabsorbed.  This is an active process.  Another hormone that plays a role in the distal convoluted tubule is aldosterone.  When it is present more sodium is reabsorbed and more potassium is secreted.  In addition, hydrogen ions and ammonium are also secreted.

The collecting tubule/duct is normally impermeable to water, but in the presence of vasopressin, becomes permeable.  Urea can also cross the collecting duct.

http://en.wikipedia.org/wiki/Nephron

The liquid that flows out of the collecting duct into the renal pelvis is the final filtrate, or urine.  It is usually about 2L per day, but its quantity and composition are dependent upon hydration, what the individual has consumed and hormone production.  Standardly urine contains a mixture of water, salts, especially NaCl, and metabolic wastes and toxins.  It should not contain any glucose or proteins.

The urine flows through the ureters, which are formed of smooth muscle and carry out peristaltic motion, to the bladder.  The bladder can hold up to 300-350ml of urine.  Pressure receptors in the bladder wall send signals to the brain about how full it is.   From the bladder, urine exits the body via the urethra.  Emptying of the bladder is controlled by sphincter muscles that are under concious control.

Disorders:

Kidney stones:  crystalized minerals found in the kidneys or the tracts of the urinary system.  If the stones are small, they simply pass through the system, but as their size increases they can cause obstruction.  Kidney stones are known to cause excruciating pain.  Calcium-based stones are the most common, but other minerals can also form stones.  Risk factors for stone formation include dehydration, high dietary intake of protein and sugar, and certain metabolic conditions.

Infections:  Urinary tract infections are typically bacterial.  They are associated with fever, lower back pain, pus in urine.  Chronic and/or serious infections can lead to problems with kidney function.

Bladder cancer: characterized by blood in the urine.  Usually found early and successfully treated.  Smokers have higher rates of bladder cancer.

Kidney failure:  characterized by a reduction in the rate of glomerular filtration

1.  Acute kidney failure -  it usually progresses rapidly, but with appropriate treatment is also usually reversible.  Possible causes include loss of body fluids, infections, hypertension, toxins, chemicals, etc

2.  Chronic kidney disease - it usually progresses less rapidly, but is not reversible.  Possible causes include metabolic disorders, toxin accumulation, hypertension and it often shows an inherited predisposition.  Treatment includes dialysis (filtration of blood through a machine - not painful, but time-consuming) or kidney transplant.