Urinary System: Anatomy, Function, and Control

Learning Outcomes

• You should be able to identify parts of the urinary tract.
• Recognize anatomical features of the kidney.
• Understand forces involved in glomerular filtration.
• Be able to explain systems affecting glomerular filtration rate.
• Know about absorptive and secretory processes in the kidney nephron.
• You should be able to relate the effective functioning of the nephron.
• You should also be able to discuss hormonal control of the nephron.
• Including hormone mechanisms that alter absorption and secretion.
• Influencing urine composition.
• You must consider anatomical features of the urinary bladder.
• Include micturition process.
• Be able to explain how the nervous system controls the cycle.
• Know the stimuli for the release of hormones by the kidney.
• Be able to explain the effects of kidney hormones.

Anatomy

• The urinary system consists of kidneys, ureters, bladder, and urethra.
• Micturition involves a filling phase and an emptying phase.

Kidneys

• Kidneys regulate body fluid osmolality and volume.
• They regulate electrolyte and acid-base balance.
• Kidneys excrete metabolic products and foreign substances.
• They produce and excrete hormones, and are gluconeogenic.
• Kidneys changes in electrolytes leads to disruptions in acid-base balance.

Renal Anatomy

• Kidneys are a pair of organs located on either side of the spinal column.
• They are behind the lower abdomen.
• Kidneys measure around 11 cm in length, 6 cm wide, and 3 cm thick.
• The cortex is the outer layer.
• The medulla is inner.
• The pelvis of the ureter leads to the ureter.
• The renal artery and veins provide blood supply.
• Medullary pyramids take materials stored to the bladder.

The Nephron

• The functional unit of the kidney is the nephron.
• Key parts of the nephron include afferent arteriole, glomerulus, Bowman's capsule, proximal tubule, distal tubule, collecting duct, renal artery, Loop of Henle and Vasa Recta.

Glomerular Filtration

• 180 liters of filtrate enter nephrons daily.
• 1-2 liters of urine get produced.
• Filtrate (99+%) is reabsorbed.
• 20% of cardiac output goes to kidneys.

Structural Considerations

• Key parts contributing to structural considerations include the afferent arteriole of capillary network, Bowman’s Capsule, Capsular epithelium, podocyte, lumen of Bowman's capsule, glomerular capillary, thick ascending limb of loop of Henle, efferent arteriole and proximal tubule.
• The thick ascending limb of the loop of Henle produces paracrines to change the blood flow, and it alters the flow to the nephron.
• Podocytes have microvilli for absorption.

Ultrafiltration

• Approximately 20% of the blood passing through the glomerulus is filtered.
• Filtration depends on size.
• Water always moves passively.
• Pores exist in the endothelium.
• Negatively charged glycoproteins and albumins are part of the capillary lumen.

Forces that Influence Filtration

• Filtration hydrostatic pressure (PH) in blood pressure.
• Colloid osmotic pressure (π) occurs due to proteins in plasma, but not in Bowman’s capsule.
• Fluid pressure (Pfluid) is created by fluid in Bowman’s capsule.
• Net filtration pressure is equal to PH - π - Pfluid.
• Net Filtration pressure equals 10 mm Hg, as calculated by 55 mm Hg – 30 mm Hg – 15 mm Hg.
• Protein concentration increases as blood flows through capillaries.

Autoregulation of GFR

• The autoregulation of glomerular filtration rate (GFR) occurs when the mean arterial blood pressure is between 80 and 180 mm Hg.

Resistance Changes in Renal Arterioles

• Resistance changes in renal arterioles alter GFR and renal blood flow.
• An initial situation, blood flows to other organs.
• Constricting resistance in the afferent arteriole during exercise restricts entry to glomerulus.
• Increased hydrostatic pressure, decreased renal blood flow, and decreased GFR will result.

GFR regulation

• GFR can be increased or decreased.
• It can be increased by dilating afferent arteriole and constricting efferent arterioles.
• Myogenic response, similar to autoregulation in systemic arterioles, is one method of GFR regulation.
• Other mechanisms include tubuloglomerular feedback, hormones and autonomic neurons.
• GFR can be changed by altering resistance in arterioles and filtration coefficient.

