 Homeostasis is the maintenance of steady internal conditions despite fluctuations in the external environment.  Examples of homeostasis include –thermoregulation—the maintenance of internal temperature within narrow limits (Ch. 20 and discussion endotherms), –osmoregulation—the control of the gain and loss of water and solutes, and –excretion—the disposal of nitrogen-containing wastes. Chapter 25 Control of Body Temperature (Ch. 20) and Water Balance=Urinary System © 2012 Pearson Education, Inc.

25.6 The urinary system plays several major roles in homeostasis  The urinary system –forms and excretes urine and –regulates water and solutes in body fluids.  In humans, the kidneys are the main processing centers of the urinary system. © 2012 Pearson Education, Inc.

 Nephrons –are the functional units of the kidneys, –extract a fluid filtrate from the blood, and –refine the filtrate to produce urine.  Urine is –drained from the kidneys by ureters, –stored in the urinary bladder, and –expelled through the urethra The urinary system plays several major roles in homeostasis © 2012 Pearson Education, Inc.

Figure 25.6 Aorta Kidney The urinary system Inferior vena cava Renal artery (red) and vein (blue) Ureter Urethra Urinary bladder Renal cortex Renal medulla Renal pelvis Ureter The kidney Proximal tubule Bowman’s capsule Tubule Collecting duct To renal pelvis Branch of renal artery Branch of renal vein Renal cortex Renal medulla Capillaries Glomerulus Distal tubule From another nephron Bowman’s capsule Arteriole from renal artery Arteriole from glomerulus Branch of renal vein Loop of Henle with capillary network Detailed structure of a nephron Orientation of a nephron within the kidney Collecting duct

25.6 Overview: The key processes of the urinary system are filtration, reabsorption, secretion, and excretion  Filtration –Blood pressure forces water and many small molecules through a capillary wall of glomeruli surrounded by Bowman’s capsule into the start of the kidney tubule.  Reabsorption –In the tubules filtrate is refined, –reclaims valuable solutes (such as glucose, salt, and amino acids) from the filtrate, and –returns these to the blood. © 2012 Pearson Education, Inc.

Figure 25.6 Filtrate From renal artery Bowman’s capsule Filtration Nephron tubule H 2 O, other small molecules Interstitial fluid Capillary Urine To renal vein Reabsorption (Removal from filtrate) Secretion (Addition to filtrate) Excretion Key Filtration Reabsorption Secretion Excretion

 Substances in the blood are transported into the filtrate by the process of secretion.  By excretion the final product, urine, is excreted via the ureters, urinary bladder, and urethra Overview: The key processes of the urinary system are filtration, reabsorption, secretion, and excretion © 2012 Pearson Education, Inc.

Figure 25.6 Filtrate From renal artery Bowman’s capsule Filtration Nephron tubule H 2 O, other small molecules Interstitial fluid Capillary Urine To renal vein Reabsorption (Removal from filtrate) Secretion (Addition to filtrate) Excretion Key Filtration Reabsorption Secretion Excretion