Blood Cleaning by the Kidneys

1. Blood Cleaning by the Kidneys • The processes performed by the kidneys in order to filter (clean) blood. are: • Glomerular Filtration also called "Ultra-filtration", • Tubular Reabsorption also called "Selective Re-Absorption" and • Tubular Secretion.

2. 1) Glomerular Filtration • Blood enters the kidney via the renal artery. This seperates many times (Renal Artery -> Segmental Arteries -> Interlobar Arteries -> Arcuate Arteries -> Interlobular Arteries -> Afferent Arterioles), eventually forming many afferent arterioles, each of which delivers blood to an individual kidney nephron

3. Filtrating membrane

4. The diameter of the afferent (incoming) arteriole is greater than the diameter of the efferent arteriole (by which blood leaves the glomerulus). The pressure of the blood inside the glomerulus is increased due to the difference in diameter of the incoming and out-going arterioles. This increased blood pressure helps to force the following components of the blood out of the glomerular capillaries:

5. Most of the water; • Most of the salts; • Most or all of the glucose; • Most or all of the urea. • The above are filtered in preference to other components of blood based on particle size. (Water and solutes of relative molecular mass less than 68,000 form the filtrate.) Blood cells and plasma proteins are not filtered through the glomerular capillaries because they are relatively larger in physical size.

6. The water and salts that have been forced out of the glomerular capillaries pass into the Bowman's Capsule and are called the glomerular filtrate. This glomerular filtrate is formed at a rate of above 125 cm3 per minute in humans. This volume is approx. 20% of the plasma delivered during that time. (Again: It contains all the materials present in the blood except blood cells and most proteins - which are too large to cross the basement membrane of the glomerulus.) • The glomerular filtrate passes from the renal corpuscle to the renal tubule:

7. Controlling the GFR: Filtration depends on adequate circulation to the glomerulus & the maintenance of normal filtration pressure. • ***Three interacting levels of control stabilize your GFR: • Auto regulation at the local level. • Hormonal regulation initiated by the kidney. • Autonomic regulation by sympathetic division of ANS

8. (2) Tubular Reabsorption • Only about 1% of the glomerular fitrate actually leaves the body because the rest (the other 99%) is reabsorbed into the blood while it passes through the renal tubules and ducts. This is called tubular reabsorption and occurs via three mechanisms. They are: • Osmosis • Diffusion, and • Active Transport. • Reabsorption varies according to the body's needs, enabling the body to retain most of its nutrients.

9. Most reabsorbed materials are nutrients that the body can use. Reabsorption is a selective process. Solute reabsorption may involve simple diffusion or the activity of carrier proteins in the tubular epithelium. • - water reabsorption occurs passively through osmosis, active absorption as in sodium ions are actively pumped in the proximal tubule. • - glucose and amino acids are absorbed by secondary active transport. • - calcium, potassium, chloride, phosphate ions are also actively absorbed.

10. Reabsorption in the different tubular segments: • *Proximal convoluted tubules: highly active for about 65% of all absorptive and secretory functions. Water & Na are reabsorbed at this segment (65%), also Cl are absorbed passively. K, Ca, Mg and sulfate are absorbed actively. Proteins and amino acids are absorbed by pinocytosis.

11. Reabsorption in the PCT

12. Thin segment of Henle: the descending portion is highly permeable to water and very low to Na, Urea and others leading to a hypertonic fluid, while the ascending thin portion is impermeable to water. The thick ascending is also impermeable to water and permeable to ions through Na-K-2Cl co-transport mechanism. Na ions are absorbed by active co-transport system & by counter-transport system through Na-H system, all this lead to hypotonic fluid.

13. reabsorption in the Loop of Henle

14. Distal tubule: its divided into the diluting segment which functions as active transport of Na and Cl (Na-K-2Cl) and is impermeable to water and urea. The late distal segment functions for active transport of Na and Cl under the effect of aldosterone, and also very impermeable to water and urea.

15. The collecting duct: the cortical and medullary segments, both portions function for active transport of Na, K, H, Ca and others. The cortical segment is impermeable to urea, while the medullary is permeable to urea under ADH. Water permeability is under ADH control.

16. 3)Tubular secretion: • The third process by which the kidneys clean blood (regulating its composition and volume) is called tubular secretion and involves substances being added to the tubular fluid. This removes excessive quantities of certain dissolved substances from the body, and also maintains the blood at a normal healthy pH . • The substances that are secreted into the tubular fluid (for removal from the body) include:

17. Potassium ions (K+), • Hydrogen ions (H+), • Ammonium ions (NH4+), • creatinine, • urea, • some hormones, and • some drugs (e.g. penicillin).

18. Tubular secretion occurs from the epithelial cells that line the renal tubules and collecting ducts. It is the tubular secretion of H+ and NH4+ from the blood into the tubular fluid (i.e. urine - which is then excreted from the body via the ureter, bladder, and urethra) that helps to keep blood pH at its normal level. The movement of these ions also helps to conserve sodium bicarbonate (NaHCO3).The typical pH of urine is about 6.

19. ... and finally • Urine formed via the three processes outlined above trickles into the kidney pelvis. At this final stage it is only approx. 1% of the originally filtered volume but includes high concentrations of urea and creatinine, and variable concentrations of ions. • The typical volume of urine produced by an average adult is around 1.5 - 2.0 liter per day.

20. Micturition or urination (Voiding) • Micturition: is the process where the urinary bladder empties after being filled. • -Micturition reflex: mediated through the pelvic nerves (S2-S3) which contain sensory fibers for the detection of bladder wall stretching and the motor fibers (parasympathetic) ending on bladder wall ganglion cells that send post ganglionic fibers to the detrusor muscles of the bladder. • The urinary bladder begins to fill up to 100-150 ml and issuing a first desire to urinate due to stretch receptors in the wall of the urinary bladder causing a reflex contraction. After some time, the contraction disappears.

21. When the volume reaches 400-500 ml, the reflex contraction increases & if the pressure builds more than the external sphincter (voluntary control), opening of the bladder neck occurs and leads to another reflex through the pudendal nerve (S1-S2) to inhibit the external sphincter. If this inhibition overcomes the voluntary control, urination occurs.