Kidney anatomy and physiology

This area of ​​the site is dedicated to kidney cancer . To introduce this topic, some basic information on the urinary tract is proposed, with a greater depth on anatomy and physiology of the kidney . There will also be a general mention of the main diseases of this organ .

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The urinary tract

Blood filtration, with the production and elimination of urine, is the main function of the urinary tract and is an essential function for survival . In fact, when the action of the organs that make up this apparatus is prevented by damage or functional alterations, it is possible to resort only to kidney transplantation or hemodialysis, a procedure for the disposal of waste and toxic substances that can be implemented with special devices external to the organism. In addition to the kidney function, with the production of urine, the kidney has other functions, such as hormone secretion. The urinary tract can be distinguished in the kidneys and urinary tract. Below is the complete list of the organs that compose it. In this sheet we will provide some details of anatomy and physiology of the kidney.

The organs that make up the urinary system are:

  • Kidney
  • Ureter
  • Bladder
  • Urethra

Kidney

Anatomy

Organs like the kidneys are called “equal” because there are two. The kidneys are arranged on the sides of the spine , in a space called retroperitoneal. This means that the kidneys are not located in the same cavity, surrounded by the peritoneum , in which the organs of the gastrointestinal system are located , but outside it, between the peritoneum and the posterior wall of the abdomen. The kidney has the shape of a bean and its upper margin, or pole, reaches the height of the 12th thoracic vertebra and the lower pole at the level of the 3rd lumbar vertebra . The right kidney is positioned slightly lower than the left. In the adult male a normal kidney weighsfrom 125 to 170 grams, in the female between 115 and 155 grams. Referring to the kidney bean shape, its concave margin is turned inwards and has a crack, called the hilum , through which the renal artery and nerves reach the organ and the renal vein and the first part of the urinary tract called pelvis or renal pelvis.

Structure of the kidney

Looking at a longitudinal section of the kidney, you can see two different areas. One, more external, is called cortical, and the innermost one called medulla. The medullary portion consists of numerous roughly triangular shaped structures, called renal pyramids, which convey urine to structures called renal calyxes. These, in turn, converge towards the renal pelvis. From the pelvis, urine is channeled into the ureter. The microscopic observation of the kidney tissue allows to identify the nephron, which is the functional unit of the organ, in the sense that each nephron is able to perform the function of producing urine and the whole activity of the organ depends on the sum of the functions of all the nephrons. There are about a million nephrons in a kidney. Each nephron is made up of a glomerulus and a tubule. This, in turn, ends in the collector ducts or tubules. The glomerulus is formed by a ball of capillaries, which are formed from an arteriole defined as afferent and flow into the efferent arteriole.

The glomerulus

The glomerulus is contained in a cup-shaped capsule, called Bowman’s capsule, which collects a liquid, pre-urine, which filters through the glomerulus, and pours it into the renal tubules. The renal tubule is formed by a first tract, proximal convoluted tubule, and by a final segment, the distal convoluted tubule. Between the proximal and distal convoluted tubule there is another conduit, the loop of Henle, in which it is possible to distinguish a descending and an ascending branch. The distal tubule of each nephron goes back to the Bowman’s capsule and converges in the angle formed by the afferent and efferent arterioles. In the area located at the intersection of the afferent and efferent arterioles and the distal convoluted tubule there is a structure, called the juxtaglomerular apparatus, which includes thejuxtaglomerular cells , which secrete renin, and the so-called dense macula. All of these structures represent the control center of the nephron function, capable of detecting alterations and implementing the necessary corrective actions. The distal convoluted tubule enters the collector tubule. Several collector tubules from different nephrons gather and form the renal pyramids.

