Lesson # 21

1. The Urinary System 2 Lesson # 21 Chapter 26 Objectives: 1- Describe the basic processes responsible for urine formation. 2- Discuss the factors that influence glomerular filtration pressure. 3- Describe the hormone influence in the volume and concentration of urine.

2. Basic Processes of Urine Formation Different segments of the nephron form the urine by three different processes: Fenestrated endothelium of capillaries 1- Filtration • It is produced in the renal corpuscle when the blood pressure (hydrostatic pressure) forces water through the filtration membrane. Reabsorption Secretion Filtration • Small solute molecules pass through pores and larger solutes and suspended materials are retained. Blood pressure 2- Reabsorption • It is the removal of water and useful solutes from the filtrate. It is a selective process. • It occurs after filtrate has left the renal corpuscle. Blood Peritubular fluid Renal tubule • Most of the reabsorbed materials are nutrients that the body can use. 3- Secretion • It is the transport of solutes from the peritubular fluid to the filtrate. • Waste that did not pass by filtration.

3. Almost any molecule smaller than 3 nm can pass freely through the filtration membrane: • 1- Water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, and vitamins. 2- Some substances of low molecular weight are bound to the plasma proteins and cannot get through the membrane (most calcium, iron, and thyroid hormone). Kidney infections and trauma can damage the filtration membrane and allow albumin or blood cells to filter. Turned back: - Blood cells. - Plasma proteins. - Protein-bound minerals and hormones. - Most molecules > 8 nm in diameter. Passed through filter: - Water - Electrolytes - Glucose - Aminoacids - Fatty acids - Vitamins - Urea - Acid uric - Creatinine Proteinuria (albuminuria): It is the presence of protein in the urine. Hematuria: It is the presence of blood in the urine.

4. BLOOD PCT DCT REABSORPTION SECRETION • It is the removal of water and useful solutes from the filtrate. It is a selective process. • Secretion of ions, acids, drugs, toxins. REABSORPTION and SECRETION Water, ions and organic nutrients. COLLECTING DUCT • Variable reabsorption of water and reabsorption or secretion of sodium and potassium. Under control of Aldosterone and ADH. Peritubular fluid Renal corpuscle • Reabsorption or secretion of hydrogen, and bicarbonate. It is an important site for the control of body fluid pH. FILTRATION • Small solute molecules pass through pores and larger solutes and suspended materials are retained. H+, CO3H- Na+, K+ H+, CO3H- Na+, K+ Water Water Water Sodium, Chloride Sodium, Chloride PAPILLARY DUCT • Delivery of urine to the minor calyx. NEPHRON LOOP REABSORPTION • Further reabsorption of water, sodium and chloride ions.

5. Hydrostatic Pressure Filtration Pressures Capsular Space Plasma proteins Colloid Osmotic Pressure Blood in glomerular capillaries Net Hydrostatic Pressure: 50 – 15 = 35 mm Hg 50 mm Hg 10 mm Hg PCT Efferent arteriole 15 mm Hg 25 mm Hg Net Filtration Pressure: 35 – 25 = 10 mm Hg Afferent arteriole

6. The concentration of sodium and chloride in the peritubular fluid produce an osmotic flow of water from the Distal Convoluted Tubes and Collecting Ducts. The presence of ADH causes the appearance of special water channels (aquaporins) in the membranes of the DCT and collecting ducts. H2O H2O H2O H2O H2O H2O H2O H2O H2O The Distal Convoluted Tubes and the Collecting Ducts are relatively impermeable to water when ADH is not present. Small volume of concentrated urine Large volume of dilute urine

7. The Control of Urine Volume and Osmotic Concentration 1- Aldosterone It is a steroid secreted by the adrenal cortex. • It stimulates the reabsorption of more Na+ and secretion of K+.Water and Cl- follow the Na+. The net effect is that the body retains NaCl and water and blood pressure rises. 2- Natriuretic Peptides (ANP and BNP) They are secreted by the myocardium of the heart in response to high blood pressure. They reduce thirst and block the release of Aldosterone and ADH, thus increasing the diuresis and decreasing blood volume and pressure.

8. 3- Antidiuretic Hormone (ADH) It is secreted by posterior lobe of pituitary in response to dehydration and rising blood osmolarity. It makes the collecting duct more permeable to water. Water in the tubular fluid reenters the tissue fluid and bloodstream rather than being lost in urine. Consequently, less volume of more concentrated urine is produced. ADH stimulates the hypothalamic thirst center. 4- Parathyroid Hormone (PTH) It is secreted from parathyroid glands in response to calcium deficiency • It acts on PCT to increase phosphate excretion, and acts on the thick segment of the ascending limb of the nephron loop, and on the DCT to increase calcium reabsorption.