Renal System
Ureter
- Modified fluid leaves kidney via ureter
- moves urine into bladder via peristalsis
Urinary Bladder
- Ureters lead to urinary bladder
- contains detrusor muscles
Urethra
- allows for expulsion of urine
- micturition is process by which urine is excreted
Kidneys
- site of urine formation
- bulk of kidney tissue is composed of nephrons
- nephrons help modify water and solutes from blood into urine
Kidney Functions
1. Regulation of ECF volume and BP
2. Regulation of osmolarity of blood
3. regulation of ion balance
4. homeostasis of pH
5. excretion of wastes
6. production of hormones
Nephron
- functional unit of the kidney
- smallest structure that performs all functions
- 1 mill per kidney
Cortical Nephrons
- 85% of the nephrons
- perform excretory and regulatory functions
- Short loop of henle that penetrates only the outer renal medulla
Juxtamedullary Nephrons
- 15% of nephrons
- concentrates or dilutes urea
- long loop of Henle
Anatomy of Nephron: Glomerulus
- specialized bundle of capillaries
- fluid in capillaries passes directly into the glomerulus
- regilates selective filtration of blood; then passes through series of tubules within nephron
Anatomy of Nephron: Bowman's Capsule
- cup-like sack surrounding the glomerulus
- encloses space of the proximal convoluted tubule of nephron
Anatomy of Nephron: Renal Corpuscle
- glomerulus + bowman's capsule
- open into a tubule
Anatomy of Nephron: Proximal Convoluted Tubule
- segment of nephron between bowman's capsule and loop of henle
- reabsorbs substances within filtrate based on what the body needs
- protein in urine is a sign of kidney dmg; AA not abosorbed into blood properly
Anatomy of Nephron: Loop of Henle
- segment that dips down toward medulla of kidney, then back up
- has thin and thick segments
- actively pumps out Na to make medulla salty
- descending limb permeable to water
- ascending limb actively pumps Na
Anatomy of Nephron: Distal Tubule
- between collecting duct and loop of Henle
- reabsorption of ions of Na, Ca
- Dumps waste products/water no longer needed into collectign duct
Anatomy of Nephron: Collecting Duct
- collects byproducts/waste from nephrons
- dips back into medulla (salty part) and permeable to water
- hormones dictact how porous tube is (more porous = more water leaves)
Antidiuretic Hormone
- Diuretics excrete more urine and cause dehydration
- ADH prevents dehydration by reabsorbing water into blood
Vascular Structures Around Nephrons
1) Afferent Arteriole
- blood that flows into ball-like network of capillaries (glomerulus)
2) Efferent Arteriole
- blood that flows out of glomerulus
3) Peritubular Capillaries
- vessels that surround the tubule
- In Juxtamedullary nephrons (vasa recta)
4) Vase Recta
- Juxtamedullay nephrons the peritubular capillaries
5) Renal Venules
- joins with vasa recta
Juxtaglomerular Apparatus
- ascending limb of the loop of Henle passes between afferent and efferent arterioles
- allows for comm between 2 structures
- helps kidneys perform its autoregulatory function
Basic Functions of the Kidney
1) filtration
- movement of fluid from blood to lumen
- only in the renal corpuscle
- filtered fluid (filtrate)
2) Reabsorption
- movement of fluid from the lumen back to the pertubular capillaries
3) secretion
- movement of selected molecules from the blood and adds them to the filtrate
- selective process and requires various cellular membrane proteins
4) Excretion
- removal of any fluid/substances that are not reabsorbed into the body
Excretion Formula
amt filtered F - amt reabsorbed R + amt secreted S = amt excreted E
Filtration Fraction
1) plasma VOL entering afferent arteriole = 100%
2) 20 % of VOL filters
3) >19% of fluid is reabsorbed
4) >99% of plasma entering kidney returns to systemic circulation
5) <1% of VOL is excreted to external environment
3 Filtration Barriers
