Excretion and the Kidney

1. Excretion and the Kidney HL (Paper 1 and 2)

2. Excretion • What is excretion? • Elimination of waste from the metabolic processes, to maintain homeostasis • What are the products we excrete? • Undigested material (egested – feces) • Metabolites • CO2 • Urea • Why do we eliminate waste? • Stop toxins from circulating in the body • Maintain homeostasis

3. The Kidney • Pair of reddish organs, smaller than your fist, just under the last rib • Blood flow • Each has a renal vein and renal artery, which branches to a network of capillaries, forming the pyramid • To empty urine, each has a ureter emptying into the bladder

4. The Kidney • Parts of the Kidney • Cortex – outer portion • Medulla – inner portion • Pyramid – collection of capillaries • Pelvis – urine empties from the collecting ducts, and collects before emptying into ureter to bladder • Functional unit, called the nephron, microscopic structure

5. Functions of the Kidney • The main functions of the kidney are: • To maintain water balance in the body. • To remove nitrogenous metabolic wastes and salts. You call this ‘pee’, we call it Urea. Urine is the whole thing together. • To allow the body to reabsorb substances it needs.

6. Urea • Made from the breakdown of proteins, from the more toxic ammonia (NH3) • Broken down in the liver • Transported from liver, in blood to kidneys to be filtered and eliminated

7. Nephron – functional unit of the Kidney • Principles of the functioning of the nephron: • Ultra-filtration • Reabsorption • Secretion

8. Ultrafiltration • Achieved by pressure built up in the glomerular vessels, due to the efferent vessel being more narrow exiting the Bowman’s capsule than the afferent vessel entering the Bowman’s capsule • Fluid pushed into the Bowman’s capsule, is called the filtrate, consisting of ions, glucose and urea

9. Ultrafiltration • Accomplished by: • Wall of the glomerulus – fenestrations • Basement membrane of the glomerulus • Outer wall of the glomerulus - Podocytes

10. Glomerulus and Bowman’s Capsule

11. Osmoregulation and Reabsorption • Osmoregulalation– the control of water balance of the blood, tissue and cytoplasm of a living organism. • Achieved by the Proximal Convoluted Tubule and Loop of Henle • Using osmosis, diffusion and active transport

12. Osmoregulation and Reabsorption • Since the blood leaving the Bowman’s capsule is more concentrated (low water), as the capillary comes close to the PCT: • osmosis drives the reabsorption of water • Sodium, glucose and other ions, are move by active transport • Once the salts move into, more water moves by osmosis • Large surface area is achieved by microvilli and large amounts of mitochondria in microvilli cells, for active transport

13. Osmoregulation and Reabsorption • Filtrate leaving the PCT, is mostly water, sodium, and urea, and goes into the Loop of Henle • As filtrate goes into the descending loop, it goes into the medulla, which is concentrated with sodium • More water is absorbed, into the capillaries along side the loop, along with some sodium, which stays in the tissue of the medulla

14. Osmoregulation and Reabsorption • Ascending Loop is impermeable to water, and salts are removed by active transport • The sodium remains in the medulla tissue, to maintain a concentration gradient • The concentration is maintained by the vasa recta, vessels that run along the loop, exchanging salts • This process is called the vasa recta counter current exchange • Filtrate leaving has only water and urea, metabolites and some sodium

15. Secretion • Filtrate goes into the distal convoluted tubule and collecting duct. • DCT and CT are permeable to water, only if ADH is present • If water is needed in body (ie. Low blood volume, High Osmotic Pressure) • ADH is secreted and water goes from the filtrate to the blood • Concentrated Urine

16. Secretion • If there is excess water (ie. High blood volume, low Osmotic Pressure) • no ADH will be secreted • Dilute Urine • If the urine is too concentrated, this can lead to kidney stones