Regulation of Blood Volume

Regulation of Blood Volume

Blood volume within the body is determined by the blood pressure that depends on several factors. The force of the heart’s contraction will affect how much the heart is enlarged by the blood flow. The level of constriction of the arteries also determines the blood flow resistance and creates a high blood pressure. The kidney also influences blood volume and pressure by causing the constriction of arteries and veins. The regulation of blood volume within the human body is done through the coordination of numerous enzymes and organs. The kidney is the main organ that coordinates the blood volume and composition in the body.

Blood volume refers to the amount of red blood cells and plasma that is present in the circulatory system of human beings. Normal human beings have an approximate blood volume of between 4.5 and 5 liters. Blood regulation is controlled by the kidneys. Diagnostic technology that has been developed to monitor the blood volume measures the blood volume analysis. Constant analysis of blood pressure is important in the diagnosis of cognitive heart failure, renal complications and chronic hypertension. The kidney plays a central role in blood volume regulation by controlling the quantity of plasma and red blood cells. By coordinating the volumes of these blood components, the kidney regulates the hematocrit (Heitz, 2005 pp 36).

This function of the kidney is not merely reduced to the production of erythropoietin but the regulation of hematocrit. The kidney also has the unique ability of making reports on the blood volumes as tissue oxygen signals. The kidney does this by detecting the minute changes in the tension of the tissue oxygen for erythropoietin production at the critmeter. The production of erythropoietin is regulated by angiotensin II that is released as part of the effector signals of the blood volume regulation loops. These functions shift the understanding of the kidney’s function from the simple job of releasing erythropoietin to that of regulating water and sodium (Thibodeau, 2012 pp 127). The main purpose of blood volume regulation is to capitalize on the efficiency of the cardiovascular system. The regulation of blood volume and hematocrit can be separated into four major areas.

Regulation of Plasma volume

            Normal human beings have limited amounts of extra cellular fluid (ECF). ECF distribution into the plasma and interstitial compartments is controlled by Starling forces in the capillaries. The renal excretion of water and sodium changes the volumes of ECF in order to maintain an optimal cardiovascular level. The ECF, on its part, is regulated by a feedback loop of sensors and effectors. Signals are transmitted to the central nervous system (CNS) which integrates the signals of the sensor sites. Efferent signals are sent from the CNS to effector organs that act to bring back the blood volume to normalcy to vascular capacitance balance. Efferent organs consist of mechanisms that influence the excretion of sodium and water.

Effectors in the simplest of sense communicate to the plasma constituent of the blood only. Efferent limbs have low and high-pressure sensors of which low pressure sensors spot blood volume changes such as renal, hepatic and cardiopulmonary changes. Cardiopulmonary receptors detect atrial wall stretches that are an indicator of plasma volume. This triggers vagal afferent signals that lower the secretion of antidiuretic hormone (ADH). A drop in ADH lowers the water resorption and produces a water diuresis. Atrial stretch receptors also trigger secretion of atrial naturetic peptide (ANP) which is responsible for increasing the excretion of sodium (Stec, 2011 pp 98).

Blood volume is determined by the quantity of sodium and water that is consumed, excreted into the urine by the kidneys and lost through the skin, gastrointestinal tract and lungs. The amounts of water and sodium lost vary considerably and to maintain the amount of these two elements, the kidneys would regulate how much sodium and water that are passed out with the urine. The main method through which the kidneys control the blood volume is by changing the excretion of sodium and water into the urine. This happens in several ways. Increased blood volume results in the increase of renal perfusion, glomerular filtration rates and arterial pressure. Therefore, there is an increase in the excretion of sodium and water in a process called pressure natriuresis. Renal diseases sometimes affect this process so that there far less excretion of water and sodium through the kidneys at a specified pressure which ultimately increases the blood volume.

Regulation of red blood cells mass

            Blood flow, oxygen content of blood and the dissociation of the oxygen from hemoglobin determine the delivery of oxygen to the peripheral tissues. Consequently, blood flow is dependent on the flow through local vascular beds and the cardiac output. The oxygen content within the blood flow depends on the hemoglobin concentration. Adequate oxygenation of the tissues is ensured by blood flow changes or oxygen changes. Changes in blood flow can occur in all the tissues through a change in the cardiac output. The red blood cells mass is altered by production of renal erythropoietin. Erythropoietin stimulates the generation of red blood cells. The homodimerization of the receptors and the binding of erythropoietin triggered a signal that resulted in the expression of genes that encourage proliferation. With the increase of the red blood cells, they are increased which boosts the delivery of oxygen thereby relieving the tissue oxygen tension. As the red blood flow is automatically regulated, the changes in the blood flow that moderate oxygen delivery are controlled in the kidney (Taal et al, 2012 pp 32)

 

 

Regulation of hematocrit

            The need for hematocrit regulation is significant to the proper delivery of oxygen to hematocrit. When the hematocrit levels are low, the relay of oxygen to the hematocrit is direct while at high hematocrit levels, the relation of delivery of oxygen to hematocrit is reversed as it is derived from the viscosity in the blood flow. Similar to other psychological aspects, the regulation of hematocrit is done by sensors and effectors. Hematocrit is made up of plasma and red blood cells both of which have different mechanisms of control. Effector signals therefore, adjust the volumes of each component to come up with the optimal hematocrit.

