The document summarizes the process of urine formation through three main steps: glomerular filtration, tubular reabsorption, and tubular secretion. It also discusses how the kidneys regulate water and electrolyte balance, as well as pH balance, through processes like selective reabsorption and secretion and hormones like antidiuretic hormone and aldosterone. The kidneys play an important role in maintaining homeostasis by precisely regulating the composition and volume of urine.

1.  URINE FORMATION
OVERVIEW
 Blood Plasma  Urine
 Three Major steps
 Glomerular filtration
 Tubular reabsorption
 Tubular secretion
Water and Electrolyte balance
Two important components of blood plasma and other body fluids are:
Water- the solvent of body fluids in which chemical reactions of life occur.
Electrolytes- substances that form ions when dissolved in water
-Named because they can conduct an electric current when dissolved in water.
* The concentration of water and electrolytes in body fluids are interrelated because the concentration of one affects
the concentration of the other.
 Diuretics
 Are substances that increase the production of urine.
 An example of this is caffeine, which is a common diuretic.
 Water Balance
 The intake of water is largely regulated by the thirst
center located in the hypothalamus of the brain.
 Thirst center is activated when it detects a
decrease in water concentration in the blood.
 Angiotensin II also activates the thirst center when
blood pressure declines significantly.
 An awareness in thirst stimulates water intake to
replace water loss from body fluids.
 Water intake must balance water loss.
 The volume of water lost in urine varies with both
the volume of water lost by other means and the
volume of water intake.
 The more water is lost through other means, the
less water that is lost in urine. The result= smaller
volume of more concentrated urine.
 If water loss through other means is minimal, water
reabsorption is reduced. Result= larger volume of more dilute urine is produced.
 The greater the water intake, the less water is reabsorbed. Result= larger volume of more diluted urine

2.  The less water intake, the more water is reabsorbed. Result= smaller volume of more concentrated urine is
produced.
 Cellular Dehydration
 If water loss significantly exceeds water intake for several days, extracellular fluids may become
more concentrated, causing water to move out of the cells by osmosis.
 May lead to serious complications unless the water loss is quickly restored.
 In serious cases, dehydration may result in fever, mental confusion, or coma.
 Antidiuretic hormone (ADH)
 Secreted by the posterior lobe of the pituitary gland
 Promotes water reabsorption
 Excessive water concentration of blood, decrease ADH secretion, less water is reabsorbed, and a greater
volume of urine is produced.
 When the water concentration of blood decreases, ADH secretion is increased, more water is reabsorbed,
and a smaller volume of urine is produced.
 Minimizes water loss in urine, but it cannot prevent it.
 Diabetes Insipidus
 A result in the deficiency of the ADH secretion
 Excessive production of dilute urine due to inability to reabsorb sufficient water from filtrate.
 Up to 15 liters of urine may be produced in a single day which triggers extreme thirst and the
almost continuous water intake.
 Without treatment, it may lead to severe dehydration and electrolyte imbalances.
 Electrolyte Balance
 Important electrolytes in body fluid:
 Sodium ions
 Potassium ions
 Calcium ions
 Chloride ions
 Phosphate ions
 Sulfate ions
 Bicarbonate ions
 Obtained through fluid and food intake.
 Regulated largely by active reabsorption of positively charged ions, which secondarily controls the passive
reabsorption of negatively charged ions by electrochemical attraction.
 Sodium ions
 Most important ions to be regulated

3.  Compose about 90% of the positively charged ions in extracellular fluids.
 Aldosterone
 Hormone secreted by the adrenal cortex
 Regulates the balance of sodium and potassium ions in the blood by stimulating the reabsorption of
sodium ions and the secretion of potassium ions by the distal tubule.
 Causes an exchange of sodium and potassium ions between the filtrate and the blood until the
blood concentrations of these two ions returns to normal.
 The adrenal cortex is stimulated to secrete aldosterone by:
+
 An increase of potassium ions (K ) in the blood
+
 A decrease of sodium ions (Na ) in the blood
 Angiotensin II
 Not secreted as long as blood concentrations of Sodium and Potassium ions are normal.
++
 Calcium ions (Ca )
 Mainly regulated by the antagonistic actions of parathyroid hormone and calcitonin.
 Parathyroid hormone (PTH)
 Secreted by the parathyroid glands when the blood calcium concentration declines.
 Promotes an increase in blood calcium by stimulating 3 different processes:
 Reabsorption of calcium ions from renal tubules
 Resorption of calcium from bones into the blood
 Absorption of calcium in foods by the small intestine
 When the blood calcium level returns to normal, parathyroid hormone secretion is
curtailed/reduced.
 Calcitonin
 Secreted by the thyroid gland
 Secreted when the level of blood calcium is too high
 Promotes deposition of calcium in bones, which reduces the blood calcium level
Hormones acting on the kidneys (table)
Hormone Source Action
+
Aldosterone Adrenal Cortex Stimulates reabsorption of Na from the filtrate into the blood;
+
stimulates secretion of K from blood into the filtrate

4. Antidiuretic hormone Posterior Pituitary Stimulates the reabsorption of water from the filtrate into the blood
by making the distal tubule and collecting ducts more permeable to
water; decreases volume of urine produced.
++
Parathyroid hormone Parathyroid glands Stimulates the reabsorption of Ca from the filtrate into the blood;
stimulates secretion of PO4
 Nitrogenous Wastes
 Kidneys’ major function: removal of excess nitrogenous wastes.
 They do not remove all of these wastes, but keep their concentrations in the blood within tolerable limits.
 Primary nitrogenous wastes produced by cellular metabolism:
 Urea
 Uric Acid
 Creatinine
 Nitrogenous Wastes (cont.)
 Urea
 Waste product of amino acid metabolism.
 The liver removes the amine (-NH2) groups from them, forming ammonia, which is converted to
less toxic urea.
 About 40% of urea in the filtrate is passively reabsorbed by diffusion, and the remainder is excreted
in urine.
 Uric Acid
 Waste product of nucleic acid metabolism.
 Much of the uric acid in the filtrate is reabsorbed by the renal tubules, but enough is actively
secreted into the filtrate to keep blood concentration of uric acid within normal limits.
 Creatinine
 Waste product of muscle metabolism.
 Actively secreted by the distal tubule.
 Gout
 A disorder characterized by an abnormally high concentration of uric acid in the blood and the…
 Deposition of uric acid crystals in joints
 Without treatment, it may lead to severe dehydration and electrolyte imbalances.
 pH Balance
 Normal blood pH: 7.35-7.45 in order for body cells to function properly.
 Cellular metabolism produces products that tend to upset the pH balance.

5.  Products include:
 Lactic acid
 Phosphoric acid
 Carbonic acid
 They tend to make the blood more acidic
 Acids
+
 Substances that release hydrogen ions (H ) when they are in water.
 Lowers the pH and increases the acidity of the liquid
 Strong acids release more hydrogen ions than weak acids.
 Bases
 Substances that release ions that can combine with hydrogen ions:
-
 Hydroxyl (OH ) ions
-
 Bicarbonate (HCO3 ) ions
 Buffers
 Prevent significant changes in pH.
 Are able to combine with or release hydrogen ions to stabilize pH.
 If the hydrogen ion concentration is excessive, buffers combine with some hydrogen ions to
increase their concentration to within normal pH.
 Help keep the pH of the blood relatively constant.
 The removal of excess hydrogen ions by the kidney prevents the buffers from being overwhelmed.
 By excreting excess hydrogen ions in urine, kidneys help to maintain the normal pH of body fluids.