Urinary System: Functions and Anatomy

Urinary System Overview

• Consists of the kidneys, ureters, bladder, and urethra
• Maintains fluid homeostasis by managing the volume and composition of fluid reservoirs like blood

Kidney Functions

• Regulates blood ionic composition specifically Na+, K+, and Cl-
• Regulates blood pH, H+, and HCO3-
• Regulates blood volume, H₂O
• Regulates blood pressure
• Maintains blood osmolarity
• Produces hormones like calcitriol and erythropoietin
• Excretes metabolic wastes and foreign substances like drugs or toxins
• Regulates blood glucose level

Renal Anatomy

• The kidneys are retroperitoneal and partly protected by the lower ribs
• The indented area of each kidney is called the renal hilum, which allows entrance for the renal artery, renal vein, ureter, nerves, and lymphatics

External Layers of Kidney (Superficial to Deep)

• Renal fascia anchors to other structures
• Adipose capsule protects and anchors
• Renal capsule is continuous with the ureter

Internal Renal Anatomy

• Renal cortex is the outer layer
• Renal medulla is the inner region
• Renal pyramids are the secreting apparatus and tubules
• Renal columns anchor the cortex

Blood and Nerve Supply to the Kidneys

• Kidneys receive 20-25% of resting cardiac output, despite making up less than 0.5% of total body mass.
• Renal nerves primarily carry sympathetic outflow that regulate blood flow in the kidneys

Nephron Structure

• Renal corpuscle consists of the glomerulus and glomerular (Bowman's) capsule
• The glomerulus is a mass of capillaries fed by the afferent arteriole, draining into the efferent arteriole
• Bowman's capsule has a visceral layer of podocytes that wrap around the capillaries, collecting filtrate between visceral and parietal layers

Renal Corpuscle Filtration

• Glomerular endothelial cells contain large pores (fenestrations) and are leaky
• The basal lamina lies between the endothelium and podocytes
• Podocytes form pedicels with filtration slits between them

Cortical Nephrons

• Cortical nephrons comprise of 80-85% of nephrons
• They have renal corpuscles in the outer cortex
• Short loops of Henle extend only into the outer region of the medulla
• They produce urine with osmolarity similar to blood

Juxtamedullary Nephrons

• Renal corpuscles are deep in the cortex with long nephron loops
• Receives blood from peritubular capillaries and the vasa recta
• Ascending limb has thick and thin regions
• Enables the kidney to secrete dilute or concentrated urine

Juxtaglomerular Apparatus

• The ascending loop contacts the afferent arteriole at the macula densa
• The wall of the arteriole contains smooth muscle cells called juxtaglomerular cells
• This apparatus regulates blood pressure in the kidney with the autonomic nervous system

Glomerular Filtration

• Glomerular Filtration is the first step in Urine Formation
• It is the process where blood plasma and dissolved substances smaller than most proteins are filtered into the glomerular capsule
• Excretion = Glomerular Filtration + Secretion - Reabsorption
• Glomerular filtration is driven by blood pressure and opposed by capsular hydrostatic pressure and blood colloid osmotic pressure
• The Glomerular Filtration Rate (GFR) equals 125mL/min in males and 105mL/min in females

Glomerular Filtration Rate Regulation

• Renal Autoregulation is completed by way of the Myogenic mechanism and Tubuloglomerular feedback
• Neural Regulation is completed by sympathetic stimulation of nerves
• Hormonal Regulation is completed by Angiotensin II and Atrial natriuretic peptide (ANP)

Tubular Reabsorption and Secretion

• Much of the glomerular filtrate is reabsorbed, especially water, glucose, amino acids, and ions
• Secretion helps to manage pH and rid the body of toxic and foreign substances

Composition of Filtrate, Plasma and Urine

• Water is filtered at 180 liters, reabsorbed at 178-178.5 liters, and secreted at 1.5-2 liters
• Proteins are filtered at 2.0 g, reabsorbed at 1.9g and secreted at 0.1g
• Sodium ions are filtered at 579g, reabsorbed at 575g, and secreted at 4g
• Chloride ions are filtered at 640g, reabsorbed at 633.7g, and secreted at 6.3g
• Glucose is filtered and reabsorbed at 162g, with no secretion
• Urea is filtered at 54g, reabsorbed at 24g, and secreted at 30g
• Creatinine is filtered and secreted at 1.6g, with no reabsorption

Reabsorption Routes

• Paracellular reabsorption occurs via passive fluid leakage between cells
• Transcellular reabsorption happens directly through the tubule cells

Transport Mechanisms for Reabsorption

• Primary active transport utilizes ATP
• Secondary active transport is driven by the ion’s electrochemical gradient, can be Symporters which move substances in the same direction, or Antiporters which move substances in opposite directions

