Urine

This is an amber colored fluid of slight acidic reaction excreted by the kidneys.

Volume: The volume of urine excreted in a normal adult varies between 1000-2000 ml/day.

Color:

Pale yellow or amber, depends upon the pigment known as urochrome.

pH: 

Mean: 6.0, range (4.6-8.0)

Chemical nature: Acidic.


Normal constituents of urine or Composition of urine


A, Organic substances:

1. Urea                                                    2. Uric Acid                                 3. Creatine                            

4. Creatinine                                            5. Ammonia                                 6. Hippuric acid

7. Oxalic Acid                                          8. Amino acid                               9. Allanton

10. Vitamins                                           11. Enzymes                                 12. Hormones


B, Inorganic substances:

1. Sodium                                                 2. Chloride                                    3. Phosphate

4. Sulphate                                                5. Potassium                                6. Calcium

7. Magnesium                                           8. Iodine


Abnormal constituents of urine:

1. Proteins: Albumin, globulin

2. Sugar: Glucose, fructose, galactose, lactose, and pentose

3. Blood: RBCs, WBCs

4. Ketone bodies: acetone, acetoacetic acid, and (R)-3-hydroxybutyric acid

5. Indicant: Indole production due to our growth of anaerobic bacteria in intestine

6. Pigments: Bilirubin, urochromogen, porphyrin and melanin

7. Casts: Cell

8. Pus: liquor puris, dead leukocytes

9. Renal calculi: Stances from kidney

10. Microbes: Bacteria, viruses


Formation of urine

Fluid is filtered from glomerular capillary into Bowman's capsule; this fluid is known as "glomerular filtrate".


Basic transport mechanisms through tubular membrane

There are two transport mechanisms.


1. Active transport:    (Absorptive/Secretory)

i. Primary active absorptive transport:

e.g. Na+ & Ca++ absorption

ii. Secondary active absorptive transport:

e.g. glucose & amine acid absorption

iii. Secondary active secretory transport:

e.g. H+, K+ & urate ion 


2. Passive transport:

i. Osmosis:            e.g. Water

ii. Diffusion:         e.g. Cl⁻ ion & urea


Formation of Dilute/Concentrated Urine

The kidneys have an ability to alter the composition of urine in response to the body's daily needs. Thereby, they maintain the osmolality of the body fluids. When it is necessary to conserve body water, the kidneys excrete urine with a high solute concentration. When it is necessary to rid the body of excess water, the kidneys excrete  urine with a dilute solute concentration.


Principle regulator:

The principle regulator of urine composition is antidiuretic hormone (ADH). In the absence of (ADH) the kidneys excrete a large volume of dilute urine. When (ADH) is present in high concentrations, the kidneys excrete a small volume of concentrated urine.


Components of concentrating and diluting system

The formation of urine that is dilute (hypo-osmotic to plasma) or concentrated (hyper osmotic to plasma) is achieved by the counter current system of the nephron. This system consists of a:

  1. Descending limb of the loop of Henle
  2. Thin and thick components of the ascending limb of the loop of Henle
  3. Medullary interstitium
  4. Distal convoluted tubule
  5. Collecting duct
  6. Visa recta, which are the vascular elements of  the juxtamedullary nephrons.

Summary for dilute urine formation

  1. Decreased osmolality of ECF (extra cellular fluid)
  2. Decreased secretion of ADH
  3. Decreased reabsorption of water in late distal tubule, cortical collecting tubule and in collecting ducts
  4. More water is excreted in urine
  5. Dilute urine