Endocrine function of the kidneys. What do the kidneys provide? The function of the kidneys in the body includes

Endocrine function of the kidneys

The kidneys produce several biologically active substances, which make it possible to consider it as an endocrine organ. Granular cells of the juxtaglomerular apparatus release renin into the blood when blood pressure in the kidney decreases, sodium content in the body decreases, and when a person moves from a horizontal to a vertical position. The level of renin release from cells into the blood also varies depending on the concentration of Na+ and C1- in the area of ​​the macula densa of the distal tubule, providing regulation of electrolyte and glomerular-tubular balance. Renin is synthesized in granular cells of the juxtaglomerular apparatus and is a proteolytic enzyme. In the blood plasma, it splits off from angiotensinogen, located mainly in the α2-globulin fraction, a physiologically inactive peptide consisting of 10 amino acids, angiotensin I. In the blood plasma, under the influence of the angiotensin-converting enzyme, 2 amino acids are split off from angiotensin I, and it turns into an active vasoconstrictor substance angiotensin II. It increases blood pressure due to the constriction of arterial vessels, increases the secretion of aldosterone, increases the feeling of thirst, and regulates sodium reabsorption in the distal tubules and collecting ducts. All of these effects help normalize blood volume and blood pressure.

The kidney synthesizes plasminogen activator - urokinase. Prostaglandins are produced in the renal medulla. They participate, in particular, in the regulation of renal and general blood flow, increase the excretion of sodium in the urine, and reduce the sensitivity of tubular cells to ADH. Kidney cells extract from the blood plasma the prohormone formed in the liver - vitamin D3 - and convert it into a physiologically active hormone - active forms of vitamin D3. This steroid stimulates the formation of calcium-binding protein in the intestines, promotes the release of calcium from bones, and regulates its reabsorption in the renal tubules. The kidney is the site of production of erythropoietin, which stimulates erythropoiesis in the bone marrow. The kidney produces bradykinin, which is a strong vasodilator.

Metabolic kidney function

The kidneys are involved in the metabolism of proteins, lipids and carbohydrates. The concepts of “kidney metabolism,” i.e., the metabolic process in their parenchyma, through which all forms of kidney activity are carried out, and “metabolic function of the kidneys,” should not be confused. This function is due to the participation of the kidneys in ensuring the constant concentration of a number of physiologically significant organic substances in the blood. Low molecular weight proteins and peptides are filtered in the renal glomeruli. Cells in the proximal nephron break them down into amino acids or dipeptides and transport them across the basal plasma membrane into the blood. This helps restore the amino acid pool in the body, which is important when there is a deficiency of proteins in the diet. With kidney disease, this function may be impaired. The kidneys are capable of synthesizing glucose (gluconeogenesis). During prolonged fasting, the kidneys can synthesize up to 50% of the total amount of glucose produced in the body and entering the blood. The kidneys are the site of synthesis of phosphatidylinositol, an essential component of plasma membranes. The kidneys can use glucose or free fatty acids for energy expenditure. When the level of glucose in the blood is low, kidney cells consume fatty acids to a greater extent; with hyperglycemia, glucose is predominantly broken down. The importance of the kidneys in lipid metabolism is that free fatty acids can be included in the composition of triacylglycerol and phospholipids in the kidney cells and enter the blood in the form of these compounds.

Principles of regulation of reabsorption and secretion of substances in renal tubular cells

One of the features of the kidneys is their ability to change the intensity of transport of various substances over a wide range: water, electrolytes and non-electrolytes. This is an indispensable condition for the kidney to fulfill its main purpose - to stabilize the basic physical and chemical indicators of internal fluids. The wide range of changes in the rate of reabsorption of each of the substances necessary for the body filtered into the lumen of the tubule requires the existence of appropriate mechanisms for regulating cell functions. The action of hormones and mediators that affect the transport of ions and water is determined by changes in the functions of ion or water channels, carriers, and ion pumps. There are several known variants of the biochemical mechanisms by which hormones and mediators regulate the transport of substances by the nephron cell. In one case, the genome is activated and the synthesis of specific proteins responsible for the implementation of the hormonal effect is enhanced; in the other case, changes in permeability and pump operation occur without the direct participation of the genome.

