Organs of the Urinary System

1. Organs of the Urinary System • Kidneys • Ureters • Urinary Bladder • Urethra

2. The Kidneys • Perform most of the work of the urinary system • Continuously cleanse the blood and filter the fluids from the bloodstream • Helps dispose of wastes and excess ions • Produces the enzyme renin, which regulates blood pressure • Produces the hormone erythropoietin, which stimulates red blood cell production • Coverts Vitamin D to its active form

3. The other organs • Ureters – slender tubes that carry urine by peristalsis from the kidney to the bladder • Urinary Bladder – a smooth, collapsible muscular sac that stores urine temporarily • Urethra – thin walled tube that carries urine by peristalsis from the bladder out of the body. It has two sphincter muscles to help with bladder control. • Internal urethral sphincter – an involuntary muscle that keeps the urethra closed when urine is not being passed • External urethral sphincter – a voluntary muscle (skeletal) near the floor of the pelvic region

4. Micturition • Also known as voiding • This is the act of emptying the bladder • Ordinary, the bladder will hold urine until about 200mL is gathered. At this point, the bladder begins to stretch, activating stretch sensors. • This causes contractions which forces urine through the internal urethral sphincter. But we can choose whether or not to allow the urine past the external urethral sphincter. If we relax this muscle, urination occurs.

5. So, what is urine? • Urine results from three processes • Filtration • Tubular reabsortion • Tubular secretion

6. Filtration • Kidneys contain millions of tiny structures called nephrons, which contain two structures, the glomerulus and renal tubule. • The glomerulus, or knot of capillaries, acts as the filter • Filtration occurs when a force is applied to the fluids in the glomerulus. This force pushes the smaller molecules through the glomerulus, but bigger particles are too large to pass through and are retained.

7. Tubular Reabsorption • Some of the small ions that are removed are still needed, so the body must reclaim them. • This is completed by a portion of the renal tubule. • The tubule cells act as transporters that take up needed substances from the filtrate and taking them to the extracellular space, where they are reabsorbed by blood capillaries. Most of this occurs by active transport and therefore requires energy. • Needed molecules: glucose, some ions, and amino acids • Unneeded molecules: urea, uric acid, and creatinine

8. Tubular Secretion • Essentially tubular reabsorption in reverse. • Certain larger molecules are moved into the tubule to be excreted • This could be certain drugs or certain molecules that help maintain the blood’s pH

9. Characteristics of Urine • Urine is the end result of cleaning the filtrate and returning the nutrients and most of the water to the blood stream. • It mainly contains nitrogenous wastes and unneeded substances. • Urine can be clear to deep yellow. • Yellow is caused mainly by the pigment urochrome, which results from the destruction of hemoglobin and other dissolved solutes • It is slightly acidic (pH of about 6) but can change with diet • Is more dense than distilled water

10. Blood Composition • The composition of the blood depends on three major factors • Diet • Cellular metabolism • Urine output • The kidneys help maintain the blood by • Excreting nitrogen containing wastes • Maintaining water levels • Maintaining electrolyte balances • Ensuring proper blood pH

11. Maintaining Water Levels • If massive amounts of water is taken in, the kidneys will allow a large amount of it to pass out of the body with the urine. • If water is scarce, the kidneys will reabsorb the water from urine and make the urine more concentrated.

12. Maintaining electrolyte balances • Water will follow salt, so it is important to maintain the correct balance of electrolytes (salts dissolved into water) • As a result, the kidneys respond to hormones to either reabsorb or release ions in the blood with the urine • Egg video

13. Ensuring proper blood pH • The body uses blood buffers to maintain a pH between 7.35 and 7.45. • The main buffer system in the body is the bicarbonate buffer system and made by mixing the weak carbonic acid and its salt sodium bicarbonate. • The salt can act as a base an help to neutralize acids, while the carbonic acid can help to neutralize bases.