Urinary System and Excretion Chapter 16 p. 303-316

Functions of the Urinary System Maintain homeostasis excretes urine, which contains metabolic wastes products, out of the body Maintains salt and water balance Maintains blood acid-base balance Secretes hormones

Urinary System Organs involved: Path of urine Kidneys (Y) Ureters (W) Urinary bladder (Z) Urethra (X) Path of urine Kidney  ureters  urinary bladder  urethra

Organs of the Urinary System Kidney Paired Bean-shaped Reddish-brown in colour Connected to the renal artery/vein Connected to ureter

The Kidney W – renal cortex (contains nehprons) X – renal medulla (contains collecting ducts of nehprons) Y - renal pelvis Z – ureter

Organs of the Urinary System Ureter Conduct urine from kidney to bladder Small – 25cm long 5mm in diameter Urinary bladder Stores urine until expulsion Bladder is expandable Sphincters control the backflow of urine

Organs of the Urinary System Urethra Remove urine from the body Small tube that extends from the urinary bladder to an external opening

Urination Bladder fills up to 250ml with urine Stretch receptors send sensory nerve impulses to the spinal cord. Motor neurons cause the bladder to contract, sphincters relax Urination occurs Brain controls this reflex until a suitable time

The Nephron A kidney is composed of over a million nephrons. Renal cortex Renal medulla A kidney is composed of over a million nephrons.

The Nephron Proximal convoluted tubule Efferent arteriole Distal convoluted tubule Afferent arteriole Bowman’s capsule Peritubular capillary network Glomerulus Renal vein Collecting Duct Loop of Henle

Parts of a Nephron Nephron Glomerulus the functional unit of the kidney involved in filtration and selective reabsorption of blood Glomerulus a cluster of capillaries enclosed by the Bowman’s capsule.

Parts of a Nephron Bowman’s capsule the bulbous unit of the nephron, surrounds the glomerulus. Blood is under hi- pressure

Parts of a Nephron Afferent (incoming) Arteriole leads into Glomerulus Efferent (outgoing) Arteriole exits glomerulus Tubular excretion (secretion) Peritubular capillaries  venule  vein

Parts of a Nephron Peritubular capillary network takes up reabsorbed material, and drains into the Renal vein helps maintain concentration gradient in the medulla for reabsorption Proximal Convoluted Tubule (PCT) Tubular (Selective) Reabsorption Lined by microvilli to increase surface area Glucose (100%), a.a. (100%), bicarbonate (80-90%), water (65%), Na+ (65%)

Parts of a Nephron Distal Convoluted Tubule (DCT) Tubular Secretion (move molecules from blood into the tubule) Not specialized in reabsorption

Parts of a Nephron Loop of Henle Collecting Duct U-shaped Connects the Proximal and Distal Convoluted Tubules Collecting Duct Receives secretion by several distal convoluted tubules Carry urine to renal pelvis

Urine Formation – A 3 Step Process Excretion - release of metabolic wastes and excess water General picture: 1. Filter by pressure 2. Reabsorb by active and passive transport 3. Secrete Urine FiREs – when you REALLY have to pee, it feels as urgent as escaping from FIRE.

Urine Formation 1. Filtration Pressure Filtration / Glomerular Filtration Takes place at the glomerulus and Bowman’s capsule Blood enters from the renal afferent arteriole and slows down as it enters capillaries High BP in the glomerulus forces molecules to move from blood into the Bowman’s Capsule

filtrate Movement of molecules is drive by a pressure gradient Only molecules that are small enough to fit through the walls of the glomerular capillaries move into the nephron H2O, urea, glucose, AA, uric acids, salts filtrate

What if BP is too low? If BP is too low: A hormone-like substance called renin is released from a specialized tissue in the glomerulus called juxtaglomerulus apparatus Renin Ensures adequate pressure filtration is happening by constricting the glomerulus, therefore increase BP

Urine Formation 2. Tubular Reabsorption Takes place in the proximal Convoluted Tubule Involves transferring molecules from filtrate in nephron back to peritubular capillaries Occurs by active and passive transport

Tubular Reabsorption Involves 2 processes: Selective Reabsorption Water Reabsorption

Selective Reabsorption Molecules are actively transported acorss the nephron membrane into the peritubular capillaries (Eg. glucose, AA, Na+, vitamins) Note: Cl- passively follows Na+ by charge attraction; therefore NaCl actually leaves the nephron in this step This increases the salt content of blood compared to the filtrate

Selective Reabsorption Active transport requires carrier protein (thus, ATP and Mitochondria) Once all carrier proteins are used, excess molecules in the filtrate will appear in urine Diabetes Mellitus: when excess glucose occurs in the blood

Water Reabsorption Water is reabsorbed into the peritubular capillaries Solutes (glucose, AA, salt, vitamins) that are actively pumped out of the nephron create an Osmotic Gradient Blood in peritubular capillaries is hypertonic to the interstitial fluid, water is passively drawn from the nephron by osmosis

So what’s in the proximal convoluted tubule? Materials NOT reabsorbed Some water A lot of nitrogenous / metabolic waste Excess salt ions And what is in the peritubular capillaries? (reabsorbed materials) Most water Nutrients Required salt ions for the blood

Urine Formation 3. Tubular Secretion Takes place at the Distal Convoluted Tubule Secretion Movement of materials from nephron back to the blood via active transport H+ is excreted from the nephron if blood pH is high (too basic). H+ acts as a buffer to neutralize pH. Excretion Movement of excess wastes from blood into nephron via active transport Materials are added to urine (Eg. Drugs, ions (H+, NH3) If blood pH is low (too acidic), excreting excess H+ ions to the nephron will help raise pH back to homeostasis

Urine Made! Once filtrate reaches the collecting duct, 99% of water has been reabsorbed into the blood Collecting duct is also permeable to urea (diffuses out) Water continues to reabsorb in the collecting duct (when ADH (hormone) is present) Filtrate is now called urine!

Defecation (or elimination) - the release of unabsorbed wastes (e. g Defecation (or elimination) - the release of unabsorbed wastes (e.g., feces) from the digestive tract