Biology 30S Excretory System Role of the Nephrons Biology 30S Excretory System

More on The Kidneys… The kidneys filter 120ml of blood per minute! only 1 ml (1%) of this turns into urine that will eventually leave the body. That leaves 119ml of fluid called filtrate to be returned back to the blood stream.

If that didn’t happen…. A person would have to pee once every 3 minutes And drink 1 Litre of fluid every 10 minutes in order to maintain Homeostasis!

The Kidneys are therefore very efficient at… Returning good stuff back to the blood AND Removing wastes by making urine

Urine Production by Kidneys This happens in 3 main steps: Filtration Reabsorption Secretion

Filtration Blood enters glomerulus under high pressure Pressure causes 20% of blood plasma (120 ml of 600 ml) to be forced into the Bowman’s Capsule (BC) Water, salt, glucose, amino acids (good stuff) and urea (bad stuff) all pass through into BC Blood Cells and proteins do NOT enter the BC – they are too large to leave the capillaries Filtrate is ISOTONIC to Plasma

Filtration Renal Veins  Efferent Arterioles Renal Artery  Afferent Arteriole  Glomerulus  BC _____________________________________________________ _________ BLOOD Filtrate FILTRATION

Filtration Bowman’s Capsule separates materials in plasma Filtrate! Bowman’s Capsule separates materials in plasma little stuff goes through big stuff doesn’t

Reabsorption Useful materials such as sugars and salt ions are reabsorbed back into the blood stream. “Good” materials that can still be used by the body are sent back to the blood. Reabsorption happens as filtrate passes sequentially through nephron. Reabsorption takes place in all regions of the nephron but one: the ascending loop of Henle.

Reabsorption: PCT As filtrate enters the PCT ~80% of the salt ions (Na+ & K+), glucose and amino acids are actively transported out & back into the blood stream. Cl- ions will flow passively out of the PCT following the positively charge K+ & Na+. As the [ ] of these molecules drops inside the PCT, water will diffuse out of the PCT passively by osmosis.

Reabsorption: Descending Loop of Henle Both sodium (Na+) and potassium (K+) diffuse back into the Loop of Henley. Water continues to move out of the Loop of Henle into the capillary network. At this point, filtrate is more concentrated (hypertonic) with respect to salt ions that it was in the PCT due to water moving out and salts being more added. This causes a concentration gradient to form

Reabsorption: Ascending Loop of Henle As the filtrate proceeds up into the ascending loop of Henle, the Cl- ions are actively pumped back out of nephron. Na+ ions passively follow Cl- out of tubule and into the medulla. Since the Loop of Henle is impermeable to water, water cannot leave this part of the nephron. Filtrate will become more dilute again.

Counter Current Mechanism This is a “current” created by the ascending L of H Purpose: to get huge amounts of water to be reabsorbed back to the blood Method: pumping salt to the medulla, which then draws water to the medulla. From the medulla, water goes back to the blood Causes less water to be lost in the urine and more water to be returned to the blood

Reabsorption: DCT Water continues to diffuse out of the nephron and back into the blood. The medulla is hypertonic to the filtrate in regards to salt concentration Due to continued re-absorption of water, the filtrate becomes more and more concentrated with wastes, mainly urea. The hormone ADH can regulate the amount of water that diffuses. The amount of water that diffuses from the DCT back into the blood depends on bodily needs.

Reabsorption: CT The same thing that happens in the DCT happens in the CT

Good stuff goes back to blood Reabsorption Good stuff goes back to blood

Secretion Wastes and toxins are actively transported from the blood into the Nephron. This “bad stuff” is “fast tracked” into the PCT, DCT, and CT of the Nephron. Excess things like H+ ions, OH- ions, glucose and salt and Toxins like ammonia, and drugs

Secretion Bad stuff moves from blood into nephron

Filtration, Reabsorption & Secretion What you Need to Know! Filtration: movement of plasma from the glomerulus capillaries to the Bowman’s Capsule Little things pass through Big things do not Reabsorption: the movement of materials from the nephron tubules back into the blood stream. Occurs in the PCT, L of H, DCT & CT

Filtration, Reabsorption & Secretion What you Need to Know! Counter Current Mechanism: a “current” created by the ascending L of H Purpose: to get huge amounts of water to be reabsorbed back to the blood Secretion: Wastes and toxins are fast tracked from the blood for excretion Happens in the PCT, DCT, and CT of the Nephron.

The Nephron! www.youtube.com/watch?v=vNvZaGcLzEo

Plasma & Filtrate Alike Both are clear, slightly yellow liquids Both can be found in the kidney Both contain water, salts, nutrients (sugars) & nitrogenous wastes Plasma that is forced from the glomerulus through Bowman’s Capsule becomes filtrate Filtrate used to be plasma

Plasma & Filtrate Differences

Filtrate & Urine Alike Both are clear, yellow liquids Both can be found in the kidney Both contain water, salts and nitrogenous wastes Both pertain to the excretion of Nitrogenous wastes Once filtrate drips out of the collecting tubules of the nephron and into the renal pelvis, it is now called urine Urine used to be filtrate

Filtrate & Urine Difference

Plasma Filtrate Urine Filtrate, filtered plasma, in nephron Urine, modified filtrate, in collecting tubule Plasma, in blood vessels