What is the Difference Between Renal Plasma Flow and Renal Blood Flow

The main difference between renal plasma flow and renal blood flow is that the renal plasma flow is the volume of plasma delivered to the kidneys per unit time whereas the renal blood flow is the volume of blood delivered to the kidneys per unit time. Furthermore, renal plasma flow only measures the amount of plasma that passes through the kidney while renal blood flow measures both plasma and hematocrit that pass through the kidney. 

Renal plasma flow (RPF) and renal blood flow (RBF) are two measurements applied for the arterial blood delivered to the kidney.  

Key Areas Covered 

1. What is Renal Plasma Flow
     – Definition, Measuring, Importance
2. What is Renal Blood Flow
     – Definition, Measuring, Importance
3. What are the Similarities Between Renal Plasma Flow and Renal Blood Flow
     – Outline of Common Features
4. What is the Difference Between Renal Plasma Flow and Renal Blood Flow
     – Comparison of Key Differences

Key Terms 

Arterial Blood, eRPF, Hct, Kidney, PAH Clearance, Renal Blood Flow (RBF), Renal Plasma Flow (RPF)

What is Renal Plasma Flow 

Renal plasma flow (RPF) is the volume of plasma delivered to the kidney per unit time period. It is important to quantify the volume of venous blood exiting the kidney. However, according to the Fick principle, the volume of blood that enters the kidney or the arterial flow is equal to the volume of blood which exists from the kidney or the venous flow.  

Figure 1: Renal Blood Supply

In practice, measuring the plasma volume is difficult. Instead, this value is estimated with the use of effective renal plasma flow (eRPF). Here, eRPF is the amount of plasma cleared of p-aminohippuric acid (PAH) per unit time. Significantly, PAH is freely filtered by the kidney, and its reabsorption does not occur at any part of the nephron. But, the epithelial cells take up the unfiltered PAH from blood capillaries and secrete them into the proximal convoluted tubules. That means; almost all the PAH is cleared out from the blood during a single pass of blood through the kidney.  

What is Renal Blood Flow 

Renal blood flow (RBF) is the volume of blood delivered to the kidney per unit time period. Generally, kidneys receive roughly 25% of the blood pumped by the heart. Importantly, renal blood flow includes both blood plasma volume and hematocrit, which enter the kidney since it accounts for the volume of blood that enters the kidney. Here, hematocrit (Hct) is the fraction of blood accounted for red blood cells. On that account, the fraction for blood plasma is 1- Hct. Therefore, the RBF x (1-Hct) should be equal to eRPF.  

Figure 2: Measuring RPF and RBF

Similarities Between Renal Plasma Flow and Renal Blood Flow 

• Renal plasma flow and renal blood flow are two measurements of arterial blood delivered to the kidney. 
• Both are important in estimating the volume of venous blood existing from the kidney per unit time. 
• However, the flow into the kidney is equal to the flow out of the kidney. 

Difference Between Renal Plasma Flow and Renal Blood Flow 

Definition 

Renal plasma flow refers to the amount of plasma that flows through the kidneys per unit of time while renal blood flow refers to the measure of blood that passes through the kidneys per unit time. Thus, this is the main difference between renal plasma flow and renal blood flow.

Type of Measurement 

Moreover, renal plasma flow only measures the amount of blood plasma which passes the kidney while renal blood flow measures both blood plasma and hematocrit values which passes the kidney. 

Measurement 

Another difference between renal plasma flow and renal blood flow is that renal plasma flow is estimated from the effective renal plasma flow while renal blood flow is estimated with the use of renal plasma flow and hematocrit. 

Conclusion 

Renal plasma flow is the measurement of the amount of blood plasma which passes through the kidney per unit period of time. It can be estimated with the plasma clearance rate of the p-aminohippuric acid. In comparison, renal blood flow is the amount of blood plasma and hematocrit, which pass through the kidney per unit period of time. Renal plasma flow can measure the renal blood flow along with the hematocrit. Both types of measurements are important to determine the volumes of arterial and the venous blood flow in and out of the kidney. However, the main difference between renal plasma flow and renal blood flow is the type of measurement.

References:

1. Dalal R, Sehdev JS. Physiology, Renal, Blood Flow and Filtration. [Updated 2018 Oct 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2018 Jan-. Available Here
2. “Renal Function.” U.Arizona, Available Here

Image Courtesy:

1. “2611 Blood Flow in the Nephron” By OpenStax College – Anatomy & Physiology, Connexions Web site. Available here, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia