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Kidney – Structure, Anatomy, Urine Formation, acid-base balance, and Function

What is Kidney?

  • The kidneys are a pair of reddish-brown bean-shaped organs located on each side of the vertebral column and in front of the last ribs. Situated on the posterior abdominal wall, they are slightly asymmetric, with the right kidney positioned slightly lower than the left due to the presence of the liver, which occupies more space on the right side. The kidneys are embedded in a mass of fat and are approximately 10 to 13 cm (4-5 inches) in length, 6 cm (2 ½ inches) in width, and 3 cm (1 ½ inches) in thickness. In adult males, the average weight of a kidney is about 150 grams, while in females, it is around 135 grams.
  • These vital organs play a crucial role in the human excretory system, responsible for filtering minerals from the blood, maintaining fluid balance, excreting waste products, and regulating blood volume, among other functions. Roughly one-third of all blood leaving the heart passes through the kidneys for filtration before being distributed to the body’s cells and tissues.
  • When the kidneys malfunction or fail, it can lead to various complications. Kidney failure can result in fluid retention, causing edema or swelling in the extremities and fluid accumulation in the lungs, known as pulmonary edema. Other complications include hyperkalemia (increased potassium levels in the blood), anemia, heart disease, and pericarditis.
  • Anatomically, the kidneys are reddish-brown, bean-shaped organs found in vertebrates. They are located in the retroperitoneal space, with the left and right kidneys on the respective sides. In adult humans, they measure approximately 12 centimeters (4 ½ inches) in length. Blood is supplied to the kidneys through the paired renal arteries, and blood exits through the paired renal veins. Each kidney is connected to a ureter, a tube that carries excreted urine to the bladder.
  • The kidneys are responsible for regulating the volume of various body fluids, fluid osmolality, acid-base balance, electrolyte concentrations, and toxin removal. Filtration occurs in the glomerulus, where one-fifth of the blood volume entering the kidneys is filtered. Important substances such as solute-free water, sodium, bicarbonate, glucose, and amino acids are reabsorbed, while hydrogen, ammonium, potassium, and uric acid are examples of substances that are secreted. The functional and structural unit of the kidney is called the nephron, and an adult human kidney contains around 1 million nephrons, whereas a mouse kidney has only about 12,500 nephrons. In addition to the nephrons, the kidneys also perform other functions, such as converting a precursor of vitamin D to its active form (calcitriol) and synthesizing the hormones erythropoietin and renin.
  • Chronic kidney disease (CKD) is a significant global health issue, with an estimated prevalence of 13.4%. Renal replacement therapy, such as dialysis or kidney transplantation, is necessary when kidney function drops below 15%. Procedures used to manage kidney disease include urinalysis, measuring kidney function through estimated glomerular filtration rate (eGFR), kidney biopsy, and CT scans to evaluate abnormalities. Nephrectomy is often performed to treat renal cell carcinoma.
  • Renal physiology focuses on the study of kidney function, while nephrology is the medical specialty that addresses diseases related to kidney function, including CKD, nephritic and nephrotic syndromes, acute kidney injury, and pyelonephritis. Urology, on the other hand, deals with diseases involving kidney and urinary tract anatomy, such as cancer, renal cysts, kidney stones, ureteral stones, and urinary tract obstruction.
  • The term “renal” is an adjective used to describe things related to the kidneys, originating from French or late Latin. There are differing opinions regarding the use of “renal” versus “kidney” in scientific writing. While some argue for replacing “renal” with “kidney” in certain contexts, others advocate for preserving the use of “renal,” including in phrases like “renal artery.”
Kidney
Image Source: https://courses.lumenlearning.com/wm-biology2/chapter/kidneys/

Features of Kidney

The kidney is a remarkable organ with several notable features. Here are some key features of the kidney:

