The Urinary System Your pee will become a

The Urinary System Your “pee” will become a little more clear! 1

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We can all learn a little something from Homer Simpson! 10: 20 3

The Urinary System • Consists of the kidneys and accessory structures • Ureters carry urine from the kidneys to the bladder 4

Why are those kidneys so important? • Structure – They are bean-shaped and ~10 cm long – Consists of three parts: • Medulla (inside) • Cortex (outside) • Renal Pelvis (inside) 5

Kidneys: Importance • Function – to filter and clean blood – Remove wastes (i. e. nitrogenous wastes) – Osmoregulation – regulation of water excretion and reabsorption – Absorption of vitamins, organic molecules, nutrients, potassium, and salt – Maintain p. H level by controlled secretion/absorption of hydrogen (H+) and bicarbonate ions (HCO 3 -) 6

Kidney Structure • 3 Main Parts: – Cortex – the outer layer of connective tissue that encircles the kidney – Medulla – the inner layer found beneath the cortex – Renal Pelvis – the hollow chamber that joins the kidney with the ureters 7

The Kidney: Interesting Facts • Only 1% of body mass but processes 20% of the body’s blood with each heart beat • 99% of water is reabsorbed to keep the body hydrated • Nitrogenous wastes (mostly from proteins) are expelled in the form of urea • Works 24 hours a day even if you don’t drink you will still urinate from extracted water • Nephrons are the functional unit (approx. 1 -1. 25 million!) of the kidney they accomplish the main functions of the kidney 8

Nephron Structure 9

Follow the Flow of Fluid! • Small branches from the renal artery, afferent arterioles, arterioles supply the nephrons with blood • These arteries branch into a capillary bed, called the glomerulus • Blood leaves the glomerulus through efferent arterioles 10

Follow the Flow • Blood is carried from the efferent arterioles to a net of capillaries, peritubular capillaries, capillaries that wrap around the kidney tubule 11

Follow the Flow • The glomerulus is surrounded by a funnel-like part, called Bowman’s capsule • The capsule, the afferent arteriole, and the efferent arteriole are located in the cortex 12

Follow the Flow • Fluids enter the Bowman’s capsule from the blood and it tapers to a thin tubule, called the proximal tubule • Urine is carried from the proximal tubule to the loop of Henle – Descends into the medulla 13

Follow the Flow • Urine moves through the distal tubule, tubule the last segment of the nephron, and into the collecting ducts • The collecting ducts collect the urine from several nephrons 14

Urine Formation Depends on three functions: 1. Filtration • • Process by which blood or body fluids pass through a selectively permeable membrane Movement of fluids from the blood into the Bowman’s capsule 2. Reabsorption 3. Secretion 15

Urine Formation Depends on three functions: 1. Filtration 2. Reabsorption • Transfer of glomerular filtrate (essential solutes and water) from the nephron back into the capillaries 3. Secretion 16

Urine Formation Depends on three functions: 1. Filtration 2. Reabsorption 3. Secretion n Movement of materials, such as ammonia and drugs/toxins, from the blood back into the nephron 17

Pathway of Filtrate through the Nephron • Filtrate = plasma (no white/red blood cells, platelets, hormones, or plasma proteins) • Nephron pathway begins in cortex Bowman’s Capsule proximal tubule loop of Henle (medulla) distal tubule (cortex) collecting tubule (medulla) • Like a roller coaster! 18

Filtration • Each nephron has an independent blood supply • The glomerulus is a high-pressure filter – Capillary bed ~ 25 mm Hg – Glomerulus ~ 65 mm Hg – The greater the blood pressure, the more nephric filtrate forms 19

Filtration contd. • Dissolved solutes pass through the walls of the glomerulus into the Bowman’s capsule • So why do you think we feel a greater urge to urinate when we are cold? – When we are cold, our blood vessels constrict and narrow to increase blood pressure so more urine forms! 20

Filtration • Not all materials enter the Bowman’s capsule: Solute Water Na. Cl Glucose Amino Acids H+ ions Plasma Proteins Erythrocytes (blood cells) Platelets Glomerulus Bowman’s Capsule yes. Why can’t plasma yes proteins, blood yescells, and platelets yes pass through the yes glomerulus and yesenter the nephron? yes no 21

Reabsorption • On average, 600 m. L of fluid flow through the kidneys every minute • About 120 m. L is filtered into the nephron • Only 1 m. L of urine is formed If we didn’t reabsorb fluids, we would have to drink 1 L of water every 10 minutes! – The remaining 119 m. L of fluids and solutes are reabsorbed 22

Selective Reabsorption • Occurs by both active and passive transport • Na+ is actively transported out, causing negatively charged ions to follow • Glucose and amino acids attach to specific carrier molecules, which shuttle them out of the nephron and into the blood 23

Proximal Tubule • Solutes actively transported out to the interstitial fluid and so creates an osmotic gradient • This draws water from the nephron (water always flows toward a hypertonic/hypotonic environment? ) • The osmotic gradient is strengthened by the proteins that are not filtered into the nephron – Proteins in the blood draw water from the interstitial fluid 24

Selective Reabsorption • What are actively transported? What are passively transported? • Na. Cl, H+ (to balance p. H) glucose, and nutrients are actively transported out of the nephron and to the interstitial fluid • Water and K+ are passively transported to the interstitial fluid for reabsorption • Water tends to follow the direction of Na. Cl – why? 25

Loop of Henle • As water is reabsorbed, the remaining solutes become more concentrated • Descending loop of Henle is impermeable to solutes and ascending the ascending loop is impermeable to water • Water passively leaves the nephron to the interstitial fluid thereby increasing the solute concentration 26

Loop of Henle • The ascending limb of the Loop of Henle is impermeable to water • Na. Cl passively leaves into the interstitial fluid 27

Distal Tubule • HCO 3 - is reabsorbed in the distal tubule (to regulate p. H) by active transport • Na. Cl, K+ and H+ are actively transported • Water is passively reabsorbed 28

Collecting Duct • In the collecting duct, urea is passively transported and reabsorbed into the interstitial fluid • Water is reabsorbed since the membrane is permeable to it • This results in making the urine more concentrated 29

Other Notes on Reabsorption The amount of solutes reabsorbed is limited! • Reabsorption occurs until the threshold level of a substance is reached – The maximum amount of material that can be moved across the nephron • Once it is reached, excess material stays in the nephron and is excreted as urine 30

Secretion • This is the movement of wastes from the blood into the nephron – i. e. Nitrogen-containing wastes, excess H+ ions, minerals (e. g. K+) • Like reabsorption, tubular secretion occurs by active transport but molecules are shuttled from the blood into the nephron 31