Renal System 1 PDF

# Renal Physiology ## Urinary System * Principal organs are the kidneys. * They process blood and form urine as a waste to be excreted. * The excreted urine travels from the kidneys to the outside of the body via accessory organs: ureters, urinary bladder, and urethra. * The urinary system regul...

# Renal Physiology ## Urinary System * Principal organs are the kidneys. * They process blood and form urine as a waste to be excreted. * The excreted urine travels from the kidneys to the outside of the body via accessory organs: ureters, urinary bladder, and urethra. * The urinary system regulates the contents of the blood plasma so that the homeostasis of the entire internal fluid environment can be maintained within normal limits. * Water content is adjusted. * Blood content of important ions such as potassium and sodium is adjusted. * pH of blood can be altered to match the set point level. ## Anatomy of the Urinary System * The image shows a cross-section of the kidney, highlighting several key components: * **Calyces:** These cup-like structures collect urine from the renal pyramids in the medulla. * **Renal artery:** This blood vessel brings oxygenated blood into the kidney. * **Renal vein:** This blood vessel carries deoxygenated blood out of the kidney. * **Renal pelvis:** A funnel-shaped structure that collects urine from the calyces. * **Ureter:** This tube carries urine from the kidney to the bladder. * **Cortex:** The outer region of the kidney. * **Medulla:** The inner region of the kidney. ## Blood Vessels of the Kidneys * Kidneys are highly vascular organs. * 1.2L of blood flows through the kidneys each minute, about one fifth of the blood pumped by the heart. * Kidneys process blood in important ways before returning it to general circulation. * **Path of blood flow:** A large branch of the abdominal aorta, the renal artery, brings blood into the kidneys. The renal artery divides into segmental arteries, which divide into lobar arteries, then interlobar arteries. These become afferent arterioles, which extend into the cortex and form the glomerular capillaries. These capillaries are called the glomerulus. From the glomerulus blood flows through the efferent arterioles. ## Nephrons * Nephrons are microscopic and functional units of the kidney. * There are about 1.25 million nephrons per kidney. * Their structure is uniquely suited for their function of blood plasma processing and urine formation. * About 85% of all nephrons are located in the cortex and are called **cortical nephrons**. * The remaining 15% are in the medulla areas and are called **juxtamedullary nephrons**. * Each nephron contains the following structures, in the order in which fluids flow through them: * Renal capsule * Bowman's capsule * Proximal convoluted tubule * Loop of Henle * Distal convoluted tubule * Collecting duct ## Physiology of the Urinary System * The basic functional unit of the kidney is the nephron. * It has two main parts - the renal capsule and renal tubules. * These form urine by a series of three processes: * **Filtration:** Movement of water and protein-free molecules from plasma in the glomerular capsule membrane into the capsular space of Bowman's capsule. * **Tubular reabsorption:** Movement of molecules out of the various segments of the tubule and into the peritubular blood. * **Tubular secretion:** Movement of molecules out of the peritubular blood and into the tubule for excretion. ### Filtration * Filtration is the first step in blood processing. * As blood flows through the glomerular capillaries, water and small ions filter out of the blood into the Bowman's capsules. * Blood cells and most plasma proteins remain in the blood. * About 180L of glomerular filtrate is formed each day. * Filtration into the Bowman's capsule is a result of the pressure gradient. * The main factor establishing the pressure gradient between the blood in the glomeruli and the filtrate in the Bowman's capsule is the hydrostatic pressure of the glomerular blood. This pressure tends to cause filtration out of the glomerular blood into the Bowman's capsules. * The intensity of glomerular hydrostatic pressure is influenced by systemic blood pressure and the resistance of blood flow through the glomerular capillaries. ### Reabsorption * Reabsorption is the second step in urine formation. * Most of the 180L of filtrate is reabsorbed back into the blood stream before reaching the end of the renal tubule. * Reabsorption takes place from all parts of the renal tubule, though most reabsorption takes place from the proximal tubule. * Reabsorption happens by passive and active transport. #### Reabsorption from the Proximal Tubule * Sodium is transported by active transport out of the tubule fluid and into blood. * Glucose and amino acids are reabsorbed along with sodium ions, by means of sodium co-transport mechanism. * Chloride ions passively move into blood plasma because of an imbalance in electrical charge. * Movement of sodium and chloride ions out of the tubule fluid into blood creates an osmotic imbalance: blood becomes hypertonic to the filtrate, and water is obliged by the principle of osmosis to passively move into blood. * Half the urea present in the tubule fluid passively moves out of the tubule into blood, the other half moves to the loop of Henle. * The total content of the filtrate has greatly reduced by the time it leaves the proximal tubule. #### Reabsorption in the Loop of Henle * The Loop of Henle and its vasa recta participate in a unique way by the process called **countercurrent mechanism**. * A counter current structure is any set of parallel passages in which the contents flow in opposite