Urinary System: Functions and Components

Introduction to the Urinary System

• The urinary system components are kidneys, ureters, bladder, and urethra
• Kidneys filter blood, remove waste products, and convert filtrate into urine
• Ureters transport urine from kidneys to the urinary bladder
• The bladder is an expandable muscular sac and stores as much as 1 L urine
• The urethra eliminates urine from the body

Urinary System Functions

• Elimination of metabolic wastes occurs
• Regulation of ion levels, such as Na+, K+, and Ca2+
• Acid-base balance is regulated by altering levels of H+ and HCO3-
• Blood pressure is regulated
• Biologically active molecules like hormones and drugs are eliminated
• Calcitriol, the active form of vitamin D, is formed
• Erythropoietin is produced and released in response to low blood oxygen
• Erythropoietin (EPO) secreted stimulates red bone marrow to increase erythrocyte production
• The urinary system has the potential to engage in gluconeogenesis during prolonged fasting or starvation
• Glucose is produced from non-carbohydrate sources to maintain glucose levels

Urinary System (Posterior)

• The left kidney is situated between the T12 and L3 vertebrae
• Rib cage partially protects the kidneys
• Right kidney sits slightly lower than the left kidney due to the large size of the liver, approximately 2 cm lower

Position and Stabilization of the Kidneys

• Each kidney is surrounded and supported by several tissue layers, from innermost to outermost
• The fibrous capsule adheres directly to the kidney and helps maintain kidney shape and protect from pathogens and trauma
• Perinephritic fat provides cushioning and stabilization to the kidney
• Renal Fascia anchors the kidney to surrounding structures
• Paranephritic fat (outermost layer) provides cushion and stability

Clinical View: Kidney Variations and Anomalies

• Renal agenesis is a failure of a kidney to develop
• Often asymptomatic if unilateral, but fatal if bilateral
• Pelvic kidney is when a developing kidney fails to migrate from the pelvic cavity
• Horseshoe kidney is when the inferior parts of left and right kidneys are fused
• Supernumerary kidney is when an extra kidney develops
• All three typically asymptomatic

Kidney Parenchyma

• The renal cortex is the outer aspect of the kidney is the renal column
• Renal columns are extensions of the cortex projecting into the medulla to subdivide it into renal pyramids
• The renal medulla is the inner aspect of the kidney
• It contains the renal pyramids

Kidney Structure

• The renal sinus is the urine drainage area, organized into minor calyces, major calyces, and a renal pelvis
• Each minor calyx is associated with a renal pyramid
• Several minor calyces converge into a major calyx
• The major calyces merge into the renal pelvis, which is continuous with the ureter
• Renal artery, renal vein, lymph vessels and nerves are housed within the space around the renal pelvis

Nephron Structure

• The nephron is the microscopic functional unit of the kidney, mostly residing in the renal cortex
• Each nephron consists of the renal corpuscle and renal tubule
• The renal corpuscle contains the glomerulus and glomerular capsule
• The glomerulus is a tangle of capillary loops where blood enters via an afferent arteriole and exits via an efferent arteriole
• The glomerular capsule (Bowman's capsule) surrounds the glomerulus and receives the filtrate from the capillaries, which is then modified to form urine
• The renal tubule extends from the glomerular capsule in three continuous sections
• Proximal Convoluted Tubule
• Nephron Loop
• Distal Convoluted Tubule

Renal Tubule

• The renal tubule extends from the glomerular capsule and has three continuous sections
• The proximal convoluted tubule originates from the glomerular capsule and has lots of microvilli
• The nephron loop, also known as the Loop of Henle, has two limbs, an ascending and a descending limb, and each limb contains both thick and thin segments
• The distal convoluted tubule begins after the ascending loop and ends at the collecting tubule, but does not contain as many microvilli as the proximal tubule
• Several nephrons drain into each collecting tubule, then empty into larger collecting ducts

Two Types of Nephrons

• Cortical Nephrons have short nephron loops that barely penetrate the renal medulla
• The bulk of cortical nephrons reside in the renal cortex
• 85% of nephrons are cortical nephrons
• Juxtamedullary Nephrons have long nephron loops that extend deep into the medulla, helping establish a salt gradient in the interstitial space outside the nephron loop
• This salt gradient helps regulate urine concentration by antidiuretic