Comparison of the features of the action of aldosterone and vasopressin allows us to reveal the essence of both variants of regulatory influences. Aldosterone increases Na+ reabsorption in renal tubular cells. From the extracellular fluid, aldosterone penetrates through the basal plasma membrane into the cell cytoplasm, connects with the receptor, and the resulting complex enters the nucleus (Fig. 12.11). In the nucleus, DNA-dependent tRNA synthesis is stimulated and the formation of proteins necessary to increase Na+ transport is activated. Aldosterone stimulates the synthesis of components of the sodium pump (Na +, K + -ATPase), enzymes of the tricarboxylic acid cycle (Krebs) and sodium channels through which Na + enters the cell through the apical membrane from the lumen of the tubule. Under normal physiological conditions, one of the factors limiting Na+ reabsorption is the permeability of the apical plasma membrane to Na+. An increase in the number of sodium channels or the time of their open state increases the entry of Na into the cell, increases the Na+ content in its cytoplasm and stimulates active Na+ transport and cellular respiration.

The increase in K+ secretion under the influence of aldosterone is due to an increase in potassium permeability of the apical membrane and the entry of K from the cell into the lumen of the tubule. Enhanced synthesis of Na+, K+-ATPase under the action of aldosterone ensures increased entry of K+ into the cell from the extracellular fluid and favors the secretion of K+.

Let us consider another version of the mechanism of cellular action of hormones using the example of ADH (vasopressin). It interacts from the side of the extracellular fluid with the V2 receptor, localized in the basal plasma membrane of the cells of the terminal parts of the distal segment and collecting ducts. With the participation of G-proteins, the enzyme adenylate cyclase is activated and 3,5"-AMP (cAMP) is formed from ATP, which stimulates protein kinase A and the insertion of water channels (aquaporins) into the apical membrane. This leads to increased water permeability. Subsequently, cAMP is destroyed by phosphodiesterase and converted into 3"5"-AMP.

The kidneys serve as a natural “filter” of the blood, which, when working properly, remove harmful substances from the body. Regulating kidney function in the body is vital for the stable functioning of the body and immune system. For a comfortable life you need two organs. There are cases that a person remains with one of them - it is possible to live, but he will have to depend on hospitals for the rest of his life, and the protection against infections will decrease several times. What are the kidneys responsible for, why are they needed in the human body? To do this, you should study their functions.

Kidney structure

Let's delve a little into the anatomy: the excretory organs include the kidneys - this is a paired bean-shaped organ. They are located in the lumbar region, with the left kidney being higher. This is nature: above the right kidney there is a liver, which prevents it from moving anywhere. Regarding the size, the organs are almost the same, but note that the right one is slightly smaller.

What is their anatomy? Externally, the organ is covered with a protective shell, and inside it organizes a system capable of accumulating and removing fluid. In addition, the system includes parenchyma, which creates the medulla and cortex and provides the outer and inner layers. Parenchyma is a set of basic elements that are limited to the connective base and the membrane. The storage system is represented by a small renal calyx, which forms a large one in the system. The union of the latter forms the pelvis. In turn, the pelvis is connected to the bladder through the ureters.

Main activities

Throughout the day, the kidneys and liver process and cleanse the blood of impurities and toxins, and remove decay products. More than 200 liters of blood are pumped through the kidneys per day, ensuring its purity. Negative microorganisms penetrate the blood plasma and are sent to the bladder. So what do the kidneys do? Considering the amount of work that the kidneys provide, a person could not exist without them. The main functions of the kidneys are:

• excretory (excretory);
• homeostatic;
• metabolic;
• endocrine;
• secretory;
• hematopoietic function.

Excretory function - as the main responsibility of the kidneys

The excretory function is to remove harmful substances from the internal environment. In other words, this is the ability of the kidneys to correct the acid state, stabilize water-salt metabolism, and participate in maintaining blood pressure. The main task lies with this function of the kidneys. In addition, they regulate the amount of salts and proteins in the liquid and ensure metabolism. Violation of the excretory function of the kidneys leads to a terrible result: coma, disruption of homeostasis and even death. In this case, a violation of the excretory function of the kidneys is manifested by an increased level of toxins in the blood.

The excretory function of the kidneys is carried out through nephrons - functional units in the kidneys. From a physiological point of view, a nephron is a renal corpuscle in a capsule, with proximal tubules and a storage tube. Nephrons perform important work - they control the correct execution of internal mechanisms in humans.

