Seeley\'s Essentials of Anatomy & Physiology Chapter 18 PDF

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67Ch. 18: Urinary System | Fluid Balance ● Functions of the Urinary System ○ Excretion ○ Regulation of blood volume and blood pressure ○ Regulation of solute concentration in the blood ○ pH regulation ○ Regulation of RBC synthesis ○ Regulation of Vitamin D synthesis ● Components of Urinary System ○ 2 kidneys ○ 2 ureters (connect kidney to bladder) → carry urine from renal pelvis of kidney to bladder ○ 1 urinary bladder → stores urine (1000 mL) ○ 1 urethra → exits bladder, carries urine from bladder to outside of body ● Upper Urinary Tract → kidneys, ureters, bladder ● Pararenal Fat (around kidneys) → energy, cushioning, insulation ● Anatomy of the Kidneys ○ Kidneys (bean-shaped) lie on posterior abdominal wall → retroperitoneal organs ■ Renal capsule (connective tissue) surrounds each kidney → protection barrier ● Thick adipose tissue surrounds renal capsule on each kidney ■ Hilum (indentation) ● Contains renal artery, veins, nerves, and ureter ■ Each kidney has an outer cortex and inner medulla ■ Renal Sinus ● Contains renal pelvis, blood vessels, fat ■ Renal Pyramid ● Junction between cortex and medulla ● Calyx: tip of pyramid ■ Renal Pelvis ● Where calyces join ● Narrows to form ureter ■ Nephron: functional unit of kidney ● Consists of: ○ Renal Corpuscle: structure that contains a Bowman Capsule and Glomerulus ■ Bowman Capsule ● Enlarged end of nephron

● Opens into proximal convoluted tubule (urine collection) ● Contains Podocytes (specialized cells around glomerular capillaries) ■ Glomerulus ● Contains capillaries wrapped around it ■ Filtration Membrane ● In renal corpuscle ● Includes glomerular capillaries, podocytes, basement membrane ■ Filtrate: fluid that passes across filtration membrane ○ Proximal Tubule ■ Where filtrate passes first ○ Loop of Henle ■ Contains descending and ascending loops ■ Water and solutes pass through thin walls by diffusion ○ Distal Tubule ■ Between Loop of Henle and Collecting Duct ■ Empty into collecting duct which empties into papillary duct which empty into calyx ● Carry fluid from cortex through medulla

○ Arteries and Veins ● Urine Production ○ Primary function of kidney is to regulate body fluid composition ■ Sorts the substances from blood for either removal via urine or return to the blood ● Waste products are removed from body, whereas other substances are conserved to maintain homeostasis ○ Filtration: occurs when blood pressure forces water and other small molecules out of glomerular capillaries and into Bowman Capsule, forming filtrate ○ Tubular Reabsorption: movement of substances from filtrate across wall of nephron back into blood of peritubular capillaries ■ Some solutes/ions are reabsorbed via active transport and cotransport

○ Tubular Secretion: active transport of solutes across nephron walls into filtrate ○ Urine Production-Reabsorption ■ 99% of filtrate is reabsorbed and reenters circulation ■ Proximal tubule is primary site for reabsorption of solutes and water ■ Descending Loop of Henle concentrates filtrate ■ Reabsorption of water and solutes from distal tubule and collecting duct is controlled by hormones ○ Urine Production-Secretion ■ Water, small ions, by-products of metabolism, drugs, and urea are found in urine ○ Filtration ■ Nonspecific process in which materials are separated by size or charge ● Filtration membrane allows some substances (water and small solutes), but not others(blood cells and proteins), to pass from blood into Bowman Capsule ■ Formation of filtrate depends on pressure gradient → Filtration Pressure ● Forces fluid from glomerular capillary across filtration membrane into Bowman Capsule ○ Glomerular Capillary Pressure: blood pressure in glomerular capillary ■ Glomerular Capillary Pressure is the major force causing fluid to move from glomerular capillary across filtration membrane into Bowman Capsule ● 2 major opposing forces to Glomerular Capillary Pressure: ○ Capsular Pressure: caused by pressure of filtrate already present in Bowman Capsule ○ Colloid Osmotic Pressure: within glomerular capillary ○ Filtration pressure forces fluid from glomerulus into Bowman Capsule because glomerular capillary pressure is greater than both the capsular and colloid osmotic pressure ○ An increase in blood protein concentration encourages movement of water by osmosis back into glomerular capillaries to reduce filtration pressure ○ A decrease in blood protein concentration inhibits movement of water by osmosis back into glomerular capillaries to increase filtration pressure ○ Regulation of Filtration ■ Filtration pressure and rate of filtrate formation are maintained within a narrow range of values usually ● Can change dramatically under some conditions

