11.1 Renal anatomy and nephron function
4 min read•august 7, 2024
The kidneys are vital organs that filter blood, remove waste, and maintain body balance. They contain millions of tiny structures called nephrons, which are responsible for urine production. Understanding anatomy is crucial for grasping how our bodies regulate fluids and eliminate toxins.
Nephrons have several key parts, each with a specific job. The filters blood, while the tubules reabsorb important substances and secrete waste. This process allows the kidneys to fine-tune our body's internal environment, keeping us healthy and functioning properly.
Renal Anatomy
Kidney structure and function
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- Kidneys are bean-shaped organs located in the upper abdominal cavity on either side of the spine
- Primary function is to filter blood, remove waste products (urea, creatinine), excess water, and maintain homeostasis of electrolytes and pH
- Receive blood supply from renal arteries and return filtered blood to circulation via renal veins
- Produce urine which is transported to the for storage and eventual elimination via ureters
Nephron as the functional unit
- Nephrons are the microscopic functional units of the kidney responsible for blood and urine production
- Each kidney contains approximately 1 million nephrons
- Nephrons consist of a renal corpuscle (glomerulus and ) and a (proximal tubule, , distal tubule, and )
- Blood enters the nephron through the afferent arteriole, is filtered in the glomerulus, and exits through the efferent arteriole
Renal corpuscle components
- Glomerulus is a network of capillaries surrounded by Bowman's capsule where blood filtration occurs
- Bowman's capsule is a cup-shaped structure that surrounds the glomerulus and collects the filtrate (ultrafiltrate) from the blood
- Filtrate moves from the Bowman's capsule into the proximal tubule for further processing
- Collecting ducts receive urine from multiple nephrons and transport it to the renal pelvis and eventually the (minor and major calyces)
Nephron Segments
Proximal tubule
- Proximal tubule is the first segment of the renal tubule following the Bowman's capsule
- Responsible for reabsorbing approximately 65% of the filtrate, including glucose, amino acids, and electrolytes (sodium, potassium, chloride)
- Actively secretes organic acids, bases, and drugs from the peritubular capillaries into the tubular lumen
- Characterized by a brush border of microvilli to increase surface area for efficient and
Loop of Henle
- Loop of Henle is a U-shaped segment of the nephron that extends into the medulla and consists of descending and ascending limbs
- Descending limb is permeable to water but not solutes, allowing for concentration of the filtrate
- Ascending limb is impermeable to water but actively pumps out sodium, chloride, and potassium, contributing to the concentration gradient in the medulla
- Countercurrent multiplication system in the loop of Henle helps to concentrate urine and maintain medullary osmotic gradient (sodium gradient)
Distal tubule and collecting duct
- Distal tubule is the segment following the loop of Henle and is responsible for fine-tuning urine composition
- Regulates sodium, potassium, and calcium reabsorption under the influence of hormones (, parathyroid hormone)
- Collecting duct receives urine from multiple nephrons and is the final site of water reabsorption under the control of antidiuretic hormone (ADH or vasopressin)
- Urine concentration is adjusted in the collecting duct before it is transported to the renal pelvis and ureter for storage in the bladder (urine osmolality ranges from 50-1200 mOsm/kg)
Nephron Functions
Filtration in the glomerulus
- Filtration is the first step in , occurring in the glomerulus
- Blood pressure forces fluid and solutes out of the glomerular capillaries and into the Bowman's capsule
- Glomerular filtration barrier (endothelial cells, basement membrane, podocytes) allows passage of water, glucose, amino acids, and small proteins while restricting cells and large proteins
- is approximately 125 mL/min in a healthy adult, with 180 L of filtrate produced daily
Reabsorption along the tubule
- Reabsorption is the process by which essential nutrients, electrolytes, and water are selectively transported from the tubular lumen back into the peritubular capillaries
- Occurs primarily in the proximal tubule (65% of filtrate reabsorbed) and to a lesser extent in the loop of Henle, distal tubule, and collecting duct
- Glucose and amino acids are completely reabsorbed in the proximal tubule via secondary active transport (sodium-glucose cotransporter SGLT, sodium-amino acid cotransporter)
- Sodium reabsorption drives the reabsorption of other solutes and water through various transport mechanisms (Na+/K+ ATPase, Na+/H+ exchanger, sodium-chloride cotransporter)
Secretion into the tubular lumen
- Secretion is the process by which substances are actively transported from the peritubular capillaries into the tubular lumen
- Occurs primarily in the proximal tubule and distal tubule
- Helps to eliminate waste products, excess potassium, and regulate blood pH
- Organic acids (uric acid), bases (creatinine), and drugs (penicillin) are secreted into the tubular lumen for excretion in the urine
- Hydrogen ions (H+) are secreted into the tubular lumen to maintain blood pH and contribute to the acidification of urine (pH range 4.5-8.0)
Key Terms to Review (21)
Aldosterone: Aldosterone is a steroid hormone produced by the adrenal cortex that plays a key role in regulating sodium and potassium levels in the body, primarily influencing kidney function. By promoting sodium reabsorption and potassium excretion in the nephrons, aldosterone helps maintain blood pressure and fluid balance, connecting crucially to renal anatomy and urine formation.
