A&P Lecture Urinary and Reproductive Systems

Features of kidneys

Homeostatic organs, filter and maintain 200L fluid/day (all blood 40x/day), remove toxins/wastes/excess ions, regulate fluid volume/pH/salts, endocrine function, produce renin (regulates BP) and erythropoietin (stimulates RBC production), metabolize/activate vitamin D

Anatomy of kidneys

Hilus: vertical cleft on medical surface, entry site for blood vessels, lymphatics, nerves

Cortex: outer granular tissue

Medulla: red/brown cone-shaped masses/pyramids, appear striped due to parallel bundles of tubules

Columns: cortical tissue between pyramids

Lobe: pyramid+surrounding tissue, 8

Pelvis: Funnel-shaped tube continuous with ureter, branches into major→minor calyces, enclose papillae/apex of pyramid

Calyces: peristalsis propels urine to pelvis→ureter→bladder

Renal artery

Delivers 25% of total cardiac output/min, 5 segmental arteries→interlobar arteries between pyramids→arcuate arteries→cortical cradiate arteries

Renal plexus

Kidney nerve network controlled by sympathetic fibers

Features of nephrons

1 million per kidney, forms urine, several connect to 1 collecting duct

Consist of glomerulus, renal tubule, renal corpuscle

Endothelium of glomerulus

Fenestrated to allow large volumes of fluid (60mmHg) to filter from blood into glomerular capsule, filtrate=unprocessed urine, gets processed in kidney tubules to urine

Endothelium of glomerular capsule

Outer parietal layer is simple squamous epithelium, inner visceral layer has unique epithelial cells- podocytes: legs/pedicles, interdigitate with pedicles of adjacent podocytes with filtration slits/pores in between

Glomerulus filtrate

Fluid/filtrate comes out

RBCs, WBCs, platelets, and proteins don’t come out bc too big

Proximal convoluted tubule

Cuboidal epithelial cells, actively reabsorb solutes, secrete molecules, dense microvilli

Loop of Henle thin segment

Simple squamous epithelia, freely permeable to water

Loop of Henle thick segment and distal convoluted tubule

Cuboidal, no microvilli, secrete solutions into filtrate, little solute absorption

Cortical nephrons

85% of nephrons in kidneys, located in cortex except for tip of loop of Henle

Juxtamedullary nephrons

Located close to medulla, loop of Henle goes deep into medulla, very long thin segments, role is to concentrate urine

Afferent arterioles

Arise from interlobar arteries, feeds glomerulus, high bp, easily forces fluids and solutes out of glomerulus

Peritubular capilaries

Arise from efferent arterioles draining glomeruli, cling to renal tubules, adapted for absorption, low pressure, porous, readily absorb solutes and water, reclaims most filtrate produced by glomerulus

Vasa recta

Formed by efferent nephrons from juxtamedullary nephrons, important role in forming concentrated urine, reclaims most filtrate produced by glomerulus

Juxtaglomerular apparatus

Portion of the distal tubule is nestles between afferent and efferent arterioles of glomerulus

In the walls of arterioles are JG cells: large smooth muscle cells containing renin, sense bp in afferent arteriole (diameter controlled by lack of ATP)

Renin increases bp

Glomerular filtration rate

Normal level=60mmHg

High=lose too much fluid/increased urine production

Low=retain waste/not filtering blood

Macula densa

Columnar cells in distal tubule next to JG cells, chemoreceptors, sense filtrate flow, regulate rate of filtration in kidneys

Urine formation processes

Glomerular filtration, tubular reabsorption, secretion

Unprocessed filtrate is like plasma but without proteins, as it percolates through tubules,most water/nutrients/ions/glucose are reclaimed, regulated by renal and hormonal controls

Urine formation- glomerular filtration

Passive, non-selective, due to high hydrostatic pressure (55mmHg)

Rate depends on forces that increase/decrease filtration: glomerular hydrostatic pressure increases it, osmotic pressure of blood and hydrostatic pressure in glomerular capsule decrease it (back pressures)