  1. Filtration: One of the primary functions of the kidney is to filter waste products, toxins, excess water, and electrolytes from the blood. This filtration process occurs in specialized structures called nephrons.
  2. Nephrons: The kidney is composed of millions of tiny functional units called nephrons. Each nephron consists of a renal corpuscle (including the glomerulus and Bowman’s capsule) and a renal tubule. Nephrons play a vital role in filtering the blood, reabsorbing essential substances, and producing urine.
  3. Regulation of Fluid and Electrolyte Balance: The kidneys play a crucial role in maintaining the balance of fluids and electrolytes in the body. They regulate the concentration of ions such as sodium, potassium, calcium, and phosphate in the blood, helping to maintain proper hydration and electrolyte levels.
  4. Acid-Base Balance: The kidneys help regulate the pH level of the blood by selectively reabsorbing and excreting hydrogen ions and bicarbonate ions. This function is essential for maintaining the body’s acid-base balance.
  5. Blood Pressure Regulation: The kidneys contribute to the regulation of blood pressure through the renin-angiotensin-aldosterone system. When blood pressure decreases, the kidneys release renin, which initiates a cascade of reactions leading to vasoconstriction and increased water and sodium reabsorption.
  6. Erythropoiesis: The kidneys produce a hormone called erythropoietin, which stimulates the production of red blood cells in the bone marrow. This hormone helps maintain adequate oxygen-carrying capacity in the blood.
  7. Vitamin D Metabolism: The kidneys are involved in the synthesis of active vitamin D (calcitriol) from its inactive precursor. Calcitriol is essential for maintaining calcium and phosphate homeostasis and promoting bone health.
  8. Waste Excretion: The kidneys remove waste products, such as urea, uric acid, and creatinine, from the blood and excrete them in the form of urine. This helps eliminate metabolic waste and toxins from the body.
  9. Osmolality Regulation: The kidneys regulate the concentration of solutes in the blood by adjusting the reabsorption and excretion of water and solutes. This ensures the body’s fluid balance and prevents dehydration or overhydration.
  10. Detoxification: The kidneys help eliminate various waste products, drugs, and foreign substances from the body, contributing to the detoxification process.

These features highlight the vital role of the kidneys in maintaining homeostasis and overall health.

Structure of Kidney

The structure of the kidney is characterized by its bean-like shape and various anatomical features. Here are the key structural aspects of the kidney:

  1. Shape: The kidneys have a bean-shaped appearance, with a convex outer edge and a concave inner side.
  2. Hilum: The concave side of the kidney contains a depression called the hilum or hilus. This is the entry point for the renal artery (which brings blood to the kidney), the renal vein (which carries filtered blood away from the kidney), and the ureter (which transports urine from the kidney to the bladder).
  3. Ureters: The ureters, which are the ducts of the kidneys, emerge from the hilum and pass downward to open into the urinary bladder. These muscular tubes constantly transport urine from the kidneys to the bladder.
  4. Bladder: The bladder is a muscular bag located in the pelvis that serves as a storage site for urine. When the bladder fills up, it contracts to expel urine through the urethra.
  5. Urethra: The urethra is a small tube that opens to the outside of the body and allows the passage of urine. It is kept closed by a ring of muscular fibers, which relaxes when urine needs to be eliminated.
  6. Location: In humans, the kidneys are positioned high in the abdominal cavity on either side of the spine. They are located in a retroperitoneal position, slightly oblique and at a higher level than the waistline. The right kidney is usually slightly lower and smaller than the left kidney due to the presence of the liver on the right side.
  7. Adrenal Glands: On top of each kidney, there is an adrenal gland. These glands produce hormones that play essential roles in various bodily functions.
  8. Protection and Surrounding Fat: The kidneys are partially protected by the 11th and 12th ribs. Each kidney, along with its adrenal gland, is surrounded by layers of fat. The perirenal fat is situated between the renal fascia and renal capsule, while the pararenal fat is located above the renal fascia.
  9. Size and Weight: The weight of the kidneys can vary depending on factors such as sex. In males, the standard reference range for the weight of the right kidney is between 80 and 160 grams, and for the left kidney, it is between 80 and 175 grams. In females, the range is 40 to 175 grams for the right kidney and 35 to 190 grams for the left kidney.
  10. Measurements: The median length of the kidneys in adults has been measured to be approximately 11.2 cm on the left side and 10.9 cm on the right side. The median renal volumes are around 146 cm3 for the left kidney and 134 cm3 for the right kidney.

These structural features contribute to the overall functioning of the kidneys in filtering waste products, maintaining fluid balance, regulating blood pressure, and other vital physiological processes.