Excretory function. Stages of work

The excretory function of the kidneys goes through the following stages:

• secretion;
• filtration;
• reabsorption.

During secretion, the metabolic product, the remainder of electrolytes, is removed from the blood. Filtration is the process of a substance entering the urine. In this case, the fluid that has passed through the kidneys resembles blood plasma. Filtration has an indicator that characterizes the functional potential of the organ. This indicator is called the glomerular filtration rate. This value is needed to determine the rate of urine excretion for a specific time. The ability to absorb important elements from urine into the blood is called reabsorption. These elements are proteins, amino acids, urea, electrolytes. The reabsorption rate depends on the amount of fluid in food and the health of the organ.

What is the secretory function?

Let us note once again that our homeostatic organs control the internal mechanism of work and metabolic rates. They filter blood, monitor blood pressure, and synthesize biological active substances. The appearance of these substances is directly related to secretory activity. The process reflects the secretion of substances. Unlike the excretory function, the secretory function of the kidneys takes part in the formation of secondary urine - a liquid without glucose, amino acids and other substances useful to the body. Let us consider the term “secretion” in detail, since in medicine there are several interpretations:

• synthesis of substances that will subsequently be returned to the body;
• synthesis of chemicals that saturate the blood;
• removal of unnecessary elements from the blood by nephron cells.

Homeostatic work

The homeostatic function serves to regulate the water-salt and acid-base balance of the body.

Water-salt balance can be described as follows: maintaining a constant amount of fluid in the human body, where homeostatic organs influence the ionic composition of intracellular and extracellular waters. Thanks to this process, 75% of sodium and chloride ions are reabsorbed from the glomerular filter, while anions move freely and water is reabsorbed passively.

Regulation of acid-base balance by the body is a complex and confusing phenomenon. Maintaining a stable pH value in the blood occurs thanks to the “filter” and buffer systems. They remove acid-base components, which normalizes their natural amount. When the pH value of the blood changes (this phenomenon is called tubular acidosis), alkaline urine is formed. Tubular acidoses pose a threat to health, but special mechanisms in the form of h+ secretion, ammoniogenesis and gluconeogenesis stop urine oxidation, reduce enzyme activity and are involved in the conversion of acid-reacting substances into glucose.

Role of metabolic function

The metabolic function of the kidneys in the body occurs through the synthesis of biological active substances (renin, erythropoietin and others), as they affect blood clotting, calcium metabolism, and the appearance of red blood cells. This activity determines the role of the kidneys in metabolism. Participation in protein metabolism is ensured by the reabsorption of the amino acid and its further excretion by body tissues. Where do amino acids come from? They appear after the catalytic breakdown of biologically active substances such as insulin, gastrin, parathyroid hormone. In addition to the processes of glucose catabolism, tissues can produce glucose. Gluconeogenesis occurs within the cortex, and glycolysis occurs in the medulla. It turns out that the conversion of acidic metabolites into glucose regulates blood pH levels.

Determination of the magnitude of renal plasma and blood flow

Indirect methods for measuring the amount of renal blood flow are based on assessing the ability of renal tubular cells to secrete - almost completely extract from the peritubular

of the liquid (and, accordingly, from the blood plasma) of a number of organic acids and their secretion into the lumen of the tubule. For this purpose, they use PAG or diodrast, which are secreted by the cells of the renal tubules so effectively that, at low concentrations in the arterial blood, it is completely cleared of these substances during a single passage through the kidney (see Fig. 12.5). Using the same notation, you can calculate the purification from PAG using the formula:

Cran = V*Upah/Ppah.

This makes it possible to measure the magnitude of the effective renal plasma flow, i.e., the amount of plasma that flows through the vessels of the renal cortex and washes the cells of the proximal segment of the nephron. Since red blood cells do not contain PAG, to calculate the effective renal blood flow (ERBF) it is necessary to enter into the formula a value that takes into account the ratio between red blood cells and blood plasma (hematocrit - Ht):

ERBF= C PAH /(1-Ht).

Above we discussed effective plasma flow and blood flow. To determine total blood flow and plasma flow through the kidneys, it is necessary to know how much PAG remains in the renal blood. Since it is believed that PAG is completely extracted from the blood flowing through the renal cortex, the presence of a small amount of PAG in the renal vein is due to the fact that part of the blood bypasses the renal cortex and enters the vessels of the medulla. The proportion of blood flow through the renal medulla is about 9%, and the blood flow in the inner medulla (papilla) is equal to only 1% of the total renal blood flow.

