The urinary system
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The Urinary System. Removing waste, balancing blood pH, and maintaining water balance. The Importance of Removing W astes. The cells of the body obtain energy by converting complex organic compounds into simpler compounds.

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The urinary system

The Urinary System

Removingwaste, balancing blood pH, and maintaining water balance

The importance of removing w astes
The Importance of Removing Wastes

  • The cells of the body obtain energy by converting complex organic compounds into simpler compounds.

  • Many of these simpler compounds can be harmful and must be eliminated

  • The lungs eliminate carbon dioxide

  • Thelarge intestine removes toxic wastes from the digestive system

Removing waste
Removing Waste

  • The liver transforms ingested toxins, such as alcohol and heavy metals, and the hazardous products of protein metabolism into soluble compounds that can be eliminated by the kidneys

  • The kidneys play a crucial role in removing waste, balancing blood pH, and maintaining water balance

Metabolic waste
Metabolic Waste

  • When protein is used for energy, it is deaminated before it can enter the Krebs cycle

  • The byproduct of deamination is ammonia

  • Ammonia is extremely toxic— a buildup of as little as 0.005 mg can kill humans

  • In the liver, two molecules of ammonia combine with carbon dioxide, to form urea.

  • Urea is 100 000 times less toxic than ammonia.

The kidney
The Kidney

  • Renal arteries branch from the aorta and carry blood to the kidneys.

  • Mass: about 0.5 kg

  • May hold as much as 25% of the body’s blood at any given time.

The urinary system1
The Urinary System

  • Wastes are filtered from the blood and conducted to the urinary bladder by ureters.

  • A urinary sphincter muscle located at the base of the bladder acts as a valve, permitting the storage of urine.

  • Urine is voided through the urethra

Structure of the kidney
Structure of the Kidney

  • The cross section of the kidney reveals three structures:

    • An outer layer of connective tissue: the cortex

    • An inner layer: the medulla,

    • A hollow chamber: the renal pelvis, joins the kidney with the ureter


  • Approximately one million slender tubules, called nephrons, are the functional units of the kidneys

Parts of the nephron
Parts of the Nephron

  • Afferent arterioles: small branches that carry blood to the glomerulus

  • Glomerulus: high-pressure capillary bed that is the site of filtration

  • Efferent arterioles: small branches that carry blood away from the glomerulus to a capillary net

  • Peritubular capillaries: network of small blood vessels that surround the nephron

Parts of a nephron
Parts of a Nephron

  • Bowman’s capsule: cuplike structure that surrounds the glomerulus

  • Proximal tubule: section of the nephron joining the Bowman’s capsule with the loop of Henle

  • Loop of Henle: carries filtrate from the proximal tubule to the distal tubule

Parts of the nephron1
Parts of the Nephron

  • Distal tubule: conducts urine from the loop of Henle to the collecting duct

  • Collecting duct: tube that carries urine from nephrons to the pelvis of a kidney


  • Small branches from the renal artery, the afferent arterioles, supply the nephrons with blood.

  • The afferent arterioles branch into a capillary bed, called the glomerulus.


  • Blood leaves the glomerulus by way of the efferent arterioles.

  • Bloodis carried from the efferent arterioles to a net of capillaries called peritubular capillaries that wrap around the kidney tubule.

The nephron
The Nephron

  • The glomerulus is surrounded by a funnel-like part of the nephron, called the Bowman’s capsule.

  • The Bowman’s capsule, the afferent arteriole, and the efferent arteriole are located in the cortex of the kidney.

The nephron1
The Nephron

  • Fluids to be processed into urine enter the Bowman’s capsule from the blood.

  • The capsule tapers to a thin tubule, called the proximal tubule.

  • Urine is carried from the proximal tubule to the loop of Henle, which descends into the medulla of the kidney.

The nephron2
The Nephron

  • Urine moves through the distal tubule, the last segment of the nephron, and into the collecting ducts.

  • As the name suggests, the collecting ducts collect urine from many nephrons that, in turn, merge in the pelvis of the kidney.

The formation of urine
The Formation of Urine

  • Urine formation depends on three functions:

    • Filtration

    • Reabsorption

    • Secretion


  • Each nephron of the kidney has an independent blood supply

  • Blood moves through the afferent arteriole into the glomerulus which is a high-pressure filter.

  • Normally, pressure in a capillary bed is about 25 mm Hg

  • The pressure in the glomerulus is about 65 mm Hg


  • Dissolved solutes and water pass through the walls of the glomerulus into the Bowman’s capsule.

  • Plasma protein, blood cells, and platelets are too large to move through the walls of the glomerulus.

  • Smaller molecules such as Na+ , H+ glucose, amino acids, vitamins, minerals, urea, and uric acid pass through the cell membranes and enter the nephron.


  • On average, about 600 mL of fluid flows through the kidneys every minute.

  • Approximately 20% of the fluid, or about 120 mL, is filtered into the nephrons.

  • Only 1 mL of urine is formed for every 120 mL of fluids filtered into the nephron.

  • The remaining 119 mL of fluids and solutes is reabsorbed.

Reabsorption in the proximal tubule
Reabsorption in the Proximal Tubule

  • 85% of the water in the filtrate is reabsorbed here

  • Selective reabsorption of nutrients such as glucose and amino acids, vitamins and minerals occurs by active and passive transport.

  • pH is controlled by secretion of hydrogen ions (H+) and reabsorption of bicarbonate ions (HCO3- )

Reabsorption in the descending loop of henle
Reabsorption in the Descending Loop of Henle

  • Descending limb of loop of Henle is permeable to water, resulting in loss of water from filtrate by osmosis.

  • Salt (NaCl) becomes concentrated in filtrate as descending limb penetrates inner medulla

Reabsorption in the ascending loop of henle
Reabsorption in the Ascending Loop of Henle

  • The ascendinglimb of loop of Henleis permeable to salt, resulting in diffusion of salt out of ascending limb.

Reabsorption in the distal tubule
Reabsorption in the Distal Tubule

  • Selective reabsorption of penicilin and other medications from blood into nephron occurs by active transport.

  • Distal tubule helps regulate potassium (K+) and salt (NaCl) concentration of body fluids.

Reabsorption in the c ollecting t ubule
Reabsorption in the Collecting Tubule

  • As water is reabsorbed from the nephron, the remaining solutes become more concentrated.

  • Molecules such as urea and uric acid will diffuse from the nephron back into the blood, although less is reabsorbed than was originally filtered.


  • Secretion is the movement of wastes from the blood into the nephron.

  • Nitrogen-containingwastes, excess H+ ions, and minerals such as K+ ions are examples of substances secreted.

  • Even drugs such as penicillin can be secreted.

  • Tubular secretion occurs by active transport

  • Molecules are shuttled from the blood into the nephron