The internal structure of the kidney is shown.
The internal structure of the kidney is shown. | Image Source: https://courses.lumenlearning.com/wm-biology2/chapter/kidneys/

Gross anatomy

Gross anatomy refers to the visible structures and organization of the human kidney. Here are the key aspects of the gross anatomy of the kidney:

  1. Renal Cortex and Renal Medulla: The functional substance, or parenchyma, of the kidney is divided into two major regions: the outer renal cortex and the inner renal medulla. These regions can be visually distinguished.
  2. Renal Lobes: The kidney is composed of eight to 18 cone-shaped renal lobes. Each renal lobe consists of renal cortex that surrounds a portion of the inner renal medulla called a renal pyramid.
  3. Renal Columns: Between the renal pyramids, there are projections of renal cortex known as renal columns. These columns help in dividing and supporting the renal structures.
  4. Nephrons: Nephrons are the functional units responsible for producing urine. They span both the renal cortex and medulla. A nephron begins with a renal corpuscle, located in the cortex, which is responsible for the initial filtration of blood. It is followed by a renal tubule that extends deep into the medullary pyramids.
  5. Medullary Rays: Medullary rays are collections of renal tubules that drain into a single collecting duct. These structures are part of the renal cortex.
  6. Renal Papilla and Calyces: The tip, or papilla, of each renal pyramid empties urine into a minor calyx. Multiple minor calyces converge to form major calyces, which, in turn, combine to create the renal pelvis.
  7. Renal Pelvis and Ureter: The renal pelvis is the expanded upper end of the ureter, which serves as the drainage system for urine from the kidneys. The ureter carries urine from the renal pelvis to the urinary bladder.
  8. Hilum: At the hilum of the kidney, the ureter and renal vein exit the kidney, while the renal artery enters. The hilum is also where the blood vessels and the ureter are connected to the kidney.
  9. Hilar Fat and Renal Sinus: Surrounding the structures at the hilum, there is a layer of adipose tissue known as hilar fat. The renal sinus is a fat-filled cavity that includes the renal pelvis and calyces. It separates these structures from the renal medullary tissue.

It’s important to note that the kidneys do not possess any overtly moving structures. The primary function of the kidneys is to filter blood and produce urine, and the gross anatomy of the kidney reflects its role in the excretory system.

The nephron is the functional unit of the kidney.
The nephron is the functional unit of the kidney. | Image Source: https://courses.lumenlearning.com/wm-biology2/chapter/kidneys/

What is the Internal Structure of Kidney?

The internal structure of the kidney can be described as follows:

  1. Outer Cortex: The outer cortex is the solid, dark-colored, and granular portion of the kidney. It forms the outer layer of the kidney.
  2. Medulla: The medulla is located beneath the cortex and is made up of 15 to 16 pyramid-shaped structures called renal pyramids. Each pyramid consists of numerous fine tubules arranged in a conical shape. The base of each pyramid faces the cortex, while the pointed end extends into cup-like branches of the pelvis. When pressure is applied to a pyramid, urine can be observed flowing from the tubules.
  3. Pelvis: The pelvis is a funnel-shaped structure that serves as a large cavity within the kidney. At its upper end, the pelvis is divided into several branches known as calyces (singular: calyx). On average, there are around 10 calyces. The pelvis collects urine and transports it downward through the ureters. Alternatively, the ureter widens out into a cavity with multiple short, wide branches. The funnel-shaped cavity is referred to as the pelvis of the ureter, while its branches are called calyces. In other words, the pelvis is formed by the union of various calyces.

Overall, the internal structure of the kidney consists of the outer cortex, the medulla composed of renal pyramids, and the pelvis that collects urine and connects to the ureters. This intricate arrangement enables the kidneys to perform their crucial functions in filtering blood and producing urine.

What is the Microscopic Structure of Kidney?

The microscopic structure of the kidney can be described as follows:

(A) Malpighian Body or Renal Corpuscles:

The cortex of the kidney contains long, coiling tubules. Each tubule begins in the cortex with a small cup-like expansion called the Malpighian body or renal corpuscle. After following a coiled course, the tubule eventually joins one of the larger tubes and runs straight through a pyramid before opening into the funnel-shaped pelvis.