Water Reabsorption

• Obligatory water reabsorption makes up 90% and follows solutes
• Facultative water reabsorption makes up 10%, is regulated by ADH

Reabsorption and Secretion in PCT

• Na+-glucose symporters
• Na+-H+ antiporters
• Aquaporin-1: membrane protein permeable to water

Reabsorption in the Loop of Henle

• Relatively impermeable to water, specifically the ascending limb
• Little obligatory water reabsorption
• Na+-K+-2Cl- symporters

Reabsorption in early DCT

• Na+-Cl- symporters reabsorb ions
• PTH stimulates reabsorption of Ca2+ and inhibits phosphate reabsorption

Late DCT and Collecting Duct

• Principal cells reabsorb Na+ through Na+-K+ pumps and water through Aquaporin-2 stimulated by ADH
• Intercalated cells reabsorb K+ and HCO3−, and secrete H+

Regulation of Water Reabsorption by ADH

• ADH increases water reabsorption in the kidneys
• This occurs by making principal cells more permeable to water, which in turn increases facultative water absorption

Factors in Hormonal Regulation of Tubular Reabsorption and Secretion:

• Angiotensin II stimulates activity of Na+-H+ antiporters in proximal tubule cells to increase reabsorption of Na+ and water
• Aldosterone enhances activity of sodium-potassium pumps/Na+ channels to increase reabsorption of Na+ and water and increases secretion of K+
• ADH stimulates insertion of water channel proteins (aquaporin-2) to increase facultative water reabsorption
• ANP suppresses reabsorption of Na+ and water, increasing their excretion
• Parathyroid hormone (PTH) stimulates opening of Ca2+ channels to increase reabsorption of Ca2+

Urine Production

• High intake of fluid results in dilute urine of high volume
• Low intake of fluid results in concentrated urine of low volume
• Urine concentration varies with ADH

Formation of Dilute Urine

• Glomerular filtrate and blood have the same osmolarity at 300mOsm/Liter
• Tubular osmolarity changes due to a concentration gradient in the medulla
• When dilute urine is formed, osmolarity in the tubule Increases in the descending limb then decreases again in the ascending limb and collection duct.

Formation of Concentrated Urine

• Juxtamedullary nephrons with long loops that create an osmotic gradient using the countercurrent multiplier
• Solutes are pumped to the ascending limb but the water stays in the tubule
• Medulla osmolarity is increased

Countercurrent Exchange

• Loop and duct cells require nutrients and oxygen from blood supply
• Capillaries (vasa recta) form loops similar to nephron loops
• Incoming and outgoing blood have similar osmolarity to maintain the medulla concentration gradient

Characteristics of Urine

• The regular volume of urine is one to two liters in 24 hours
• Urine is yellow or amber, with color varying from urine concentration ad diet
• Color is due to urochrome and urobilin, with concentrated urine being darker in color
• Urine is transparent when freshly voided; becomes turbid (cloudy) on standing
• Urine has a mildly aromatic odor, becoming ammonia-like on standing
• pH ranges between 4.6 and 8.0, with high-protein diets increasing acidity and vegetarian diets increasing alkalinity
• Specific gravity ranges from 1.001-1.035, depending on the concentration of solutes

Abnormal Constituents in Urine

• Albumin indicates increased permeability of filtration membranes, increased blood pressure, or irritation of kidney cells.
• Glucose (glucosuria) indicates diabetes mellitus
• Red blood cells (hematuria) generally indicates some pathological condition
• Ketone bodies (ketonuria) may indicate diabetes mellitus, anorexia, or starvation
• Bilirubin (bilirubinuria) indicates abovenormal level of bilirubin
• Urobilinogen (urobilinogenuria) may be due to hemolytic/pernicious anemia, or some other infection/disease
• Casts are name for the cells or substances that compose them. or by their appearance
• Unusual bacteria indicate urinary tract infections

Micturition

• The discharge of urine involves voluntary and involuntary muscle contractions
• Stretch receptors trigger a spinal reflex, which we learn to control in childhood
• The urethra carries urine from the internal urethral orifice to the other side of the body
• Males discharges semen as well as urine too in this process

Summary of Urinary System Organs

• Kidneys are solid, reddish, bean-shaped organs that regulate, synthesizes and excrete waste products in urine
• Ureters are thick, muscular walled tubes that transport urine from the kidneys to the bladder
• The Urinary bladder is a hollow, distensible organ that is a storage organ for the urine produced
• Finally the Urethra is a thin-walled tube that drains the urine to the exit point

Renal Procedures

• Kidney transplant - where you will receipt of kidney from healthy donor
• Cytoscopy is where direct examination of urethra mucosa and urinary bladder

Aging and the Urinary System

• Kidneys shrink in size, losing 1/3 their mass
• Decrease in blood flow occurs as vessels are damaged or decrease in number
• A decrease in filtration results in a decrease in GFR