Chapter 25 - Kidneys Flashcards
Functions of Kidneys
Regulating total water volume and solute concentration in water
Regulating ECF ion concentrations
Long-term acid-base balance
Removal of metabolic wastes, toxins, drugs
Endocrine functions
Renin - regulation of blood pressure
Erythropoietin - regulation of RBC production
Activation of vitamin D
Gluconeogenesis during prolonged fasting
3 layers of Kidneys
Renal cortex
Renal medulla
Renal Pelvis
Nephrons
Structural and functional units that form unrine
Renal Corpuscle
Glomerulus: Tuft of capillaries; fenestrated endothelium, porous, filtrate formation. Specialized for filtration, Different from other capillary beds – fed and drained by arteriole. Blood pressure in glomerulus high:
-Afferent arterioles larger in diameter than efferent arterioles and arterioles are high-resistance vessels
Glomerular capsule (Bowman’s capsule): Hollow structure surrounding glomerulus
Proximal convoluted tubule (PCT)
Proximal convoluted tubule (PCT): Cuboidal cells with dense microvilli (surface area); large mitochondria
Functions in reabsorption and secretion
Confined to cortex
Distal convoluted tubule (DCT)
Distal convoluted tubule (DCT): Cuboidal cells with very few microvilli, Function more in secretion than reabsorption, Confined to cortex
Collecting Duct
Principal cells: Sparse, short microvilli, Maintain water and Na+ balance
Intercalated cells: Cuboidal cells; lots of microvilli; maintain acid-base balance
Peritubular capillaries:
Peritubular capillaries: Low-pressure, porous capillaries adapted for absorption Arise from efferent arterioles
Cling to adjacent renal tubules in cortex
3 Main parts of Renal tubule
Proximal Convoluted Tubule
Nephron Loop
Distal Convoluted Tubule
3 Steps of Urine formation
- Glomerular filtration: produces cell and protein-free filtrate
- Tubular reabsorption: Selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts
- Tubular secretion: Selectively moves substances from blood to filtrate in renal tubules and collecting ducts
What types of molecules can pass through filteration membrane
Molecules smaller than 3 nm to pass
Water, glucose, amino acids, nitrogenous wastes
Plasma proteins remain in blood maintains colloid osmotic pressure prevents loss of all water
Chief force pushing water, solutes out?
Hydrostatic pressure in glomerular capillaries
Net Filtration Pressure
Pressure responsible for filtrate formation
Main controllable factor determining glomerular filtration rate (GFR)
Glomerular Filtration Rate (GFR)
Volume of filtrate formed per minute by both kidneys (120–125 ml/min)
GFR directly proportional to
NFP: primary pressure is hydrostatic pressure in glomerulus
Total surface area for filtration: mesangial cells control by contracting
Filtration membrane permeability – much more permeable than other capillaries
Regulation of Glomerular Filtration
Constant GFR allows kidneys to make filtrate and maintain homeostasis
Goal of intrinsic controls - maintain GFR in kidney
GFR affects systemic blood pressure
GFR urine output blood pressure, and vice versa
Goal of extrinsic controls - maintain systemic blood pressure
Intrinsic Control
Myogenic Mechanism
Smooth muscle contracts when stretched
Raised BP - muscle stretch - constriction of afferent arterioles - restricts blood flow into glomerulus
Decreased BP - dilation of afferent arterioles
Tubuloglomerular Feedback Mechanism
Flow-dependent mechanism directed by macula densa cells; respond to filtrate NaCl concentration
If GFR raised - filtrate flow rate increased - decreased reabsorption time high filtrate NaCl levels constriction of afferent arteriole, decreased NFP & GFR more time for NaCl reabsorption
Opposite for decreased GFR
Extrinsic Control
Sympathetic Control
If extracellular fluid volume extremely low, norepinephrine and epinephrine released
Systemic vasoconstriction increased blood pressure
Constriction of afferent arterioles GFR increased blood volume and pressure
Renin-Angiotensin- Aldosterone Mechanism
Main mechanism for increasing BP
Three pathways to renin release
Direct stimulation of granular cells by sympathetic nervous system
Stimulation by activated macula densa cells if filtrate NaCl concentration low
Reduced stretch of granular cells
Tubular Reabsorption
Most of contents reabsorbed to blood
Selective transepithelial process
~ All organic nutrients reabsorbed
Water and ion reabsorption hormonally regulated and adjusted
Transcellular Route
Transport across the apical membrane
Diffusion through cytosol
Transport across the basolateral membrane
Movements through interstitial fluid and into capillary
Paracellular
Movement through leaky tight junctions, particularly in the PCT.
Movement through interstitial fluid and into capillary
Steps of Reabsorption in Proximal Convoluted Tubular
At the basolateral membrane, Na+ is pumped into the interstitial space by the Na+-K+ ATPase. Active Na+ transport creates concentration gradients that drive
“Downhill” Na+ entry at the apical membrane.
Reabsorption of organic nutrients and certain ions by cotransport at the apical membrane.
Reabsorption of water by osmosis through aquaporins. Water reabsorption increases the concentration of the
solutes that are left behind. These solutes can then be reabsorbed as they move down their gradients:
Lipid-soluble substances diffuse by the transcellular route.
Various ions (e.g., Cl−, Ca2+, K+) and urea diffuse by the paracellular route.
How is water reabsorbed
Movement of Na+ and solutes creates osmotic gradient for water.
Water reabsorbed by osmosis, aided by water-filled pores called aquaporins
-Aquaporins always present in PCT obligatory water
reabsorption
-Aquaporins inserted in collecting ducts only if ADH
present facultative water reabsorption
Hormones that affect reabsorption
ADH - released by posterior pit gland
Aldosterone - Targets collecting ducts and distal PCT
Atrial Natriuretic peptide - Reduces blood Na+, decreases blood volume, and blood pressure, Released by cardiac atrial cells.
Parathyroid Hormone - Acts on DCT to increase Ca2+ Reabsorption.
Tubular secretion
Reabsorption in reverse; almost all in PCT
Disposes of substances (e.g., drugs) bound to plasma proteins
Eliminates undesirable substances passively reabsorbed (urea and uric acid)
Rids body of excess K+ (aldosterone)
Controls blood pH by altering amounts of H+ or HCO3– in urine
Diuretics
ADH inhibitors (alcohol)
Na+ reabsorption inhibitors (and resultant H2O reabsorption) (caffeine, hypertension or edema drugs)
Loop diuretics inhibit medullary gradient
Osmotic diuretics - substance not reabsorbed so water remains in urine, (high glucose of diabetic patient)
Chemical Composition of Urine
95% water and 5% solutes Nitrogenous wastes Urea (amino acid breakdown) Uric acid (nucleic acid metabolism) Creatinine (metabolite of creatine phosphate)
3 events must happen for urination or micturition
Contraction of detrusor muscle by ANS
Opening of internal urethral sphincter by ANS
Opening of external urethral sphincter by somatic nervous system