The Malpighian body consists of two parts:

  1. Glomerulus: Small branches of the renal artery run along the straight tubes in the medulla. Each branch gives rise to a small branch that enters the Malpighian body and breaks up into a rounded bunch of blood capillaries called the glomerulus. The glomerulus is a network of blood capillaries formed by branching afferent arterioles. The efferent vessels then emerge as small efferent arterioles. They further break up to form a second capillary network around the urinary tubules. These capillaries reunite to form the renal vein, which carries blood away from the kidney.
  2. Bowman’s Capsule: This is the expanded end of each tubule toward the cortex. Bowman’s capsule is a cup-like, double-walled cavity that contains a network of blood vessels called the glomerulus. The function of the Malpighian body is the filtration of plasma.

(B) Renal Tubules:

The renal tubules are approximately 3 cm long and 20-60 microns wide. There are numerous renal tubules in the kidney. Each renal tubule consists of the following parts:

  1. Bowman’s capsule: The cup-like part at the beginning of the tubule.
  2. Proximal coiling tubule.
  3. Loop of Henle: A U-shaped portion of the tubule that extends into the medulla.
  4. Distal coiling tubule.
  5. Junctional tubule.

Nephron:

The nephron is the basic functional unit of the kidney. The kidney is composed of numerous nephrons, with each human kidney containing approximately 1-2 million nephrons. Each nephron is made up of two main parts: the Malpighian body and the renal tubule.

(C) Blood Vessels:

The two important blood vessels in the kidney are the renal artery and renal vein. The renal artery enters the kidney through the hilum and carries blood for filtration. It breaks up into afferent arterioles, forming a network of capillaries in the Malpighian body known as glomeruli. These glomerular capillaries unite to form efferent arterioles, which in turn unite to form the renal vein, carrying the filtered blood back into the general circulation.

Within the kidney, the small branches of the renal artery and renal vein are connected by minute renal capillaries that surround the kidney tubules. The renal efferent vessels contain the purest blood in the body, as waste products have been filtered out during the process.

Blood supply in Kidney

The blood supply to the kidneys can be described as follows:

  • The kidneys receive blood from the renal arteries, which branch directly from the abdominal aorta. There is a left renal artery and a right renal artery. The kidneys receive approximately 20-25% of the cardiac output in an adult human, indicating the high blood supply required for their function.
  • Each renal artery branches into segmental arteries, which further divide into interlobar arteries. These interlobar arteries penetrate the renal capsule and extend through the renal columns located between the renal pyramids. The interlobar arteries then supply blood to the arcuate arteries, which run along the boundary of the renal cortex and medulla.
  • The arcuate arteries give rise to several interlobular arteries that feed into the afferent arterioles. The afferent arterioles are responsible for supplying blood to the glomeruli, which are the filtering units of the kidney.
  • After filtration occurs in the glomeruli, the filtered blood moves through a network of small veins called venules. These venules converge to form interlobular veins, which follow a similar distribution pattern as the arterioles. The blood from the interlobular veins then flows into the arcuate veins and subsequently into the interlobar veins. Finally, the blood exits the kidney through the renal veins, which ultimately drain into the inferior vena cava.
  • This blood supply ensures that the kidneys receive a continuous flow of blood, allowing for the filtration and processing of waste products, regulation of electrolyte balance, and maintenance of overall fluid balance in the body.

Nerve supply in Kidney

  • The kidney is innervated by the nervous system through the renal plexus, a network of nerve fibers that course along the renal arteries to reach each kidney. The renal plexus allows for communication between the kidney and the nervous system.
  • The sympathetic nervous system plays a significant role in the regulation of kidney function. Input from the sympathetic nerves triggers vasoconstriction in the kidney, leading to a reduction in renal blood flow. This response is part of the body’s physiological mechanisms for regulating blood pressure and maintaining overall cardiovascular homeostasis.
  • The parasympathetic nervous system also provides input to the kidney through the renal branches of the vagus nerve. However, the exact function of this parasympathetic input in kidney regulation is not fully understood.
  • Sensory input from the kidney is transmitted through sensory nerves and travels to the T10-11 levels of the spinal cord. This means that sensations originating from the kidney, such as pain or discomfort, are sensed in the corresponding dermatome, which is an area of skin innervated by a particular spinal nerve. As a result, pain or discomfort originating from the kidney may be referred to the flank region, corresponding to the T10-11 dermatome.
  • Overall, the nerve supply to the kidney allows for communication between the kidney and the nervous system, enabling regulation of renal blood flow, blood pressure, and sensation.