directions. * The Loop of Henle is a countercurrent structure. The contents of the ascending limb travel in the opposite direction to contents in the descending limb. * The kidneys countercurrent mechanism functions to keep the solute concentration of the medulla extremely high. * NOTE: The thin-walled descending limb allows water and urea to diffuse freely into and out of the tubule, depending on their concentration gradient. * The thick-walled ascending limb limits the diffusion of most molecules, including water, sodium, chloride, and urea, while actively transporting selected molecules out of the tubule and into the interstitial fluid. * The primary function of the loop of Henle can be summarized as follows: * The Loop of Henle reabsorbs water from the tubule fluid and picks up urea in the interstitial fluid in the descending limb. * It reabsorbs salt from the tubule fluid in the ascending limb. * By reabsorbing salt from the ascending limb, it makes the tubule fluid dilute. * Reabsorption of salt also creates and maintains a high osmotic pressure, or a high solute concentration, of the medulla's interstitial fluid. #### Reabsorption in the Distal Tubules and Collecting Duct * The distal tubule also reabsorbs sodium by active transport, but in much smaller amounts. * Cells forming the distal tubule are relatively impermeable to water. * This means sodium can be removed, but water cannot follow osmotically. * The dilute concentration of tubule fluid continues to decrease. * The collecting duct prevents water from leaving the filtrate by osmosis. * The above circumstances would make the kidneys to produce very dilute urine, which would soon lead to dehydration and possibly death. * A regulatory mechanism ensures that excessive loss of water is prevented. * This mechanism involves the hormone **ADH**. * ADH targets cells of the distal and collecting tubules and causes them to be more permeable to water. * Water moves out of the tubule to achieve equilibrium. * The more ADH present, the more water will move out of the tubule going into blood. * The concentration of urine secreted depends in large part on the amount of ADH present. * Some urea is also reabsorbed due to the effects of ADH. ### Tubular Secretion * Tubular secretion means the movement of substances out of the blood and into the tubular fluid. * The distal and collecting tubules secrete potassium, hydrogen, and ammonium ions. * Potassium ions or hydrogen ions are actively transported out of the blood into the tubule fluid in exchange for sodium ions, which diffuse back into the blood. * Potassium secretion increases when the blood aldosterone concentration increases. * Aldosterone targets distal and collecting tubule cells and causes them to increase the activity of the sodium-potassium pump, which moves sodium out of the tubule and potassium into the tubule. * Hydrogen ion secretion increases when blood levels of this ion increase. ### Formation of Dilute or Concentrated Urine * An osmotic gradient is used to raise urine concentration to conserve water. * **Overhydration** leads to a large volume of dilute urine. * ADH production is low, and urine concentration is around 100 mOsm. * If aldosterone is present, additional ions are removed, bringing the urine down to about 50 mOsm. * **Dehydration** leads to a small volume of concentrated urine. * Maximal ADH is released, and urine osmolality is around 1200 mOsm. * In severe dehydration, 99% of water is reabsorbed. ### Regulation of Urine Volume * ADH plays a central role in the regulation of urine volume. * Control of the solute concentration of urine translates into control of urine volume. * If no water is reabsorbed by the distal and collecting tubules, urine volume is relatively high (and water loss from the body is high). * As water is reabsorbed under the influence of ADH, the total volume of urine is reduced by the amount of water removed from the tubule. * **Aldosterone** also tends to decrease the urine volume - and thus conserve water. * It increases distal and collecting duct tubule absorption of sodium. * This in turn causes an osmotic imbalance that drives the reabsorption of water from the tubule. ## Urine Composition ### Physical Characteristics * **Color and Transparency:** * Urine is usually clear. * Cloudy urine may indicate a urinary tract infection. * The normal color of urine varies from pale to deep yellow due to **urochrome**, a pigment from hemoglobin breakdown. More concentrated urine has a deeper color. * Abnormal colors (pink, brown, smoky) can be caused by food ingestion, bile pigments, blood, or drugs. * **Odor:** * Fresh urine is slightly aromatic. * Urine develops an ammonia odor upon standing as bacteria metabolize solutes. * Some drugs and vegetables can alter the odor of urine. * **pH:** * Urine is slightly acidic (around pH 6, with a range of 4.5 to 8.0). * An acidic diet (protein, whole wheat) decreases pH, while an alkaline diet (vegetarian), prolonged vomiting, or urinary tract infections increase pH. * **Specific Gravity:** * The specific gravity of urine ranges from 1.001 to 1.035, depending on solute concentration. ### Chemical Composition * Urine is composed of approximately 95% water and 5% solutes. * **Nitrogenous Wastes:** * **Urea:** This is the largest solute component and is produced from amino acid breakdown. * **Uric acid:** This is produced from nucleic acid metabolism. * **Creatinine:** This is a metabolite of creatine phosphate. * **Other Normal Solutes:** * Na+, K+, PO43-, and SO42-, Ca2+, Mg2+ * HCO3- * Abnormally high concentrations of any constituent, or abnormal components, such as blood proteins, white blood cells, or bile pigments, may indicate pathology.