Excretion Objectives Describe production of urea by the liver Describe kidney structure Explain the formation of urine Explain control of blood concentration by ADH.

Functions of the Urinary System Slide 15.1a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Elimination of waste products  Nitrogenous wastes  Toxins Regulate aspects of homeostasis Water balance Electrolytes Blood pH Blood pressure

Production of urea Excess amino acids are broken down by the liver. This involves removal of the amino group forming ammonia. Ammonia is converted into urea which is less toxic. Urea is carried in the bloodstream to the kidneys for excretion.

The excretory system ei_0273.gif

Nephrons The structural and functional units of the kidneys Responsible for forming urine Main parts of the nephrons  Glomerulus  Renal tubule A million nephrons in each kidney

Functional Unit of the Kidney is the NEPHRON Glomerulus Proximal Tubule Loop of Henle Distal Tubule Collecting Duct

Arcuate artery Arcuate vein Thin ascending limb of the loop of Henlé Thick ascending limb of the loop of Henlé Distal convoluted tubule Proximal convoluted tubule Collecting duct Descending limb loop of Henlé Vascular supply to the nephron Vasa recta Glomerulus Afferent arteriole Efferent arteriole

A specialized capillary bed Maintains high pressure  Large afferent arteriole  Narrow efferent arteriole Glomerulus Glomerulus

Proximal convoluted tubule Capsule space Efferent arteriole Pedicel Podocyte Endothelium of glomerulus Afferent arteriole Juxtaglomerular cell Parietal layer of glomerular capsule Structure of the Bowman’s (glomerular) capsule

Bowman’s capsules - with glomerulus

Urine Formation Filtration Reabsorption Secretion

Filtration High hydrostatic pressure in glomerulus Water and solutes smaller than proteins are forced through capillary walls Blood cells cannot pass out to the capillaries Filtrate is collected in the capsule and leaves via the renal tubule

The filtration barrier - podocytes fenestrated endothelium fenestrated endothelium primary process podocyte cell body secondary process (pedicel) filtration slit basal lamina podocyte pedicel filtration slit basal lamina

Detailed structure of the filtration system Capillary Basement membrane Basement membrane Fenestrations Podocyte process Capillary BM Endoth cell nucleus Endoth cell nucleus E F

The filtration barrier - pedicels Bowman’s space capillary pedicel filtration slit

Selective reabsorption occurs in proximal tubule Capillaries around the PCT reabsorb:  Some water  All glucose  All amino acids  Some ions Some reabsorption is passive, most is active

Substances which remain in tubule are: Nitrogenous waste products  Urea  Uric acid  Creatinine Excess water Salts Plus  Hydrogen and potassium ions can be secreted into the tubule This becomes urine

Loop of Henle Enables us to concentrate urine if necessary As tube descends water comes out and contents become more concentrated Ascending limb impermeable to water, but sodium actively pumped out of tube. Tube contents become more dilute as you go up. High salt concentrations found at base.

Collecting ducts Collecting ducts pass through this region. Water can leave the collecting ducts by osmosis making the urine more concentrated.

Bladder Smooth, collapsible, muscular sac Temporarily stores urine

Urethra Thin-walled tube that carries urine from the bladder to the outside of the body by peristalsis Release of urine is controlled by two sphincters  Internal urethral sphincter (involuntary)  External urethral sphincter (voluntary)

Urethra Length  Females – 3–4 cm (1 inch)  Males – 20 cm (8 inches) Location  Females – along wall of the vagina  Males – through the prostate and penis Function  Females – only carries urine  Males – carries urine and is a passageway for sperm cells

Water balance Water intake must equal water output Sources for water intake  Ingested foods and fluids  Water produced from metabolic processes Water is lost As we breathe out.  in perspiration  in the faeces  In urine

Maintaining water balance Dilute urine is produced if water intake is excessive Less urine (concentrated) is produced if large amounts of water are lost

Acid- base balance Blood pH must remain between 7.35 and 7.45 to maintain homeostasis  Alkalosis – pH above 7.45  Acidosis – pH below 7.35 Most acid-base balance is maintained by the kidneys Other acid-base controlling systems  Blood buffers  Respiration

The kidneys can: Excrete bicarbonate ions if needed Conserve or generate new bicarbonate ions if needed Urine pH varies from 4.5 to 8.0