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus comprises a close association with Bowman's capsule.
• Macula densa cells can sense distal tubule flow and release paracrines.
• Paracrines released affect the afferent and efferent arteriole diameters.
• Key parts include the afferent and efferent arteriole, ascending limb of Loop of Henle, distal tubule, granular cells, macula densa, proximal tubule, glomerular capillaries and Bowman's capsule.

Tubuloglomerular Feedback

• GFR increases.
• Flow through tubule increases.
• Flow past macula densa increases.
• Paracrine diffuses from macula densa to afferent arteriole.
• Afferent arteriole constricts, resulting in decreased blood entering glomerulus.
• Resistance in afferent arteriole increases.
• Hydrostatic pressure in glomerulus decreases, which reduces GFR.

Glomerular Filtrate

• Substances can be freely filtered depending on molecular size and shape.
• Freely filtered substances have a size of <7KD.
• Includes H2O, Na+, K+, Cl-, HCO3-, Ca2+, Mg+, PO4, etc.
• The large molecules like proteins are not usually filtered.
• Large molecules have a size of ≤70KD, such as immunoglobulins and ferritin.
• Cells are also unable to be filtered.
• Inulin gets used artificially to look at kidney functioning.

Reabsorption in the Proximal Tubule

• The proximal tubule is the site of first absorption.
• It functions as a mass absorber, reabsorbing bulk of filtered solutes and fluid.
• The processes in the proximal tubule are iso-osmotic.
• Brush border increases surface area.
• Transporting cells contain many mitochondria.
• This supplies the energy for active transport and secondary active transport.
• Tight junctions facilitate transport of small solutes and water between cells (paracellular transport), by not being very tight.

Proximal Tubule Specifics

• Two-thirds of filtrate is reabsorbed.
• Glucose, amino acids, and other organic solutes are completely absorbed.
• Significant amounts of phosphate gets reabsorbed.
• Calcium and water are absorbed in parallel with sodium.
• Secretion of H+ occurs.
• Process supports resorption of bicarbonate.
• Secretion of organic acids, such as uric acid and drugs.
• For example, penicillin.

Transepithelial Transport

• Steps involve tubular lumen, luminal membrane, filtrate, basolateral membrane, interstitial fluid, passage between cells, capillary wall.
• It includes Bowman's capsule.

Sodium Reabsorption in PCT (Proximal Convoluted Tubule)

• Sodium reabsorption in PCT utilizes Na+ - K+ ATPase carriers.
• Process plays a pivotal role in re-absorption of glucose, amino acids, water, and Cl- plus others.
• Active transport carries sodium.
• A tubular epithelial cell, interstitial fluid, and peritubular capillary are involved.
• Sodium concentration in tubule is relatively low, due to sodium removal.

More on Sodium Transport

• Nat into the cell is largely active diffusion.
• Potassium is also moved.
• Sodium diffusion occurs into cell.
• This provides energy.
• Pumps will contribute to the entry via ATP energy, which favors entry into the capillary.

Sodium Absorption

• Electrical gradient created also draws Cl across.
• H2O follows Na+ due to osmotic force.
• Fluid left in tubule is concentrated.

Potassium Reabsorption in PCT

• Potassium reabsorption in PCT is largely passive with paracellular absorption.
• Potassium transport follows the movement of Na+ and fluid
• Potassium circulates through the pump.
• Chloride channels contribute to the transportation.

High Intracellular Potassium

• The high intracellular levels are not lost into the lumen, but transported back into the interstitial fluid.

Bicarbonate Absorption

• Bicarbonate absorption (HCO3-) is important for the regulation of the pH of the body.
• It also allows for adjustment by the kidneys.

Bicarbonate Reabsorption in PCT

• Na+/H+ exchanger is involved.
• Also carbonic anhydrase (CA).
• Basolateral Na+- HCO3- co-transporter also participates.

Principal Sodium Transport

• Mechanisms summary in the proximal convoluted table.
• Sodium-coupled co-transporters are involved.
• A sodium-H+ exchanger is also involved.
• Apical membrane, lumen, and basolateral membrane are involved.

Absorption in tubules

• In electrical gradient also draws Cl- across.
• H2O follows Na+ due to osmotic force.
• The fluid that's left in the tubules is concentrated.