Juxtaglomerular Apparatus

• The JG apparatus contains two types of cells and helps regulate blood filtrate formation and systemic blood pressure
• Granular cells are modified smooth muscle cells of the afferent arteriole near the entrance to the renal corpuscle, contracting when stimulated by stretch or sympathetic stimulation, and synthesize, store, and release renin
• Macula Densa is a group of modified epithelial cells in the distal convoluted tubule where it contacts the granular cells that detect changes in sodium chloride concentration in the tubular fluid

Blood Flow Through the Kidney

• Blood moves through the kidney via specific arteries and veins
• Blood flows into the kidney through interlobar artery, before entering the nephron
• Blood then travels through Interlobular Vein and then returns out of the kidney

Filtrate, Tubular Fluid, and Urine Structures

• Blood flows through the glomerulus
• Both water and solutes filtered from blood plasma occurs, and the filtrate moves across the wall of glomerular capillaries and into the capsular space
• Filtrate then forms
• Tubular fluid in the Proximal Convoluted Tubule
• The tubular fluid flows though

1. Proximal Convoluted Tubule
2. Nephron loop
3. Distal Convoluted Tubule
4. Enters collecting tubules
5. Empties into larger collecting ducts
6. Now called urine

• Urine enters the papillary duct located within the renal papilla and flows within the renal sinus of the kidney
• Urine follows through

1. Minor calyx
2. Major calyx
3. Renal pelvis

Urine Formation

• Urine is formed through three interrelated processes:
• Filtration
• Reabsorption
• Secretion
• Glomerular filtration occurs in glomerular capillaries, separating water and dissolved solutes from blood plasma
• Water and solutes enter the capsular space of the renal corpuscle
• Filtrate is the separated fluid
• Tubular reabsorption is where components within the tubular fluid move by diffusion, osmosis, or active transport from the lumen of tubules and collecting ducts across walls
• The components return to the blood within peritubular capillaries and vasa recta
• All vital solutes and most water are reabsorbed
• Leftover excess solutes, waste products, and some water remain in tubular fluid
• Tubular secretion is the movement of solutes, usually via active transport, out of blood within peritubular and vasa recta capillaries and into tubular fluid
• Materials are selectively moved into tubules to be excreted

Filtration Membrane

• 180 L of filtrate is produced daily, which is filtered plasma with certain solutes and minimal protein
• The filtrate is caught within the capsular space and funneled into the PCT
• Materials that are not filtered remain in blood and exit through efferent arteriole
• Some filtered material is trapped and phagocytized within the basement membrane by mesangial cells
• The three categories of substances in blood are:
• Freely filtered (small substances, easily pass through)
• Not filtered (formed elements and large proteins, cannot pass through)
• Limited filtration (proteins of intermediate size usually blocked)

Pressures Associated with Glomerular Filtration

• Net filtration pressure is calculated if pressures promoting filtration are greater than pressures opposing
• Glomerular hydrostatic (blood) pressure (HPg) is the blood pressure in the glomerulus, which pushes water and some solutes out into the capsular space, and is higher than other systemic capillaries,
• Larger afferent arteriole diameter and smaller efferent arteriole diameter required for filtration to occur
• HPg is opposed by blood colloid osmotic pressure (OPg), the osmotic pressure of dissolved solutes like plasma proteins, which opposes filtration and draws fluid back into the glomerulus
• Capsular hydrostatic pressure (HPc) is pressure in the glomerular capsule, impeding additional fluid movement

Glomerular Filtration Rate

• The rate at which the volume of filtrate is formed is called the Glomerular filtration rate (GFR)
• GFR is the volume per unit of time (usually 1 min)
• GFR is tightly regulated in order to control urine production effectively based on hydration status, etc
• There are multiple things that influence GFR:
• Changing luminal diameter of afferent arteriole
• Altering surface area of filtration membrane
• Processes within kidney itself (intrinsic controls)
• Processes external to kidney (Extrinsic Controls)
• Myogenic responses

