15. Urinary System 2 Flashcards
Explain reabsorption and secretion in the tubules (3)
- Usually more than 99% of the water in the
glomerular filtrate is reabsorbed as it
passes through the tubules - Therefore dissolved substances that are not
reabsorbed are greatly concentrated (e.g.
creatinine) - Some dissolved substances are almost
completely reabsorbed so their concentration
decreases (e.g. glucose and amino acids)
What kind of border is on the luminal surface and basal channels in base of proximal tubule? (1)
Brush border to increase surface area
Steps of sodium reabsorption in the proximal tubule (8)
- Sodium (Na⁺) is actively transported from tubular epithelium cells into the intercellular spaces.
- Sodium then moves into the peritubular capillaries, reducing the intracellular Na⁺ concentration.
- Active transport creates an electrochemical gradient, causing Na⁺ diffusion from the tubular lumen into the tubular epithelium.
- For every Na⁺ reabsorbed, an anion (commonly Cl⁻) follows or a cation (commonly H⁺) is exchanged.
- Secondary Active Transport: Glucose, amino acids co-transport with Na⁺ into the cell using carrier proteins at the brush border.
- This is done against the concentration gradient for these other molecules - Countertransport: Na⁺ enters the cell while H⁺ is exchanged out of the cell (Na⁺/H⁺ antiport).
- Water Reabsorption: Solute movement creates an osmotic gradient, leading to water reabsorption into peritubular capillaries.
- Concentration of Urea: As water is reabsorbed, urea and other solutes become more concentrated, facilitating further reabsorption or excretion.
Explain the proteins in the proximal tubule (1)
Pulled into the cells by endocytosis – then degraded in cells to amino acids which are moved into the peritubular space via facilitated diffusion
Explain the small peptides in the proximal tubule (2)
- Hydrolyzed in brush border cells to amino acids
- The amino acids are reabsorbed via cotransport
What is the loops of henle important for? (1)
Functions to reduce the volume of the
urine
Describe the descending loop in the loops of Henle (4)
- Highly permeable to water
- Does not contain the mechanism to reabsorb Na+
- Thus filtrate becomes more concentrated as it descends the loop – due to water leaving the loop
- Reaches maximum concentration at the tip of the loop
Describe the ascending loop in the loops of Henle (5)
- Impermeable to water
- Contains Na+ pumps which actively reabsorb Na+ into the medullary tissue and capillaries
- Contains Cl- pumps
- This is what creates the osmotic gradient that draws water out of the descending loop
- By the time the filtrate reaches the top of the ascending loop it has the same concentration as when it entered but the volume is less
What occurs in the distal convoluted tubule? How does this happen? (7)
- Final adjustments are made to the chemical
constituents of the urine - Sodium reabsorption
- Potassium excretion
- Reabsorption of water (Varies with the level of the hormone aldosterone)
- Secretion of hydrogen ions
- Increases if blood pH is low (acidic)
- Stops if blood pH is high (alkaline)
What occurs in the collecting ducts? (2)
- Permeability to water
- Varies with the level of the Antidiuretic hormone (ADH)
What is the counter-current mechanism for concentrating urine? (2)
- The countercurrent multiplier: the loops of Henle
- The countercurrent exchanger: the vasa recta
Explain this photo for the countercurrent multiplier and exchanger (2)
- Urine can be less concentrated than the original filtrate
- Occurs in the absence of ADH – the collecting duct is then impermeable to water – increased volume of urine excreted
How does concentrated urine occur? (3)
- Need to excrete solutes while conserving water
- ADH causes the entire collecting duct to become highly permeable to water
- Then as tubular fluid passes through the collecting duct water is pulled by osmosis into the concentrated medullary interstitium
What factors stimulate/inhibit the release of ADH (5)
Stimulated by
1. Hypertonicity of hypothalamic ECF
2. Low blood pressure
3. Trauma, pain
Inhibited by
1. Cold
2. Alcohol
Explain the structure juxtaglomerular apparatus (6)
- Distal tubule (DCT) passes in the angle between the afferent (1) and efferent (2) arterioles.
- The cells of the DCT that contact the arterioles are more dense than other DCT cells – called the macula densa (4)
- The smooth muscle cells of the arterioles are swollen and contain dark granules where they contact the macula densa – called the juxtaglomerular cells (3)
- Lacis and mesangial cells (5 and 6) fill the space between the DCT and the arterioles
- Secrete prostaglandins
- Act as macrophages.
Describe angiotensin II in the renin-angiotensin-aldosterone system
Angiotensin II
1. Constricts resistance vessels thereby increasing arterial blood pressure
2. Acts on the adrenal cortex to release aldosterone
3. Stimulates the release of ADH from the posterior pituitary
4. Stimulates thirst centers in the brain