Properties of Molecules

• Molecules/ions are reabsorbed using transporter proteins.
• All have maximum transport capacities (Tm) where transport saturates.
• Above a certain saturation value, the excess is excreted in urine.
• Valuable indications of disease.
• Amino acids also have a high T™ value because human bodily systems try to preserve as much of the nutrients as possible.

Loop of Henle

• Comprises ascending and descending limb, macula densa.
• 15% of volume absorption occurs here.
• 25% of sodium chloride absorption occurs.
• The differential absorption of water and sodium chloride enables the loop to concentrate or dilute urine.
• Ca2+ is absorbed in thick ascending limb.
• The medullary part is not regulated by parathyroid hormone (PTH).
• The cortical part of the loop is regulated by PTH.
• Passive absorption of water from descending limb occurs.
• Active absorption of sodium from thick ascending limb occurs maintaining interstitial hypertonicity.
• Cortex has an osmolality of 300 mOsm.
• Medulla has an osmolality of 1200 mOsm/kg H2O.

Nephrons

• There are two kinds of nephrons, one deep in the medulla, and a short one in the medulla.

Properties of the Loop

• The descending limb is permeable to water.
• The thin ascending limb is permeable to NaCl.
• The thick ascending limb pumps NaCl.
• The loop concentrates.

Thick Ascending Limb

• Sodium / Potassium ATPase is here.
• It allows absorption between interstitial fluid and basolateral membrane.

Osmolarity

• As the descending limb is permeable to water, water leaves so concentration in tubule increases.

More Details on Transport in the Loop

• Active Na+ transport occurs in the thick ascending limb.
• Passive NaCl leakage from the thin ascending limb occurs.

Osmolality

• Osmolality varies throughout the kidney nephron.
• The concentrated part is the hypertonic region.

Distal Nephron

• The distal nephron comprises the distal convoluted tubule and the collecting ducts.
• It is specialized to allow regulation of reabsorption and secretion.
• Brush border is less prominent than in the PCT, with fewer mitochondria.
• Tight junctions are "tight.”
• Distal nephron contain receptors for hormones that regulate solute transport.
• Water reabsorption does not always follow solute absorption.

Distal Convoluted Tubule

• The DCT in cortex is water impermeable during times of high water retention.
• It contains Active Na+ absorption, K+ and H+ secretion.
• Calcium absorption happens under the influence of parathyroid hormone (PTH).

Collecting Ducts

• Primary site of ADH (antidiuretic hormone / vasopressin) action occurs here.
• Water absorption influenced by ADH and (medullary hypertonicity) occurs here.
• Na+ is reabsorbed in the DCT where it is influenced by aldosterone.
• New bicarbonate is generated for acid-base imbalance.

Sodium Absorption

• Potassium(K+) secretion is largely passive and follows the movement of Nat and fluid.
• The electrolytes move in and out based on the electrical symporter.
• High Nat levels are in the interstitial fluid.

Water

• Sodium absorption and potassium secretion occur in the distal convoluted tubule (DCT).
• Also, water crosses into extracellular space.

DCT

• The high intracellular K+ levels are lost into the distal convoluted tubule (DCT) lumen through leak channels on the luminal membrane.

Later Na+ absorption

• Sodium absorption later in the distal tubule depends on symporters for Nat but there is also an ion channel.

Importance of Urea

• Protein breakdown produces ammonia which turns into urea.
• It helps to maintain the high osmolarity gradient within the medullary region.
• It is criticaly in the kidney to drive water reabsorption.

Urea

• Some 50% of the filtered urea is reabsorbed.
• If urea concentration still has 110% or original filtered conc.
• Antidiuretic hormone (ADH) assists in absorption of water, which into vasa recta.
• The "high" (lol) urea cycle keeps urea concentration high, and 40% of urea is excreted.

More on Urea

• Sodium and chloride account for >90 % of the ECF's osmotic activity.
• The Nat load in the body is reflected by the ECF's volume.
• When Na+ load is above normal the ECF's osmotic activity is increased.
• The extra Nat holds extra H2O expanding the ECF volume.
• When Na+ load is below normal the ECF osmotic activity is reduced.
• Plasma is part of the ECF.
• Change in ECF volume results and relates to the matching volume of plasma.
• Leads to an increase or decrease in blood pressure.