Renal blood supply/Renal circulation

  • Renal blood supply, also known as renal circulation, is responsible for delivering blood to the kidneys to support their vital functions. The kidneys receive their blood supply through the renal arteries, which branch directly from the abdominal aorta, the main artery of the body.
  • Despite their relatively small size, the kidneys receive a significant amount of blood flow, accounting for approximately 20% of the cardiac output. This highlights the importance of efficient blood supply for the proper functioning of the kidneys.
  • The renal arteries divide into segmental arteries, which further branch into interlobar arteries. These interlobar arteries penetrate the renal capsule, the outer protective layer of the kidney, and extend through the renal columns located between the renal pyramids. This branching pattern ensures that blood is distributed throughout the kidney.
  • The interlobar arteries give rise to arcuate arteries, which run along the boundary between the cortex (outer region) and the medulla (inner region) of the kidney. The arcuate arteries supply blood to the glomeruli, which are specialized structures involved in the filtration of blood and the formation of urine. From the arcuate arteries, several interlobular arteries arise and further divide into smaller vessels called afferent arterioles. The afferent arterioles are responsible for supplying blood to the glomeruli, where the process of filtration takes place.
  • After the filtration of blood in the glomeruli, the filtered blood moves into a network of small veins called venules. These venules converge to form interlobular veins, which follow a similar distribution pattern as the arterioles. Blood from the interlobular veins drains into arcuate veins, which in turn merge into interlobar veins. Finally, the interlobar veins join together to form the renal vein, which exits the kidney and carries the filtered blood back to the general circulation for distribution throughout the body.
  • The renal blood supply is crucial for maintaining the proper functioning of the kidneys. It ensures that an adequate amount of blood reaches the kidneys for filtration, waste removal, regulation of fluid and electrolyte balance, and other essential renal processes.
Structures of the kidney
1. Renal pyramid • 2. Interlobular artery • 3. Renal artery • 4. Renal vein 5. Renal hilum • 6. Renal pelvis • 7. Ureter • 8. Minor calyx • 9. Renal capsule • 10. Inferior renal capsule • 11. Superior renal capsule • 12. Interlobular vein • 13. Nephron • 14. Renal sinus • 15. Major calyx • 16. Renal papilla • 17. Renal column | Piotr Michał Jaworski; PioM EN DE PL, CC BY-SA 3.0, via Wikimedia Commons

Structure of Renal circulation

The structure of renal circulation involves the arteries and veins that supply and drain blood from the kidneys. The pathway of blood flow through the renal circulation can be described as follows:

Arteries:

  1. Abdominal aorta: The blood supply to the kidneys originates from the abdominal aorta, the main artery of the body.
  2. Renal artery: The renal artery branches off from the abdominal aorta and delivers oxygenated blood to the kidneys. It also provides a branch to the inferior suprarenal artery, which supplies the adrenal gland.
  3. Segmental arteries: The renal artery further divides into segmental arteries, which are named superior, inferior, anterior, anterior superior, anterior inferior, and posterior.
  4. Interlobar arteries: The segmental arteries give rise to interlobar arteries. These arteries run within the renal columns, which are the regions of the cortex located between the renal pyramids.
  5. Arcuate arteries: The interlobar arteries penetrate the renal capsule and extend through the renal columns. Once inside the kidney, they curve and run along the boundary between the cortex and the medulla, forming arcuate arteries.

Arterioles and Glomerulus: 6. Afferent arterioles: The arcuate arteries give rise to afferent arterioles, which supply blood to the glomeruli. The glomeruli are specialized capillary networks involved in the filtration of blood in the nephrons.

  1. Glomerulus: Each afferent arteriole enters a glomerulus, where filtration occurs. The glomerulus is a tuft of blood capillaries surrounded by Bowman’s capsule, a structure in the nephron.

Veins: 8. Efferent arterioles: After filtration, blood exits the glomerulus through efferent arterioles. It is important to note that the efferent arterioles do not directly drain into the interlobular vein.