Antidiuretic Hormone (ADH): Antidiuretic hormone (ADH), also known as vasopressin, is a peptide hormone produced by the hypothalamus and released from the posterior pituitary gland. It plays a crucial role in regulating water balance in the body by promoting water reabsorption in the kidneys, influencing urine concentration, and assisting in osmoregulation across different environments.
Bladder: The bladder is a hollow, muscular organ that stores urine produced by the kidneys before it is excreted from the body. It plays a crucial role in the urinary system, allowing for the controlled release of urine through the urethra during urination, which is facilitated by coordinated contractions of its muscular wall.
Bowman's Capsule: Bowman's capsule is a cup-shaped structure located at the beginning of the nephron in the kidney, responsible for enclosing the glomerulus and initiating the filtration process. This structure plays a crucial role in the formation of urine by allowing blood plasma to be filtered while retaining larger molecules like proteins and blood cells, which are essential for maintaining bodily functions.
Chronic Kidney Disease: Chronic kidney disease (CKD) is a progressive condition characterized by the gradual loss of kidney function over time, often resulting from underlying health issues such as diabetes and hypertension. This condition affects the kidneys' ability to filter waste and excess fluid from the blood, leading to a buildup of toxins in the body. Understanding CKD is crucial, as it involves both renal anatomy and nephron function, which are essential in assessing how the kidneys fail to perform their normal regulatory roles.
Collecting duct: The collecting duct is a crucial component of the nephron in the kidney, responsible for the final concentration of urine. It plays a key role in regulating water and electrolyte balance by reabsorbing water and certain ions from the filtrate, ultimately influencing blood pressure and fluid homeostasis. The collecting duct connects the distal convoluted tubule to the renal pelvis, serving as a passage for urine before it enters the ureter.
Cortical nephron: A cortical nephron is a type of nephron located mainly in the renal cortex, characterized by a shorter loop of Henle that only extends into the outer medulla. These nephrons play a crucial role in the overall function of the kidney, particularly in the process of filtration and reabsorption, making them essential for maintaining homeostasis and regulating urine concentration.
Filtration: Filtration is the process by which blood plasma is filtered through the glomerulus in the kidneys, resulting in the formation of a filtrate that contains water, electrolytes, and small molecules while retaining larger molecules and blood cells. This crucial first step in urine formation allows for the removal of waste products and excess substances from the bloodstream, playing a vital role in maintaining homeostasis in the body.
Glomerular filtration rate (GFR): Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal glomeruli into the Bowman's capsule per unit time, typically measured in milliliters per minute. GFR is a crucial indicator of kidney function, reflecting how well the kidneys are filtering blood and removing waste products. It provides insight into nephron performance, as the nephron is the functional unit of the kidney responsible for filtration and reabsorption processes.
