Urinary tract structure and function 2

• Urinary tract structure and function are explored.
• Kidney physiology can be applied and expanded upon through the cases presented in the lecture.
• Hydrostatic pressure and colloid osmotic pressure are recapped.
• The blood supply of the kidneys informs how the kidneys filter plasma.
• An overview of the function of each main section of the nephron is given.
• The glomerulus and glomerular filtration rate are discussed.
• Move onto the tubules and gives an overview on how they function.

Learning Outcomes

• Develop an overview of the nephron, including describing kidney functions and blood supply.
• Apply knowledge of hydrostatic and colloid osmotic pressure to the nephron's functions.
• Outline how kidneys form urine.
• Explain glomerular filtration, tubular reabsorption, and secretion.

Kidney Functions

• Excretion of metabolic waste products and chemicals.
• Regulation of electrolyte and water balance.
• Regulation of body fluid osmolality and electrolyte concentration.
• Regulation of arterial pressure.
• Regulation of acid-base balance.
• Secretion, metabolism and excretion of hormones.
• Gluconeogenesis.

Blood Supply of Kidneys

• Renal artery enters the kidney through the hilus.
• Then to interlobar artery, arcuate artery, interlobular artery, and afferent arteriole.
• Then the glomerular capillaries, and the efferent arteriole.
• Lastly into the peritubular capillaries, interlobular veins, and arcuate vein.
• Renal circulation is unique as it has two capillary beds.

Kidney Capillaries

• Renal circulation features two capillary beds: glomerular and peritubular.
• These are arranged in series and separated by the efferent arteriole.
• The efferent arteriole regulates hydrostatic pressure in both sets of capillaries.

Capillary Hydrostatic Pressure

• High hydrostatic pressure in glomerular capillaries enables rapid filtration.
• Lower hydrostatic pressure in peritubular capillaries allows rapid fluid reabsorption.
• Resistance adjustment in afferent and efferent arterioles allows kidneys to regulate hydrostatic pressure in both glomerular and peritubular capillaries.
• Rate of glomerular filtration and tubular reabsorption can change in response to the body's needs.

Urine Formation

• Urine formation results from glomerular filtration, tubular reabsorption, and tubular secretion.
• Tubular Reabsorption is the reabsorption of substances from the renal tubules into the blood.
• Tubular Secretion is the secretion of substances from the blood into the renal tubule.
• Kidneys cannot generate new neurons, renal injury, disease, and ageing leads to declining neuron numbers.
• The kidney's can express the relationship between filtration rate, reabsorption rate, and secretion rate using + and - signs to explain urinary excretion rate.

Plasma Composition

• Plasma is made up of water, proteins, electrolytes, dissolved gases, nutrients, waste products and hormones.
• Plasma proteins primarily consist of primarily albumin, globulin, and fibrinogen.
• Plasma electrolytes include Na+, Cl-, HCO3-.
• Plasma nutrients are glucose, lipids, amino acids, and vitamins.
• Waste products include urea, creatinine, and uric acid.
• Promotes blood clotting, also known as coagulation.

Urine Formation Intro

• Urine formation begins with fluid filtration from glomerular capillaries to Bowman's capsule.
• The Bowman's capsule is virtually protein-free.
• Most plasma substances, except proteins, are freely filtered.
• The concentration of most substances in Bowman's capsule is similar to plasma.
• As the fluid leaves Bowman's capsule and passes through the tubules they are modified by reabsorption and secretion.

Glomerular Filtration

• Glomerular filtration is the kidney filtering the blood.

Structure of Capillaries

• Blood vessels have an internal lining of endothelial cells
• Capillary walls consist of a single layer of endothelial cells supported by a basement membrane.
• Pericapillary cells, such as fibroblasts, macrophages, and smooth muscle cells, lie between basement membrane and endothelial cells.
• Capillaries are classified via permeability: Continuous with no gaps and Fenestrated that have a high content of fenestrae.

Structure of the Glomerulus

• Glomerular capillaries have three major layers: endothelium, basement membrane, and epithelial cells.
• Endothelium is perforated with fenestrae, but does not allow passage of proteins because endothelial cells have fixed negative charges.
• Basement membrane consists of collagen and proteoglycan fibrillae, allowing water and solute passage.
• Epithelial cells have footlike processes (podocytes) encircling capillaries separated by slit pores for glomerular filtrate movement.

Composition of Glomerular Filtration

• Urine formation begins with large fluid amounts filtered through glomerular capillaries.
• Glomerular filtrate is protein-free, devoid of cellular elements (including RBCs) due to impermeability of glomerular capillary membrane.
• The glomerular filtrate concentration has similar constituents to plasma.

