What is the Difference Between Filtrate and Urine

The main difference between filtrate and urine is that filtrate is the liquid filtered out from the blood into Bowman’s capsule whereas urine is the nitrogenous liquid formed by the nephron, the functional unit of the kidney. 

Filtrate and urine are two types of liquids produced inside the kidney as a result of the kidney function. Furthermore, the filtrate is similar in composition to blood plasma, but it does not contain globular proteins and other large molecules while urine mainly contains the waste products of the body, excess ions, and water.

Key Areas Covered

1. What is Filtrate
     – Definition, Formation, Composition, Importance
2. What is Urine
     – Definition, Formation, Composition, Importance
3. What are the Similarities Between Filtrate and Urine
     – Outline of Common Features
4. What is the Difference Between Filtrate and Urine
     – Comparison of Key Differences

Key Terms

Bowman’s Capsule, Filtrate, Kidney, Nephron, Nitrogenous Waste, Osmotic Balance, Urine

What is Filtrate

Filtrate or glomerular filtrate is the resultant liquid of the glomerular filtration, which is the first step of the formation of urine. Glomerular filtration occurs at the beginning of a nephron called the Bowman’s capsule. Here, blood plasma filters into the Bowman’s capsule due to the hydrostatic pressure generated between the afferent and efferent arteriole of the glomerulus. More importantly, blood cells, globular proteins including hemoglobin, albumin, globulin, etc., and other large molecules do not get filtered into the Bowman’s capsule. In general, 20% of the total blood volume is filtered into the Bowman’s capsule while the remaining 80% volume flows back to the body.

Figure 1: Filtration

Furthermore, 99% of the filtrate contains water. Also, it contains other small molecules including hormones, amino acids and nitrogenous wastes such as uric acid, urea, and ammonia. Besides, it contains electrolytes including sodium, potassium, chloride, and bicarbonate.

What is Urine

Urine is the yellowish color fluid formed as a result of the kidney function. The three steps in the formation of urine in the kidney include filtration, reabsorption, and secretion. Filtration step, which forms the glomerular filtrate, is described in the above section. Reabsorption is the second step involved in the re-uptake of water, small molecules, and ions into the circulatory system. It occurs at the proximal and distal convoluted tubules, the loop of Henle, and the collecting duct. Moreover, the osmolarity of the filtrate changes as a result of reabsorption.

Figure 2: Urine Formation

The final step of the urine formation is the secretion; this involves secretion of some molecules including creatinine, drugs, and hydrogen ions into the filtrate at the proximal and distal convoluted tubules.

Figure 3: Urine

Furthermore, 95% of the urine contains water. Also, it contains urea (9.3 g/L), chloride (1.87 g/L), sodium (1.17 g/L), potassium (0.750 g/L), and creatinine (0.670 g/L). In other words, urine is the main excretory medium of nitrogenous wastes from the body. In addition, it is responsible for the elimination of excess water and electrolytes from the body, contributing to the osmotic balance. Ultimately, this urine travels into the bladder for storage and is discharged through the urethra.

Similarities Between Filtrate and Urine

• Filtrate and urine are two types of liquids formed inside the nephron as a result of the kidney function.
• The main component of both liquids is water.
• Also, both liquids contain glucose, creatinine, urea, uric acid, and various electrolytes like sodium, potassium, chloride, and bicarbonate ions.
• However, they generally do not contain globular proteins and other large molecules in the blood.

Difference Between Filtrate and Urine

Definition

Filtrate refers to the fluid that passes from the blood through the capillary walls of the glomeruli of the kidney. While, urine refers to the watery typically yellowish fluid, which is one of the primary means through which the body eliminates excess water and salt. Urine also contains nitrogen compounds such as urea and other waste substances removed from the blood by the kidneys. Thus, this is the main difference between filtrate and urine.

Formation

Their formation is also a major difference between filtrate and urine. While filtrate forms inside the Bowman’s capsule, urine forms at the end of the nephron. 

Composition

The filtrate is similar in composition to the blood plasma without globular proteins and other large molecules while urine contains water, urine, creatinine, electrolytes including sodium, potassium, and chloride, and other small organic molecules. Hence, this is another difference between filtrate and urine.

Sequence

Moreover, the filtrate is formed at the first step of the urine formation while urine is formed at the end of the filtration, reabsorption, and secretion steps.

Importance in Analysis

Their importance in the analysis is one other difference between filtrate and urine. Glomerular filtration rate (GFR) is a measurement of kidney function while the color, odor, pH, turbidity, and volume of urine are important parameters in diagnosing diseases.

Conclusion

The filtrate is the liquid formed at the Bowman’s capsule in a process known as filtration from the blood. Its composition is mostly similar to the composition of the blood plasma. But, it does not contain globular proteins usually present in the plasma. On the other hand, urine is the ultimate result of the kidney function. It is formed at the end of the three steps: filtration, reabsorption, and secretion, which occur through the nephron. Urine contains urea, creatinine, excess electrolytes, small organic molecules, and water. Therefore, the main difference between filtrate and urine is the composition, formation, and the importance in the analysis.

References:

1. “Physiology of Urine Formation|Anatomy and Physiology II.” Lumen Learning, Lumen, Available Here

Image Courtesy:

1. “Glomerular Physiology” By Tieum – (CC BY-SA 4.0) via Commons Wikimedia  
2. “Kidney nephron molar transport diagram” By Nephron-urine.svg: M•Komorniczak -talk-, polish wikipedist.derivative work: Juvo415 (talk) – Nephron-urine.svg (CC BY 3.0) via Commons Wikimedia     
3. “Weewee” By en:User:Markhamilton – English Wikipedia, User:Markhamilton (Public Domain) via Commons Wikimedia  

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