Lecture 5 Flashcards
Why is high amounts of K+ in the ECF bad ?
High amounts of potassium in the ECF affects the RMP, decreasing it → causing depolarization
- poses a special risk to cardiac muscle [ sensitive to fluctuations in plasma K+ levels ]
- plays an essential role in acid-base balance within cells by moving in the opposite direction of H+ correspondingly to maintain cation balance
with acidosis - K+ levels in the ECF rise as K+ leaves cells in exchange for H+ moving into cells, interfering with the activity of excitable cells
What occurs with LOW K+ levels in the ECF ?
This would cause ↑ RMP, leading to hyperpolarization and non-responsiveness
with alkalosis - K+ levels in the ECF decrease as a result of K+ entering into cells (ICF) as H+ leaves them to enter the ECF
How is K+ handled by the renal system ?
Usually, 10 % of K+ in filtrate is lost, with 65 % of it reabsorbed in the PCT and 25 % in the ascending limb of the nephron loop. This is often more than the body needs back, so then tubular secretion of K+ kicks in by the DCT and the principal cells of the CD
How does this compare to Na+ handling ?
The amount of Na+ reabsorbed in the kidney tubules is precisely tailored to need and it is NEVER secreted into filtrate
Why is dietary K+ important ?
Since the kidneys are mostly focused on K+ excretion, it will eventually need to be replaced to maintain intracellular (ICF) stores
What are the 2 factors that determine the rate & extent of K+ excretion ?
-
Plasma (K+) Concentration — ↑K+ (or ↓Na+) concentration in the ECF from a high-potassium diet favours the entry of K+ into the proposal cells, prompting them to secrete K+ into the filtrate so more of it is excreted
- Conversely, ↓K+ (or ↑Na+) concentration in the ECF from a low-potassium diet or accelerated K+ loss depresses its secretion (& promotes its limited reabsorption) - Aldosterone — Stimulates principal cells to reabsorb Na+ & simultaneously enhances K+ secretion. Adrenal cortical cells are directly sensitive to K+ content in the ECF, and an ↑ would cause the adrenal cortex to release aldosterone to increase its secretion. Additionally, aldosterone is secreted in response to renin-angiotensin mechanism
Where is the most of body’s Ca2+ found ?
99 % found in bone
What is the importance of Ca2+ ?
MOST reabsorbed (98 %) for the use of …
- release of NT via calcium channels
- muscle contraction
- secondary messenger for hormones
!!! must be closely regulated (cannot be too high / low) !!!
What hormones regulate Ca2+ levels ?
-
PTH (Parathyroid Hormone) — Primary regulator, secreted by parathyroid glands, stimulated by declining Ca2+ levels in plasma to promote on increase in Ca levels by targeting :
- bone — osteoclasts will dissolve to move Ca2+ back into blood
- SI — triggers activation of Vit D to assist intestinal Ca2+ absorption from diet
- kidneys — ↑Ca2+ reabsorption by renal tubules while ↓HPO4 (opposite effects) - Calcitonin — Secreted by parafollicular cells, NOT important in adults but can ↓ plasma Ca2+ in children by stimulating bone deposition & inhibiting bone resorption
How is phosphate reabsorbed ?
About 75 % of filtered phosphate ions reabsorbed by PCT via active transport
- when there is NO PTH, phosphate is reabsorbed until the Tm is reached
- with ↑ PTH secretion, there is reduced active transport of phosphate
How is phosphate reabsorbed ?
About 75 % of filtered phosphate ions reabsorbed by PCT via active transport
- when there is NO PTH, phosphate is reabsorbed until the Tm is reached
- with ↑ PTH secretion, there is reduced active transport of phosphate
Why is acid-base (pH) balance important ?
Activity of functional proteins depends on pH
Regular arterial pH :
7.4
Regular venous pH :
7.35
Regular intracellular pH :
7.0
