Urinary System Anatomy and Function

Urinary System

• The presentation outlines the structure and function of the urinary system.

Learning Outcomes

• The series of lectures aims to cover the identification of urinary tract parts, including kidney anatomy.
• Accurately identifying forces in glomerular filtration, and explaining related systems is a key goal.
• Another aim is explaining the absorptive and secretory processes in different parts of the kidney nephron, and relating this to the nephron's effective functioning and discussing hormonal control of the nephron, including how hormones change absorption and secretion, influencing urine composition is a key goal
• Anatomical features of the urinary bladder, the micturition process and the nervous system controlling the cycle will be covered.
• Also, stimuli for hormone release by the kidney, and hormone effects will be explained.

Anatomy

• The urinary system consists of the kidneys, ureters, bladder, and urethra.
• The micturition cycle has a filling phase (urine storage) and an emptying phase.

Kidneys

• Kidneys are the primary organs discussed.

Kidney Functions

• The kidneys regulate body fluid osmolality and volume, electrolyte balance, and acid-base balance.
• Kidneys excrete metabolic products and foreign substances and produce and excrete hormones.
• Also, the kidneys perform gluconeogenesis (cortex function).

Renal Anatomy

• The kidneys are paired organs located on either side of the spinal column, behind the lower abdomen.
• A typical kidney is about 11 cm long and 6 cm wide, with a 3cm thick cortex.
• The kidney structures include the renal artery, renal veins, pelvis of the ureter, ureter, capsule, cortex, medulla, and medullary pyramid.
• The medullary pyramid collects materials stored for the bladder.

Nephron

• The nephron is the functional unit of the kidney.
• The nephron components are the afferent arteriole, glomerulus, Bowman's capsule, proximal tubule, distal tubule, collecting duct, renal artery, loop of Henle, and vasa recta.

Glomerular Filtration

• It begins with glomerular filtration.
• 180 liters of filtrate enter the nephrons daily, producing 1-2 liters of urine, with 99+% of the filtrate being reabsorbed.
• Approximately 20% of cardiac output goes to the kidneys.

Glomerular Filtration Structure

• Important structural considerations include the thick ascending limb of the loop of Henle, afferent and efferent arterioles, Bowman's capsule, capsular epithelium, podocytes, proximal tubule, glomerular capillary, and the lumen of Bowman's capsule.
• Paracrines produced in this process change blood flow to the nephron.

Ultrafiltration

• Ultrafiltration of the blood involves approximately 20% of the blood passing through the glomerulus being filtered.
• Water always moves passively.
• Filtration structures include pores in the endothelium, capillary lumen, foot process of podocyte, filtration slit, basal lamina, and the lumen of Bowman's capsule.
• Filtered material size determines filtration.
• Glycoproteins and albumin are also factors in the process.

Filtration Forces

• Filtration is influenced by hydrostatic pressure (blood pressure), colloid osmotic pressure (due to proteins in plasma but not in Bowman's capsule), and fluid pressure (created by fluid in Bowman's capsule).
• Net filtration pressure (overall pressure) is calculated using these forces.

Autoregulation

• Autoregulation of glomerular filtration rate (GFR) occurs over a wide range of blood pressures.
• Autoregulation maintains a constant GFR when mean arterial blood pressure is between 80 and 180 mm Hg.

Afferent/Efferent Arterioles

• Changes in renal arterioles alter GFR and renal blood flow (RBF).
• Increased resistance in the afferent arteriole decreases GFR. The hydrostatic pressure causes a renal blood flow to the glomerulus which then goes to the Bowman's capsule, thus increasing the GFR
• Constricting efferent arterioles due to exercise can lead to decreased GFR.

GFR Regulation

• GFR can be increased and is impacted by myogenic response, tubuloglomerular feedback, hormones, and autonomic neurons.
• Changing resistance in arterioles, and altering the filtration coefficient can affect GFR.

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus's role in controlling the afferent arteriole's diameter allows control in the kidneys.
• The macula densa cells sense distal tubule flow and release paracrines affecting the afferent arteriole diameter.

Tubuloglomerular Feedback

• GFR increases, leading to increased flow through the tubule and past the macula densa.
• Paracrine diffuses to the afferent arteriole, causing constriction, thus decreasing GFR. A low hydrostatic pressure in the glomerulus decreases the GFR

Glomerular Filtrate

• Freely filtered substances depend on molecular size and shape; those <7KD are easily filtered up to ≈70KD.
• Freely filterable substances include H2O, Na+, K+, Cl-, HCO3-, Ca2+, Mg+, and PO4, and others are glucose, urea, creatinine, and inulin.
• Not easily filtered are Large molecules ≤70KD, proteins and Cells.

