HAPII Exam 3

Kidneys

Principle organ of the urinary system, creates filtrate (urine)

Ureters

Transports urine from kidneys to bladder

Urinary bladder

Temporary storage reservoir for urine, stores 600-1000 ml

Urethra

Carries urine out from body, has two sphincters

Methods by which the kidneys maintain the body’s internal environment

• Regulating total water volume and total solute concentration in water

• Regulating ion concentrations in extracellular fluid (ECF)

• Ensuring long-term acid-base balance

• Excreting metabolic wastes, toxins, drugs

• Producing erythropoietin (regulates RBC production) and renin (regulates blood pressure)

• Activating vitamin D

Location of kidneys

In the retroperitoneal space, located between T12 and L3

Renal hilum

Located on the concave medial surface, place of entry and exit for the ureters, renal blood vessels, lymphatics, and nerves

Renal fascia

Anchoring outer layer of dense fibrous connective tissue surrounding the kidney

Perirenal fat capsule

Fatty cushion around the kidney

Fibrous capsule

Transparent capsule that prevents spread of infection to kidney

Three distinct regions of the internal kidney

• Renal cortex

• Renal medulla

• Renal pelvis

Renal cortex

Granular-appearing superficial region

Renal medulla

• Deep to cortex, composed of cone-shaped medullary (renal) pyramids

  • Papilla: Tip of pyramid, points internally

  • Renal pyramids separated by renal columns

  • Lobe: Medullary pyramid and surrounding cortical tissue; approx 8 lobes per kidney

Renal pelvis

Funnel-shaped tube continuous with ureter, contains the major and minor calyces

Minor calyces

• Cup-shaped areas that collect urine draining from pyramidal papillae

Major calyces

• Areas that collect urine from minor calyces

• Empty urine into renal pelvis

Urine flow in the kidneys

• Renal pyramid → minor calyx → major calyx → renal pelvis → ureter

Nephrons

The structural and functional units that form urine

Renal corpuscle

Composed of the glomerulus and glomerular capsule

Glomerulus

Tuft of capillaries composed of fenestrated endothelium. Highly porous capillaries allow for efficient filtrate formation.

Filtrate

Plasma-derived fluid that renal tubules process to form urine

Glomerular capsule

AKA Bowman’s capsule: Hollow structure surrounding glomerulus. Consists of two layers:

• Parietal layer: Simple squamous epithelium

• Visceral layer: Clings to glomerular capillaries; branching epithelial podocytes. Filtration slits between foot processes allow filtrate into capsular space.

Renal tubule

Single layer of simple cuboidal epithelial cells with three major regions: PCT, nephron loop, and DCT.

Proximal convoluted tubule (PCT)

Cuboidal cells with dense microvilli increase surface area. Also have large mitochondria due to high activity and movement requiring lots of ATP. Functions in reabsorption and secretion. Confined to cortex.

Nephron loop (Loop of Henle)

U-shaped structure consisting of two limbs:

• Descending limb:

  • Proximal part continuous with proximal tubule

  • Distal portion called descending thin limb; simple squamous epithelium

• Ascending limb:

  • Thick ascending limb with cuboidal or columnar cells.

Distal convoluted tubule (DCT)

Cuboidal cells, very few microvilli. Function more in secretion than reabsorption. Confined to cortex. Distal convoluted tubule drains into collecting duct.

Collecting ducts

Maintain water and Na+ balance. Cuboidal cells with microvilli help maintain acid-base balance of blood. Receive filtrate from many nephrons. Run through medullary pyramids, giving pyramids their striped appearance. Ducts fuse together to deliver urine through papillae into minor calyces.

Cortical nephrons

Make up 85% of nephrons, very little are found in medulla.

Juxtamedullary nephrons

Loops deeply into medulla. Ascending limbs have thick and thin segments. Important in concentrating urine.

Blood circulation through kidneys

• Aorta → Renal artery → Segmental artery → interlobar artery → arcuate artery → cortical radiate artery → afferent arteriole → glomerulus (capillaries) → efferent arteriole → peritubular capillaries or vasa recta → cortical radiate vein → arcuate vein → interlobar vein → renal vein → inferior vena cava

Kidneys and blood

• Kidneys cleanse blood and adjust its composition, so it has a rich blood supply

• Renal arteries deliver about one-fourth (1200 ml) of CO to kidneys each minute

Kidneys arterial flow

• renal → segmental → interlobar → arcuate → cortical radiate (interlobular)

Kidneys venous flow

• cortical radiate → arcuate → interlobar → renal veins

Nerve supply to the kidneys

Nerves are supplied to the kidneys via sympathetic fibers from renal plexus

Glomerulus

A capillary bed specialized for filtration; fed by afferent arteriole and drained by efferent arteriole.

Peritubular capillaries

Low-pressure, porous capillaries adapted for absorption of water and solutes; arise from efferent arterioles.