  1. Peritubular capillaries: The efferent arterioles of the cortical nephrons supply blood to the peritubular capillaries, which surround the renal tubules and participate in the exchange of substances.
  2. Interlobular veins: Blood from the peritubular capillaries drains into interlobular veins.
  3. Interlobar veins: The interlobular veins converge to form interlobar veins, which run through the renal columns.
  4. Renal vein: The interlobar veins combine to form the renal vein, which exits the kidney and carries deoxygenated blood back to the general circulation via the vena cava.

The structure of renal circulation ensures that blood is efficiently distributed to the kidneys, allowing for the filtration of waste products and the regulation of fluid and electrolyte balance.

Mechanism of Urine Formation in Kidney

The mechanism of urine formation involves several processes in the kidney:

  1. Glomerular Filtration: Blood enters the glomerulus, and through the process of glomerular filtration, fluid and small solutes are filtered from the blood into the Bowman’s capsule. This filtration occurs due to the pressure exerted by the blood in the glomerular capillaries.
  2. Reabsorption: After filtration, the filtrate moves through the renal tubules, where the majority of the filtered substances, such as water, glucose, amino acids, ions, and other valuable molecules, are selectively reabsorbed back into the bloodstream. Reabsorption occurs primarily in the proximal tubule, loop of Henle, and distal tubule.
  3. Secretion: Along with reabsorption, substances that were not adequately filtered or need to be eliminated further are actively secreted from the peritubular capillaries into the renal tubules. This process allows the removal of additional waste products, drugs, and excess ions from the blood.
  4. Excretion: The final step is excretion, where the processed filtrate, now referred to as urine, flows through the collecting ducts and into the ureters for elimination from the body. The urine consists of waste products, such as urea, creatinine, uric acid, as well as excess water, ions, and other substances that were not reabsorbed.
  5. Osmoregulation: The kidney plays a crucial role in maintaining the balance of water and electrolytes in the body. Through the process of reabsorption and secretion, the kidney regulates the concentration of solutes and the volume of water in the urine, helping to maintain proper osmotic pressure and fluid balance.
Four main processes are involved in the creation of urine.
Four main processes are involved in the creation of urine. | Image Source: Madhero88, CC BY 3.0, via Wikimedia Commons

Overall, the mechanism of urine formation involves filtration of blood in the glomerulus, reabsorption of valuable substances, secretion of additional waste products, and the final excretion of urine. This intricate process helps maintain homeostasis by regulating fluid volume, electrolyte balance, and the elimination of waste products from the body.

Key Points on Urine Formation and Osmoregulation

Here are the key points on urine formation and osmoregulation:

  1. Urine Formation: Urine formation involves three main steps: glomerular filtration, reabsorption, and secretion. During glomerular filtration, blood is filtered in the glomerulus, resulting in the formation of a filtrate that eventually becomes urine. Reabsorption occurs when substances from the filtrate are transported back into the bloodstream. Secretion involves the movement of certain substances from the blood into the filtrate.
  2. Composition of Urine: Urine is composed of approximately 95% water and 5% waste products. The waste products include ions such as sodium, potassium, and calcium, as well as nitrogenous wastes like creatinine, urea, and ammonia. These waste products are eliminated from the body through the urine.
  3. Osmoregulation: Osmoregulation is the process by which the body maintains homeostasis of body fluids, including the osmotic pressure. It ensures that the concentrations of solutes and the overall water balance in the body remain within a narrow range. The kidneys play a vital role in osmoregulation by regulating the osmotic pressure of the blood.
  4. Osmotic Balance: Osmoregulation helps maintain osmotic balance by facilitating the diffusion of solutes and water across semi-permeable membranes. The kidney plays a key role in regulating the osmotic pressure of the blood through filtration and purification processes. By selectively reabsorbing water and solutes, the kidneys maintain the proper concentration of substances in the body fluids.

Regulation of acid-base balance

The regulation of acid-base balance in the body involves the coordinated efforts of the respiratory and renal systems.

Respiratory Regulation

The respiratory system plays a crucial role in maintaining acid-base homeostasis by controlling the levels of carbon dioxide (CO2) in the blood. When the body experiences an acidic condition, such as an increase in hydrogen ions (H+), the respiratory rate increases. This increased breathing rate helps eliminate excess CO2 from the body through expiration, which in turn reduces the concentration of H+ ions in the blood, raising the pH and restoring a more alkaline balance. Conversely, in alkaline conditions, the respiratory rate decreases to retain more CO2, increasing the concentration of H+ ions in the blood and lowering the pH.

