D4.2 - Formation of Urine (Updated 2024) PDF

D4.2 - Formation of Urine I can… Identify the major and associated structures of the nephron, including the glomerulus, Bowman’s capsule, tubules, loop of Henle, collecting duct, afferent and efferent arterioles, and capillary net Explain the function of the nephron in maintaining b...

D4.2 - Formation of Urine I can… Identify the major and associated structures of the nephron, including the glomerulus, Bowman’s capsule, tubules, loop of Henle, collecting duct, afferent and efferent arterioles, and capillary net Explain the function of the nephron in maintaining blood composition (water, pH and ions) Describe the function of the kidney in excreting metabolic waste Blood Movement Terminology Afferent arteriole: A small branch of the renal artery that carries blood to the glomerulus Glomerulus: The high-pressure capillary bed that is the site of filtration Efferent arteriole: A small branch of the renal artery that carries blood away from the glomerulus to the peritubular capillaries Peritubular capillary: A member of the network of small blood vessels that surround the tubule of the nephron Movement of Blood Through Nephrons Kidneys are composed of millions of slender tubules called nephrons Small branches from the renal artery, known as the afferent arterioles, supply the nephrons with blood The afferent arterioles branch into a group of capillaries known as the glomerulus ○ The glomerulus acts to filter out certain components of the blood, including water and salts Blood leaves the glomerulus through the efferent arterioles ○ It then moves through the peritubular capillaries and eventually exits the kidney via the renal vein Urine Movement Terminology Bowman’s capsule: The cuplike structure that surrounds the glomerulus Proximal tubule: The section of the nephron joining the Bowman’s capsule with the loop of Henle Loop of Henle: The section of the tubule that carries filtrate from the proximal tubule to the distal tubule Distal tubule: Conducts urine from the loop of Henle to the collecting duct Collecting duct: a tube that carries urine from nephrons to the renal pelvis Movement of Urine Through Nephrons The glomerulus is surrounded by a funnel-like structure called the Bowman’s capsule The fluid or “filtrate” that is filtered out of the blood by the glomerulus (again, mostly water and salts) enters the Bowman’s capsule ○ The capsule tapers to a thin tubule, called the proximal tubule Filtrate is carried from the proximal tubule to the loop of Henle, which descends into the medulla of the kidney Movement of Urine Through Nephrons Urine moves through the distal tubule, the last segment of the nephron, into the collecting ducts ○ These ducts gather urine from many nephrons, which then merge in the pelvis of the kidney Formation of Urine Formation of urine is the result of the following steps: 1. Filtration Accomplished by the movement of fluid from the blood into the Bowman’s capsule 2. Reabsorption Involves the transfer of essential solutes and water from the nephron back into the blood 3. Secretion Involves the transport of materials from the blood into the nephron 1. Filtration As blood moves through each nephron of the kidney, it must pass through the glomerulus into the Bowman’s capsule The glomerulus acts like a high-pressure filter, allowing only certain substances to enter the Bowman’s capsule and move on into the proximal tubule Water, salts, glucose, amino acids, hydrogen ions and urea are able to pass through this filter Blood plasma proteins, erythrocytes, and platelets are not (they stay in the bloodstream and exit the nephron via the efferent arteriole) The activity of the glomerulus explains why red blood cells are not found in the urine of a healthy person What’s that smell? Urine gets its distinct smell from the urea that passes through the glomerulus Urea is derived from excess protein. When you obtain more protein than your cells require, these proteins are broken down into amino acids in your liver ○ This results in the production of ammonia (NH4) that can become toxic if it builds up In the liver, two molecules of ammonia are combined with a molecule of CO2 to form urea, which is eventually released into the bloodstream and excreted through the kidneys into your urine 2. Reabsorption Reabsorption of water, salt, glucose, potassium, amino acids, and urea begins in the proximal tubule The majority of water reabsorption occurs in the descending limb of the Loop of Henle ○ The Loop of Henle extends down into the extremely salty medulla, allowing water to be absorbed via osmosis. 2. Reabsorption The ascending limb of the Loop of Henle is not permeable to water ○ Salt (NaCl) is reabsorbed through diffusion (in the lower portion) and active transport (in the upper portion) In the distal tubule, additional water and salt can be reabsorbed if necessary ○ This requires the action of hormones 3. Secretion Secretion is the movement of wastes from the blood into the proximal tubule, distal tubule, or collecting duct of a nephron Ammonia, excess H+ ions, and minerals are examples of substances that are secreted pH is controlled by the secretion of excess hydrogen ions and restoration of bicarbonate ions in the blood