Osmoregulation and Excretion

1. Osmoregulation and Excretion Chapter 44 – Excretory System Show Campbell video – Ch 44

2. The Urinary System • Osmoregulation: Mammals must account for water gain/loss to maintain homeostasis = keep tolerable limits of water AND solute in interstitial fluid • Water gain = ingestion/absorption from gut (87%) = metabolism (13%) • Water loss = urinary excretion (58%) = evaporation from lungs, skin and sweating (35%) = elimination of feces (7%)

3. Urinary Excretion • Osmoregulation : Provides control of water loss and eliminates excess water and solutes as urine • Osmolarity : total solute concentration expressed as molarity or moles of solute/liter • Solute gain = absorption from gut = secretion from cells (ex – CO2) = respiration (O2 and CO2) = metabolism (O2 and CO2) • Solute loss = urinary excretion = respiration = sweating

4. Components to Urine • A form of nitrogenous waste: • Ammonia (fish) = excreted mostly as NH4+ but need lots of excess water due to high toxicity levels • Urea (mammals, adult amphibians, sharks, turtles) = made from ammonia and CO2 in liver = (NH2)2CO = low toxicity but high energy demands to make it • Uric acid (birds, insects, land snails, reptiles) = “pasty”, low water loss but VERY high energy cost to make • Excess water and solutes • Hemoglobin breakdown products • Drugs/toxins • Food additives

5. Organs of the human urinary system and functions • 2 kidneys • 2 ureters • 1 urinary bladder • 1 urethra

6. The Kidney • Size of a fist • Outer capsule of connective tissue • 2 regions = cortex (outside) and medulla (inside) • Filter water, mineral ions, organic wastes from the blood • Help to maintain homeostasis • All but 1% of the filtrate is returned to the blood • 180L of initial filtrate and only 1.5L excreted as urine/day

7. The Nephron • Intro Video • The functional unit of the kidneys which filter water and solutes from blood, make adjustments as needed • 1,000,000 nephrons per kidney (80 km)

8. Urine Forms in 3 ways • Filtration: water and solutes are forced from glomerular capillaries into Bowman’s capsule  into the proximal tubule • Tubular Reabsorption: most of the water and solutes are reclaimed by peritubular capillaries. Hormonal controls later adjust these conserved/excreted amounts • Tubular Secretion: excess ions, urea, neurotransmitters, histamine, drugs or toxins from interstitial fluid around nephron now enter the nephron through membrane proteins; they are excreted (nephrons and capillaries don’t actually touch…they move through interstitial fluid)

9. Bowman’s Capsule • Bowman’s Capsule Video

10. Reabsorption • After the hairpin turn (Loop of Henle), Na+ and Cl- are pumped out, making interstitial fluid really salty (hypertonic), drawing more water out of the proximal tubule before the turn  water is reabsorbed in the blood • Loop of Henle video • Collecting Duct Video

11. Hormones • Induced adjustments of hormone to influence the concentration of urine • ADH – antidiuretic hormone – makes walls of the distal tubule and collecting ducts more permeable to water, so more water is absorbed by the body and urine becomes MORE concentrated in response to osmolarity of blood • Effect of ADH on reabsorption video • Aldosterone – promotes reabsorption of sodium and water by the body (which increases blood volume and pressure) • Angiotensin II– promotes feelings of thirst when your body needs more water and increases blood pressure (you want to block this hormone, or its production, if you have hypertension)

12. Other body controls… • pH buffer system – helps keep the pH of blood around 7.4 • Eliminates excess H+ from the body through urine when blood is filtered in the kidneys • Remember : H2O + CO2 H2CO3  HCO3- + H+ • Body temperature – see ch. 40 notes on endotherms and ectotherms • Temperature regulation in mammals • Heat: vasodilation (open capillaries – red face), sweating • Cold: vasoconstriction (close capillaries – conserve heat), shivering, hair stands up