Reabsorption

• Reabsorption occurs in the proximal tubule and is the first absorption stage.
• The proximal tubule is a mass absorber, reabsorbing most filtered solutes and fluid.
• Solute and water reabsorption are coupled and iso-osmotic.
• Transporting cells contain many mitochondria and tight junctions with high permeability to small solutes and water, facilitating paracellular transport.

Proximal Tubule

• Approximately 2/3 of the filtrate is reabsorbed in the proximal tubule.
• Glucose, amino acids, and other organic solutes are completely absorbed.
• A significant amount of phosphate is reabsorbed.
• Calcium and water are absorbed along with sodium.
• Active secretion of H+ and secretion of organic acids such as uric acid and drugs like penicillin also occur.

Transepithelial Transport

• Transepithelial transport steps involve movement through the tubular lumen, luminal membrane, filtrate, capillary wall, tight junction, basolateral membrane, interstitial fluid, and plasma.

Sodium Reabsorption

• Plays a pivotal role in reabsorption of glucose, amino acids, water, and Cl- plus others.

Na+ Reabsorption in PCT

• Relative to the Na+ concentration inside the cells, a Na+ - K+ ATPase carrier causes relative differences with K+ high inside, and Na+ high outside.
• Movement of Na+ into the cell involves an active diffusion.
• The concentration of Na+ in the interstitial fluid is high and requires energy.

Na+ and Water

• Electrical gradients draw Cl- across, and H2O follows Na+ due to osmotic force. The water is reabsorbed when the Na+ moves.
• The fluid left in the tubule is concentrated.

Molecule/Ion Reabsorption

• Many molecules and ions are reabsorbed using transporter proteins to their maximum capacity.
• Amino acids have a high transport maximum (TM) to preserve as much nutrients as possible.
• Above its value, you excrete substances, its useful in kidneys and other systems.

Loop of Henle

• The loop of Henle is comprised of the descending and ascending limbs, and macula densa where the water and sodium are absorbed.
• About 15% of volume absorption and 25% of sodium chloride absorption happens.
• Differential absorption of water and sodium chloride enables the loop to concentrate or dilute urine.
• Active sodium absorption helps maintain interstitial hypertonicity.
• Cortex osmolality is 300 mOsm and the medulla has a 1200 mOsm/kg H2O reading.

Nephron Types

• The two kinds of nephrons are deep in the medulla, and short in the medulla

Loop of Henle Functions

• Key functions include: descending limb permeable to H2O
• Thin ascending limb permeable to NaCl
• Thick ascending limb pumps NaCl.
• It does concentrate.

Thick Ascending Limb

• Within the thick ascending limb of the loop of Henle, sodium, potassium, and chloride ions are transported across the cell membrane into the interstitium, maintaining a concentration gradient with the lumen. Sodium/Potassium ATPase is key.

Descending Limb

• The descending limb is permeable to water, so H2O leaves.
• The ascending limb is impermeable to water, and it is imperative the limb is concentrating through movement of the solutes.

Osmolality

• Osmolality varies throughout the kidney nephron.

Distal Nephron

• Comprises the distal convoluted tubule and collecting ducts, specialized for regulation of reabsorption and secretion.
• The brush border is less prominent than in the PCT, and fewer epithelial cells have less mitochondria than previous, and tight junctions exist.
• Contains receptors for hormones and water retention as well as for retention of several solutes.
• Water reabsorption does not always follow solute absorption.

Distal Convoluted Tubule

• Operates in the cortex and is water impermeable.
• Has active Na+ absorption, K+ and H+ secretion.
• Involves calcium absorption under parathyroid hormone (PTH) influence.
• A key function in the collecting ducts is Primary site of ADH (antidiuretic hormone / vasopressin) action.
• Influenced by water absorption which then causes aldosterone.

Distal Tubule Ions

• In the distal convoluted tubule (DCT), K+ secretion is largely passive and follows the movement of Na+ and fluid. Na+ channels and KCl symporter, are key cogs

Distal Cell Channels

• Nat absorbs and K+ secretes throughout cells with Nat+ - K+ ATPase channels throughout.

Urea

• Urea is a product of protein breakdown.
• Urea helps maintain high osmolarity gradient in the medullary region and drives water reabsorption.

Urea Cycling

• Urea concentration is still 110% higher than its original filtered concentration in the DCT
• Antidiuretic hormone allows absorption of water into vasa recta.
• This allows urea to be reabsorbed at varying rates.
• Keeps urea concentration high in interstitial fluid.. 40% is excreted through the urine.

