Ch. 44- The Excretory System (Osmoregulation & Excretion)

1. Ch. 44- The Excretory System (Osmoregulation & Excretion)

2. HOMEOSTASIS **Recall… HOMEOSTASIS is the steady-state physiological condition of the body. It includes: • Thermoregulation: maintaining internal temperature; • Osmoregulation: regulating solute balance and gain/loss of water; • Excretion: getting rid of waste products of metabolism

3. The Excretory System Three main functions: 1) Remove wastes (ex: NH3) from blood 2) Help control blood pH, [salt], Na+, K+, H+, Mg2+, Ca2+, Cl-, HCO3- 3) Regulate body’s H2O content 

4. Toxic waste products of metabolism include ammonia (NH3): proteins  NH3 (ammonia)  urea  excreted nucleic acids

5. Evolutionary perspective on H2O balance Problems faced by… a) freshwater fish: always taking in H2O; excrete lots of dilute urine b) saltwater fish: lose H2O; drinks a lot of seawater; gills dispose of NaCl; excrete a very small amt. of water c) terrestrial animals: need to get rid of N-wastes without losing much H2O; NH3 is very toxic & it is converted to urea which is less toxic

6. When we talk about water in the body, we speak of “compartments” – places where water can go -Intracellular Fluid (ICF): includes water in the cell cytoplasm -Extracellular Fluid (ECF): includes water between cells (interstitial fluid) or in blood plasma

7. The main organs in the Excretory system include the KIDNEYS & associated “plumbing”: • Ureters • Bladder • Urethra (the skin & lungs play minor roles in excreting N waste)

8. Excretory System Processes: • Filtration: blood (or other fluid) is exposed to a filtering device (selectively permeable membranes); produces the FILTRATE • Reabsorption: selective transport of water and other valuable solutes back into the body fluids (ex: blood) • Secretion: solutes (excess salts and toxins) are removed from the blood and added to the filtrate • Excretion: final solution containing wastes is expelled from body

9. Excretory System…the basics: • Renal artery: brings blood to each kidney • Renal vein: drains blood from each kidney • Ureter: duct through which urine exits each kidney • Urinary bladder: structure into which both ureters drain • Urethra: tube that drains the urinary bladder (empties near the vagina in females; through the penis in males)

10. **KIDNEY STRUCTURE/FUNCTION: • kidneys make urine from filtered blood • very unlike how feces are made!!

11. **KIDNEY STRUCTURE/FUNCTION: • the renal artery brings blood to be filtered • between 1,100 & 2,000 L of blood flows through a pair of kidneys each day! • the renal vein returns filtered blood to the heart (this blood contains the good stuff that the body doesn’t want to urinate away)

12. **KIDNEY STRUCTURE/FUNCTION: • Kidney has two distinct regions: -RENAL CORTEX: outer region -RENAL MEDULLA: inner region • Packing both regions: about 1 million microscopic excretory tubules & associated blood vessels called NEPHRONS (approx. 50 miles worth!)

13. NEPHRONS • Each NEPHRON consists of a single long TUBULE and a ball of capillaries called the GLOMERULUS • The blind end of the tubule forms a cup-shaped swelling called BOWMAN’S CAPSULE. • Bowman’s capsule surrounds the glomerulus

14. Pathway of Filtrate: • From Bowman’s capsule: • Proximal tubule; • Loop of Henle (descending & ascending); • Distal tubule; • Collecting duct; • Renal pelvis; • Ureter…

17. GLOMERULAR FILTRATION: • occurs as blood pressure forces fluid from the capillaries into the lumen (space) of Bowman’s capsule • Capillaries are fenestrated (have many pores) which will allow any size molecule through • Outside of the capillary has a basement membrane, composed of glycoproteins which only allow a certain size molecule to pass into Bowman’s capsule • Result of ultrafiltration: Nonselective passing of water & small molecules pass into Bowman’s capsule • the composition of filtrate small molecules is similar to that of plasma (water, salts, amino acids, nitrogenous wastes, etc.)

