...................... THE HUMAN URINARY SYSTEM AND THE EXCRETION

Activities to do. Read the explanation on the urinary system and perform the first Multiple Choice Test and the Mute Picture 10.1. Then read the explanations about the process of excretion and perform the second Multiple Choice Test, the second Mute Picture and the Crossword.

1. The corporal excretion in humans. Excretion is the expulsion to the outside of harmful or useless products that are in the blood and intercellular plasma. The main excretion products are the urea, the mineral salts and substances that cannot be degraded by our cells, such as specific drugs and food additives. The main part of these substances is eliminated by the urinary system (urine), and the rest is removed through the skin (sweat) and eyes (tears). There is also another very harmful substance in the blood, which is the carbon dioxide produced in the mitochondria during cell respiration. Its excess is eliminated by the lungs during corporal breathing or ventilation. Because of that some authors consider that the lungs have an excretory function, but it is the best to consider that the removal of CO2 is done during the respiration process and the excretion only covers the removal of other undesirable substances in the blood.

2 . The human urinary system. It is the apparatus consisting of the kidneys, the ureters, the urinary bladder and the urethra.

a) The kidneys. Are two bean-shaped organs, about 12 cm in length, which filter the blood and separate the urea and excess of the salts, resulting in the urine.

b) The ureters. Are two conduits about 25 cm in length.

c) The urinary bladder. Is an expandable bag with a capacity of 350 to 1500cm3.

d) The urethra. Is a conduit of about 6 cm in lenght in females and about 15 cm long in men.

3. The macroscopic anatomy of the kidney. The human kidney has on its outside a layer of tissue called the renal capsule, below lays a granular area called cortical area, deeper there is an area with numerous fiber bundles (called renal pyramids or pyramids of Malpighi) called medulla zone, and the innermost zone is a funnel-shaped structure, called the renal pelvis, which includes a series of small funnels called chalices which is where the urine enters to the Malpighi pyramids.

4 . The microscopic anatomy of the kidney. If they are made very thin cuts of a kidney and observed under a microscope, it can be seen that the human kidney is made of approximately one million nephrons, which are structures having a globose head called Malpighian Corpuscle (all together constitute the cortex, which is why it presents grainy aspect) followed by a long conduict bent into a U-shape called renal tubule (together constitute Malpighian pyramids of the medullary area, which therefore presents fibrous).

Malpighian corpuscle. Within it can be differentiated a dense net of capillaries called glomerulus of Malpighi and a kind of cup that surrounds it, called Bowman's capsule.

Renal tubule. Within it can be differentiated a sinuous segments called convoluted tubules (the one near the corpuscle is called proximal and the one which is far from it is called distal) and straight segments called straight tubules that form an U called loop of Henle, with a descending limb and ascending limb. In each branch a thick segment and thin segment can be differentiated.

5 . The formation of the urine. In this process we can distinguish 4 stages which are:

1.) Filtration. Because of the pressure inside the blood capillaries of glomeruli the water and dissolved substances of low molecular weight goes out of them; such as sodium ion (Na +) from the dissolution of the salt (NaCl), urea, glucose and amino acids, but not the red blood cells nor large molecules such as proteins.

2.) Reabsorption of the solutes. In the proximal convoluted tubule, due to the special proteins of the membrane of its cells, from the inside, the sodium ions (Na +), glucose and amino acids are extracted, which then return to the blood remaining inside the urea.

3.) Reabsorption of the water. In the straight tubule descended from the loop of Henle, by being permeable to water and sodium ion and running through the high salnity zone, there is produced the water outlet, through a process called osmosis, and the entry of sodium ion.

4.) Second extraction of sodium ions and secon reabsorption of water. The second extraction of sodium ions from the urine is performed on the rest of the route (straight tubular ascending from the loop of Henle, distal convoluted tubules and collecting tubule), thanks to special proteins of membrane of its cells. The straight tubule ascending from the loop of Henle is impermeable to water but the two next and last sections are permeable to water. In them during the osmosis the second reabsorption of water occurs, thereby the formed urine is highly concentrated. The result is that the final urine is a liquid rich in urea and uric acid, the two substances that are very toxic to our body.

6 . The other forms of excretion. The principal is sweating, and with a much lesser importance, secretion of bilirubin into the biles and salts in the tears.

The formation of the sweat. The sudoriparous glands produce sweat from the water that has left from the capillaries during the filtration, so its composition is similar to that of a very dilute urine, ie it also contains urea, dissolved salts and uric acid. Because of this the sweating process also contains a certain degree of excretion.

The excretion of the sweat depends on the temperature and humidity. In our country about 600 to 900cm3 of sweat daily is produced. The composition of sweat is:

• 99,00% of water
• 00,60% of mineral salts (NaCl)
• 00,40% of organic substances (urea, creatinine and uric acid)

However it is necessary to emphasize that the main function of sweating is not the excretion but the refreshment of the body temperature. This can be achieved because as the water needs the heat in order to evaporate it picks it from the skin, which due to this process is being cooled.

7 . The main diseases of the urinary system.

• Kidney failure. It is the diminished ability of the kidney to remove the urea from the blood. The patient require regular hemodialysis sessions with an artificial kidney.
• Renal colic. Involves painful spasms of the ureter while on its walls the solid precipitates are being stroked (kidney stones) that abnormally can form within the urine.
• Urethritis. It is an inflammation of the walls of the urethra caused by a bacterial infection or by certain chemicals substances. It can cause narrowing of the opening of the urethra.
• Urinary tract infection. Inflammation of the walls of the bladder caused by bacterial infection. It is usually accompanied by urinary incontinence (involuntary and frequent disposal of the urine in small amounts).
• Prostatitis. Inflammation of the prostate (a gland only in the male reproductive system that secrets one of the components of semen) which presses the urethra and interferes with micturition (the act of urinating).

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