Urine Formation

Urine Formation The process of urine formation occurs in the nephron. It can be divided into three processes: 1) Glomerular Formation 2) Tubular Reabsorption 3) Tubular Secretion

These steps are found in a particular part of the kidney, the nephron. Use the diagram at the end of your notes to follow along as the parts of the nephron are mentioned. We will go through this a few times, so bear with me!

Glomerular Filtration: What it is: When blood from the afferent arteriole enters the glomerulus, molecules pass from the glomerulus into the Bowman’s capsule. - The filtrate (stuff that passes through) is pushed into the Bowman’s capsule by the blood pressure in the glomerulus

Where it occurs: Glomerulus and Bowman’s Capsule

Substances Exchanged: - Water and small molecules (nitrogenous wastes, nutrients, salts) - Not all blood components pass through. Some are too large or are part of the blood itself (blood cells and platelets).

Tubular Reabsorption: What it is: The filtrate from the Bowman’s capsule flows into the proximal convoluted tubule where portions of it are reabsorbed into the blood of the pertubular capillary network. - Some of the filtrate that enters the proximal convoluted tubule cannot be reabsorbed. The substances that are not reabsorbed continue to pass through the Nephron to be further processed into urine.

Where it Occurs: Proximal convoluted tubule and pertubular capillary network

Substances Exchanged: - The components of the filtrate that are reabsorbed include mostly water, nutrients and required salts (ions). The components that cannot be reabsorbed. Include some water, most nitrogenous wastes, and excess salts (ions).

Tubular Secretion:   What it is: Some substances from the blood in the peritubular capillary network are added to the fluid inside the convoluted tubules. These substances join the components that were not reabsorbed into the bloodstream in step #2 to form urine. The urine formed from several nephrons enters one collecting duct which carries the urine to the renal pelvis and then to the ureter.

Where it occurs: convoluted tubules, collecting duct, minor/major calyx, renal pelvis and then to the Ureter.

Substances Exchanged: These substances include hydrogen ions, potassium ions, creatinine, and drugs (ie. penicillin).

Final Contents of Urine The urine contains: substances that have undergone glomerular filtration but have not been reabsorbed. Substances that have undergone tubular secretion These substances include: Water Excess Salts (ions) Nitrogenous wastes (urea, uric acid, ammonium, creatinine)