Section iii disturbance of potassium balance
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Section III.Disturbance of Potassium Balance. Potassium Balance (1) Content and distribution (2) Function of potassium (3) Regulation of K + balance Hypokalemia Hyperkalemia. 1. Potassium Balance (1) Content and distribution.

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Section III.Disturbance of Potassium Balance

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Section iii disturbance of potassium balance

Section III.Disturbance of Potassium Balance

Potassium Balance

(1) Content and distribution

(2) Function of potassium

(3) Regulation of K+ balance

Hypokalemia

Hyperkalemia


1 potassium balance 1 content and distribution

1. Potassium Balance(1) Content and distribution

  • The adult body contains about 45 mmol/Kg of BW.

  • About 98% of potassium is within the cells,

  • [K +] i= 140~160 mmol/L.

  • About 2% of K + is in the ECF,

  • [K +]e = 3.5~5.5 mmol/L.

  • 3/4 of the intracellular K+ is in muscle cells.


Section iii disturbance of potassium balance

  • There are two forms of existence:

  • (1) free state of potassium, K+,

  • (2) the K combining to protein and glycogen.

  • Both are exchangeable.

  • (Na+)


2 function of potassium

(2) Function of potassium

  • 1) Metabolism

  • 2) Membrane potential

  • 3) Regulation of pH

  • 4) Osmotic pressure of ICF


1 metabolism

1) Metabolism

  • (a) K+ is required for the activity of some intracellular enzymes e.g. the enzyme for ATP production.

  • (b) K+ is involved in anabolism.

  • 1 g of glycogen contents 0.33~0.45 mmol,

  • The K+ moves into cells with glucose, during the synthesis of glycogen.

  • 1 g of protein needs 30 mmol of K+.


2 membrane potential

2) Membrane potential

  • The ratio of the intracellular to the extracellular potassium concentration

  • ([K+]i∕[K+]e) constitutes the cellular resting membrane potential.

  • Minimal change of [K+]e may affect the membrane potential obviously.

  • K+ is important for normal neuromuscular irritability.


3 regulation of ph

3) Regulation of pH

  • Exchange of K+ and H+ crossing the cell membrane is important for acid-base balance.

  • Changes of K+ concentration will lead to the changes of pH.

  • When K+ moves out of the cells, H+ will move into the cells as an exchange for electrical neutrality. More H+ will lead to acidosis.


4 osmotic pressure

4) Osmotic pressure

  • Potassium ion is the major intracellular cation, so K+ is important in the formation of osmotic pressure in the cell.

  • To keep the volume of ICF.


3 regulation of k balance

(3) Regulation of K+ balance

  • 1) Equilibrium of K+ in ICF and ECF

  • 2) Balance of intake and excretion

  • K excretion in kidney

  • K excretion in colon


1 equilibrium of k in icf and ecf transcellular potassium movement

1) Equilibrium of K+ in ICF and ECF (transcellular potassium movement)

  • Equilibrium means to keep

  • [K+]i= 140~160 mmol/L;

  • [K+]e = 3.5~5.5 mmol/L


Significance

Significance :

After intake a bottle of orange juice (40 mmol/L of K+),

if all K+ stays in ECF, the [K+]e=4.5+2.4=6.9 mmol/L, which will lead to abnormal ECG. Abnormal ECG means the heart muscle is injured.

If all K+ is transported into ICF, the [K+]i=160+1.2=161.2 mmol/L.

Then the excessive K+ will be eliminated within several hours in the urine.


Mechanism to keep the equilibrium

Mechanism to keep the equilibrium:

The basic mechanism to the balance is “leak and pump”.

(Integrity of cell membrane and function of Na+-K+ pump)


Section iii disturbance of potassium balance

钾的跨细胞转移调节

细胞内液[K+]

140-160mmo1/L

Na+ - K+泵(泵)

K+

K+

细胞外液[K+]

4.2mmol/L

Na+

Na+

K+

K+

K+通道(漏)

K+细胞内外移动的泵一漏机制

(Pump-leak mechanism)


Section iii disturbance of potassium balance

钾代谢紊乱

【影响钾的跨细胞转移的主要因素】

可直接刺激Na+-K+泵活性,影响钾转移主要激素。

胰岛素

肾上腺素通过cAMP机制激活Na+-K+泵活性,肾上腺能神经激活是促K+自细胞内移出。

儿茶酚胺

ECF [H+]↑, H+入细胞内,细胞内K+外移。ECF每0.1 pH变化大约引起0.6mmol/L血清钾变化

ECF [K+]

ICF [K+]

140-160mmo1/L

酸碱平衡状态

渗透压

ECF 渗透压↑ ↑ ,使细胞内K+外移。

运动

ECF [K+] 4.2mmol/L


A changes of ph

(a) Changes of pH

A decrease in pH of 0.1 units usually elevates the serum [K+] by about 0.6 mmol/L.

(alkalosis: pH ↑ 0.1, [K+] ↓ 0.6 mmol/L)


B extracellular k concentration

(b) Extracellular K+ concentration

  • A high [K+]e will stimulate the activity of Na+-K+ pump.

  • A low [K+]e will do in the converse way. 


C total k quantity in the body

(c) Total K quantity in the body

  • When the total quantity of K in the body reduces, the loss of intracellular K+ is more than extracellular K+, but the ratio of extracellular K+ loss is more than intracellular K+ loss.

  • When the total quantity of K in the body increases , the increase of intracellular K+ is more than extracellular K+, but the ratio of extracellular K+ increase is more than intracellular K+ increase.