18. Figure 1.The structure of the glomerulus.The normal glomerulus (right) is a capillary tuft supported on a basement membrane lined by podocytes on the urine-space aspect. Along its route from the blood space to the urine space, the plasma ultrafiltrate passes sequentially through the fenestrated glomerular capillary endothelium, the collagenous network of the GBM and the filtration slits and slit diaphragm formed by the podocyte foot processes. The foot processes show effacement and fusion in nephrotic syndrome (left). GBM, glomerular basement membrane; MC, mesangial cells; US, urine space.

19. GLOMERULAR FILTRATION: *FILTRATION RATE varies with FILTRATION PRESSURE

20. TUBULAR REABSORPTION: • substances are selectively reabsorbed from the glomerular filtrate back into the blood • Examples of reabsorbed molecules (through active or passive transport): • Water, HCO3-, Glucose, Amino acids, K+ • substances that remain in the filtrate become more concentrated as water is reabsorbed

21. REABSORPTION- Proximal Convoluted Tubule: • most of the NaCl and water from the huge initial filtrate volume • (Na+ actively transported Cl- passively follows as a result  water then follows by osmosis) • glucose & amino acids • pumps in the membrane reabsorb useful substances through active transport • high surface area of Proximal Tubule is due to microvilli projecting into the lumen

23. REABSORPTION- Descending Limb, Loop of Henle: • Water • Epithelium is freely permeable to water but not to salt or other solutes • For water to move by osmosis, the interstitial fluid must be hyperosmotic to the filtrate • Osmolarity of the interstitial fluid becomes more concentrated as move deep into the medulla from the cortex

24. REABSORPTION- Ascending Limb, Loop of Henle: • Salt • Movement of salt (diffusion) out of the filtrate into the interstitial fluid near the tip of the loop increases the osmolarity of the medulla • Movement of salt (active transport) out of the filtrate near the top of the loop causes the filtrate to become more dilute

25. REABSORPTION- Distal Convoluted Tubule: • Variable NaCl and HCO3- reabsorption • Key in regulating homeostasis

26. REABSORPTION- Collecting Duct: • variable amounts of water • Under hormonal control • Filtrate becomes increasingly concentrated • Inner medulla: permeable to urea • High concentrations of urea lead to “leakage”/diffusion of some urea into the interstitial fluid (keeps osmolarity high) • Carries filtrate through medulla to the renal pelvis

28. SECRETION • substances are selectively secreted from the blood into the glomerular filtrate • Proximal tubule: H+, drugs/other poisons processed by the liver; • Distal tubule: K+, H+

29. In summary… You begin with a liquid a lot like blood plasma; by the time it travels through the nephron, your body has taken out the “good stuff” and put it back into the veins (stuff like water); what you do not reclaim goes out as urine. *For every 100 mL of filtrate you start with, 99 mL are reabsorbed & only 1 mL urine is made! Countercurrent multiplier

30. Regulation of Kidney Function: • Key feature of kidney: ability to adjust both the volume & osmolarity of urine depending on the organism’s water and salt balance & the rate of urea production • Hormones involved: • ADH • Renin / angiotensin II / Aldosterone (RAAS) • Atrial natriuretic factor (ANF)

31. Hormonal Regulation of Water Balance: 1) ADH (antidiuretic hormone): made in hypothalamus & stored/released from posterior pituitary gland • helps you conserve water (& prevent dehydration) • Osmoreceptor cells in hypothalamus monitor osmolarity • Above set point  release ADH • ADH binds to receptors in distal tubules & collecting ducts • ADH increases water permeability  more concentrated urine

32. Hormonal Regulation of Water Balance: 2) RAAS (renin / angiotensin II / aldosterone): enzymes/peptides that function as hormones; work together to increase blood pressure and blood volume • Initiates due to drop in blood pressure or volume (ex: decreased salt intake or blood loss)

33. Control of Water Content/Urination: 3) ANF: opposes RAAS • Walls of the atria release ANF in response to increased blood volume and pressure • ANF inhibits the release of renin, inhibits NaCl reabsorption, reduces aldosterone release from adrenal glands