Renal Autoregulation

• Increase in blood pressure:
• Narrowed arteriole lumen allows less blood into glomerulus offsetting the higher blood pressure
• Normal systemic blood pressure:
• Normal arteriole lumen has normal systemic pressure
• Decrease in systemic blood pressure:
• Widened arteriole lumen allows more blood into glomerulus offsetting the lower blood pressure

Myogenic Response Limits

• GFR is maintained by Renal Autoregulation
• Normal glomerular pressure between 80-180 mm Hg
• Decreased blood pressure below 80 mm Hg has maximum dilation, decrease glomerular blood pressure and GFR
• If extremely low, no waste elimination in urine
• Increased blood pressure above 180mm Hg has arteriolar maximum constriction, increased glomerular blood pressure and GFR, and increase urine formation

Increased Net Filtration Pressure

• Increases GFR and solutes and water remaining, which then increases substances. ultimately leads to decreased filtrate reabsorption

Transport Maximums

• Transport maximum (Tm) or tubular maximum is the maximum rate of substance that can be reabsorbed or secreted across tubule epithelium per a certain time, depending on transport proteins in membrane
• With no more than 375 mg/min, glucose in tubule all reabsorbed
• If its greater than 375 mg/min, excess glucose excreted in urine
• Renal threshold is the max plasma concentration of a substance that can be transported in the blood without seeing it in the urine
• If Tm exceeded, substance excreted in urine

Clinical View: Glucosuria

• Glucosuria is the excretion of glucose in urine with plasma levels above 300mg/dL
• Glucose acts as an osmotic diuretic
• Pulls water into tubular fluid and cause loss of fluid in urine
• Classic symptom of diabetes, along with frequent urination and thirst

Sodium Reabsorption

• Aldosterone (steroid hormone made by adrenal cortex) stimulates protein synthesis of Na+ channels and Na+/K+ pumps, while embedding in plasma membranes of principal cells for increased Na+ reabsorption, and water follows by osmosis
• Atrial natriuretic peptide inhibits reabsorption of Na+ in PCT and collecting tubules as well as release of aldosterone, with more Na+ and water excreted in urine and increased GFR

Sodium Reabsorption Overview

• 65% of Na+ is reabsorbed in PCT
• 25% of Na+ is reabsorbed in nephron loop
• Regulate percent of Na+ reabsorbed in DCT, CT, and CD

Water Transport Overview

• 180 L of water is filtered daily, all but 1.5 L reabsorbed
• 65% of the filtered water is reabsorbed in PCT by aquaporins constant number where water follows Na+ by osmosis (obligatory water reabsorption), and 10% in Nephron loop
• Reabsorption in DCT and collecting tubules are controlled by aldosterone and antidiuretic hormone

Antidiuretic Hormones

• Water Transport is related to antidiuretic hormones
• Aldosterone increases Na+/K+ pumps and Na+ channels causing water reabsorption
• Antidiuretic hormone (ADH) binds to principal cells and adds channels
• Water reabsorption near the end of tubule is regulated near by this hormone and concentrates urine

Potassium Movement

• Potassium movements is a combination of Reabsorption and secretion
• It is unique because of its process to secrete and reabsorb
  • 60-80% in tubular movement (Na+ Dependent)
  • 10-20% in thick segment loops

Bicarbonate Movement

• Bicarbonate ions, hydrogen ions, and pH are all intertwined
• It moves across filtration membrane
• pH of both urine and blood regulated in collecting tubules -Type B intercalated is active depending on the scenario to either secrete or reabsorb -Increase/decrease H+ to maintain homestasis of urine

Substances Eliminated as Waste

• Nitrogenous waste is the metabolic waste in bodies (Nitrogen)
• There are 4 main sources of these
  • Ure, is reabsorbed and secreted (50% excreted)
  • Uric helps establish the concentration gradient of interstitial fluid
  • *Creatinine helps produce from creatine (Only secreted)

Elimination of Drugs and Bioactive Substances

• Most of secretion comes from drugs and bioactive materials
  • Most secretion occurs within PCT
• It is responsible to clear penicillin, sulfonamides, and aspirin materials
• it eliminates Urobilin and Gonadotropin substances