Urinary System Anatomy

Urinary System Anatomy

• The urinary system comprises the kidneys, ureters, bladder, and urethra
• The kidneys filter blood and produce urine
• The ureters transport urine from the kidneys to the bladder
• The bladder stores urine
• The urethra excretes urine from the body

Learning Outcomes

• Identify urinary tract components including anatomical features of the kidney.
• Identify forces involved in glomerular filtration and explain systems affecting glomerular filtration rate.
• Explain absorptive and secretory processes in the kidney nephron and relate these to nephron functioning.
• Discuss hormonal control of the nephron, including the mechanisms through which hormones change urine composition by affecting absorption and secretion.
• Consider the anatomical features of the urinary bladder and describe the micturition process, explaining nervous system control of the cycle.
• Describe stimuli for hormone release by the kidney, and explain the effects of those hormones.

Renal Anatomy

• The kidneys are a pair of organs located on either side of the spinal column, just behind the lower abdomen
• Kidneys are approximatley 11 cm long, 6 cm wide, and 3 cm thick
• Key components include; the renal artery and veins, the cortex and medulla, a capsule, Medullary pyramid, and the ureter leading to the bladder

Kidney Functions

• Kidneys regulate body fluid osmolality and volume, electrolyte and acid-base balance
• They excrete metabolic products and foreign substances
• Kidneys produce/secrete hormones and perform gluconeogenesis
• Disruptions in electrolyte balance can cause issues in the acid-base balance

The Nephron

• The nephron is the functional unit of the kidney
• Key compartments of the nephron include: the afferent arteriole, glomerulus, Bowman's capsule, proximal and distal tubules, the loop of Henle, the collecting duct, and the renal artery
• Glomerular filtration is the first step in urine formation
• 180 liters of filtrate enter the nephrons each day resulting in the production of 1-2 liters of urine
• 99+% of filtrate is reabsorbed

Ultrafiltration

• Approximately 20% of blood that passses through the glomerulus gets filtered
• Ultrafiltration occurs through pores in the endothelium
• The thick ascending limb of the Loop of Henle produce paracrines to change blood flow within the nephron
• Podocytes have lots of microvilli for surface area, aiding in absorption
• Water always moves passively; negatively charged glycoprotiens and albumin are also key to filtration

Filtration Forces

• Key forces in the filtration process: hydrostatic, colloid osmotic and fluid pressures
• Net filtration pressure is 10 mm Hg; Mean arterial blood pressure should remain between 80-180 mm Hg

GFR

• Autoregulation maintains constant Glomerular Filtration Rate if mean arterial blood pressure is between 80 and 180 mm Hg
• Resistance changes in renal arterioles alter GFR and blood flow
• Increased resistance in afferent arterioles decreases GFR
• Reduced constriction enables increased GFR
• Hormones and the autonomic nervous system also affect GFR
• GFR affected by alterations to surface area available for filtration

Juxtaglomerular Apparatus

• Macula densa cells sense distal tubule flow and release paracrines that alter afferent arteriole diameter

Tubuloglomerular Feedback

• GFR increases, as does flow through the tubule, increasing the liquid flowing past the macula densa
• The macula densa releases paracrine which communicates to the afferent arteriole, causing it to constrict
• Constriction of the afferent arteriole then reduces resistance; The hydrostatic pressure and GFR decrease

Renal Filtration

• Freely filtered molecules depend on size and shape
• Molecules under 7KD can freely enter, whereas ones above 70KD generally cannot
• H2O, Na+, K+, Cl-, HCO3-, Ca 2+, Mg2+, and PO4 etc. are freely filtered
• Proteins are not generally filtered
• Glucose, urea, and creatinine also freely filter through the nephron

Reabsorption

• Occurs in the proximal tubule, which reabsorbs the bulk of filtered solutes and water (first absorption)
• Water and solute reabsorption are coupled, and the process is iso-osmotic
• Brush borders increase surface area, cells contain large numbers of mitochondria to help absorb molecules
• This part of the nephron has relatively high permeability to small solutes to and water and facilitates transport between cells

PCT

• 2/3 of filtrate is reabsorbed, inclucing glucose, amino acids, and other organic solutes
• Significant amounts of phosphate is reabsorbed
• Calcium and water are absorbed in parallel with sodium
• Active secretion of H+ secretion and resorption of bicarbonate
• There is secretion of organic acids like uric acid and drugs like penicillin

Transepithelial Transport

• Water may enter the filtrate from the tubular lumen
• Next it has to travel through the luminal membrane, the epithelial cell, the basolateral side and then the capillary wall to reach the plasma
• Water moves through tight juncitons and interstitial fluid to make this happen

Sodium Reabsorption

• Sodium reabsorption in PCT requires active transport; Plays pivotal role in reabsorption of glucose, amino acides etc
• A Na+/K+ ATPase carrier is required for this, thus high concentrations of Sodium must exist in interstitial fluid than in the cell or the tubular lumen

PCT Ion Reabsorption

• Electrical gradients drive Cl reabsorption across
• H2O follows Na+ due to osmotic force
• Fluid left in the tubule is concentrated

Transporters

• Molecules/ions are reabsorbed using transporter proteins, each has a maximum transport (Tm) capactiy
• Over Tm, you excrete excess in urine
• Amino acids have a high Tm value because the kidney perseverves these nutrients
• Tm capacity for glucose is 10mmol/l, the high glucose levels lead to excretion