Vasa recta

Long, thin-walled vessels associated with juxtamedullary nephrons; serve to concentrate urine.

Juxtaglomerular Complex (JGC)

A nephron structure involving modified portions of the distal ascending limb of the nephron loop and afferent arteriole, essential for regulating glomerular filtrate formation and blood pressure, comprising macula densa cells (chemoreceptors sensing NaCl) and granular cells (mechanoreceptors releasing renin).

Renin-Angiotensin-Aldosterone System (RAAS)

A hormone system that regulates blood pressure and fluid balance; initiated by the enzyme renin, which converts angiotensinogen into angiotensin I. Angiotensin I is then converted into angiotensin II, promoting vasoconstriction and stimulating aldosterone secretion to increase sodium reabsorption in the kidneys, thereby increasing blood volume and pressure.

How much fluid flows through the kidney daily?

180 L

How much urine is formed each day?

~1.5 L

How much oxygen do the kidneys consume at rest

20-25%

Filtrate

Formed by glomerular filtration, blood plasma minus proteins. Produces urine.

Urine

<1% of original filtrate, contains metabolic wastes and unneeded substances

Three processes of urine formation and blood composition adjustment

• Glomerular filtration: Produces cell-free and protein-free filtrate

• Tubular reabsorption: Selectively returns 99% of substances from filtrate back to blood in renal tubules and collecting ducts

• Tubular secretion: Selectively moves substances from blood to filtrate in renal tubules and collecting ducts

Glomerular filtration

A passive process where no metabolic energy is required. Hydrostatic pressure forces fluids and solutes through the filtration membrane into the glomerular capsule, and no reabsorption into capillaries of the glomerulus occurs.

Filtration membrane

A porous membrane between blood and the interior of the glomerular capsule that allows water and solutes smaller than plasma proteins (glucose, amino acids, nitrogenous wastes) to pass. Normally, no cells can pass. It contains three layers:

• Fenestrated endothelium of glomerular capillaries

• Basement membrane: fused basal laminae of two other layers

• Foot processes of podocytes with filtration slits; slit diaphragms repel macromolecules.

Where do proteins remain during glomerular filtration

Proteins remain in the blood, maintaining osmotic pressure and preventing the loss of all water to the capsular space.

Outward pressure

Promotes filtrate formation

Glomerular blood pressure

Higher than other capillary beds, 55 mm Hg

Glomerular Filtration Rate (GFR)

• Volume of filtrate formed per minute by both kidneys (normal = 120-125 ml/min)

Why is GFR important?

it allows kidneys to form filtrate and maintain homeostasis. An increased GFR leads to increased urine output, which decreases blood volume and consequently lowers blood pressure.

Tubular reabsorption

A selective process that swiftly returns most of the filtrate contents to the blood. Almost all organic nutrients are reabsorbed, while water and ion reabsorption are hormonally regulated and adjusted. This process includes both active and passive reabsorption.

Proximal convoluted tubule

The site of most reabsorption in the nephron. Almost all nutrients, including glucose and amino acids, are reabsorbed here. Approximately 65% of Na and water is reabsorbed in this section, along with various ions such as Ca²⁺, Mg²⁺, K⁺, and some Na⁺. Additionally, lipid-soluble substances, water, and vitamins are also reabsorbed.

Nephron loop

• Descending limb: H20 reabsorbed, solutes cannot leave filtrate

• Ascending limb: H20 cannot leave, solutes can

  • Thin segment is passive to Na movement

  • Thick segment transports Na into renal medulla

Distal convoluted tubule and collecting duct

Reabsorption is hormonally regulated in these areas

Antidiuretic hormone

Hormone that is released by the posterior pituitary gland and causes the direct reabsorption of water by increasing ADH levels to increase water reabsorption

Aldosterone

Hormone released by the adrenal gland that targets collecting ducts and DCT. Promotes active Na+ reabsorption via Na+/K+ pump. It increases blood pressure and decreases blood K+ levels.

What would happen without aldosterone?

• daily loss of Na+ would be 2% = DEATH

Atrial natriuretic peptide (ANP)

A hormone released by cardiac atrial cells when blood volume or pressure is elevated. It decreases blood sodium (Na+) levels, which in turn reduces blood volume and lowers blood pressure.