D regulation of hormone

(d) Regulation of hormone

Insulin promotes the movement of K+ into the liver cells and skeletal muscle cells by increasing sodium-potassium ATPase activity.

The β-adrenergic agonists also elevate Na+ -K+ pump activity to enhance K+ entry.

The α-adrenergic agents enhance the K+ transport out of cells.

Epinephrine stimulates α-recepter first,then β-recepter.

(K+ )


E metabolism anabolism catabolism

(e) Metabolism(anabolism; catabolism)

Increased anabolism (AA protein, glucose glucogen) elevates the [K+]i.

Increased catabolism (protein AA, glucogen glucose) reduces the [K+]i.


F increased smotic pressure of ecf leads to increased k e

f) Increased smotic pressure of ECF leads to increased [K+]e.

  • Increased osmolality of extracellular fluid draws the water out of cells with K+.

    Decreased water in the cells elevates the [K+]i. The difference [K+]iand [K+]e increases, which leads to the shift of K+ out of cells increases.


G exercise

(g) Exercise

  • Exercise can promote K shift out of cells through:

    (1) opening of ATP-dependent K+ channels

    与电压依赖型的钾离子通道不同,也与依赖钙离子的钾离子通道不同,,KATP通道主要受细胞内的ATP浓度调节。在生理条件下细胞内ATP浓度约为3-4 mmol.L-1, KATP通道基本处于关闭状态。只有当心肌细胞发生缺血缺氧,能量耗竭,胞内ATP浓度低于0.2 mmol.L-1时通道开放,K+外流,

    游离ATP是KATP通道最强而有效的内源性阻断剂,其主要功能有:(1)舒张血管,包括外周血管和冠状动脉。主要由于KATP激活,K+外流,细胞复极化加速,使动作电位时程缩短,Ca2+内流减少,血管舒张。 (2)Ca2+内流减少,使心肌收缩力减弱,降低心肌氧耗,产生心脏保护作用。

    (2) decrease Na+ -K+ ATPase activity due to ATP depletion.


H integrity of cellular membrane

(h) Integrity of cellular membrane

  • “Leak” indicates the moving of K+ out of the cell according to the gradient of [K+ ] between ICF and ECF, without expending ATP.

  • Leaking leads to the tendency to reduce the [K+]i. When the cell membrane is injured, the permeability of cell membrane to K+ is increased. More K+ move from cells into ECF.


2 balance of intake and excretion

2) Balance of intake and excretion

  • (a) Intake: The common foods, like lean meat, milk and fruits content a lot of potassium.

  • The average diet contains 60~100 mmol of potassium per day, which is enough for the daily body requirement. 90% of potassium in food is absorbed in small intestine.


B excretion

(b) Excretion

  • ① Via kidney

  • About 90% or more potassium is eliminated from kidney. (12字)

  • The more K we eat, the more K is eliminated from kidneys. When the intake of potassium is decreased, the elimination from urine is decreased.

  • If no potassium intake, the kidneys will still secrete small amount of potassium (20~15 mmol/day). (Na+?)


Section iii disturbance of potassium balance

  • Potassium is freely filtered at the glomerulus. Almost all the potassium filtered is reabsorbed in proximal tubules via active transport. In loop of Henle: via Na+-K+-2Cl-contransporter.

  • Most of the potassium in the urine is secreted from distal tubules and collecting ducts.

  • No decrease of K+ filtration except severe reduction of GFR.


Section iii disturbance of potassium balance

  • a) aldosterone

  • Aldosterone activates pump (Na+ /K+ pump) in basolateral membrane, the K+ transport from peritubular interstitial fluid into renal tubular cells will increase.

  • Aldosterone increases the permeability of lumenal membrane, the K+ transport from renal tubular cells into tubules (urine) will increase.

  • (C)Regulation of renal loss in renal distal tubules and collecting ducts,


B high k e

b) High [K+]e

High [K+]e activates pump (Na+ /K+ pump) in basolateral membrane, the K+ transport from peritubular interstitial fluid into renal tubular cells will increase.

High [K+]e increases the permeability of lumenal membrane, the K+ transport from renal tubular cells into tubules (urine) will increase.

High [K+]edecreases the [K+] difference between renal tubular cells and peritubular interstitial fluid, then decrease the back-flow of K+ from tubule.


C volume and flow rate of urine in distal tubules and collecting ducts

c) Volume and flow rate of urine in distal tubules and collecting ducts

Increased volume and flow rate of urine in distal tubules and collecting ductsreduce the [K+], increase the difference between the [K+] in urine and in tubule cells, increase the excretion of K.


D acid base balance

d) Acid-base balance

  • In acute acidosis, increased [H+] suppresses Na+-K+-ATPase, and the excretion of K+ decreases.

  • In alkalosis the excretion of K+ increases.

  • In chronic acidosis, the dominant effect is that increased [H+] suppresses the reabsorption of water and sodium in proximal tubule cells, urine volume increases, excretion of K+ increases.


E electric field

e) Electric field

If more negative charges in tubular fluid, more K+ will be excreted.


Via intestinal tract k excretion in colon

② Via intestinal tract K excretion in colon

  • 10% of K is excreted through the colon. The epithelial cells are just as the principal cells in the collecting duct, and affected by aldosterone.

  • K+ excretion via colon will increase in renal failure.

  • GFR↓↓↓ K excretion in colon to 1/3 of intake .


Section iii disturbance of potassium balance

  • ③ Via sweating

  • Generally speaking, the loss of K+ with sweat is neglectful (5~10 mmol/L). This kind of loss may be significant some time (in plenty of sweat).


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