Urine Formation Chapter 15

Urine Formation Occurs in nephron Molecules exchanged between blood vessels (glomerulus and peritubular) and renal tubule 3 steps Pressure Filtration Selective Reabsorption Tubular Secretion

Pressure Filtration Occurs inside the Bowman’s capsule High blood pressure in the GLOMERULUS forces SMALL MOLECULES [such as nitrogenous wastes, *H2O, *nutrients, *ions (salts)] into BOWMAN'S CAPSULE. The AFFERENT ARTERIOLE supplies the glomerulus with blood.

Pressure Filtration Large, non-filterable molecules are unable to pass (i.e. blood cells, platelets, proteins). These remain in the blood and leave the glomerulus via the EFFERENT ARTERIOLE. (Efferent – “E” for “E”xit.) The small, filterable molecules that are forced into Bowman's capsule form FILTRATE

Selective Reabsorption If the kidneys only did pressure filtration, we would quickly die from water and nutrient loss. Once the original filtrate is made, the next task of the kidneys is to reabsorb molecules in the filtrate that the body cannot afford to lose. (e.g. water, nutrients, some salts)

Selective Reabsorption Takes place along renal tubule Proximal convoluted tubule Loop of Henle Distal convoluted tubule

Reabsorption and Secretion at the Proximal Convoluted Tubule CARRIER MOLECULES determine what is reabsorbed and what passes through the tubule; This is done by ACTIVE transport except for the water, which is reabsorbed by OSMOSIS. The molecules that are reabsorbed move from the proximal convoluted tubule to the peritubular capillary network. Most of the glomerular filtrate gets reabsorbed!

Reabsorption and Secretion at the Proximal Convoluted Tubule WHAT GETS REABSORBED?: most H2O, nutrients (glucose, amino acids, vitamin C, potassium ions…) some salts (NaCl) A balanced salt concentration in the blood must be maintained. The process of selective reabsorption ensures this by actively reabsorbing sodium ions while chloride ions follow passively. WHAT DOES NOT GET REABSORBED and therefore SECRETED?: some H2O, wastes, excess salts. Non-reabsorbed material continues through Loop of Henle.

Reabsorption and Secretion at the Loop of Henle and the Distal Convoluted Tubule Filtrate now enters the LOOP OF HENLE and, eventually, the distal convoluted tubule Primary role of Loop of Henle and distal convoluted tubule is REABSORPTION OF WATER. Over 99% of the water in original filtrate is reabsorbed by the nephron during urine formation. Much of this reabsorption is done by OSMOSIS at the Loop of Henle. This CONCENTRATES THE URINE

Reabsorption and Secretion at the Loop of Henle and the Distal Convoluted Tubule Also, the loop of Henle secretes NaCl into the surrounding tissue (the renal medulla) to ensure that the neighbouring tissue is hypertonic to the filtrate. By doing this, the loop of Henle creates an osmotic gradient. Na+ ions are actively reabsorbed as their uptake is associated with water retention (remember, the body does not want to lose or waste water!)

Tubular Secretion Although urine formation occurs primarily by selective reabsorption, a supporting mechanism, called tubular secretion, is also involved. This is an ACTIVE PROCESS by which other non-filterable wastes (i.e. those wastes that cannot be added to the filtrate at Bowman’s capsule) can be added to the tubular fluid so that these wastes will also be excreted in the urine. Tubular secretion occurs along the distal convoluted tubule Actively secreted substances include some chemicals (e.g. penicillin, histamine) H+ ions, NH3.

TUBULAR SECRETION Fluid now enters the COLLECTING DUCT H2O PASSIVELY DIFFUSES OUT OF COLLECTING DUCT AND STAYS IN THE BODY Also, both H+ and K+ ions are secreted INTO the filtrate exchanging it with Na+. This ensures that water is reabsorbed as well. Tubular excretion is important in maintaining the pH of blood The tubular fluid, which we can now “OFFICIALLY” call URINE passes from the collecting duct into the pelvis of kidney, and enters the ureter for transport to the bladder.

Characteristics of Urine Clear to yellow in color Due to urochrome (a pigment resulting from destruction of hemoglobin) Sterile and slightly aromatic when formed If allowed to stand, takes on ammonia odor caused by action of bacteria on urine solutes Slightly acidic but pH can vary from 4.5 to 8.0

Urine Water plus solutes therefore, more dense than water Specific gravity compares how much heavier urine is than distilled water Pure water – specific gravity of 1.0 Urine – specific gravity of 1.001 (diluted) to 1.035 (concentrated)

Urine Substance found in urine: Nitrogeneous wastes Water Ions like sodium and potassium

Urine Substances NOT found in urine and their presence can indicate a medical condition: Glucose Blood proteins Blood Pus Bile

Micturition Act of emptying bladder Web link Video Clip

Micturition Steps: Bladder collects about 200 ml of urine Stretching of bladder activates stretch receptors Impulses from receptors sent to spinal cord and cause bladder to contract

Micturition As contractions become stronger, stored urine is force past internal sphincter into upper part of urethra. At this point, the urge to void is felt The external sphincter is controlled voluntarily, so it can be kept closed

Micturition When one chooses to not relax the external sphincter, the contractions will stop and urine will continue to accumulate in the bladder After 200-300 ml more have been collected, micturition occurs whether one wills it or not

Urethra Sphincters Internal urethral sphincter Thickening of smooth muscle at the bladder urethral junction Involuntary, keeps urethra closed when urine not being passed External urethra sphincter Made of skeletal muscle Voluntary control

Incontinence Unable to voluntarily control the external sphincter Common in children under 2 older children who sleep very soundly Pregnancy due to pressure Stroke or spinal injury patients

Urinary Retention Unable to expel urine Often occurs after surgery in which anesthesia has been given, because it takes time for the smooth muscles to regain activity A slender drainage tube called a catheter can be inserted through the urethra to drain the urine and prevent bladder trauma