Vertebrate kidney structure and function ap biology chapter 44

Human Excretory System  Kidneys (2)  Ureters (2)  Urinary bladder  Urethra

Nephron  Functional unit of the kidney  3 main jobs:  Filtration of water and solutes from blood  Reabsorption of water and molecules back into blood or interstitial fluid  Secretion of ions and other waste products from capillaries  Remember: the idea is to concentrate the fluids (filtrate) in the tubes of the kidney to produce urine that has higher osmolarity than that of blood

 Filtration:  Glomerulus  Bowman’s capsule  Reabsorption  Proximal convoluted tubule  Reabsorbs 75% of the water, salts, glucose, and amino acids  Reabsorption/Secretion  Loop of Henle  Countercurrent exchange, which maintains the gradient  Distal convoluted tubule  Tubular secretion of H ions, potassium, and certain drugs

Overview: Flow of Filtrate  Bowman’s capsule  proximal convoluted tubule  descending loop of Henle  ascending loop of Henle  distal convoluted tubule  collecting duct  renal pelvis  ureter  bladder  urethra

Urine production through filtration, secretion, and reabsorption  1. Blood pressure in glomerulus forces fluid into Bowman’s capsule (BC) [example of bulk flow!]  Blood is now called filtrate and is composed of glucose, salts, vitamins, nitrogenous wastes (urea)  The osmolarity of blood and filtrate are about equal at this point since both water and salt are removed

 2. As the blood flows from BC to proximal convoluted tubule (PCT) solutes are reabsorbed from the tubule  Water and salts are reabsorbed back into interstitial fluid  Ammonia, drugs, ions (H + ) are secreted into tubule  Some ions are actively transported  Water flows passively out of PCT

 3. From the PCT filtrate moves down the descending loop of Henle  Water is reabsorbed by the interstitial fluid out of the tubule  This section is permeable to water but not to solutes  As water is reabsorbed, the concentration within the tubule increases and is at its maximum at the elbow of the loop

 4. As the filtrate makes the turn at the bottom of the loop, the epithelium is change and becomes permeable to salt but not to water  Salt is transported out of the ascending loop of Henle, which makes the interstitial fluid on the inner medulla of the kidney very saline  The filtrate within the loop is less concentrated  Through the thick portion of the loop, salt is actively pumped out and the filtrate continues to decrease in osmolarity

 5. Filtrate enters the distal convoluted tubule (DCT) where ions are secreted into the tubule; water and bicarbonate are reabsorbed by the interstitial fluid  Concentration of the filtrate is at its lowest point along the DCT

 6. From the DCT the filtrate again turns toward the inner medulla down through the collecting duct  Water is reabsorbed and the filtrate becomes increasingly more concentrated  Interstitial fluid increases in osmolarity from the cortex to the inner medulla so what is constantly diffusing out of the tube back into the interstitial fluid  This creates a urine that is hypertonic to most of the body fluids (since urea is present), becoming more concentrated as the water diffuses out of the duct

Hormone Control  Antidiuretic  Any chemical that prevents excessive urine production  Example: antidiuretic hormone (ADH)  Diuretic  Any chemical that increases urine production  Example: coffee, tea, alcohol  Alcohol inhibits secretion of ADH, increasing urine production