One of the main functions of the kidneys is to form the urine for elimination of waste metabolites and regulation of water and electrolytes balance.
The urine is formed through the nephrons, which are the structural and functional unit of the kidney. Nearly 1 million nephrons are present in each kidney.
The first step in urine formation is the filtration of blood through glomerular capillaries into the bowman’s capsule. Then, the filtrate passes along the nephron structures where various substances are reabsorbed or secreted according to their levels in the body.
Rate of urine excretion = Filtration rate – Reabsorption rate + Secretion rate
What is Primary Urine?
The primary urine corresponds to the glomerular filtrate. The pressure difference between glomerular capillaries and bowman’s capsule facilitates the movement of blood plasma from arterioles into the nephron.
Composition of Primary Urine
The composition of primary urinary is blood plasma, which is devoid of proteins. It contains glucose, amino acids, water, sodium, etc. The minute pores in the filtration membrane do not let large proteins like formed blood elements and molecules pass through it.
Normal Primary Urine Volume
In normal healthy adults, around 180 liters of primary urine is formed per day. But most of it is reabsorbed in the proximal convoluted tubules, loop of Henle, and distal convoluted tubules.
What is Secondary Urine?
Secondary urine is formed through the primary urine. In the tubules of nephrons, the primary urine undergoes processing, and substances like water, sodium, and potassium are reabsorbed or secreted according to their normal levels.
Secondary Urine Composition
The main constituent of secondary urine is water. It is present up to 95% in the normal adult. The normal concentration of Urea is 9.3 g/L. Normal values of other elements like Chloride is 1.87 g/L, Sodium = 1.17 g/L, Potassium = 0.750 g/L, Creatinine = 0.670 g/L.
Changes in the normal composition like the presence of glucose (glycosuria) in the urine indicate diabetes or other pathological conditions.
Normal Secondary Urine Volume
On average, around 1 to 1.5 liters of secondary urine is formed and excreted out of the body.
If less than 500 mL of urine is produced in a day, then it is termed as oliguria. It may be due to dehydration, infection, obstruction, renal stones, kidney failure, etc.
If more than 2500 mL of urine is excreted in 24 hours, then this condition is known as polyuria. It occurs in the case of diabetes insipidus, hypertension, nephrotic syndrome, ingestion of alcohol or drugs, endocrinal disorders.
The complete absence of urine is called anuria and indicates obstruction, kidney failure, stenosis.
Primary Urine vs Secondary Urine
The initial unprocessed waste formed by glomerular filtration is known as primary urine. It is passed into tubules of nephrons where substances like water, sodium, and potassium move back into the blood or are secreted into the tubules. This processing of raw urine led to the formation of secondary urine, which is excreted out of the body.
Also Read : Short notes on Anatomy of Digestive System