■ Sympathetic stimulation (and cardiovascular shock) constricts arteries, decreasing renal blood flow and filtrate formation ● Therefore, only a small amount of urine is produced ● Intense physical activity and/or trauma also increase sympathetic stimulation, resulting in a small amount of urine production ■ Increased blood pressure decreases sympathetic stimulation, increasing urine volume ■ Decreased concentration of plasma proteins increases filtration pressure, increasing urine volume ○ Tubular Reabsorption ■ As filtrate flows from Bowman Capsule through proximal convoluted duct, loop of Henle, distal convoluted tube, and collecting duct, many of the solutes are reabsorbed ● Only 1% of original filtrate volume becomes urine ■ The proximal convoluted tubule is primary site for reabsorption of ions/water ● Cuboidal cells of proximal convoluted tubule have microvilli and mitochondria → well-adapted to transport molecules/ions across nephron wall by active transport and cotransport ○ Proteins, amino acids, glucose, fructose, Na +, K+, Ca2+, HCO3-, and Cl- are transported from proximal convoluted tubule ■ Most of the useful solutes that pass through the filtration membrane into the Bowman capsule are reabsorbed in the proximal convoluted tubule ● However, little water is removed from the filtrate ○ Filtrate becomes dilute ○ Tubular Secretion ■ Substances, including by-products that become toxic, are secreted into nephron from peritubular capillaries ■ Can be active or passive ● Ammonia passively diffuses into lumen of nephron ● H+, K+, creatinine, histamine, and penicillin are actively transported into nephron

● Regulation of Urine Concentration and Volume ○ Hormonal Mechanisms ■ Renin-Angiotensin-Aldosterone Mechanism (conserve water/ions) ● Renin and Angiotensin help regulate Aldosterone secretion ○ Renin: produced by liver; converts Angiotensinogen to Angiotensin I when blood pressure is low ■ Angiotensin-Converting Enzyme (ACE) converts Angiotensin I to Angiotensin II ■ Angiotensin II causes constriction, and forces adrenal cortex to secrete aldosterone ● Aldosterone increases rate of active transport of Na+ in the distal tubules and collecting ducts (reabsorption from nephrons) → volume of water in urine decreases→ increased water/ion retention → increased blood pressure ○ If aldosterone is missing, large amounts of Na+ remain in nephron and causes water to remain which increases urine volume ■ Urine would then contain high concentration of Na+ and would take Cl- with it ■ Antidiuretic Hormone Mechanism (ADH or Vasopressin, AVD) ● ADH is secreted by posterior pituitary gland when blood pressure is low to increase water retention and blood pressure ○ When ADH levels rise, increased permeability of distal tubules and collecting ducts to water increases ■ Causes a greater reabsorption of water from filtrate

● Increases production of small volume of concentrated urine ○ When ADH levels decrease, less water is reabsorbed and a large volume of dilute urine is produced ● Release of ADH from posterior pituitary gland is regulated by hypothalamus (sensitive to changes in solute concentration) ○ Baroreceptors that monitor blood pressure also influence ADH secretion ■ Atrial Natriuretic Hormone (ANH) Mechanism → DECREASES BP ● ANH is secreted from cardiac muscle cells in right atrium when blood pressure in right atrium increases ● ANH acts on kidneys to decrease blood pressure ○ Decreases Na+ and water reabsorption, causing ions and water to stay in nephron to become urine ■ Increased loss of Na+ and water ■ Increased urine decreases blood volume and decreases blood pressure ● Alcohol interferes w/ this process! ● Urine Movement ○ Anatomy and Histology of Ureters, Urinary Bladder, and Urethra ■ Ureters: tube that carries urine from kidney to bladder; lined w/ transitional epithelium (stretchy) ● Urinary bladder (lined w/ transitional epithelium) can hold up to about 1000 mL of urine ■ Urethra: tube that carries urine from bladder to outside of body ■ Internal Urinary Sphincter (males only): junction between bladder and urethra) ● Contracts to keep semen from entering bladder during sex ■ External Urinary Sphincter (males and females): skeletal muscle that surrounds urethra which allows person to start/stop flow of urine through urethra ● Voluntary control ○ Micturition Reflex ■ Activated by stretching of bladder wall ● As bladder fills w/ urine, pressure increases which stimulates stretch receptors in wall of bladder ■ The Micturition Reflex is an automatic reflex, but can be inhibited or stimulated by higher brain centers