Glomerular filtration pressure

Determined by net effect of HPg (55mmHg), OPg (30mmHg), and HPc (15mmHg)=55mmHg-(33mmHg and 15mmHg)=10mmHg=GFR 180L/day

15% drop in HPg stops filtration

Regulation of glomerular filtration

Greater filtrate form- increase flow through tubules, substances can’t be reabsorbed fast enough

Reduced filtration form- decrease flow, many substances including waste reabsorbed

Autoregulation regulates diameter of afferent arterioles

Myogenic regulation and tubuloglomerular feedback are intrinsic mechanisms that directly regulate GFR despite moderate changes in bp (80mmHg-180mmHg mean arterial pressure)

Myogenic regulation

Smooth muscle contracts when stretched

If bp increases→vessels stretch→vasoconstriction→decrease flow→maintain GFR

If bp decreases→decreased blood flow→decrease stretch of vessels→vasodilation→increase flow→maintain GFR

Tubuloglomerular feedback

Macula densa cells detect filtrate flow and osmotic levels

If filtrate has low flow or osmolarity→macula densa cells don’t release vasoconstrictive chemical ATP→vasodilation of afferent arteriole→more blood enters glomerulus→increase GFR

If filtrate has high flow or osmolarity→macula densa cells release vasoconstrictive chemical ATP→vasoconstriction of afferent arteriole→less blood enters glomerulus→decrease GFR

Renin

Angiotensin mechanism, vasoconstriction restores GFR, aldosterone lowers osmolarity

Sympathetic nervous system control of renal flow

Only during extreme stress (shock), vasoconstriction, shunts blood to vital organs, reduces fluid loss, helps maintain bp

Tubular reabsorption- primary convoluted tubule

Mostly transcellular, Na+ most abundant ion in filtrate, actively transported from tubule cell by Na+/K+ ATPase pumps (produces electrochemical gradient that pulls Na+ into cells from filtrate), inside of tubule cell left with small negative charge, obligatory water reabsorption by diffusion/osmosis

Molecules in primary convoluted tubule concentration gradient

Cations, fatty acids, urea, lipid-soluble drugs and environmental toxins

Gradient created by Na+/K+ pump draws glucose, amino acids, lactate, vitamins, and cations across tubule cells luminal membrane by symport

Transcellular route across proximal convoluted tubule

Transport across apical membrane, diffusion through cytosol, transport across basolateral membrane (involves lateral intercellular spaces), movement through interstitial fluid and into capillary

Paracellular route across proximal convoluted tubule

Movement through leaky tight junctions, movement through interstitial fluid and into capillary

Loop of Henle- water

Water reabsorbed by osmosis, can leave descending limb only (increase concentration 300-1200mOsm), solutes can leave ascending limb (decreases concentration 1200-100mOsm) only via sodium/potassium/2chloride symporter

Vasa recta gradient

Highly permeable to water and solutes, countercurrent exchanges occur between each section and its surrounding fluid, blood remains isosmotic with surrounding fluid, able to reabsorb water and solutes into general circulation without undoing osmotic gradient created by countercurrent multiplier

Distal convoluted tubule and collecting ducts

Na/Cl symporters reabsorb Na and Cl, water also reabsorbed, most reabsorption regulated by hormones, water controlled by ADH released from posterior pituitary when blood is too concentrated/high osmolarity, ADH makes collecting ducts more permeable to water, Na reabsorption controlled by aldosterone from adrenal cortex, targets collecting ducts t open more Na channels to increase Na reabsorption→increase BV and bp

Tubular secretion

Some substances (H, K, creatinine, ammonium ions, penicillin, phenobarbital) move from blood of peritubular capillaries into filtrate, most secretion occurs in primary convoluted tubule, important for excreting K ions, controlling blood pH

Concentrating urine

Kidneys must keep solute concentration around 300mOsm, regulate urine concentration via countercurrent mechanism: filtrate flows in 1 direction through loop of Henle and flows in opposite direction through vasa recta, establishes and maintains osmotic gradient

Mechanism for concentrating urine