Parathyroid hormone

Hormone that acts on DCT and increases Ca2+ reabsorption

Tubular secretion

The process where selected substances are moved from peritubular capillaries into the filtrate, including potassium (K⁺), hydrogen (H⁺), ammonium (NH₄⁺), creatinine, organic acids, and bases. Additionally, substances synthesized in renal cells are also secreted. Tubular secretion is important for:

• Disposing of substances, such as drugs or metabolites, that are bound to plasma proteins

• Ridding the body of excess potassium (K⁺) due to aldosterone's effect

• Controlling blood pH by adjusting the amounts of hydrogen (H⁺) or bicarbonate (HCO₃⁻) in urine

Main function of kidneys

Make adjustments to maintain body fluid osmotic concentration (water & salt balance)

Amount of urine required daily

400 ml

Countercurrent mechanism

Mechanism in the nephron loop by which the kidney adjusts body fluid concentrations

Descending limb of nephron loop

• Freely permeable to H2O

• Impermeable to solutes

• Causes filtrate osmolality to increase to ~1200 mOsm

Ascending limb of nephron loop

• Impermeable to H2O

• Selectively permeable to solutes

• Na+ and Cl- are actively reabsorbed

Countercurrent multiplier system

• Flow in opposite directions in the ascending and descending limbs

• Close proximity of the two limbs allows interaction

If we were so dehydrated we have maximal ADH:

• Increased osmolality of extracellular fluids

• Increased ADH release from posterior pituitary

• Increased number of aquaporins (H2O channels) in collecting duct

• Increased H2O reabsorption from collecting duct

• Small volume of concentrated urine produced (~1200 mOsm)

If we were so overhydrated we had no ADH:

• Decreased osmolality of extracellular fluids

• Decreased ADH release from posterior pituitary

• Decreased number of aquaporins in collecting duct

• Decreased H2O reabsorption from collecting duct

• Large volume of diluted urine produced

Micturition

Process of urination

What is the main composition of urine?

Urine is made up of approximately 95% water and 5% solutes.

What is the largest solute component found in urine?

Urea, which is a product of amino acid breakdown.

Which nitrogenous wastes are found in urine?

Urea, uric acid (from nucleic acid metabolism), and creatinine (a metabolite of creatine phosphate).

What are some normal solutes found in urine?

Normal solutes include Na+, K+, PO4³-, SO4²-, Ca2+, Mg2+, and HCO3-.

What can high concentrations of constituents or abnormal components in urine indicate?

They may indicate pathology, such as the presence of blood proteins, WBCs, or bile pigments.

Normal max volume of urinary bladder

500 ml

Infant body water content

73% (low body fat, low bone mass)

Adult male body water content

~60%

Total body water volume

40 L, composed of ICF and ECF combined

ICF body water volume

25 L, 40% of body weight

ECF body water volume

15 L, 20% of body weight. Composed of interstitial fluid (12 L, 80%) and plasma (3 L, 20%)

Intracellular fluid (ICF) compartment

Fluid inside cells, accounts for 2/3 of total body fluid

Extracellular fluid (ECF) compartment

• compartment: Fluid in two main ECF compartments outside the cells

  • = 1/3 of total body fluid:

    • Plasma: 3 L (separate of the IF, found in blood vessels)

    • Interstitial fluid (IF): 12 L in spaces between cells

      • Also considered part of IF: Lymph, CSF, humors of the eye, synovial fluid, serous fluid, and gastrointestinal secretions

Universal solvent

Water

Solutes

Substances dissolved in water; classified as nonelectrolytes and electrolytes

What are nonelectrolytes?

Nonelectrolytes are primarily organic molecules that do not dissociate in water, meaning they do not create charged particles. Examples include glucose, lipids, creatinine, and urea.

What are electrolytes?

Electrolytes are substances that dissociate into ions when dissolved in water. This includes inorganic salts, all acids and bases, and some proteins. Ions can conduct electrical current and have greater osmotic power compared to nonelectrolytes, which allows them to cause fluid shifts due to their ability to dissociate into two or more ions. Most abundant solutes in body fluids and determine most chemical and physical reactions

ECF electrolyte patterns

• Electrolyte contents are all similar except for higher protein, lower Cl- content of plasma

  • Major cation: Na+

  • Major anion: Cl-

ICF electrolyte patterns

• Contains more soluble proteins than plasma

  • Low Na+ and Cl-

  • Major cation: K+

  • Major anion: HPO4^2–

Majority of dissolved solutes

consist of proteins, phospholipids, cholesterol, and triglycerides

Osmolality

The concentration of solutes in a solution

What regulates the continuous exchange and mixing of fluids in the body?

Osmotic and hydrostatic pressures regulate the continuous exchange and mixing of fluids.

How does water move in relation to osmotic gradients?

Water moves freely along osmotic gradients.

Is body fluid osmolality equal across all compartments?

Yes, all body fluid osmolality is almost always equal.

Increase in ECF osmolality

Water leaves cell

Decrease in ECF osmolality

Water enters cell

Exchanges between plasma and IF

Occurs across capillary walls. Fluid leaks from the arteriolar end of the capillary and is reabsorbed at the venule end. Lymphatics pick up any remaining fluid and return it to the blood.

Exchanges between IF and ICF

Exchanges occur across the cell membrane, allowing for a two-way osmotic flow of water. Ions move selectively into or out of the cell, while nutrients, wastes, and gases have a unidirectional flow.

Water balance

• Water intake must equal water output: ~2500 ml/day