Ch. 25- The Urinary System

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The Urinary System and Kidney Functions

1) Urine flows from each kidney, down its ureter to the bladder and to the outside via the urethra 2) System filters the blood and returns most of the water and solutes to the bloodstream 3) Regulation of blood ionic composition (Na+, K+, Ca2+, Cl- and phospate ions) 4) Regulation of blood pH, osmolarity & glucose 5) Regulation of blood volume (conserving or eliminating water) 6) Regulation of blood pressure (Secreting the enzyme renin, Adjusting renal resistance) 7) Release of erythropoietin (hormone that controls RBC production) & (controls reabsorption of Calcium) 8) Excretion of Nitrogen wastes (urea and uric acid) & foreign substances

External Anatomy of the Kidney

1) Paired kidney-bean-shaped organ 2) 4-5 in. long, 2-3 in. wide 3) Found just above the waist between the peritoneum & posterior wall of abdomen (Retroperitoneal along with adrenal glands & ureters) 4) Protected by 11th & 12th ribs with right kidney lower 5) Blood vessels & ureter enter hilus of kidney 6) Renal capsule= transparent membrane maintains organ shape 7) Adipose capsule that helps protect from trauma 8) Renal fascia= dense, irregular connective tissue that holds against back body wall

Other Urinary System Organs

1) Urinary bladder- provides a temporary storage reservoir for urine 2) Paired ureters- transport urine from the kidneys to the bladder 3) Urethra- transports urine from the bladder out of the body

Internal Anatomy of the Kidney

A frontal section shows three distinct regions: 1) Cortex- the light colored, granular superficial region 2) Medulla- exhibits cone-shaped medullary (renal) pyramids (Pyramids are made up of parallel bundles of urine-collecting tubules, Renal columns are inward extensions of cortical tissue that separate the pyramids, The medullary pyramid and its surrounding capsule constitute a lobe) 3) Renal pelvis- flat, funnel-shaped tube lateral to the hilus within the renal sinus (Major Calyces- large branches of the renal pelvis that collect urine draining from the papillae and empty urine into the pelvis, Urine flows through the pelvis and ureters to the bladder)

Blood and Nerve Supply of the Kidney

1) Abundantly supplied with blood vessels (Receive 25% of resting cardiac output via renal arteries) 2) Functions of different capillary beds (Glomerular Capillaries where filtration of blood occurs, Peritubular Capillaries that carry away reabsorbed substances from filtrate) 3) Sympathetic vasomotor nerves regulate blood flow % renal resistance by altering arterioles

Blood Vessels around the Nephron

1) Glomerular capillaries are formed between the afferent & efferent arterioles 2) Efferent arterioles give rise to the peritubular capillaries and vasa recta

The Nephron

1) Kidney has over 1 million nephrons composed of a corpuscle and tubule 2) Renal corpuscle= site of plasma filtration (Glomerulus is capilliaries where filtration occurs, Glomerular/Bowman's capsule is double-walled epithelial cup that collects filtrate) 3) Renal tubule (Proximal convoluted tubule, Loop of Henle dips down into medulla, Distal convoluted tubule) 4) Collecting ducts and papillary ducts (Drain Urine to the renal pelvis and ureter)

Cortical Nephron

1) 80-85% of nephrons are cortical nephrons 2) Renal corpuscles are in outer cortex and loops of Henle lie mainly in cortex 3) Produces dilute urine because the LOH is short not time for reabsorption

Juxtaglomerular Apparatus

Structure where afferent arteriole makes contact with ascending limb of loop of Henle: 1) Macula densa is thickened part of ascending limb 2) Juxtaglomerular cells are modified muscle cells in arteriole

Number of Nephrons

1) Remains constant from birth (Any increase in size of kidney is size increase of individual nephrons) 2) If injured, no replacement occurs 3) Dysfunction is not evident until function declines by 25% of normal (other nephrons handle the extra work) 4) Removal of one kidney causes enlargement of the remaining until it can filter at 80 % of normal rate of 2 kidneys