In men, the effective renal plasma flow is about 655 ml/min (per 1.73 m 2 of body surface), the total plasma flow is 720 ml/min, and the total blood flow through the kidneys is 1300 ml/min. To determine how much fluid from blood plasma undergoes glomerular filtration, the filtration fraction is calculated (FF):

ff = C1n/s RAS.

The filtration fraction is about 0.2, i.e. equal to almost 20 % on the volume of plasma flowing through the kidney.

IN The kidneys produce certain substances that are excreted into the urine (for example, hippuric acid, ammonia) or enter the blood (renin, prostaglandins, glucose synthesized in the kidney, etc.). Hippuric acid is formed in tubular cells from benzoic acid and glycol. In experiments on an isolated kidney there was

It has been shown that when a solution of benzoic acid and glycocol is injected into an artery, hippuric acid appears in the urine. In tubular cells, when amino acids, mainly glutamine, are deaminated, ammonia is formed from amino groups. It enters primarily into the urine, partially penetrates through the basal plasma membrane into the blood, and there is more ammonia in the renal vein than in the renal artery.

The kidney is a paired organ; the main structural unit of the kidney is the nephron. In 1 minute, 1000 - 1300 ml of blood is filtered in the kidneys. Thanks to good blood supply, the kidneys are in constant interaction with other tissues and organs and are able to influence the state of the internal environment of the entire body.

KIDNEY FUNCTIONS:

1. EXCRETORY. The following are excreted from the body by the kidneys:

a) end products of catabolism (for example, products of nitrogen metabolism such as urea, uric acid, creatinine, as well as products of neutralization of toxic substances).

b) excess substances absorbed in the intestines or formed during the process of catabolism: water, organic acids, vitamins, hormones and others.

c) xenobiotics - foreign substances (medicines, nicotine).

2. HOMEOSTATIC. The kidneys regulate:

a) water homeostasis

b) salt homeostasis

c) acid-base state

3. METABOLIC.

a) participation in carbohydrate, protein, fat metabolism

b) synthesis in the kidneys of some biologically active substances: renin, the active form of vitamin D 3, erythropoietin, prostaglandins, kinins. These substances affect the processes of blood pressure regulation, blood coagulation, phosphorus-calcium metabolism, the maturation of red blood cells and other processes.

STAGES OF URINARY FORMATION

From the components of blood plasma, the kidneys form urine and can effectively regulate its composition.

1. ULTRAFILTRATION

During the process of ultrafiltration, primary urine is formed.

Blood, moving through the vessels of the kidney, is filtered into the cavity of the glomerulus through the pores of the connective tissue capsule - a special filter that consists of 3 layers. The 1st layer is the endothelium of blood capillaries, which has large pores. All blood components pass through these large pores, except for formed elements and high-molecular proteins. The 2nd layer is the basement membrane, which is built from collagen threads (fibrils) forming a molecular “sieve”. Pore ​​diameter - 4 nm. The basement membrane does not allow proteins with a molecular weight higher than 50 kDa to pass through. 3rd layer - epithelial cells of the capsule, the membranes of which are negatively charged, which does not allow negatively charged albumins in the blood plasma to penetrate into the primary urine. The shape of the three-layer pores is complex and does not correspond to the shape of the protein molecules of the blood plasma. This discrepancy prevents normal protein molecules from entering the primary urine. If the structure, shape, charge of a protein molecule is changed compared to a normal protein molecule, then such an abnormal protein can pass through the filter and end up in the urine. This is one of the mechanisms for purifying blood plasma from defective proteins and restoring its normal composition.

Thus, the ultrafiltrate (primary urine) normally contains almost no proteins and peptides (only 3-4 g/l). But the composition of low-molecular non-protein components and the content of various ions in primary urine are the same as in blood plasma. Therefore, primary urine is sometimes called “protein-free blood plasma filtrate.”

The amount of ultrafiltrate formed depends on the magnitude of the driving force of ultrafiltration - the hydrostatic blood pressure in the glomerular vessels (normally it is approximately 70 mmHg).

