Introduction
The kidneys have many essential functions for the human body's normal functioning. The kidney's main functions are the excretion of metabolic wastes, maintaining the fluid-electrolyte balance, maintaining bone integrity, etc. They also connect with the cardiovascular system to keep the hemodynamic stability. A person is said to be hemodynamically stable if their blood pressure and heart rate are normal.
What Is Renal Autoregulation?
The autoregulatory mechanisms in the kidneys maintain the renal blood flow (RBF) and the glomerular filtration rate (GFR) irrespective of the renal perfusion pressure between 80 to 180 mmHg. The amount of blood given to the kidneys at a time is called renal blood flow. The glomerular filtration rate is a test to know if the kidneys are functioning well. It is the process by which the excess fluid and wastes are removed from the blood. The pressure in the blood flow of the renal vessels is termed renal perfusion pressure.
How Was Renal Autoregulation Discovered?
In 1931, Rein discovered renal autoregulation. Bayliss had observed that the renal vasculature underwent vasoconstriction when subjected to increased pressure as early as 1902. He considered it as a part of the myogenic response. The response of the renal vasculature to increased pressure might be to serve the function of the kidney was further observed by Forster and Maes in 1947. They observed that not on the real blood flow but also the glomerular filtration was maintained constant with an acute elevation in the blood pressure.
Goormaghtigh observed a unique anatomical relationship between the early distal nephron and its glomerular vascular pole. In many mammals, the early distal nephron has direct contact with the vascular pole of the glomerulus it originates. Hársing noted that inhibiting the proximal fluid reabsorption caused a drop in the glomerular filtration rate and the renal blood flow. This led him to assume that an increase in the filling of the distal tubule might cause signaling through the macula densa to regulate vascular resistance. Further studies done in this aspect have concluded that both the mechanisms (myogenic vasoconstriction and the macula densa-tubuloglomerular feedback) are involved in normal autoregulation. However, their relative contribution to the process is still debatable. Further experiments and research in this field suggest that these exact mechanisms (responsible for renal autoregulation) also play an essential role in protecting the kidney from the damage caused by hypertension.
How Is Renal Autoregulation Maintained?
The autoregulation of the kidneys is maintained by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses. An increase in the concentration of sodium chloride at the macula densa (the macula densa are the salt sensors at the distal part of the nephrons) causes the constriction of the afferent arterioles of the glomerulus. This reduces the filtration rate in a single glomerulus. The myogenic regulation maintains a constant blood flow in the kidneys at varying renal pressure. If the pressure in the vessel is suddenly decreased, there is constriction of the vessels, thereby increasing the pressure in the vessel and maintaining the blood flow.
The two mechanisms work hand in hand to regulate the glomerular filtration rate and the distal salt delivery. Recent studies show that these mechanisms protect the excretory functions of the kidney from fluctuations in blood pressure. Hence, renal autoregulation is necessary for the normal functioning of the kidney and volume hemostasis.
What Are the Implications of the Protective and Regulatory Roles of the Renal Autoregulatory Response?
The mean blood pressure decides the renal blood flow and the glomerular filtration rate. Hence, a myogenic response to systolic blood pressure (the pressure in the arteries when the heart beats) can change the autoregulation of the mean blood pressure by making parallel changes in the diastolic blood pressure. The diastolic blood pressure refers to the pressure in the arteries during the resting phase of the heart. Suppose the glomerulus is protected by the myogenic response by responding only to the systolic blood pressure. In that case, it can be understood that the autoregulation glomerular filtration rate, or renal blood flow, can be considered a secondary consequence.
Another debate states that plasma colloid osmotic pressure, proximal tubular pressure, and filtration coefficient influence the glomerular filtration rate. Plasma colloid osmotic pressure is the pressure exerted by large molecules and helps to hold water within the vascular space. The product of a membrane's permeability to water and the surface area of the membrane is its filtration coefficient. Hence, a myogenic mechanism that responds to the changes in transmural pressure (the pressure difference across a hollow structure is called transmural pressure) alone would not be sufficient for regulating it. It is still debatable whether these systems evolved to protect the organ against injury from hypertension or to insulate the functions from fluctuations in blood pressure.
What Are the Consequences of Impaired Renal Autoregulation on the Protection of the Kidneys?
In typical cases, the kidney is protected from hypertensive injury as long as it is within the autoregulatory range. But when the autoregulation does not function properly, the vulnerability to hypertensive injury is more. BP radiotelemetry assesses the quantitative relationships between blood pressure and kidney damage. Experiments have been done in rats to study the effect of impaired renal autoregulation. They show an association between reduced autoregulatory capacity and hypertensive injury; however, whether there are specific contributions of impairments in myogenic and macula densa-tubuloglomerular feedback mechanisms.
What Are the Consequences of Impaired Renal Autoregulation on Volume Homeostasis?
There are few pieces of evidence to suggest that impaired autoregulation is linked with disturbed volume homeostasis. A potential manifestation of impaired volume regulation is hypertension. However, there is less evidence to show that it is related to a loss of autoregulation.
Conclusion
The interpretation of autoregulation of renal blood flow and glomerular filtration rate suggests that it is a necessary mechanism for maintaining the kidneys' normal excretory function and volume homeostasis. However, experiments done in animal models have shown that even though autoregulation is impaired, volume regulation might not be affected much. Autoregulation is one of the main functions of the kidneys. It is a unique combination of tubular and vascular control mechanisms.