Afferent sensing signals are responsible for reporting the state of each component within the blood using a common factor that is the tissue oxygen tension found in the kidney. There is a direct relation between the RPF, oxygen consumption and re-absorption of sodium. Renal blood flow is closely related to glomerular filtration where about 90% of the sodium is reabsorbed. Erythropoietin production at this stage is determined by the amount of oxygen tension from the balance between oxygen supplied and that used in the feedback loop. Slight changes in the oxygen tension demand that the oxygen delivery rates and the consumption rates equal each other psychologically. These similar rates may seem unlikely in the kidney.

Integration of blood volume regulation and hematocrit

Renin- angiotensin system (RAS) is one of the effector mechanisms that regulate plasma volume and consequently, blood flow. The RAS also controls the production of erythropoietin. RAS therefore affects the presence of red blood cell mass and the plasma. Angiotensin II has tubular, vascular and glomerular effectors that combine to change tissue pressure and influence the secretion of erythropoietin (Suzuki & Saruta, 2004 pp 45-9). The angiotensin II constricts the efferent arteriole and hastens the filtration fraction. An increase in the filtered sodium per unit of blood flow is reabsorbed which results in the increased oxygen consumption. Angiotensin II also causes lower blood flow to the renal medulla by constricting the vasa recta. Therefore, using various mechanisms, Angiotensin II controls the oxygen consumption per unit of blood flow and decreasing the supply of oxygen to the kidney. These opposing functions in the kidney tend to lower tissue oxygen tension. Erythropoietin production that is triggered by RAS completes the loop between the efferent and afferent neuro-hormonal indicators that regulate blood volumes.

Kidney filtration

The kidney has unique proteins called transporters that are found within the membrane of the nephron. The transporters are responsible for getting hold of the small molecules of various substances as they go by. These molecules may include glucose and sodium. Most of the sodium transporters are located at the proximal tubule while fewer ones are spread out elsewhere. Water is usually reabsorbed passively through osmosis as a natural response to the accumulation of sodium in the wall spaces of the nephron. Other molecules are also reabsorbed passively through solvent drag. Two factors affect the re-absorption rates within the kidneys. One, the concentration of different molecules within the filtrate will affect the rate. More molecules within the filtrate will result in a higher rate of absorption. This is because in the kidney, there is a limited number of transporters and number of molecules that can be grabbed at an instance.

The combination of different processes of the kidney namely filtration, re-absorption and secretion all contributes towards the blood volumes. A typical instance would be that one has consumed large amounts of sodium one sitting. The sodium will be absorbed into the blood system through the intestines. This increases the amount of sodium within the blood. The excess sodium will be filtered when it passes through the nephron. Although most of the sodium will be extracted by the transporters, most of the sodium will remain in the lumen. Consumed water will also remain because of osmosis (Klosterman, 2010 pp 128).

The excess sodium will then be passed out with the urine and eliminated from the blood and body. This is the reason why medicine is taken in repeated doses as the initial doses are normally excreted instantly. Kidneys are also responsible for wasting or conserving body water. In essence, kidneys can dictate the amount of blood volume that an individual has. Most of the water balance in the kidney is done at the loop of Henle where two processes occur. When the filtrate goes down the loop, water is reabsorbed and sodium passes on. This concentrates the amount of sodium in the lumen. When the filtrate moves up the loop of Henle, sodium is re-absorbed. This whole process is controlled by ADH that will increase or decrease the absorption of water depending on its concentration. This can explain the sudden need to urinate after drinking a large glass of water.

The kidney’s role of regulating blood composition

            The kidney can correct any imbalances in the blood volumes by either removing the concentration of bicarbonate in the blood or removing the excess acid and base in the urine. The kidney cells create constant levels of hydrogen ions and bicarbonate ions as they also have their own cellular metabolism. A carbonic anhydrase reaction that is similar to that of the red blood cells produces hydrogen ions that are secreted into the lumen in the nephron.  Bicarbonate ions are also produced and secreted into the blood system. Within the lumen, the filtered bicarbonates are mixed with the hydrogen ions that result in carbon dioxide and water. The kidney produces varying amounts of hydrogen or bicarbonate ions in the urine depending on the amount of bicarbonate that is filtered in the glomerulus as compared to the amount of hydrogen secreted in the kidney.

References

HEITZ, U. E., HORNE, M. M., & SPAHN, D. L. (2005). Pocket guide to fluid, electrolyte, and acid-base balance. St. Louis, Mo, Elsevier/Mosby.

KLOSTERMAN, L. (2010). Excretory system. Tarrytown, N.Y., Marshall Cavendish Benchmark.

STEC, D. E. (2011). Heme oxygenase and the kidney. San Rafael, Calif, Morgan & Claypool Life Sciences. Retrieved from http://dx.doi.org/10.4199/C00036ED1V01Y201107ISP024.

SUZUKI, H., & SARUTA, T. (2004). Kidney and blood pressure regulation. Basel, Karger. Retrieved from http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&db=nlabk&AN=119966.

TAAL, M. W., BRENNER, B. M., & RECTOR, F. C. (2012). Brenner & Rector’s the kidney. Philadelphia, PA, Elsevier/Saunders. Retrieved from http://www.mdconsult.com/public/book/view?title=Taal:+Brenner+&+Rector’s+The+Kidney.

THIBODEAU, G. A., PATTON, K. T., & ANTHONY, C. P. (2012). Structure & function of the body. St. Louis, Mo, Elsevier/Mosby.

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