Renal Blood Circulation

Renal blood circulation can be defined as the blood supply of the kidney from the body. The kidney receives blood from the body then performs the critical function of excretion. The renal blood flow is under tight regulation to achieve proper filtration and excretion of the waste. To understand renal blood circulation it is important to understand the location of the kidney. The kidney is in the dorsolumbar, retroperitoneal cavity. The nephron acts as the functional unit of the kidney. Nephron receives the incoming blood, performs its filtration, and then sends back the purified blood. This process of filtration of blood leads to the formation of the urine, excretory waste of humans. This article is focused on renal blood flow, factors affecting renal blood flow, and its regulation.

Nephron

The kidney is located in the dorsolumbar cavity of the body. The nephron can be defined as the basic functional unit. The nephron has the following vital parts: Bowman's capsule, tubule-like region, the loop of Henle. It is important to note that nephrons are terminally differentiated. To understand the peculiarities of renal circulation it is important to understand the basic anatomy of the nephron.

Bowman’s Capsule

It is a capsular cup-like structure, it has a blood vessel, the blood vessel takes blood into the bowman’s capsule, it is also known as the glomerulus. The vessel taking blood into the glomerulus is known as afferent arteriole. The vessel taking blood out from the glomerulus is known as efferent arteriole.

Tubule 

Tubular shaped region evolving from Bowmans is known as the convoluted tube. The tube near Bowmans is known as the proximal convoluted tubule. The tube that originates from the far end of the nephron is called the distal convoluted tube. 

Loop of Henle

Loop of Henle can be divided into ascending and descending loop of Henle. The descending segment is the thin tubular structure, the ascending segment has the anatomical segmentation into a thin and thick tubular structure.

The ascending segments that enter the medulla areas are known as DCT (distal convoluted tubule). DCT leads to a connecting tubule known as a cortical connecting tubule (CCT). Distal convoluted tubule that enters the medullary is known as MCT, medullary collecting duct. The medullary collecting duct merges with the collecting duct.

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Nephrons can be classified into two groups, one being cortical nephron and the other juxtamedullary nephron.

Difference Between the Types of Nephrons

Blood Vessels Involved in Renal Blood Flow

Renal blood circulation can be defined as the blood supply of the kidney and back to the body. The renal vessels involved in renal circulation can be divided into three major groups namely,

1. Glomerular blood vessels

2. Peritubular capillaries

3. Vasa recta

There are majorly six glomerular blood vessels involved in this renal circulation, they are as follows, renal artery, segmental artery, interlobar artery, arcuate artery, interlobular artery, and afferent artery. 

It is important to note that the normal blood vessel has three layers namely, tunica interna, tunica media and, tunica externa, whereas the blood vessel responsible for the blood supply of the kidney only has tunica interna. The tunica interna layer of the blood vessels is modified to perform the special task of ultrafiltration to produce urine. 

The Sequence of Blood Vessels Involved in the Blood Supply of Kidney

The flowchart here explains the sequence in which blood vessels work to facilitate renal blood supply.

Renal artery

↓

Segmental artery

↓     

Interlobar artery

↓

Accurate artery

↓

Interlobular artery

↓

Afferent artery

↓

Glomerulus capillaries

↓ 

Efferent arteriole

↓

Peritubular capillaries or vasa recta

↓       

Cortical veins

↓

Accurate veins

↓

Interlobar veins 

↓

Renal vein

Renal Blood Flow

Renal blood flow is commonly known as RBF. Renal blood flow can be defined as the amount of blood received by the kidney per unit of time. In other words, it is the renal blood supply per unit time. The normal renal blood flow of the human body is RBF = 1000 mL/min. Factors affecting renal blood flow include glomerular filtration rate, colloidal osmotic pressure and, capsular hydrostatic pressure. 

GFR

Glomerular filtration rate, also known as GFR, is the amount of plasma filtrate formed each minute. In simpler terms, it can be defined as the rate at which filtration occurs.  Glomerular filtration can be mathematically expressed as the 

GFR= Kf × NFP

Where NFP is net filtration pressure, 

Kf is the filtration coefficient.

NFP

It is the net filtration pressure, it is defined on the basis of the glomerular hydrostatic pressure and colloidal osmotic pressure.

GHP or glomerular hydrostatic pressure is developed because of the change in the diameter of the afferent and efferent arteriole. The afferent arteriole generally has a large diameter when compared to the efferent arteriole.it is also denoted as PG. The osmolarity increases from afferent to efferent end because of movement of solvent out from the vessel, this leads to the comparative increase in the concentration of the plasma protein. The normal average of the glomerular hydrostatic pressure  is 60 mm Hg.

COP is also known as colloidal osmotic pressure develops because of the accumulation of the plasma protein, which is denoted as the πG. It opposes filtration. The normal average value of colloidal osmotic pressure is 32mm Hg.

CHP is also known as capsular hydrostatic pressure, developed by the pressure that the bowman’s capsule exerts on the blood vessels of the glomerulus. It is denoted by PB. The normal average value of capsular hydrostatic pressure is 8mm Hg.

The net filtration in humans can be calculated by the mathematical expression which is as follows,

NFP =  PG - πG  + PB

NFP calculation in humans

NFP =  PG - πG  + PB

 60 - (32 +8)

60 - 50

10 mm Hg is the net filtration pressure in humans.

Kf

It is known as the filtration coefficient. It depends on the permeability of the glomerulus and the surface area of the glomerulus. It is the function of the mesangial cell, these are specialized macrophages. This cell performs the following functions

1. They provide structural support to the glomerulus

2. They have the ability to promote and inhibit the filtration

3. They are the only contractile cells in the glomerulus of the nephron, the contraction and relaxation of these cells control the permeability and surface area of the glomerulus. 

Factors Affecting Renal Blood Flow

Factors that affect the kidney blood flow actually affected by controlling the parameters like GFR, it can be affected by the change in glomerular hydrostatic pressure,

1.  an increase in glomerular hydrostatic pressure leads to an increase in the GFR.

2. The glomerular filtration rate decreases when the colloidal osmotic pressure increases.

3. The glomerular filtration rate decreases with the increase in the capsular hydrostatic pressure.