ECF

• Regulating on ECF volume with Na+ either has high, normal or low levels.
• Low volume impacts blood pressure.

Control/Regulatory mechanisms

• Influences events in the distal nephron.
• Allows for the production of concentrated or dilute urine.
• Influences extracellular fluid volume.

Blood Volume

• If blood volume is too low, water excretion will decrease.
• Increase NaCl absorption should increase blood volume.

NaCl absorption

• You have to detect it, have areas where you can alter absorption, signal the areas and respond to the stimulus.

Retaining Sodium

• Decreased circulating volume stimulates renin release via the juxtaglomerular apparatus.
• Juxtaglomerular apparatus releases renin in response to sympathetic nervous system stimulation.
• Also by decreased filtrate osmolality and decreased stretch (due to decreased blood pressure).

More on increased Sodium

• Effective circulating volume exists when the ACE and aldosterone contribute.
• Increased renal sodium retention decreases effective circulating volume.

Angiotensin

• JG cells produce renin.
• Angiotensin constricts, relates cardiovascular response, and also increases thirst.
• Also increases sodium reabsorption.

Aldosterone

• Aldosterone increases sodium reabsorption into blood stream.

Preventing Water loss

• To increase sodium chloride absorption, also, you need to stimulate the hypothalamus to cause thirst,
• Also you must add and add water to the body.
• And retain water already in the body.

Water Retention

• Water retention and conservation
• Water in the DCT
• Vasopressin must be present in the epithelium to make it permeable to H2O.
• Vasopressin promotes water retention to maintain ECF volume and reduce plasma osmolarity.

Effect of hypertonic medulla

• Closer to the cortex is H2O while deep in the medulla the concentration in the collecting duct increases.

Hypertonic Medulla

• The concentrate is higher towards the medulla.
• In the absence of sufficient sodium diffusion, urine will be diluted,

Lack of Diffusion

• No diffusion then the urine will be diluted,

Water Reabsorption

• Water in the absence of vasopressin in the collecting duct is impermeable to H2O.
• More fluid moves with the active transport
• Reabsorption rate increases with the water

Vasopressin

• Stimulus for secretion: ↓ Atrial stretch ↓ Osmolarity

• Vasopressin in released from the posterior pituiary gland.

                                                          reabsorption
  

ADH

• There are increasing returns for plasma osmolarity.
• More water from less sodium

Blood Volume

• If volume is too high. Increase excreation is required to maintain homeostatic

Atrial Natriuretic Hormone

• Increase blood

• Increase Stetch

  1. Inhibits vasopressin
  2. Increase GFR
  3. Less aldosterone
  4. blood pressure levels lower or stay about same

Sodium and Water Balance

The Need 4 more Na+ Macula Dena +Low Levels Deteched Lower Bp - Lower Stretch of Blood Vessel Stimulation of Sympathic Nervous system for vasoconstrict. Low Frate

The Urinary Bladder

• The urinary bladder holds around 300 – 550 ml of urine.
• People typically void at 400 ml.
• The muscles of the uretha and the pelvic cavity control flow.

Micturition cycle

- The afferent have an expansive bladder

signals

    - Efferent nerves signel when to stay shout, to
      continue  expanition
        Pudendal  Hypognatric Nerve

Nerves

The nerves are in control for if the urtheal is contracted or relaxed 1 Internal 2 External

Another Role for the Kidneys

• The kidneys also work to secrete hormones.
• Erythropoietin (EPO) gets released by the kidneys.

Erythropoietin

• Release of Eryth from kidney is due to:
  • Hypoxia due to low RBC
  • Low oxygen levels
  • Tisuue needing more Eryth increases more RBC and Ability to cary Oxygen in the Blood due to high need.

Hypoxia

• Causes Red bone marrow to increase RBC

  1. EPO causes high production of RBC

2. Bone is the cause of the inproment

Produciing EPO

1 The Kid are mostly all Renal Parenchyma but also the bass membrane. - EPO prodcues mostly in the innner cortex.

2 Hypo -Increases 3 It also increase Deno

• There is also of Detectoble Hormone withing the cells.

How they cause more EPO production

i gene contain sequence - the EPOrs activate intracellular signel to immune

• May also promote profteration.
  

  Hormone and

Result

increase Cell