Chapter 26: The Urinary System

(The Urinary System) Chapter 26 The Urinary System · includes kidneys, ureters, urinary bladder, and urethra · Functions of the urinary system: ↳ excretion of waste products ↳ elimination of organic materials ↳ homeostatic regulation - regulating blood volume and blood pressure - regulating plasma ion concentrations (Na, K, Cl etc.) - regulating blood PH - reabsorption of valuable nutrients The Kidneys · highly vascular, paired, bean-shaped organ · left kidney higher than right b/c of location of the liver · three layers of supportive tissue around each kidney: ↳ inner fibrous capsule ↳ middle adipose tissue layer ↳ outer renal fascia · adrenal gland on top of each kidney (Kidney hat!) · hilum: medial indentation where nerve fibers, lymphatics, blood, and ureters enter the kidney · blood supply: renal arteries and veins, kidneys receive 20% of cardiac output parasympathetic: constricts bladder for urination sympathetic: relaxes bladder to hold in pee Internal Anatomy of Kidney · fibrous renal capsule · renal cortex · renal medulla ↳ divided into renal pyramids ↳ base of each pyramid touches the cortex (renal papilla) ↳ Calyces (calyx) at renal papilla for urine collection · renal medulla → minor calyx → major calyx → renal pelvis → ureter Kidney Blood Flow: The Nephron · 2 kinds of nephrons: ↳ cortical nephron · each nephron has 2 sections: ↳ juxtamedullary nephron ↳ renal corpuscle: includes glomerulus (capillary network) inside the glomerular capsule (bowmans capsule) ↳ renal tubule: includes the proximal renal tubule (PCT), nephron loop, and distal convoluted tubule (DCT) Renal Corpuscle and Glomerulus · the glomerulus contains about 50 intertwined capillaries in the glomerular capsule · blood is delivered to the glomerulus by the afferent arteriole, and leaves through the efferent arteriole · glomerular capillaries are fenestrated with pores for movement of ions, water etc. · filtration membrane: there are filtration slits between podocyte cells in the glomerulus where water and small solutes are pushed out of glomerular capillaries and into the glomerular space through hydrostatic pressure ↳ the end product is filtrate ↳ filtrate leaves the glomerular capsule and enter the renal tubule at PCT Intraglomerular mesangial cells: located among glomerular capillaries and are made of smooth muscle with pores that can change their lumenal diameter to affect filtration Renal Tubule · once filtrate enters the renal tubule (PCT) it is called tubular fluid · renal corpuscle → collecting system → away from nephron · proximal convoluted tubule: reabsorption of critical ions · nephron loop: contains ascending and descending limbs · distal convoluted tubule: reabsorption of water and selected ions · basic functions of renal tubule: ↳ reabsorption and secretion of water and solutes to maintain homeostasis and produce urine ↳ reabsorption and secretion varies due to different tissues and membrane permeability The Juxtaglomerular Complex (JGC) · regulates blood pressure and forms filtrate · Contains 3 types of cells: ↳ Macula densa cells: epithelial cells in DCT that act as chemoreceptors and baroreceptors ↳ Juxtaglomerular cells: modified smooth muscle cells in afferent arteriole that secretes renin ↳ Mesangial cells: extraglomerular cells between afferent and efferent arterioles for feedback between macula densa and juxtaglomerular cells Collecting System · functions to move tubular fluid from nephron to renal pelvis and change its composition to form urine · nephron → collecting duct → larger papillary duct → minor calyx → major calyx → renal pelvis → ureter → bladder → excretion of urine Formation of Urine ① Filtration: blood pressure forces water and solutes across glomerulus capillaries and into the glomerular space by hydrostatic pressure ↳ if BP↓ filtration↓ ② Reabsorption: the removal of water and solutes across the renal tubule and collecting system as it moves back into circulation ↳ reabsorption of water is passive ↳ simple diffusion and carrier proteins ③ Secretion: Secretion of solutes back into tubular fluid (filtrate) prior to excretion Glomerular Filtration · glomerular filtration: blood plasma is forced through pores of the filtration membrane, molecules small enough are carried along and forms filtrate ↳ fenestrated endothelium (big filter) ↳ basement membrane (medium filter) ↳ foot processes of podocytes (small filter) · hydrostatic pressure · blood colloid osmotic pressure (BCOP): pressure due to materials in solution like plasma proteins · Glomerular filtration rate (GFR): the amount of filtrate the kidneys produce in one minute ↳ high creatinine levels indicate reduced GFR and reduced kidney function ↳ creatinine: the byproduct of muscle activity and