Physiology - Renal System Lecture 2

Foces Opposing Filtration

-Capsular hydrostatic pressure (CHP) -Pressure exerted by fluid already in the capsular space and renal tubules -Blood colloid osmotic pressure (BCOP) (glomerular capillary osmotic pressure) -Due to the presence of proteins in plasma

Histology of Bowman's Capsule

-Capsule is composed of two layers of squamous epithelial cells Visceral layer: -it covers the glomerulus -Podocytes: modified squamous cells, component of the glomerular filtration barrier Parietal layer: -Outer wall of the glomerular capsule -Squamous epithelial cells

Glomerular Filtration

-First step of urine production -occurs in the renal corpuscle -inorganic ions, low molecular weight solutes and water move freely across glomerular capillaries -commonly filtered substances include: ions (Na+, K+, Cl-), and neutral organic molecules such as glucose and urea -amino acids, vitamins, small proteins are also filtered -from the glomerular capsule, the filtrate moves into the renal tubule

Force Favoring Filtration

-Glomerular blood hydrostatic pressure (GBHP) -Blood pressure in the glomerular capillaries is generally high -Efferent arterioles are smaller than afferent arterioles

Tubuloglomerular Feedback

-Increased BP will increase GFR producing a decrease of Na+, Cl- and water reabsorption -Increased delivery of Na+ and Cl- and water to the macula densa -Macula densa in renal tubules provides feedback to the glomerulus inhibiting the secretion of NO (nitric oxide) vasoconstriction of afferent arteriole -It is slower than myogenic mechanism

Tubuloglomerular Feedback Mechanism

-It involves the anatomical unit known as juxtaglomerular apparatus, which is formed by the afferent arteriole, efferent arteriole and the distal convoluted tubule (DCT)

Myogenic Mechanism

-It involves the smooth muscle cells in the walls of afferent arterioles and prevents increased pressure from being transmitted to the glomerulus -Contraction of smooth muscle in walls of afferent arterioles decreases blood flow, decreases glomerular blood volume, and decreases glomerular blood pressure -Increasing systemic blood pressure increases the renal blood flow and stretches the arterioles. -Protects glomerular capillary from sudden changes in blood pressure within seconds -local mechanism that decreases the pressure in the glomerulus that decreases the filtration rate

Podocytes

-Podocytes wrap the glomerular capillaries -Pedicels of neighboring podocytes interdigitate - Filtration slits (narrow space) between pedicels -important for filtration Primary processes -cell surface extensions from cell body Secondary processes -fingerlike extensions -pedicels

How is the Glomerular Filtration Rate Regulated

-Regulatory mechanisms are important to maintain a nearly constant glomerular filtration rate -Mechanisms adjust blood flow in and out of the glomerulus Mechanisms of renal autoregulation -Myogenic mechanism -Tubuloglomerular feedback mechanism -Hormonal regulation (initiated by kidneys) -Angiotensin II -Atrial natriuretic peptides (ANP) Autonomic regulation -Sympathetic division of ANS

Macula Densa

-Tall, closely packed cells in DCT -It provides feedback to the glomerulus to modify the blood flow Cells function as: -Chemoreceptors (respond to changes in NaCl levels in the tubular fluid) -Osmoreceptors: (respond to changes in filtrare volume in DCT)

Mesangial Cells

-They are located between the macula densa and the afferent and efferect arterioles -They are supportive cells for the glomerular capillaries -They are also fagocytic cells

Granular Cells

-They are smooth muscle cells -They act as mechanoreceptors, responding to changes in blood pressure in afferent arteriole -They synthesize, store, and secrete renin and erythropoietin -Renin is secreted in response to sympathetic stimulation

Hormonal Regulation

-Vasodilation of the afferent arteriole is mediated primarily by prostaglandins

Filtration Process

-in the kidney, there is a force that helps filtration by pushing things through the membrane, and there are two forces opposing filtration

Filtration Membrane or Barrier

Components of the filtration membrane -Endothelial cells of glomerular capillaries -Basement membrane -Podocytes -filtration of water and small solutes -effective barrier for most plasma proteins (large proteins), blood cells, and platelets

Filtration Membrane

Endothelial cells -have fenestrations -they are leaky Basement membrane of basal lamina -glycoprotein matrix -collagen and proteoglycans -it is negatively charged -located between the endothelium and the podocytes Podocytes -form filtration slits

Processes of Urine Formation

Filtration -water and solutes from blood -based on size Reabsorption -it is a selective process -right amount of substances, water, electrolytes, and glucose reenter the blood -waste products and substances in excess are not reabsorbed Secretion -removes substances that must be eliminated from the blood

Hormonal Mechanism

Hormones involved: -Angiotensin II -Atrial natriuretic peptides (ANP) Three stimuli cause the juxtaglomerular complex to release renin: -Decrease in blood pressure at glomerulus (decrease in blood volume, fall in systemic pressures, or blockage in renal artery) -Stimulation of juxtaglomerular cells by sympathetic innervation -Decrease in osmotic concentration of tubular fluid at macula densa

Cells Present in the Juxtaglomerular Apparatus

Macula Densa -modified epithelial cells in distal convoluted tubule Granular or juxtaglomerular cells -smooth muscle cells mainly in the wall of afferent arteriole Mesangial cells -Between macula densa and afferent and efferent arterioles

Regulation of GFR by The SNS

Most important with extreme drop of blood pressure SNS at rest -renal blood vessels are maximally dilated Moderate SNS stimulation -both afferent and efferent arterioles constrict similarly -Slight decrease in GFR Greater SNS stimulation: -Vasoconstriction primarily of the afferent arterioles -Decreases blood ow to the glomerulus -Slows or inhibits filtration production

Net Filtration Pressure (NFP)

NFP = GBHP - CHP - BCOP NFP =10 mmHg is the pressure needed to filter through the capillaries -Filtration ceases when the glomerular blood pressure is ≤ 45 mmHg The rate of filtration depends on pressure

Neuronal Control: Sympathetic Nervous System

Sympathetic nervous system (SNS) innervates: -Smooth muscle in afferent and efferent arterioles -Granular or juxtaglomerular cells Activation of SNS -release of norepinephrine and epinephrine -stimulation of the renin-angiotensin-aldosterone system (RAAS) SNS adjusts rate of urine formation by changing blood flow and blood pressure at the glomerulus -Most important with extreme drop of blood pressure