● Body Fluid Compartments

○ 60% of male body weight consists of water ○ 50% of female body weight consists of water ■ Females have a higher percentage of body fat typically ○ Water and its dissolved ions are distributed in 2 compartments ■ Intracellular Fluid Compartment: fluid inside all body cells ● ⅔ of total body water ● Includes everything enclosed by cell membranes ■ Extracellular Fluid Compartment: fluid outside all body cells ● ⅓ of total body water ● Includes interstitial fluid, plasma in blood vessels, and fluid in lymphatic cells ○ Composition of the Fluid in the Body Fluid Compartments ■ Intracellular fluid has high concentration of ions like K+, Mg2+, PO43-, and SO42- and proteins ■ Extracellular fluid has high concentration of Na+, Ca2+, Cl-, and HCO3● Regulation of Extracellular Fluid Composition ○ Thirst Regulation ■ Thirst Center in hypothalamus has neurons that control water intake → thirst = motivation, drinking = behavior ● When blood becomes more concentrated, thirst center initiates sensation of thirst ● Likewise, when blood pressure drops, thirst center activates sensation of thirst ○ Ion Concentration Regulation ■ Maintaining extracellular fluid composition within a normal range is required to sustain life ● Regulating positively-charged ions is particularly important! ■ Sodium Ions ● Dominant ions in extracellular fluid ○ Affects osmotic pressure ● Controlled by renin-angiotensin-aldosterone mechanism, ANH mechanism, and ADH mechanism ● 2.4 g/day of Na+ is recommended intake ■ Potassium Ions ● Muscles and nerves are highly sensitive to changes in extracellular K+ concentration ● Aldosterone regulates concentration of K+ ○ Dehydration, shock, and tissue damage all increase concentration of K+, causing aldosterone secretion to increase which increases secretion of K+ ■ Calcium Ions ● Affects muscles and nerves when changes in concentration occur

○ Decreases in Ca2+ causes higher permeability for Na+ which makes cells more excitable → tetany and twitching ○ Increases in Ca2+ causes cells less excitable, inhibiting action potentials in nerves and muscle cells → paralysis ● PTH increases Ca2+ concentration ○ Degrades bone to increase blood calcium ● Vitamin D increases Ca2+ concentration ○ Increases rate of absorption of calcium in intestine ● Calcitonin decreases Ca2+ concentration ○ Reduces rate of bone breakdown and decreases release of calcium from bone ■ Phosphate and Sulfate Ions ● Slowly reabsorbed by active transport in kidneys ○ Excess is excreted in urine

● Regulation of Acid-Base Balance ○ Body fluid pH is maintained between 7.35-7.45 ■ Deviations from that range are life-threatening (acidosis or alkalosis) ○ pH of body fluids is controlled by 3 factors: buffers, respiratory system, and kidneys ○ Buffers: chemicals that resist a change in pH of a solution when either acids or bases are added to the solution ■ Contain salts of weak acids/bases that combine with H+ when H+ increases in those fluids, or release H+ when H+ decreases in those fluids ■ Tend to keep the H+ concentration in a narrow range of values ■ 3 major buffers: proteins, PO43- buffer, and HCO3- buffer ■ Proteins and phosphate combine with H+ ions ○ Respiratory System

■ Maintains blood pH by altering levels of O2 and CO2 ● Increasing levels of CO2 decreases blood pH → acidosis ○ Kidneys ■ Nephrons of kidneys (distal tubule) secrete H+ into urine, directly regulating pH of body fluids ● Responds more slowly than respiratory system...