Page 193 - 84 human physiolofy part-1

The Urinary System

Formation of Urine

Urine is formed in three steps: Filtration, Reabsorption, and Secretion.

Filtration

Blood enters the afferent arteriole and flows into the glomerulus. Blood in the glomerulus has both
filterable blood components and non-filterable blood components. Filterable blood components move
toward the inside of the glomerulus while non-filterable blood components bypass the filtration process
by exiting through the efferent arteriole. Filterable Blood components now take on plasma like form
called glomerular filtrate. A few of the filterable blood components are water, nitrogenous waste,
nutrients and salts (ions). Nonfilterable blood components include formed elements such as blood cells
and platelets along with plasma proteins. The glomerular filtrate is not the same consistency as urine, as
much of it is reabsorbed into the blood as the filtrate passes through the tubules of the nephron.

Reabsorption

Within the peritubular capillary network, molecules and ions are reabsorbed back into the blood.
Sodium Chloride reabsorbed into the system increases the osmolarity of blood in comparison to the
glomerular filtrate. This reabsorption process allows water (H2O) to pass from the glomerular filtrate
back into the circulatory system.

Glucose and various amino acids also are reabsorbed into the circulatory system. These nutrients
have carrier molecules that claim the glomerular molecule and release it back into the circulatory
system. If all of the carrier molecules are used up, excess glucose or amino acids are set free into the
urine. A complication of diabetes is the inability of the body to reabsorb glucose. If too much glucose
appears in the glomerular filtrate it increases the osmolarity of the filtrate, causing water to be released
into the urine rather than reabsorbed by the circulatory system. Frequent urination and unexplained
thirst are warning signs of diabetes, due to water not being reabsorbed.

Glomerular filtrate has now been separated into two forms: Reabsorbed Filtrate and Non-
reabsorbed Filtrate. Non-reabsorbed filtrate is now known as tubular fluid as it passes through the
collecting duct to be processed into urine.

Secretion

Some substances are removed from blood through the peritubular capillary network into the distal
convoluted tubule or collecting duct. These substances are Hydrogen ions, creatinine, and drugs. Urine
is a collection of substances that have not been reabsorbed during glomerular filtration or tubular
secretion.

Maintaining Water-Salt Balance

It is the job of the kidneys to maintain the water-salt balance of the blood. They also maintain

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