Renal Regulation

The kidneys also contribute significantly to the regulation of acid-base balance. Intercalated cells in the renal tubules, specifically type A and type B cells, play a role in this process. Type A cells are stimulated in response to acidic conditions. In the presence of high CO2 concentration, CO2 diffuses into these cells, driving the reaction HCO3 + H ↔ H2CO3 ↔ CO2 + H2O to the left. On the luminal side of the cell, there are proton pumps and a hydrogen/potassium (H/K) exchanger. These pumps actively transport H+ ions against their concentration gradient, removing H+ from the blood and transferring it to the filtrate, thereby increasing blood pH. On the basal side of the cell, there is an exchange of bicarbonate (HCO3) and chloride (Cl) ions, further raising the pH by increasing the concentration of HCO3 in the blood.

Type B cells, on the other hand, respond to alkaline conditions and function in a similar manner but with the membrane proteins reversed. The proton pumps are located on the basal side, releasing protons into the blood, and the HCO3/Cl exchanger and potassium/chloride (K/Cl) co-transporter are situated on the luminal side. These cells help decrease the pH by releasing H+ ions into the blood.

The kidneys’ ability to selectively reabsorb or excrete bicarbonate ions (HCO3-) also contributes to acid-base regulation. Bicarbonate acts as a buffer in the blood, helping to neutralize excess acids or bases. By adjusting the reabsorption or excretion of bicarbonate, the kidneys can influence the acid-base balance in the body.

Overall, the respiratory system regulates acid-base balance through changes in the respiratory rate and CO2 levels, while the kidneys play a vital role in removing or retaining hydrogen ions and regulating bicarbonate levels. Together, these systems work to maintain the pH around a value of 7.4, ensuring proper functioning of various physiological processes in the body.

Diseases related to Kidneys

There are several diseases related to the kidneys that can significantly impact their function and overall health. Some of these diseases include:

  1. Uremia: Uremia occurs when the kidneys are severely damaged, leading to the accumulation of urea and other toxins in the bloodstream. This condition is life-threatening and can result in kidney failure. Symptoms of uremia include fatigue, itching, muscle twitching, and difficulties with mental concentration. Treatment for uremia often involves hemodialysis, a process that helps remove waste products and excess fluids from the blood.
  2. Renal Calculi: Renal calculi, commonly known as kidney stones, are deposits of salt and minerals that form in the kidneys. These stones can cause severe abdominal pain, nausea, and discomfort during urination. Treatment options for renal calculi include medication to dissolve the stones or interventions to help them pass naturally through the urine. Dietary changes, such as increasing fluid intake and modifying the diet to reduce the formation of stones, are also recommended.
  3. Glomerulonephritis: Glomerulonephritis is characterized by the inflammation of the glomeruli, which are tiny filtering units in the kidneys. This condition can be caused by various factors, including infections, autoimmune diseases, and certain medications. Symptoms of glomerulonephritis may include pink-colored urine, swelling or edema in the face, and high blood pressure. Treatment for glomerulonephritis depends on the underlying cause and may involve medications to control inflammation and manage blood pressure. In severe cases, dialysis or kidney transplantation may be necessary.

Functions of Kidney

The kidney performs several important functions in the body, including:

  • Filtration and Excretion: The primary function of the kidney is to filter waste products and excess substances from the blood, such as urea, creatinine, and uric acid, and excrete them in the form of urine.
  • Regulation of Fluid and Electrolyte Balance: The kidney plays a crucial role in maintaining the balance of fluids and electrolytes in the body. It regulates the amount of water reabsorbed and the concentration of electrolytes, such as sodium, potassium, and chloride, in the blood.
  • Acid-Base Balance: The kidney helps maintain the pH balance of the body by reabsorbing bicarbonate ions and excreting hydrogen ions and other acid ions into the urine.
  • Endocrine Functions: The kidney functions as an endocrine gland by producing and releasing important hormones and enzymes. It secretes renin, which helps regulate blood pressure; erythropoietin, which stimulates the production of red blood cells in the bone marrow; and calcitriol, the active form of vitamin D, which regulates calcium absorption in the intestines.
  • Osmoregulation: The kidney plays a role in regulating osmotic pressure in the body by adjusting the reabsorption and excretion of water and electrolytes.
  • Blood Pressure Regulation: Through the secretion of renin and its effects on the renin-angiotensin-aldosterone system, the kidney helps regulate blood pressure.
  • Metabolism and Excretion of Drugs and Toxins: The kidney is responsible for filtering and excreting various drugs, toxins, and foreign substances from the blood.
  • Erythropoiesis: The kidney releases erythropoietin, a hormone that stimulates the production of red blood cells in the bone marrow, contributing to the regulation of oxygen-carrying capacity in the blood.

The functions of the kidney are essential for maintaining overall homeostasis in the body, ensuring proper fluid balance, waste removal, and the regulation of various physiological processes.

FAQ

What is the function of the kidneys in the human body?

The kidneys play a vital role in maintaining overall health by filtering waste products and excess fluid from the blood, regulating electrolyte balance, and producing hormones that control blood pressure and red blood cell production.

What are the common causes of kidney disease?

Common causes of kidney disease include high blood pressure, diabetes, chronic infections, kidney stones, autoimmune disorders, certain medications, and genetic conditions.

What are the signs and symptoms of kidney disease?

Early stages of kidney disease may not exhibit any noticeable symptoms. However, as the condition progresses, symptoms may include fatigue, swelling in the legs and ankles, changes in urination (frequency, color, or foaminess), blood in urine, persistent itching, and muscle cramps.

How is kidney disease diagnosed?

Kidney disease is typically diagnosed through a combination of medical history evaluation, physical examination, blood tests (to measure kidney function and assess levels of waste products), urine tests (to check for abnormalities), imaging tests (such as ultrasound or CT scan), and kidney biopsy in some cases.

Can kidney disease be prevented?

While some causes of kidney disease are not preventable, adopting a healthy lifestyle can help reduce the risk. Maintaining a balanced diet, controlling blood pressure and blood sugar levels, staying hydrated, avoiding excessive alcohol consumption, quitting smoking, and regular exercise can contribute to kidney health.

What are kidney stones?

Kidney stones are hard deposits formed in the kidneys when minerals and salts in urine crystallize and stick together. They can cause severe pain in the back or side, blood in urine, and frequent urination. Treatment may involve medication, increased fluid intake, or surgical intervention in severe cases.

Can kidney disease be treated?

The treatment for kidney disease depends on its underlying cause and severity. It may involve lifestyle modifications, medication to control blood pressure and manage symptoms, dietary changes to reduce the workload on the kidneys, dialysis (a process to filter blood artificially), or kidney transplantation in advanced cases.

How does high blood pressure affect the kidneys?

High blood pressure, also known as hypertension, can damage blood vessels in the kidneys, reducing their ability to function properly. Over time, uncontrolled high blood pressure can lead to chronic kidney disease or even kidney failure.

Can certain medications harm the kidneys?

Yes, some medications, especially when taken in high doses or for prolonged periods, can cause kidney damage. Nonsteroidal anti-inflammatory drugs (NSAIDs), certain antibiotics, antiviral medications, and some chemotherapy drugs are examples of medications that can be harmful to the kidneys.

Can kidney health be improved through dietary changes?

Yes, adopting a healthy diet can positively impact kidney health. A balanced diet with controlled amounts of protein, sodium, and phosphorus can reduce the workload on the kidneys. It is also important to stay hydrated and limit the intake of foods high in potassium and oxalate (which can contribute to the formation of kidney stones). Consulting a healthcare professional or a registered dietitian is recommended for personalized dietary advice.

References

  • https://www.kenhub.com/en/library/anatomy/kidney-histology
  • https://biologydictionary.net/nephron/
  • https://www.onlinebiologynotes.com/kidney-structure-anatomy-function
  • https://courses.lumenlearning.com/wm-biology2/chapter/kidneys/
  • https://www.news-medical.net/health/Anatomy-of-the-Kidney.aspx
  • https://www.notesonzoology.com/vertebrates/development-of-kidney-in-various-vertebrates-zoology/4064

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