Glomerular Filtration Rate (GFR)

• Filtration rate of glomerulus is determined by the sum of hydrostatic and colloid osmotic pressures, and the glomerular capillary filtration coefficient.
• Net filtration pressure is determined by glomerular hydrostatic pressure, and Bowman's capsule and glomerular oncotic pressure.

Determinants of GFR

• Glomerular Capillary Filtration Coefficient: Increased coefficient increases GFR.
• The coefficient is unlikely to provide regulations of GFR, due to various diseases can cause lower GFR. For example: diabetes. Bowman's Hydrostatic Pressure:
• increased Bowman's hydrostatic pressure decreases GFR.
• Not normally a primary means of regulating GFR.
• Blockage of the urinary tract markedly increases the pressure. Glomerular Capillary Colloid Osmotic Pressure:
• Increased glomerular capillary colloid osmotic pressure decreases GFR.
• As blood passes from the afferent arteriole through the glomerulus to the efferent arterioles, the colloid osmotic pressure increases.
• The pressure increase occurs because fluids pass through into Bowman's capsule, concentrating the glomerular plasma proteins.
• Factors in GFR include: Arterial colloid osmotic pressure and Fraction of plasma filtered.

Glomerular Capillary Hydrostatic Pressure

• Increased glomerular capillary hydrostatic pressure increased GFR
• Changes in the glomerular hydrostatic pressure serve as the primary means for physiological regulation of GFR.
• Glomerular hydrostatic pressure is influenced by an arterial pressure, afferent arteriolar resistance and efferent arteriolar resistance.

Tubular Reabsorption and Secretion

• Next steps in urine formation as it helps determine glomerural filtration.

Tubular Function Intro

• Glomerular filtrate enters the renal tubules and move sequentially
  • Proximal convoluted tubule, Loop of Henle, Distal convoluted tubule, collecting tubule and lastly the collecting duct.
• Along this course, substances are selective reabsorbed and secreted
• Tubular reabsorption is highly selective
• Glomerular filtration is not selective, except for proteins

Filtration, Reabsorption, and Excretion Figures

• Glucose: 180 were filtered and reabsorbed. 0 were excreted, with 100% reabsorbed.
• Sodium: 25,560 filtered, 25,410 reabsorbed, 150 secreted, 99.4% reabsorbed.

Reabsorption

• Reabsorption involves passive and active mechanisms.
• For a substance to be reabsorbed, it must
  • transported across the tubular epithelium membranes into the renal interstitial fluid.
  • transported through the peritubular capillary membrane back into the blood.

Passive Transport

• Simple diffusion is used to describe a substance that diffuses through a lipid bilayer, an example is Oxygen.
• Facilated diffusion occurs through carrier proteins and conformational changes.
• Simple diffusion can occur through the use of channel proteins that are either gated or non-gated, moving ions such as Na+

Active Transport

• Primary active transport used adenosine triphosphate or ATP against the concentration gradient. Examples include Na+/K+ and Ca2+ pumps.

Reabsorption and Secretion In PCT

• The Proximal Convoluted Tubule reabsorbs: sodium, chloride, bicarbonate, potassium, glucose and amino acids.
• The Proximal Convoluted Tubule secretes: organic acids, bases and hydrogen molecules.

Reabsorption and Secretion In Loop of Henle

• Thin Descending Segment: highly permeable to water and moderately permeable to most solutions.
• This Ascending Segment: has a lower rate of reabsorption.
• Thick Ascending Segment: Reabsorbs sodium, chloride, calcium bicarbonate, magnesium, and potassium. It secretes hydrogen molecules.

Reabsorption and Secretion In DCT

• The Early Distal Tubule consists of the first portion, which provides feedback control on the GFR in tandem with the Juxtaglomerular complex.
• This area is very highly convoluted, similar to the ascending limb of the Loop of Henle, in addition to being impermeable to water.
• In this area Sodium, Cl-, Ca++, and Mg++ are reabsorbed.

Reabsorption and Secretion In The Late DT

• The Late Distal Tubule comprises of 2 types of Dell: Principal and Intercalated.
• Principle cell Reabsorb water and sodium, and then secrete potassium.
• Intercalated Cells absorb potassium and bicarbonate and then secrete hydrogen.

Reabsorption and Secretion In Medullary CT

• Medullary CT describes the Medullary collecting duct and serves as a place for processing urine.
• It can determine the the final amount of urine output of water and solution.
• It displays the characteristic of permeability to water via the ADH and urea, and then it secretes hydrogen ions based on gradient concentrations.

Explore kidney function, blood flow, and capillary types. Learn about glomerular filtration rate (GFR) and the unique arrangement of renal circulation. Understand how these factors contribute to urine formation.