11.3.1, , Kidney and Excretion

Draw the kidney A- Renal Vein B- Renal Artery C- Ureter D- Medulla E- Pelvis F- Cortex

Overview The kidney is the most important organ responsible for excretion. Excretion is simply the removal of metabolic waste products. Urea is a nitrogenous waste product which humans remove in urine (while conserving as much water as possible.) The kidney is also responsible for water retention. Water balance is achieved by balancing salt concentration in the kidney, as well as with hormones: solutes will draw water back into the body if water is needed; otherwise, water is excreted along with waste products. The kidney is responsible for 3 processes: filtration, reabsorption, and secretion.

Structure of Kidney and Nephron The functional unit of the kidney is the These are supplied with blood by a renal artery (and a renal vein.) The nephron consists of the Each nephron also has blood vessels associated with it. Conserved products go back into the bloodstream of the body. Waste products are carried through

Glomerulus inside Bowman’s Capsule

Ultrafiltration in Glomerulus Each nephron is supplied with blood by an Filtration occurs as blood pressure forces fluid from the blood The pressure is high (why?) The fenestrated (porous) capillaries, along with cells called podocytes, are permeable This filtration is a mechanical The filtrate (20% of original volume) in Bowman’s capsule contains

11.3.5, Osmoregulation- Proximal Convoluted Tubule Secretion and reabsorption in the PCT change the filtrate. Valuable nutrients, including The PCT is single celled, and has microvilli to One of the most important functions of the PCT is reabsorption of most of the Cells actively transport Na+ out of PCT. This transfer is balanced by the passive transport of Cl- out of the tubule. As salt moves from the filtrate to the interstitial fluid, Most of the water and glucose is removed to go back in circulation, rather than be excreted in urine.

Osmoregulation- Loop of Henle Overall role is to increase the Reabsorption of water continues as the filtrate moves into the This descending limb is freely permeable to water but not very permeable to salt. For water to move out by osmosis, In contrast to the descending limb As filtrate goes up the ascending limb, NaCl is secreted (pumped) out of the permeable tubule into the interstitial fluid. By losing salt without giving up water, the filtrate becomes progressively

Osmoregulation- The Collecting Duct Like the PCT, the distal tubule plays a key role in regulating the K+ and NaCl concentrations. In the collecting duck, The duct is permeable to The filtrate becomes increasingly concentrated as it loses more and more water by osmosis to the hypertonic interstitial fluid.

Collecting Duct Regulation If the body is in need of water, more will be reabsorbed in the CD. If there is excess water, ADH will not be released, This is a negative feedback system. ADH alone only prevents further movements away from the set point If water levels are high, the urine ends up being more dilute. Alcohol inhibits ADH, which is one reason why drinking dehydrates a person.

Dialysis Machines- NO NEED TO COPY, DOOD Dialysis machines take the place of the kidney. Blood is taken from the patient, run through the machine. In the machine, dialysis fluid bathes the semi- permeable membrane containing the blood. The concentration of the solutes in the dialysis fluid determines what will be removed/added to the patient’s blood. The dialysis fluid has no urea, so urea is removed from blood by diffusion, but has some amount of solute (like dextran) to pull excess water from the blood. Glucose, ions, and vitamins are isotonic so that they remain in circulation for the patient to use. Blood minus urea and some water returns to the patient.

Renal ArteryGlom Filtrate Urine Glucose Urea Proteins Na+ ions Treat these as relative concentrations. More oxygen in renal artery, more carbon dioxide in renal vein Recall why diabetics would have glucose in their urine.