Glomerulus: The glomerulus is a network of tiny blood vessels, known as capillaries, located within the kidney that plays a crucial role in filtering blood to form urine. This structure is part of the nephron, the functional unit of the kidney, and is surrounded by the Bowman’s capsule, where the initial stages of urine formation take place. The glomerulus serves as the primary site for glomerular filtration, allowing water, ions, and small molecules to pass while retaining larger proteins and blood cells in the bloodstream.
Juxtamedullary nephron: A juxtamedullary nephron is a type of nephron characterized by its long loop of Henle that extends deep into the renal medulla, which plays a crucial role in concentrating urine and maintaining water balance. These nephrons are essential for producing concentrated urine in response to dehydration, as they allow for the efficient reabsorption of water and solutes due to their unique anatomical structure and proximity to the vasa recta.
Kidney: The kidney is a vital organ in the urinary system responsible for filtering blood, removing waste products, and regulating fluid balance and electrolytes. It plays a crucial role in maintaining homeostasis, as it not only filters toxins but also controls blood pressure and red blood cell production through the secretion of hormones like erythropoietin. Each kidney contains approximately one million functional units called nephrons, which are essential for performing the organ's complex functions.
Loop of Henle: The loop of Henle is a U-shaped portion of the nephron in the kidney that plays a critical role in concentrating urine and maintaining the body's fluid balance. It consists of a descending limb and an ascending limb, each with distinct permeability properties that facilitate the reabsorption of water and sodium chloride, ultimately influencing the concentration of urine.
Nephritis: Nephritis refers to inflammation of the kidneys, which can affect their ability to function properly. This condition can impact renal anatomy and nephron function by disrupting the filtration process, altering fluid and electrolyte balance, and potentially leading to kidney damage or failure if left untreated. Understanding nephritis is crucial in studying how kidney health affects overall bodily homeostasis.
Osmoregulation: Osmoregulation is the process by which organisms maintain the balance of water and solutes in their bodies to ensure optimal physiological function. This process is crucial for homeostasis, allowing animals to regulate their internal environment despite external changes, thereby connecting to physiological control systems and feedback mechanisms that help maintain stability.
Reabsorption: Reabsorption is the process by which the kidneys selectively reclaim water and essential solutes from the filtrate back into the bloodstream, playing a vital role in maintaining homeostasis. This process occurs mainly in the nephron, the functional unit of the kidney, and ensures that necessary substances such as glucose, ions, and water are conserved while waste products are excreted. Effective reabsorption is crucial for regulating fluid balance, electrolyte levels, and overall blood pressure.
Renal perfusion: Renal perfusion refers to the process of delivering blood to the kidneys, essential for their function of filtering blood, regulating fluid balance, and maintaining electrolyte homeostasis. Adequate renal perfusion is crucial because it ensures that the nephrons, the functional units of the kidneys, receive enough oxygen and nutrients to operate efficiently, ultimately impacting the overall health of the body.
Renal tubule: The renal tubule is a crucial component of the nephron, the functional unit of the kidney, responsible for the reabsorption and secretion of substances during urine formation. It consists of several segments, including the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct, each playing a unique role in processing the filtrate that originates from blood plasma.
Secretion: Secretion is the process by which substances are produced and released from cells or glands into the bloodstream or into specific body cavities. This process is crucial in maintaining homeostasis, regulating physiological functions, and facilitating communication between different systems in the body, particularly in the kidneys where it plays a vital role in filtering and excreting waste products.
Ureter: The ureter is a muscular tube that carries urine from the kidneys to the bladder. Each kidney has its own ureter, and these structures play a crucial role in the urinary system, facilitating the transport of urine after it has been produced by the nephrons within the kidneys. The ureters are lined with transitional epithelium, which allows them to stretch as they fill with urine, and their smooth muscle layers help propel urine downward through peristalsis.
Urine formation: Urine formation is the biological process by which the kidneys filter blood, remove waste products, and regulate water and electrolyte balance, resulting in the production of urine. This complex process involves several stages including filtration, reabsorption, secretion, and excretion, each occurring primarily within the nephron, the functional unit of the kidney. Understanding urine formation is crucial for comprehending how the kidneys maintain homeostasis and eliminate toxins from the body.