Physiology

The main function of the kidney is to filter, as selectively as possible, the components of the blood to be eliminated in the urine, reabsorbing those excreted at the first filtration, but too useful for the body to be lost in excessive quantities. Among catabolites eliminated in the urine in greater quantities there are urea, uric acid, creatinine, ammonia, in addition to drugs, their derivatives and various substances foreign to the body. Others related to this main function, such as the regulation of the acid-base balance and the hydro-electrolyte balance, the maintenance of the circulating blood volume and the regulation of blood pressure.

The kidney is also an endocrine organ, as it produces hormones such as erythropoietin and renin. Erythropoietin stimulates the production of red blood cells. Renin is produced by the juxtaglomerular cells and transforms a molecule called angiotensinogen to angiotensin I. In the liver and in the lung angiotensin I is transformed in turn, thanks to the ‘ enzyme angiotensin converting (Angiotensin Converter Enzyme: ACE) to angiotensin II. Angiotensin II has a potent direct vasocottric effect and a stimulating effect of aldosterone and cortisol secretion . Returning to renin, its production is influenced by several factors, including blood pressure.

Finally, large quantities of prostaglandins are produced in the kidney, which regulate, among other things, the physiological flowblood into the kidney. The physiological blood flow in the kidney is about 1200 ml per minute, which means that almost 25% of the total blood volume passes through the kidney in a minute. Most of this volume, about 80%, is filtered through the glomerulus and this filtration produces the aforementioned pre-urine, which is collected in the Bowman capsule and then drained into the lumen of the proximal tubule. The most important difference between the composition of pre-urine and that of plasma is that the former is almost protein-free, especially albuminand globulins, and fat. In fact, the basement membrane of the glomerulus does not allow substances with a molecular weight greater than 70,000 Daltons to pass.

Pre-urine produces 120 mI per minute, for a total of 170-180 liters in 24 hours, a very large amount, compared to the volume of blood, which is about 5 liters. On the other hand, such a large amount of water is reabsorbed from the pre-urine, to reach a final volume of urine in the 24 hours of only 1-1.5 liters. This makes clear the importance of the resorption function that occurs in the parts of the nephron downstream of the Bowman capsule. For example, in the proximal tubule, reabsorption occurs, i.e. the passage from the tubule to the capillary, of about 80-85% of the water contained in the pre-urine, together with all the glucose, of potassium and amino acids, of about 70% of sodium and 60% of urea. The sodium is reabsorbed with an active mechanism and, in the passage towards the blood, drags the chlorine with it, attracting water with an osmosis mechanism. As for the reabsorption of glucose, it should be remembered that there is a limit to the ability of the tubular cells to reabsorb it, called the renal glucose threshold.

This limit is canceled in the presence of blood glucose values above 180 mg/ dl. Under these conditions, glucose passes into the urine and glycosuria appears. In Henle’s loop there is a further concentration of urine, compared to pre-urine, which involves a modulation of the balance between elimination and retention of electrolytes, to maintain the hydroelectrolytic balance. The composition of the urine continues to undergo changes also in the distal tubule and in the collector ducts, in which there is a further recovery of sodium, exchanged with potassium. Among the systems for regulating the formation and composition of urine there is that of aldosterone. This hormone , called mineralocorticoid, is produced in the cortical area of ​​the adrenal gland, starting from cholesterol, and its synthesis is stimulated by renin. Vasopressin , produced in the pituitary gland, has a reduction effect on urine production . All the water reabsorption mechanisms active in the distal nephron tubules contribute to maintaining the acid-base balance.

pathology

The kidneys can suffer damage and alterations of various types and caused by various causes. From acute and chronic infections to inflammatory processes, of which those due to autoimmune mechanisms are frequent. Among the most characteristic functional alterations of this organ is kidney stones or lithiasis, that is, the formation of stones in the kidney or urinary tract. The most serious form of functional failure is renal failure, i.e. a reduction in kidney function as a whole, which progresses over months or years.