The driving force of ultrafiltration is counteracted by the oncotic pressure of blood plasma proteins (about 25 mmHg) and the hydrostatic pressure of the ultrafiltrate in the capsule cavity (about 15 mmHg).

Thus, the driving force of ultrafiltration is:

70 - (25+15) = 30 (mm Hg),

and is called effective filtration pressure.

ATP energy is not consumed in the ultrafiltration process.

It is clear that a decrease in blood pressure and/or an increase in hydrostatic pressure in the capsule cavity can lead to a slowdown, and with significant changes, to a complete cessation of the formation of primary urine (anuria).

As a result of the ultrafiltration process, primary urine is formed. Approximately 1500 liters of blood pass through the human kidneys per day, and about 180 liters of primary urine are formed (125 ml per minute).

Kidney filtration capacity assessed by calculating filtration clearance (purification coefficient) - for this, certain substances are introduced into the blood that are only filtered, but not reabsorbed or secreted (polysaccharide inulin, mannitol, creatinine).

Filtration clearance- this is the volume of blood plasma that is completely cleared of non-reabsorbable substances in 1 minute.

(Fig. 1). They are bean-shaped and located in the retroperitoneal space on the inner surface of the posterior abdominal wall on either side of the spinal column. Weight of each kidney adult is about 150 g, and its size is approximately the same as a clenched fist. The outside of the kidney is covered with a dense connective tissue capsule that protects the delicate internal structures of the organ. The renal artery enters the portal of the kidney, from which emerge the renal vein, lymphatic vessels and the ureter, which originates from the pelvis and carries the final urine from it to the bladder. In a longitudinal section, two layers are clearly distinguished in the kidney tissue.

Rice. 1. Structure of the urinary system: words: kidney and ureters (paired organs), bladder, urethra (indicating the microscopic structure of their walls; SMCs - smooth muscle cells). The composition of the right kidney shows the renal pelvis (1), the medulla (2) with pyramids opening into the calyxes of the pelvis; renal cortex (3); right: the main functional elements of the nephron; A - juxtamedullary nephron; B - cortical (intracortical) nephron; 1 - renal corpuscle; 2 - proximal convoluted tubule; 3 - loop of Henle (consisting of three sections: thin descending part; thin ascending part; thick ascending part); 4 - dense spot of the distal tubule; 5 - distal convoluted tubule; 6 connecting tubule; 7- collecting duct of the renal medulla.

Outer layer, or cortical gray-red substance, kidneys has a granular appearance, as it is formed by numerous microscopic structures of red color - renal corpuscles. Inner layer, or medulla, kidneys consists of 15-16 renal pyramids, the apexes of which (renal papillae) open into the small renal calyces (large renal calyces of the pelvis). In the medulla, the kidneys secrete outer and inner medulla. The parenchyma of the kidney consists of renal tubules, and the stroma is thin layers of connective tissue in which the vessels and nerves of the kidneys pass. The walls of the calyces, cups, pelvis and ureters have contractile elements that help propel urine into the bladder, where it accumulates until it is emptied.

The importance of kidneys in the human body

The kidneys perform a number of homeostatic functions, and the idea of ​​them only as an excretory organ does not reflect their true significance.

TO kidney function Their participation in the regulation includes:

• volume of blood and other internal fluids;
• constancy of blood osmotic pressure;
• constancy of the ionic composition of internal fluids and the ionic balance of the body;
• acid-base balance;
• excretion (secretion) of the final products of nitrogen metabolism (urea) and foreign substances (antibiotics);
• excretion of excess organic substances received from food or formed during metabolism (glucose, amino acids);
• blood pressure;
• blood clotting;
• stimulation of the process of formation of red blood cells (erythropoiesis);
• secretion of enzymes and biologically active substances (renin, bradykinin, urokinase)
• metabolism of proteins, lipids and carbohydrates.

Kidney functions

The functions of the kidneys are diverse and important for the functioning of the body.

Excretory (excretory) function- the main and most well-known function of the kidneys. It consists in the formation of urine and the removal from the body of metabolic products of proteins (urea, ammonium salts, creaginine, sulfuric and phosphoric acids), nucleic acids (uric acid); excess water, salts, nutrients (micro- and macroelements, vitamins, glucose); hormones and their metabolites; medicinal and other exogenous substances.