Mechanism of Urine Formation

1) The kidneys filter the body's entire plasma volume 60 times each day 2) The filtrate: (Contains all plasma components except protein, Loses water, nutrients, and essential ions to become urine) 3) The urine contains metabolic wastes and unneded substances

Overview of Renal Physiology

1) Urine formation and adjustment of blood composition involves three major processes 2) Nephrons and collecting ducts perform 3 basic processes (Gomerular filtration- a portion of the blood plasma is filtered into the kidney, Tubular Reabsorption- Water & useful substances are reabsorbed into the blood, Tubular Secretion- Wastes are removed from the blood & secreted into urine)

Glomerular Filtration

1) Blood Pressure produces glomerular filtrate 2) Filtration fraction is 20% of plasma 3) 48 gallons/day of filtrate are reabsorbed 4) Filtration is a passive, nonselective process that produces hypertonic urine (more concentrate)

Filtration Membrane

1) Fenestration layer stops all cells and platelets 2) Basal lamina stops large plasma proteins 3) Slit membrane stops medium-sized proteins, not small ones

Net Filtration Pressure (FYI)

1) Net Filtration Pressure= total pressure that promotes filtration 2) NFP= GBHP - (CHP + BCOP) = 10mm Hg

Glomerular Filtration Rate

1) Amount of filtrate formed in all renal corpuscles of both kidneys/minute (Average adult male rate is 125 mL/min) 2) Homeostasis requires Glmerular Filtration Rate that is constant (Too high & useful substances are lost due to the speed of fluid passage through nephron, Too low and sufficient wast products may not be removed from the body)

Tubular Reabsorption and Secretion

1) Normal Glomerular Filtration Rate is so high that volume of filtrate in capsular space in half an hour is greater that the total plasma volume 2) Nephron must reabsorb 99% of the filtrate (Proximal Convoluted Tubule with their microvilli do most of work with rest of nephron doing just fine-tuning) 3) Another important function of nephron is tubular secretion (Transfer of materials from blood into tubular secretion)

Sodium Reabsoption: Primary Active Transport

1) Sodium reabsorption is almost always by active transport (Na+ enters the tubule cells at the luminal membrane, Is actively transported out of the tubules by a Na+, K+ ATPase pump) 2) From there it moves to peritubular capillaries due to: (Low hydrostatic pressure, High osmotic pressure of the blood) 3) Na+ reabsorption provides the energy and the means for reabsorbing most other solutes

Nonreabsorbed Substances

1) Substances are not reabsorbed if they: (Lack carriers, Are not lipid soluble, Are too large to pass through membrane pores) 2) Urea, creatinine, and uric acid are the most important nonreabsorbed substances

Absorptive Capabilites of Renal Tubules and Collecting Ducts

1) Substances reabsorbed in Proximal Convoluted Tubule include: (Sodium, all nutrients, cations, anions, and water, Urea and lipid-soluble solutes, Small proteins) 2) Loop of Henle reabsorbs: (H2O, Na+, Cl-, K+ in the descending limb, Ca2+, Mg2+, and Na+ in the ascending limb) 3) Distal Convoluted Tubule absorbs: (Ca2+, Na+, H+, K+, and water, HCO3-, and Cl-) 4) Collecting duct absorbs: water and urea

Tubular Secretion

1) Essentially reabsorption in reverse, where substances move from peritubular capillaries or tubule cells into filtrate 2) Tubular secretion is important for: (Disposing of substances not already in the filtrate, Eliminating undesirable substances such as urea and uric acid, Ridding the body of excess potassium ions, Controlling blood pH)

Antidiuretic Hormone

1) Increases water permeability of collecting ducts so regulates facultative water reabsorption (Water molecules move more rapidly) 2) ADH is secreted by the hypothalamus and pituitary gland 3) When osmolarity of plasma & interstitial fluid decreases, more ADH is secreted and facultative water reabsorption increases 4) When osmolarity increases (drinking lots of water), ADH secretion is decreased and less water is reabsorbed

Production of Dilute or Concentrated Urine

1) Homeostasis of body fluids is maintained despite variable fluid intake 2) Kidneys regulate water loss in urine 3) ADH controls whether dilute or concentrated urine is formed (If lacking, urine contains high ration of water to solutes)