Loop of Henle

• Comprises descending and ascending limb, macula densa and absorbs 15% of volume and 25% of all sodium chloride
• Facilitates differential absorption of water and sodium chloride, which enables the loop to concentrate or dilute urine
• Calcium is absorbed with a thick ascending limb, and is medullary or cortical regulated.
• Thick descending limb absorbs water passively, while the thick ascending limb transports sodium chloride
• Cortex has 300 mOsm and medulla = 1200 mOsm/kg H2O

Ascending and Descending Limbs

• The descending limb is permeable to H2O
• The thin ascending limb is permeable to NaCl
• The thick ascending limb pumps NaCl
• The function concentrates

Nephron Types

• Deep in medulla = short in medulla
• Two types of nephrons: juxtamedullary and cortical

Thick Ascending Limb

• Sodium potassium ATPases help drive movement of ions through the tubule, ultimately into the vascular system
• Key ions include Sodium and Chloride, and Potassium

Osmolality

• Loop of henle creates the hypertonic medullary intersitium

Ascending Limb

• Na is actively transported in thick ascending limb
• Passive NacL leak from thin ascending limb drives this
• Water can freely move here depending on concentration levels

Distal Nephron

• Comprises the Distal convoluted tubule and the collecting ducts; important for the regulation reabsorption and secretion
• The brush border is less prominent than in the PCT and they have tighter tight junctions that contain receptors for hormones;
• Epithelial cells have fewer mitochondria than PCT cells
• This section controls water volume and several solutes; Water reabsorption here does not always follow water absorption

The DCT

• Cortex has better water impermeability
• Active Na+ absorption is important
• K+ and H+ secretion occur and Calcium absorption is under hormonal influence
• Distal nephron reabsorbs sodium and secretes H+ and K+ under influence of aldosterone and generates new bicarbonate for acid- base balance

DCT Ion Absorption

• Sodium absorption and potassium secretion are regulated by electrochemical gradients

Role of Urea

• Protein breakdown leads to the release of amino groups, then ammonia production, then finally urea creation
• Helps maintain high osmolality gradient within medullary region, and ultimately drives reabsorption

Tubular Urea Volumes

• 50% of tubular urea gets reabsorbed
• Anti-diuretic hormones can help kidneys recycle water and excrete any urea that remain
• Final gradient can exist at 110% of original filtered concentration

NaCI

• Na+ and Cl account for >90% of the ECF's osmotic activity that reflects load the body volume
• Extra Na+ holds extra H2O, which increases ECF volume
• An increased EFC results in increased blood pressure

Na+ Control

• To retain sodium we need to detect if it is required, have areas within nephron where you can alter the absorption, signal to these areas and respond
• The response to the volume is to produce the desired effect: increased blood pressure
• Decreased circulating volume stimulates renin release via the juxtaglomerular apparatus
• Sympathetic nervous system stimulation and decreased filtrate osmolality and stretch also drive that

How to retain fluid

• This leads to increased vasoconstriction, Angiotensin II
• The Liver, lungs, adreanl glands, kidneys and hypothalmus all communciate via fluid and hormones to increase renin and thirst

Angiotensin II

• Low blood pressure or blood volume promote Angiotensin II release in plasma after JG kidney signaling
• The blood vessels will contrict due to cardiovascular response
• The brain will create the sensation of thirst via vasopressin

Aldosterone

• The distal nephron is the target of the hormone aldosterone
• New aquaporins and channels are formed to help drive reabsorbtion and retention

Water Retention

• Stimulation of the hypothalamus leads to greater feelings of thirst
• The kidney may also retain water, which means hormones like vasopressin must increase epithelium permeability to fluid

Vasopressin

• H2O does not automatically follow Na+ reabsorption
• Vasopressin required to make epithelium permeable
• Vasopressin increases aquaporins which make epithelium cells
• Vasopressin enables osmotic balance

Hypertonic Effect

• Medulla enables a hypertonic force, which facilitates concentrated urine release

Water Reabsorption

• Maximal vasopressin promotes permeable water movement in collecting duct, enabling osmosis
• In vaspressin abscence collecting duct becomes impermeable to h2o
• Factors such as decreased atrial stretch increases vasopressin release

Water Control

• Body volume can be regulated via multiple mechanisms: Atrial Natriuretic Peptide (stimulus), effector vasopressin (tissue response)

Diuresing

• Control of Na+ & H2O Excretion is Regulated by Atrial Natriuretic Peptide (ANP)

Bladder

• Typically holds 300-550 ml of urine and efferent nerves signal need to void around 400 ml

Nerves role

• Sympathetic nerves contract the internal urethral sphincter -Parasympathetic nerves contract detrusor muscles
• The somatic nerve is voluntary

Kidneys role

• The kidneys also secrete erythropoietin (EPO) that gets released when key blood stimuli get registered

EPO

• Hypoxia from reduced RBC's enables kidneys to release hormones that stimulate: erythropoiesis (↑ RBC in circulation) & greater oxygen carrying capability in blood (+02)
• Erythropoietin facilitates increased synthesis on EPO and red bone marrow
• End-stage failure reduces renal function, but these can be resolved via treatments such as transplant

How EPO effects synthesis

• The EPO gene sequences promotes oxygen sensitivity and facilitate regulation of EPO and gene expression in cells
• This increases erythroid levels which lead to a larger red cell mass
• This process also requires iron

Dietary impact

• EPO requres; Iron, Vitamin B12 and Follic acid
• Anemia may be caused by iron deficiency
• Iron moves throughout both red and white blood cells