However, in addition to excretion, the kidneys perform a number of other important (non-excretory) functions in the body.

Homeostatic function The kidneys are closely related to the excretory system and consist in maintaining the constancy of the composition and properties of the internal environment of the body - homeostasis. The kidneys are involved in the regulation of water and electrolyte balance. They maintain an approximate balance between the amount of many substances excreted from the body and their entry into the body, or between the amount of a metabolite formed and its excretion (for example, water entered and removed from the body; electrolytes sodium, potassium, chlorine, phosphates, etc. entered and removed from the body). . Thus, the body maintains water, ionic and osmotic homeostasis, a state of isovolumia (relative constancy of the volumes of circulating blood, extracellular and intracellular fluid).

By removing acidic or basic products and regulating the buffer capacities of body fluids, the kidneys, together with the respiratory system, ensure the maintenance of the acid-base state and isohydry. The kidneys are the only organ that secretes sulfuric and phosphoric acids, which are formed during protein metabolism.

Participation in the regulation of systemic blood pressure - The kidneys play a major role in the mechanisms of long-term regulation of blood pressure through changes in the excretion of water and sodium chloride from the body. Through the synthesis and secretion of varying amounts of renin and other factors (prostaglandins, bradykinin), the kidneys take part in the mechanisms of rapid regulation of blood pressure.

Endocrine function of the kidneys - This is their ability to synthesize and release into the blood a number of biologically active substances necessary for the functioning of the body.

With a decrease in renal blood flow and hyponatremia, renin is formed in the kidneys, an enzyme under the influence of which the peptide angiotensin I, a precursor of the powerful vasoconstrictor angiotensin II, is cleaved from α2-globulin (angiotensinogen) in the blood plasma.

In the kidneys, bradykinin and prostaglandins (A 2, E 2) are formed, which dilate blood vessels and lower blood pressure, the enzyme urokinase, which is an important component of the fibrinolytic system. It activates plasminogen, which causes fibrinolysis.

When oxygen tension in the arterial blood decreases, erythropoietin is formed in the kidneys, a hormone that stimulates erythropoiesis in the red bone marrow.

With insufficient formation of erythropoietin in patients with severe nephrological diseases, with kidneys removed, or undergoing hemodialysis procedures for a long time, severe anemia often develops.

In the kidneys, the formation of the active form of vitamin D3, calcitriol, is completed, which is necessary for the absorption of calcium and phosphates from the intestine and their reabsorption from primary urine, which ensures a sufficient level of these substances in the blood and their deposition in the bones. Thus, through the synthesis and release of calcitriol, the kidneys regulate the flow of calcium and phosphates into the body and bone tissue.

Metabolic kidney function lies in their active participation in the metabolism of nutrients and, above all, carbohydrates. The kidneys, along with the liver, are an organ capable of synthesizing glucose from other organic substances (gluconeogenesis) and releasing it into the blood for the needs of the whole body. Under fasting conditions, up to 50% of glucose can enter the blood from the kidneys.

The kidneys take part in protein metabolism - the breakdown of proteins reabsorbed from secondary urine, the formation of amino acids (arginine, alanine, serine, etc.), enzymes (urokinase, renin) and hormones (erythropoietin, bradykinin) with their secretion into the blood. In the kidneys, important components of cell membranes of lipid and glycolipid nature are formed - phospholipids, phosphatidylinositol, triacylglycerols, glucuronic acid and other substances that enter the blood.

Features of blood supply and blood flow in the kidneys

The blood supply to the kidneys is unique compared to other organs.

• Large specific value of blood flow (0.4% of body weight, 25% of IOC)
• High pressure in the glomerular capillaries (50-70 mm Hg)
• Constancy of blood flow regardless of fluctuations in systemic blood pressure (Ostroumov-Beilis phenomenon)
• The principle of a double capillary network (2 capillary systems - glomerular and peritubular)
• Regional features in the organ: ratio cortical substance: outer layer of the medulla: inner layer -> 1:0.25:0.06
• The arteriovenous difference in O2 is small, but its consumption is quite large (55 µmol/min. g)

Rice. The Ostroumov-Beilis phenomenon

The Ostroumov-Beilis phenomenon- a mechanism of myogenic autoregulation that ensures the constancy of renal blood flow regardless of changes in systemic blood pressure, due to which the value of renal blood flow is maintained at a constant level.