Formation of Dilute Urine

1) Dilute= having fewer solutes than plasma (Filtrate and blood have equal osmolarity in Proximal Convoluted Tubule) 2) Water reabsorbed in thin limb, but ions reabsorbed in thick limb of loop of Henle create a filtrate more dilute than plasma 3) Principle cells do not reabsorb water if ADH is low 4) Diabetes Insipidus: (Caused by insufficient release of ADH, Causes excessive water in urine and lots of urination which causes exessive thirst, Urine will have a lower specific gravity than normal urine)

Formation of Concentrated Urine

1) Compensation for low water intake or heavy perspiration 2) Urine can be up to 4 times greater osmolarity than plasma 3) It is possible for principle cells & ADH to remove water from urine to that extent, if interstitial fluid surrounding the loop of Henle has high osmolarity (Long loop juxtamedullary nephrons make that possible, Loop of Henle produces a hypertonic condition in which all collecting ducts must pass) 4) Cells in the collecting ducts reabsorb more water & urea when ADH is increased

Diuretics

1) Chemicals that enhance the urinary output 2) Osmotic diuretics include: (High glucose levels-carries water out with the glucose, Alcohol-inhibits the release of ADH, Caffeine and most diuretic drugs-inhibit sodium ion reabsorption)

Physical Characteristics of Urine

1) Color and transparency (Clear, pale to deep yellow, Concentrated urine has a deeper yellow color, Drugs, vitamin supplements, and diet can change the color of urine, Cloudy urine may indicate infection of the urinary tract) 2) Odor (Fresh urine is slightly aromatic, Standing urine develops an ammonia odor, Some drugs and vegetables alter the usual odor) 3) pH (Slightly acidic with a range of 4.5 to 8.0, Diet can alter pH) 4) Specific Gravity (Ranges from 1.001 to 1.035, Is dependent on solute concentration)

Chemical Composition of Urine

1) Urine is 95% water and 5% solutes 2) Nitrogenous wastes include urea, uric acid, and creatinine 3) Other normal solutes include: (Sodium, potassium, phosphate, and sulfate ions, Calcium, magnesium, and bicarbonate ions) 4) Abnormally high concentrations of any urinary constituents may indicate pathology 5) Urine with high glucose levels can indicate diabetics mellitis 6) High protein levels can indicate pregnancy, bacteria, or hypertension 7) Uric acid crystals can indicate gout or kidney stones 8) Low specific gravity can indicate diabetis insipidus 9) High pH can indicate a bacterial infection

Ureters

Slender tubes that convey urine from the kidneys to the bladder: 1) They are extensions of the renal pelvis 2) Retroperitoneal- located behind the abdominal peritoneum 3) Lined with transitional epithelium 4) Ureters enter the base of the bladder through the posterior wall 5) This closes their distal ends as bladder pressure increases and prevents backflow of urine into the ureters

Urinary Bladder

1) Smooth, collapsible, muscular sac that temporarily stores urine 2) It lies retroperitoneally on the pelvic floor posterior to the pubic symphysis (Males-prostate gland surrounds the neck inferiorly, Females-anterior to the vagina and uterus, Clinically important because infections tend to persist in this region)

Urethra

1) Muscular tube that: (Drains urine from the bladder, Conveys it out of the body) 2) Differs greatly from males to females 3) The female urethra is tightly bound to the anterior vaginal wall 4) Its external opening lies anterior to the vaginal opening and posterior to the clitoris 5) The male urethra has three named regions (Prostatic urethra-runs within the prostate gland, Membraneous urethra- runs through the urogenital diaphragm, Spongy urethra-passes through the penis and opens via the external urethral orifice)

Micturition Reflex

Micturition or urination requires: 1) Stretch receptors signal spinal cord and brain (when volume exceeds 200-400mL) 2) Spinal cord reflex is triggered 3) Parasympathetic fibers cause external & internal sphincter muscles to relax 4) Contraction of the detrusor muscle 5) Filling causes a sensation of fullness that initiates a desire to urinate before the reflex actually occurs

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