Water Balance, Blood Pressure, and pH

Kidneys help balance:

• Water 💧

• Blood Pressure (BP) ❤‍🩹

• pH (Acidity/Alkalinity) 🧪

💧 Water Balance Overview

Regulated by a hormone called:

🧪 ADH (Antidiuretic Hormone)

• What it does: Tells your kidneys to hold on to water

• How: Makes the collecting ducts (in nephrons) more permeable to water (lets water pass through)

• Why: To control how salty/concentrated your blood is (osmotic pressure)

• Where it comes from:

  • Made in the hypothalamus (brain’s “thirst centre”)

  • Stored & released from the posterior pituitary gland

🧠 Analogy: ADH is like a sponge-saver telling your body “we’re low on water, reuse what we can!”

💦 How Water Balance Works (LOW WATER)

↓ Water in body → ↑ Blood solute (salts) → ↑ Osmotic pressure

Here’s what happens step-by-step:

1. Cells shrink as water leaves them to balance the salty blood

2. Osmoreceptors (in hypothalamus) detect shrinkage

3. ADH is released

4. ADH goes to the collecting ducts

5. Ducts open up to let water back into blood

6. You get thirsty

7. Water is reabsorbed → urine becomes more concentrated

🧠 Analogy: Imagine your body is a desert. ADH is like an emergency water ration system, saving every drop possible.

💦 What Happens When You Drink Water? (HIGH WATER)

1. Water is absorbed

2. Osmoreceptors swell (happy again)

3. ADH stops being released

4. Less water is reabsorbed

5. You pee out the excess → urine is lighter

🧠 Analogy: Like turning off the sprinklers when the soil isn’t dry anymore 🌧

🔄 ADH and the Nephron (Diagram likely here)

• Nephron = filter unit of the kidney

• 85% of water = automatically reabsorbed

• With ADH: Remaining 15% is reabsorbed from the collecting duct

• Without ADH: That 15% is lost → more urine

🧠 Analogy: Imagine ADH as a gatekeeper deciding whether to save or flush water.

✅ Step 1: Automatic Water Saving

• Inside your nephron, about 85% of the water is automatically reabsorbed back into the body.

• No hormones needed here — it just happens by default.

✅ Step 2: ADH Kicks In

• The last 15% of water can either be saved or flushed out.

• This is where ADH steps in and decides what to do with that leftover water.

❤ Kidneys and Blood Pressure

1. If you lose fluid (sweat, bleed) → ↓ blood pressure

2. Juxtaglomerular Apparatus in kidney detects low BP

3. Releases renin

4. Renin → converts angiotensinogen (from liver) → angiotensin

Angiotensin:

• Tightens blood vessels (↑ BP)

• Triggers aldosterone release

Aldosterone (from adrenal gland):

• Acts on distal tubule

• Increases NaCl reabsorption

• That pulls in more water

• That raises blood pressure

🧠 Analogy: Renin = firefighter 🚒 → Angiotensin = hose tightening pipes → Aldosterone = fire hydrant adding pressure to get water back in!

🥵 Step 1: You Lose Water → BP Drops

If you:

• sweat a lot ☀

• bleed 🩸

• get dehydrated

➡ Your blood pressure goes down (because there’s less fluid in your blood vessels).

---

🧠 Step 2: Kidney Detects It

There’s a sensor inside your kidney called the:

Juxtaglomerular Apparatus (say: “jucks-ta-glow-MER-you-lar”)

It checks for low blood pressure like a guard watching a pressure gauge.

---

🧪 Step 3: It Releases Renin

This sensor tells the kidney:

“Hey! We’re running low — time to help out.”

So the kidney releases a hormone called Renin (REE-nin).

---

🔄 Step 4: Renin Starts a Chain Reaction

Renin goes to the liver and says:

“Send me angiotensinogen!”

Then it changes angiotensinogen into angiotensin.

💥 What does angiotensin do?

---

🧠 What Angiotensin Does:

Action	Why It Matters
Tightens blood vessels	Makes vessels narrower → pressure rises 🧠💉
Triggers aldosterone	A hormone that makes your body hold salt and water

---

🧂 What Aldosterone Does:

It’s released from your adrenal glands (those sit above your kidneys).

Then it goes to your distal tubule (part of your nephron) and says:

🧂 “Reabsorb more salt (NaCl)!”
💧 “Keep more water too!”
📈 That causes blood pressure to go UP again!

---

🔥 Analogy: RAAS as a Firefighter System

• 🧑‍🚒 Renin = firefighter responding to low pressure

• 🚿 Angiotensin = tightens the hose to squeeze water through faster

• 🔥 Aldosterone = fire hydrant adding more water pressure from the base

So if your BP drops, your kidneys call the squad to bring it back u

⚖ pH Balance (Acidity/Alkalinity)

• Body pH = 7.3–7.5 (slightly basic)

• Too acidic/basic? Your body fixes it using buffers

Buffers:

• H+ ions = acidic

• Bicarbonate (HCO₃⁻) = base that soaks up H+

• Kidneys filter out or hold H+ to adjust pH

• Lungs also help: breathe out CO₂ → reduces acidity

• H₂O + CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

  💥 H⁺ Ions = Acidity

  • Think of H⁺ ions as acid particles.

  • The more H⁺ you have → the more acidic your blood is.

  ---

  🧴 Bicarbonate = Your Main pH Buffer

  • Bicarbonate (chemical name HCO₃⁻) is a base.

  • It soaks up H⁺ ions, neutralizing acidity.

  Analogy: Imagine H⁺ ions are like spicy chili flakes. Bicarbonate is milk — it cools things down 🌶🧊

  ---

  🧂 How Your Body Adjusts pH 🧠 Kidneys:

  • The kidneys help maintain pH balance long-term.

  • If there’s too much H⁺, kidneys can:

    • Filter them out in urine

    • Or keep them if your blood’s too basic

  ---

  🌬 Lungs:

  • Your lungs also help balance pH fast!

  • When you breathe out CO₂, it reduces acidity.

  Why?

  CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

  That’s a chemistry equation that means:

  CO₂ becomes acid in your blood. So if you breathe out more CO₂ → you remove acid!

  ---

  🔁 Final Analogy:

  Think of your body like a pool that needs just the right pH.

  Lungs = fan that blows off the bubbles (CO₂)

  Kidneys = filter that adjusts the pH long term

  Bicarbonate = baking soda keeping everything chill

✅ Recap (How kidneys help maintain balance):

Role	Hormone or Process	What Happens
Water Balance	ADH	Saves water by reabsorbing in collecting duct
Blood Pressure	Renin → Angiotensin → Aldosterone	Constricts vessels, keeps NaCl + water → raises BP
pH Balance	Buffers (HCO₃⁻), H⁺ secretion	Maintains blood acidity within normal range (7.3–7.5)

One-line memory trick:

Angiotensin tightens, aldosterone fills.
Angiotensin tightens your vessels. Aldosterone fills them with salt and water.