Urinary Elimination

Urine-Urine is composed of water, certain electrolytes, and various waste product that are filtered out of the blood system.

For the production of urine, the kidneys do not simply pick waste products out of the bloodstream and send them along for final disposal.
The kidneys' 2 million or more nephrons (about a million in each kidney) form urine by three precisely regulated processes

1. Filtration-

Blood courses through the Glomeruli, much of its fluid, containing both useful chemicals and dissolved waste materials, soaks out of the blood through the membranes (Osmosis & Diffusion)
It is then filtered and then flows into the Bowman's Capsule.
The product formed is collectively known as Glomerular Filtrate.
Glomerular Filtrate - Water, waste products, excess salts (Na+ & K+), glucose, and other chemicals that have been filtered out of the blood.

Glomerular Filtrate Rate-

2. Reabsorption-

Movement of substances out of the renal tubules back into the Blood capillaries located around the tubules
Substances reabsorbed are water, glucose and other nutrients, and sodium (Na+) and other ions
Reabsorption begins in the proximal convoluted tubules and continues in the loop of Henle, distal convoluted tubules, and collecting tubules.
About 99% of the 180L of water that leave the blood each day by glomerular filtration returns to the blood from the proximal tubule through the process of Passive Reabsorpton.
Nutrient glucose is entirely reabsorbed back into the blood from the proximal tubules. In fact, it is actively transported out of the tubules and into the Peritubular Capllary Blood
Sodium ions (Na+) and other ions are only partially reabsorbed from the renal tubules back into the blood.
Amount depends largely on how much salt we take in from the foods that we eat. ↑Na+ intake = ↓Na+ reabsorption in the blood

3. Secretion -

Substances move into the distal and collecting tubules from blood in the capillaries around these tubules
Secretion is reabsorption in reverse
Reabsorption- substance from tubules - blood
Secretion - substance from blood - tubules
Substances are secreted through either an Active Transport Mecheanism or as a result of Diffusion across the membrane
Secretion plays a crucial role in maintaining the body's Acid-Base balance , another example of an important body function that the kidney participates in.

Composition of Urine