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The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.
The bean-shaped kidneys are at the back of the abdominal wall beneath the peritoneum, protected by the lower rib cage.
The renal artery and renal vein along with ureters exit the kidney at the hilum .
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The kidneys produce urine which is conducted by two muscular tubes called ureters to the urinary bladder where it is stored before being released through the urethra .
Two urethral sphincters control the release of urine.
In females, the urethra is 4 cm long; in males, the urethra is 20 cm long and conveys both urine and sperm during ejaculation.
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As the bladder fills with urine, sensory impulses travel to the spinal cord where motor nerve impulses return and cause the bladder to contract and sphincters to relax.
With maturation, the brain controls this reflex and delays urination , the release of urine, until a suitable time.
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Excretion refers to the elimination of metabolic wastes that were cell metabolites; this is the function of the urinary system.
Kidneys play a role in homeostasis of the blood by excreting metabolic wastes, and by maintaining the normal watersalt and acid-base balances of blood.
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Kidneys excrete nitrogenous wastes , including urea, uric acid, and creatinine.
Urea is a by-product of amino acid metabolism.
The metabolic breakdown of creatine phosphate in muscles releases creatinine .
Uric acid is produced from breakdown of nucleotides.
Collection of uric acid in joints causes gout .
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Kidneys maintain the water-salt balance of the body which, in turn, regulates blood pressure.
Salts, such as NaCl, in the blood cause osmosis into the blood; the more salts, the greater the blood volume and also blood pressure.
Kidneys also maintain correct levels of potassium, bicarbonate, and calcium ions in blood.
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The kidneys regulate the acid-base balance of the blood.
Kidneys help keep the blood pH within normal limits by excreting hydrogen ions (H + ) and reabsorbing bicarbonate ions (HCO
3
) as needed.
Urine usually has a pH of 6 or lower because our diet often contains acidic foods.
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Kidneys secrete or activate several hormones:
1) They secrete the hormone erythropoietin to stimulate red blood cell production,
2) They activate vitamin D to the hormone calcitriol needed for calcium reabsorption during digestion, and
3) They release renin , a substance that leads to the secretion of aldosterone.
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The kidneys filter wastes from the blood, and thus the renal arteries branch extensively into smaller arteries and then arterioles inside each kidney.
Many venules unite to form small veins, which merge to become the renal vein .
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There are three regions to a kidney: an outer renal cortex , an inner renal medulla , and a central space called the renal pelvis .
Microscopically, each contains over one million nephrons .
The nephrons produce urine which flows into a collecting duct ; several collecting ducts merge and drain urine into the renal pelvis.
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Each nephron has its own blood supply.
An afferent arteriole approaches the glomerular capsule and divides to become the glomerulus , a knot of capillaries.
The efferent arteriole leaves the capsule and branches into the peritubular capillary network.
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The closed end of the nephron is a cuplike glomerular capsule.
Spaces between podocytes of the glomerular capsule allow small molecules to enter the from the glomerulus via glomerular filtration .
The cuboidal epithelial cells of the proximal convoluted tubule have many mitochondria and microvilli to carry out active transport (following passive transport) from the tubule to blood.
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The descending loop of the nephron allows water to leave and the ascending portion extrudes salt.
The cuboidal epithelial cells of the distal convoluted tubule have numerous mitochondria but lack microvilli.
They carry out active transport from the blood to the tubule or tubular secretion .
Collecting ducts gather in the renal medulla and form the renal pyramids.
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During glomerular filtration , small molecules including water, wastes, and nutrients are forced from the blood inside the glomerulus to the inside of the glomerular capsule.
Blood cells, platelets, and large proteins do not move across.
About 180 liters of water are filtered daily.
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Substance Amount
Filtered
Water, L 180
Amount
Excreted
1.8
Reabsorption (%)
99.0
Sodium, g
Glucose, g
Urea, g
630
180
54
3.2
0.0
30.0
99.5
100.0
44.0
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During tubular reabsorption , certain nutrients, water and some urea moves from the proximal convoluted tubule into the blood of the peritubular capillary network.
Tubular reabsorption is a selective process because only molecules recognized by carrier molecules are actively reabsorbed.
The rate of this process is limited by the number of carriers.
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During tubular secretion , specific substances such as hydrogen ions, creatinine, and drugs such as penicillin move from the blood into the distal convoluted tubule.
In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, and substances that have undergone tubular secretion.
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The kidneys maintain the water-salt balance of the blood within normal limits.
By doing so, they also maintain blood volume and blood pressure.
Most of the water and salt (NaCl) present in the filtrate is reabsorbed across the wall of the proximal convoluted tubule.
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Salt passively diffuses out of the lower portion of the ascending limb of the loop; the upper thick portion actively extrudes salt into the tissue of the outer renal medulla.
Water is reabsorbed by osmosis from all parts of the tubule.
The ascending limb of loop of the nephron establishes an osmotic gradient that draws water from the descending limb of the nephron and the collecting duct.
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The permeability of the collecting duct is under the control of antidiuretic hormone
( ADH ).
Diuresis is an increase in urine flow and antidiuresis is a decrease.
When ADH is present, more water is reabsorbed, blood volume and blood pressure rise, and there is a decreased amount of urine.
If there is insufficient water intake, the posterior pituitary releases ADH, causing more water to be reabsorbed with a decreased urine output.
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Kidneys regulate salt balance by controlling excretion and reabsorption of ions.
Two hormones, aldosterone and atrial natriuretic hormone ( ANH ), control the kidneys’ reabsorption of sodium (Na).
When the juxtaglomerular apparatus detects low blood volume, it secretes renin that eventually results in the adrenal cortex releasing aldosterone that restores blood volume and pressure through reabsorption of sodium ions.
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Reabsorption of salt increases blood volume and pressure because more water is also reabsorbed.
ANH is secreted by the atria of the heart when cardiac cells are stretched by increased blood volume.
ANH inhibits secretion of renin; the resulting excretion of sodium also causes excretion of water and blood volume drops.
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Diuretics are chemicals that lower blood pressure by increasing urine output.
Alcohol inhibits secretion of ADH; dehydration after drinking may contribute to the effects of a hangover.
Caffeine increases the glomerular filtration rate and decreases tubular reabsorption of sodium.
Diuretic drugs inhibit active transport of
Na + so a decrease in water reabsorption follows.
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Kidneys rid the body of acidic and basic substances.
If the blood is acidic, hydrogen ions (H + ) are excreted and bicarbonate ions
(HCO
3
) are reabsorbed.
If the blood is basic, H + are not excreted and HCO
3
are not reabsorbed.
Breathing also ties up H + when carbon dioxide is exhaled.
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The urinary system has organs specialized to produce, store, and rid the body of urine.
Kidneys excrete nitrogenous wastes and maintain the water-salt and the acidbase balance of the blood within normal limits.
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Kidneys have a macroscopic anatomy and a microscopic anatomy.
Urine is produced by many microscopic tubules called nephrons.
Urine formation is a multistep process.
Kidneys are under hormonal control as they regulate the water-salt balance of blood.
Kidneys excrete hydrogen ions and reabsorb bicarbonate ions to regulate the pH of blood.
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