The reduction in function is classified in 5 stages and each stage marks the transition to a lower and worse level of renal function than the previous one and is quantified on the basis of blood creatinine levels. Any kidney disease that causes nephron damage or loss of a part of it, forces the remaining nephrons to intensify their activity. In this way the initial damage is fully or partially compensated and, if it is not too serious, it does not manifest itself clinically. Meanwhile, however, the overload of work continues to damage the organ. Exceeded the threshold within which the compensation mechanisms manage to “mask” the insufficiency, the inability of the kidney to correctly perform the filter function is manifested clinically.

Glomerulonephritis are the pathologies most often at the origin of the work overload continues to damage the organ. After exceeding the threshold within which the compensation mechanisms manage to “mask” the insufficiency, the inability of the kidney to correctly perform the filter function is manifested clinically. Glomerulonephritis are the pathologies most often at the origin of the work overload continues to damage the organ. After exceeding the threshold within which the compensation mechanisms manage to “mask” the insufficiency, the inability of the kidney to correctly perform the filter function is manifested clinically. Glomerulonephritis are the pathologies most often at the origin ofchronic renal failure .

Glomerulonephritis consists of various diseases, also different from each other in some aspects, but united by the localization of the damage in the renal glomerulus. Sometimes the diagnosis of kidney failure is made on the basis of an increase in creatininemia , because until then functional failure has progressed subclinically. When present, the most frequent signs and symptoms are: high blood pressure , peripheral edema or episodes of pulmonary edema, increased concentration of nitrogen and potassium in the blood and anemia, due to the deficiency of erythropoietin. From the above, it can be understood that the most serious signs and symptoms appear only in advanced states, but that many damage mechanisms progress from the earliest stages. Two laboratory parameters, fundamental in the diagnosis and monitoring of renal failure, are: creatininemia and azotemia . Three types of therapeutic approaches to renal failure can be distinguished.

One relates to pathologies that have caused organ damage, for example autoimmune glomerulonephritis that require immunomodulatory therapy. Another approach concerns the alterations associated with renal failure: from hypertension, to atherosclerosis. Finally, renal failure per se is dealt with by dialysis or transplantation, from the moment it reaches stage 5. As for dialysis, it is measured on the basis of the quantity of urea that must be removed from the blood. The lower the effectiveness of the kidneys in eliminating urea, the higher the dialysis dose to be “administered”. The main cause of death of subjects with renal insufficiency is constituted by cardiovascular events, regardless of the stage of functional insufficiency achieved. Hemodialysis significantly prolongs life expectancy, but penalizes quality of life. Home hemodialysis maintains the survival benefits associated with a lower quality of life penalty. Transplantation improves long-term survival and quality of life, but is burdened by a fair frequency of short-term complications.

Urinary tract

Pelvis or renal pelvis: hollow structure, funnel-shaped, which collects the urine produced in the kidney and conveys it to the ureters.

Ureter: equal and symmetrical hollow organ 25-30 cm long and with a diameter ranging from 3-4 mm to 8 mm. The wall is made up of several muscle layers and the lumen is covered with a mucous membrane . The ureter opens into the bladder.

Bladder: hollow organ located in the pelvis, placed between the ureters and the urethra, the urine is collected in it before urination . In the male, it has relations with the prostate and seminal vesicles and, in the female, with the neck of the uterus and with the vagina . On the back, in the male, it contracts with the rectum and in the female with the uterus. The part of the bladder from which urine flows out to the urethra is called the bladder neck and has a specific structure that serves to prevent the involuntary leakage of urine into the urethra.

Urethra: conduit that connects the bladder with the outside of the body and therefore constitutes the last part of the urinary tract. In the male it is 16-18 cm long and, in addition to the passage of urine, it is also used for that of seminal fluid. The diameter is not uniform. The outlet of the urethra outward, called the urethral meatus, in the male is a crack located at the end of the glans penis. In the same sex, the urethra is covered, internally, by a mucosa rich in small extroflexions and glands. In the female the urethra is much shorter, about 3 cm, and the urethral meatus is in the most external part of the vagina.

 

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