1) capillary endothelium
2) basal lamina
3) Bowman's capsule endothelium
Capillary Endothelium
fenestrations
- large enough for plasma contents to pass
- small enough to stop blood cells
- neg charged so proteins are not filtered through
Basal Lamina
- acellular
- consists of extracellular matrix: collagen and glycoproteins
- between capillary and bowmans capsule endothelial layers
- acts like a sieve
- prevents proteins from entering the filtrate
Bowman's Capsule Endothelium
- consists of podocytes that wrap around capillaries and leave narrow filtration slits
- podocytes of neg charged proteins
- repel proteins in blood from being filtered
Glomerular Filtration Pressure
1) PH
- hydrostatic pressure of the blood in glomerulus
- avg 55 mmHg
2) π
- colloid osmotic pressure in glomerular capillaries
- avg 30 mmHg
3) Pfluid
- hydrostatic pressure of fluid in bowmans capsule
- avg 15 mmHg
Glomerular Filtration Rate
- constant; BP helps create hydrostatic pressure
- VOL of fluid that filters into bowman's capsule per unit time
- blood plasma vol filtered by kidneys 60x/day
- avg GFR is 125mL/min or 180L/day
Autoregulation
- kidney maintains constant GFR at MAP of 80-180 mmHg
- Important for potecting filtration barriers against high pressures
- myogenic repsonse and tubuloglomerular feedback help maintain constant GFR
Regulation of GFR: Intrinsic Controls
Renal autoregulation
1) myogenic mech
2) tubuloglomurular feedback mech
Myogenic Mechanism
- local smooth muscle contracts when stretched
- increased BP causes muscle to stretch, leading to constriction of afferent arterioles
- restricts blood flow into glomerulus
- protects glomeruli from dmging high BP
- decreases BP causes dilation of afferent arterioles
- both help maintain norm GFR
Myogenic Response Steps
1. increased BP
2. increased stretch in afferent arterioles
3. stretch-sensitive ion channels open
4. Depolarization
5. voltage gated Ca channels open
6. smooth muscle constricts
7. increased resistance in afferent arteriole bed
8. maintenance of GFR and GFP
Tubuloglomerular Feedback Mechanism
- flow dependent directed by macula densa cells
- respond to filtrates NaCl concentration
- If GFR increases, filtracte flow rate increases
- decreased reabsorption time, causing high NaCl lvls in filtrate
- feedback causes constriction of afferent arteriole
- lowers NFP and GFR, more time for NaCl reabsorption
Tubuloglomerular Feedback Steps
1. GFR increases
2. flow through tubule increases
3. flow past macula densa increases
4. paracrine from macula densa to afferent arteriole
5. afferent arteriole constricts
6. reisstance in afferent arteriole increases
7. hydrostatic pressure in glomerulus decreases
8. GFR decreases
Regulation of Glomeurular Filtration: Extrinsic Control
Neural and Hormonal Mechanisms
- regulate GFR to maintain systemic BP
- extrinsic controls override renal intrinsic controls if blood VOL needs to be increased
Regulation of Glomerular Filtration: Sympathetic Nervous System
Normal Conditions at Rest:
- renal autoregulation mechanisms prevail
Abnormal Conditions, Low BP:
- NE released
- systemic vasoconstriction, increases BP
- constriction of afferent arterioles, decreases GFR
- blood V/P increase
Regulation of Glomerular Filtration: RAA Mechanism
Renin-angiotensin-aldosterone Mechanism
- main mechanism for increasing BP
3 Pathways to renin release by JG cells
1. direct stimulation of JG cells by sns
2. stim by activated macula dens acells when filtrate NaCl con is low
3. reduced stretch of JG cells
ANS and GFR
- Sympathetic neurons innervate both aff and eff arterioles
- Alpha-receptors will cause vasoconstriction and will reduce GFR
Sympathetic innervation = alpha receptor = vasoconstriction of afferent arteriole = reduced GFR
when severe hemorrhage occurs:
- very low BP
- sns stimed
- constricts afferent arteriole reducing GFR