III] KIDNEY and WASTE EXCRETION A] Macronutrients & wastes 1. Carbohydrates a. formula is [CH 2 O] n b. break down into CO 2 and H 2 0 which are exhaled

2. Lipids a. formula involves C,H and a few O b. converted to glucose or pyruvate & breakdown into CO 2 and H 2 0  exhaled

3. Proteins a. formula involves C, H, O and N b. breakdown into amino acids & then amino group is removed before converted to pyruvate or oxaloacetate which breakdown into CO 2 and H 2 O c. the -NH 2 or amino group is a problem  it is toxic

O // -NH 2 + -NH 2 + CO 2  H 2 N – C – NH 2 d. amine wastes are combined with carbon dioxide as follows to form urea amine + amine + CO2  urea

B] Goals of the kidney 1. filter as much blood as possible– 1.2 L per minute 2. conserve the glucose and amino acids in the blood 3. retain sodium in the body 4. retain as much water as possible 5. eliminate the urea and other wastes from the blood 6. be able to adjust system to account for changes in dietary water, proteins, minerals

C] The parts of the system kidney ureter bladder urethra

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RENAL ARTERY GLOMERULUS ASCENDING ARM COLLECTING DUCT D] The parts of the nephron DISTAL TUBULE BOWMAN’S CAPSULE PROXIMAL TUBULE DESCENDING ARM of LOOP of HENLE RENAL VEIN

E] Concept of Countercurrent System 1. the countercurrent system is similar to an uncontrolled positive feedback loop 2. it depends on the fact that as water is pulled out of the decending arm of the Loop of Henle by the NaCl gradient, the concentration of the remaining solution changes 3. The development of a countercurrent system is shown below in 3 steps.

300  mol/L 300 Loop of Henle is at equillibrium No pumping or diffusion is occuring A 300

B. 300  mol/L Ascending arm is pumping out sodium The interstitial tissue surrounding the nephron now has a higher sodium concentration than the nephron

C. The higher concentration of sodium outside the nephron descending arm causes water to diffuse out of the descending arm of the nephron. 300  mol/L H2OH2O H2OH2O

D. As water diffuses out of the descending arm, the concentration of sodium remaining increases 300  mol/L H2OH2O H2OH2O This means that sodium pumping by the ascending arm creates a larger gradient outside the nephron

4. This countercurrent eventually builds up to a maximum level of 1400  mol/L H2OH2OH2OH2OH2OH2OH2OH2OH2OH2OH2OH2O

F. The value of the counter current system 1. Most of the water will be removed at the proximal tubule [85%] 2. More water will be removed at the descending arm [5 %] 3. The counter current system is designed to recover additional water from the collecting duct [10 %]

4. It does this by creating a concentration gradient in the tissues outside the collecting duct and then water will move from the area of high concentration in the urine to an area of low water concentration in the interstitial fluids.

G. Achieving the goals of the kidney 1. Filtering a. The blood from the renal artery enters the glomerulus under high pressure. b. All large molecules and cells stay in the blood, all dissolved molecules pass through. d. The NaCl concentration is 300  mol/L. c. The Bowman’s capsule therefore receives plasma + dissolved molecules now called urine.

2. Retaining glucose and amino acids b. Water follows these molecules out of urine due to the concentration gradient. c. The NaCl concentration of the urine is still 300  mol/L. a. The cells in the proximal tubule actively transport glucose & amino acids out of urine [because NaCl is also pumped out of the nephron here]

glucose amino acid H2OH2O H2OH2O NaCl

3. Retaining sodium a. Pumps in the ascending arm of the Loop of Henle pump out NaCl. c. By the top of the ascending arm, there is 300  mol/L in the urine. b.Due to the countercurrent system this means that the level of [NaCl rises] to as high as 1400  mol/L in the urine and surrounding tissue. d. Sodium / potassium pumps in the distal tubule and the collecting duct will pump additional sodium out of the urine.

area with extra sodium area very high in sodium glucose amino acid H2OH2O H2OH2O NaCl K + Na + Na + K + NaCl H20H20

4. Retaining water a. About 85% of the water leaves the urine in the proximal tubule due to glucose & amino acid gradients. b. 5 % of the water leaves the urine in the descending arm due to the sodium gradient. c. 0 – 10 % water leaves the urine in the collecting duct due to the sodium gradient. d.Remember water can never be pumped It moves due to a concentration gradient

area high in sodium glucose amino acid H2OH2O H2OH2O NaCl K + Na + Na + K + H2OH2O H2OH2O urea H2OH2O NaCl H20H20 urea + some H 2 O some Na & K other wastes

5. Removing urea and wastes from the body a. The urea is dissolved in the plasma and therefore enters through the Bowman’s capsule. b. Some additional wastes are added at the distal tubule by exocytosis. 6. Controlling the system over a range of conditions hormones d. Water moves out of the urine due to the [gradient] c. In cases of severe dehydration, the urea diffuses out of the urine

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