Importance of Sodium Reabsorption

• Na+ and Cl- account for >90% of the ECF's osmotic activity.
• Na+ load in the body is reflected by the ECF volume.
• Extra Na+ holds extra H2O expanding the ECF volume.
• Sodium chloride leads to an increase in blood pressure.

ECF impact

• Regulation on ECF volume with Na+ will keep volume within normal levels.

Control Mechanisms

• Control/regulatory mechanisms influence events in the distal nephron to produce concentrated or dilute urine and influence extracellular fluid volume.

Low Blood Volume

• If blood volume is too low, the body needs to increase it.
• One way to increase blood volume is to decrease water excretion.
• The body also increases NaCl absorption to increase blood.

Sodium Detection

• The need to retain Na+ is detected by the juxtaglomerular apparatus which is Renin
• Decreased circulating volume triggers renin release via the juxtaglomerular apparatus
• Sympathetic nervous system stimulation is key to detecting and fixing blood levels.
• Decreased filtrate osmolality causes Sodium chloride release.
• Decreased stretch (due to decreased blood pressure) is a clear indicator.

Result

• The effective circulating volume increases.
• Angiotensin II increases to increase blood and volume in various regions of the body.
• Renal retention occurs to make blood and blood flow more concise through out region.

Angiotensin functions

• Blood pressure is influenced by angiotensin and is produce in the vessels and the kidneys
• Cardiovascular control center in medulla oblongata release thirst to aid water retention throughout the cardio functions.

How hormones act

• Aldosterone functions within the intracellular fluid which leads to protein synthesis from outside the renal system.

NaCl absorption

• Stimulation of the hypothalamus leads to thirst.
• Keep water that is present as stimulation of both thirst and production occur. This then signals more absorption.

Water Reabsorption

• Requires vasopressin presence
• Medullary interstitial fluid is essential to maintaining water reabsoption at ADH.

Aquaporins

• In the presence of aquaporins, the rate of blood that leaves the capillaries is consistent through to blood that is returned.

Less Medulla, More Aquaporins

• Does not allow concentrated urea to be produced.

Water reabsorptions

• 100mosM for Cortex
• H2O absorbs back with permeable to 300 mosm A typical human will need to keep about 1200mosM reabsorbed for H2O

Vasopression

• Synthesized in the hypothalamus. Also known as AVP.
• Stored in the collecting duct in the epithelum and distal neuron. Also water pores in 2 to ensure collection.
• Controls water balance via hormones

Blood Volume

• If blood volume is too high, increase water and water excretion.

Natriuretic Peptide (ANP)

• Is regulated by atrial natriuretic peptide levels to allow homeostatis
• This leads to blood increases, and other regions increase and stimulate blood to have a consistent homeostatis.

Low Sodium

• The need to retain NA+ comes with a need to lower BP in those regions, and that is signaled when circulation is low or when BP is low in any region. Macula denses detect if more is needs. These are detected through stretching in the blood.

The Urinary Bladder

• The Urinary Bladder is a structure that expands depending on the input and out that depends on the input and out. The bladder is filled through Sphinctors signals and can store up to 800mL in many adults. It signals and is filled with nerves.

Micturition

• Micturition occurs. the signals are sent to the microturition center.
• That then signals to the nervous system to empty the bladder (Bladder Expanding)

Nerves role

• The role of the bladder in nerves will contract the the the bladder for expansion purposes to stay shut. Under the autonomic the Sphincter will stay to protect fluid levels.

Voiding

• The Voiding system will fill. and depending on expanding rates that will then signal to either excrete and contract to keep fluids consisten.

Kidneys roles

• The Erythropoietin is released by the kidneys is critical to the O2 ability which is triggered by hypoxemia, low O2 ability, Increased tissue.

EPO

• Imbalance of O2 and levels comes with key components. The kidney helps maintain homeostatis through a series of bone releases.
• Kidney then uses Erthyopoteins to ensure 02 has the capacity to travel.
• The epo synthesis is made through a cycle

EPO cells

• These are rare found and found in the renals which is the cortx and medulla/
• These are regulated as Anaemina. and the synthesied and controlled de novo in the body

synthesis

• The synthesis requires cells and genes.

Increased

• Increase erythroid. And ending and adding blood cells.

Stem cells

• Can not transfer cells and go through eirthoporesis.

Erythropoiesis

• This ensures it is needed via iron.
• This depends of diet for it be useful

Red blood cells

• Red blood cycles need new blood vessels from what is old.
• The liver absorbs as required.
• Erythropoiesis levels must rise, as blood need 120 days for cells
• Macopages are need to be sent all over the body to ensure no blockages occur.

EPO deficiencies

• There is a correlation that the renal system is always involved but not the major factor.
• There are still more defencies that are found around the human body.