digestion of animal proteins, break down of creatinine phosphate · GFR is controlled by three levels of regulation: autoregulation, hormonal regulation, and autonomic regulation Regulation of GFR ① Autoregulation: control of local blood flow via vasoconstriction and vasodilation ② Hormonal Regulation: renin-angiotensin-aldosterone system (RAAS) and natriuretic peptides ③ Autonomic Regulation: SNS stimulation can cause powerful vaso constriction of afferent arterioles to decrease GFR and slow down production of filtrate ↳ decreases blood to glomeruli, thus ↓GFR, and less urine formed, so water stays in system and BP↑ Tubular Reabsorption · Reabsorption and secretion alter the composition of the filtrate ↳ tubular reabsorption: returns nutrients from tubular fluid to blood ↳ tubular secretion: adds substances from the blood back to the tubular fluid · involves diffusion, osmosis, channel-mediated diffusion, and carrier-mediated transport · carrier-mediated transport relies on specificity and saturation · renal threshold: tubular transport maximum is different for each substance, and can only transport a certain amount (threshold) ↳ exceeding threshold means more stuff will be secreted in urine or sits in the tubules Tubular Secretion · involves transfer of selected substances from the peritubular capillaries (blood) into the tubular lumen (tubular fluid) ↳ H+ : acid-base balance ↳ K+ : maintains normal membrane excitability in muscles and nerves ↳ organic ions: elimination of foreign organic compounds (drugs, pesticides, etc.) · K+ or H+ is secreted in exchange for Na reabsorption to maintain concentration gradients Regulating Urine Volume and Concentration · hypothalamus constantly makes low levels of ADH and secretes from posterior pituitary so that we always have a degree of permeability · urine volume and osmotic concentration are controlled by water reabsorption ↳ dependent on ADH: - as ADH rises, DCT and collecting system become more permeable to water and water reabsorption increases - under max ADH, osmotic concentrations match surrounding medulla · vasa recta: long capillary beside nephron loop that carries reabsorbed water and solutes back to circulation (counter current exchange) Composition of Urine · Metabolic wastes: ↳ Urea: most abundant organic waste, by product of amino acid breakdown ↳ creatinine: the byproduct of skeletal muscle activity by breakdown of creatine phosphate ↳ uric acid: formed as body recycles nitrogenous bases from RNA · urine osmolality: measure of the osmotic concentrations of urine · urinalysis: analysis of the chemical and physical composition of urine Monitoring Kidney Function · urinalysis: looks at colour, clarity, and presence of unexpected substances · creatinine clearance: compares creatinine in urine vs. creatinine in blood to estimate GFR + Kidney function · BUN (blood urea nitrogen): measures the amounts of urea in blood Ureters · muscular tubes from renal pelvis of kidney to urinary bladder · path of ureters is different in males and females · ureters enter the back of the bladder through a small slit to prevent backflow of urine (ureteric orifices) · peristaltic contractions every thirty seconds Urinary Bladder · temporarily stores urine before excretion · rugae in mucosal lining allow for expansion ↳ transitional epithelium · detrusor muscle: When it contracts, bladder squeezes and urine enters urethra · trigone area: thick, smooth mucous layer that acts like a funnel for urine into the urethra · Internal (involuntary) sphincter and external (voluntary) sphincter Urethra · carries urine from the bladder out of the body · longer in males ↳ three sections: prostatic, membranous, spongy urethra · Internal urethral sphincter is involuntary, external is voluntary · urethra is anterior, Vagina is posterior Regulation of Urination · peristaltic contractions bring urine from kidney to bladder · urge to pee when 200mL in bladder · urination involves 2 spinal reflexes: ↳ urine storage reflex: - spinal reflex and pontine storage center - stretch receptors stimulate sympathetic activity that inhibits contraction of detrusor muscles and promotes contraction of sphincter ↳ urine voiding reflex: - spinal reflex and pontine micturation center - micturation center stimulates increased parasympathetic activity and decreases sympathetic so detrusor muscles contract and sphincter relaxes urinary incontinence: inability to voluntarily control urination ↳ issue with nerves, CNS, sphincters, stretch receptors etc. ↳ NOT a normal part of aging Age Related Changes · Infants: lack voluntary control of urination because corticospinal connections not yet developed · old age: decreased nephrons, nephrolithiasis, decreased ADH sensitivity, loss of muscle tone in sphincters