Proximal Convoluted Tubule Active Reabsorption Nutrients glucose and

Proximal Convoluted Tubule • Active Reabsorption – Nutrients (glucose and amino acids) – Ions (K+, Na+, Cl-) ~70% of Filtrate is – Small plasma proteins reabsorbed in PCT – Some urea and uric acid Na+ is actively reabsorbed: First – simple diffusion: Then – 1 o active transport:

Na+ linked 2 o Active Transport Symport with: – Glucose – Amino acids – Ions (e. g. , Ca 2+)

Passive Transport of Water: – As Na+ pumped out, H 2 O follows by osmosis. (passive) Passive Transport of Urea: – As other solutes leave lumen, [urea] higher than ECF, thus passively diffuses into ECF. Transcytosis of Proteins: – Small proteins can get into filtrate, due to size they are reabsorbed via vesicular transport.

Reabsorption of Urea

Transporter Characteristics – Specificity: glucose, fructose, tyrosine, valine, etc, all have own carriers. – Competition: maltose instead of glucose – takes a seat, but not transported. – Saturation (# of carriers): limited # of carriers to transport solutes back into body. A substance can exceed renal threshold, e. g. , glucosuria.

H 2 O Reabsorption – Loop of Henle a key site. • First, Na+ transported out of filtrate. • Osmolarity of ECF gets higher. • As loop gets deeper into renal medulla, more H 2 O is drawn out. • Filtrate becomes Very concentrated! • Collecting duct also a key site for H 2 O reabsorption – (role of ADH).

Ascending Loop of Henle • Region is impermeable to H 2 O. • Thus, H 2 O can no longer leave filtrate in this region, so Osmolarity becomes lower again at start of DCT.

Secretion – DCT a key site. • Active Transport into nephron tubules + + e. g. , K , H and HCO 3 • Fine-tuning of filtrate; getting rid of what body needs to eliminate. • All that remains in tubules is destined to be urine unless reabsorbed in collecting duct.

Final Modification: Collecting Ducts • Reabsorption of Na+ • Reabsorption of H 2 O • Under Endocrine Control – ADH (vasopressin) • After collecting duct, filtrate now called urine (no longer modified).

Mictruition Reflex

Autoregulation of Renal System

Liver Lungs Kidneys Adrenal Cortex ______________________ (inactive) ______ (activated) ________ (active) Kidneys ____________ (_______) Na+ _______ Vasoconstriction Reabsorption of H 2 O Thirst Stimulation H 2 O _______

Liver Lungs Kidneys Adrenal Cortex Aldosterone Angiotensin Converting Enzyme (ACE) Renin Angiotensinogen (inactive) Angiotensin I (activated) Angiotensin II (active) Kidneys Anti Diuretic Hormone (ADH) (Vasopressin) Na+ retention Vasoconstriction Reabsorption of H 2 O Thirst Stimulation H 2 O retention

Comparison of Fluids Substance (parameter) Blood Plasma Filtrate Urine

Renal Failure When kidney function disrupted to the point they are unable to perform regulatory and excretory functions sufficient to maintain homeostasis. Acute – sudden onset with rapid reduction in urine formation (less than 500 ml/day minimum being excreted). Chronic – slow, progressive, insidious loss of renal function. Up to 75% of function can be lost before detected.

Polycystic kidneys (16 to 18 pounds combined).

Variety of Causes: 1. Infectious organisms. - Blood borne microbes - UTI’s 2. Toxic agents. - lead, arsenic, pesticides, additives, medications - long-term exposure to high aspirin doses 3. Inflammatory immune response (allergic). - glomerulonephritis - e. g. , after strep throat (streptoccocus)

Variety of Causes: 4. Obstruction of urine flow. - Kidney stone (uric acid-calcium crystals) - Tumors - Enlarged prostate gland All create back pressure, decreasing GFR 5. Insufficient renal blood flow. - 2 o to heart failure - Hemorrhage (e. g. shock) - Atherosclerosis Leads to inadequate Filtration pressure

Potential Ramifications: 1. Uremic Toxicity - Caused by retention of toxins/waste products in blood. 2. Metabolic Acidosis - From inability of kidneys to secrete H+. 3. Potassium (K+) retention - Inability to secrete K+ (effects RMP).

4. Na+, Ca 2+ and phosphate and Imbalances - Inability of kidneys to regulate ion reabsorption and secretion. 5. Loss of plasma proteins - Result of increased leakiness of glomerulus. 6. Anemia - Inadequate erythropoiten production. 7. Depressed immune system - Increased toxic waste and acidic conditions.