urination

The rate of filtration is critical for homeostasis.
The fluid that leaves the blood is called glomerular filtrate once it is formed.

Filtrate moves through the nephron into the tubules:
- Proximal Convoluted Tubule (PCT)
- Loop of Henle
- Distal Convoluted Tubule (DCT)
- Final stage: fluid reaches minor calyx and is now referred to as urine.
A simplified view of the nephron omits twists and turns, focusing on key processes.

Tubular Reabsorption: The process of reabsorbing necessary solutes from filtrate back into the body.
- Anything filtered but not needed is reabsorbed into extracellular fluid and eventually into peritubular capillaries.
- Examples of substances reabsorbed:
- Electrolytes (sodium, potassium, chloride, calcium)
- Glucose
- Fatty acids, amino acids, vitamins
- Kidney health: normally prevents larger molecules (like proteins or blood cells) from entering filtrate.
Tubular Secretion: Movement of substances from blood into the filtrate.
- Active transport is used to remove unwanted substances (toxins, drugs) from blood to urine.
- Essential for adjusting pH levels by regulating hydrogen and bicarbonate ions.
- Goal: maintain blood pH around 7.3.

Kidney filtration: filters 50 to 60 times all blood volume daily.
Importance of reabsorbing most of the filtrate:
- Normally, 99% of filtered substances are reabsorbed.
- Average urine output: 1 to 2 liters per day.
Disturbances in GFR can lead to problems:
- GFR too high: dehydration, loss of electrolytes.
- GFR too low: accumulation of waste products in body.

Regulation of GFR is achieved by adjusting blood pressure:
- Mechanisms include:
- Autoregulation: smooth muscle in arterioles responds to blood pressure changes.
- Neural regulation and hormones impact GFR.
Renin-Angiotensin-Aldosterone System (RAAS):
- Activated by decreased blood pressure, leading to renin release from kidneys.
- Renin converts angiotensinogen (from liver) into angiotensin I (10 amino acids).
- Angiotensin I is activated to angiotensin II (8 amino acids) by Angiotensin Converting Enzyme (ACE) in lungs/kidneys.
- Angiotensin II results in:
- Increased sodium and water retention by kidneys.
- Increased blood pressure and volume, stimulating thirst.

PCT is where significant reabsorption occurs (~65% of filtrate):
- Reabsorbs all glucose and significant amounts of water and electrolytes.
- Uses passive and active transport mechanisms.

Nephron loop focuses on salt and water balance.
Regulated by hormones:
- Aldosterone regulates sodium reabsorption, which causes water retention (osmosis follows salt).
- Aldosterone release occurs during:
- Drop in blood pressure or low sodium levels.
- High potassium levels (dangerous, necessitating potassium excretion).

Natriuretic Peptides stimulate increased urine production (natriuresis) to lower blood volume:
- Released by heart in response to atrial stretch (too much blood volume).
- Promote salt excretion, hence water follows.

ADH increases water retention in kidneys:
- Inserts aquaporins in collecting ducts to aid water reabsorption when needed (e.g., dehydration).
- Regulation occurs due to increased blood osmolarity, stimulating ADH release.

Collecting Duct: critical for water conservation based on hydration needs:
- Aquaporins allow selective water reabsorption based on osmotic gradients.
- Saltier gradients promote more water reabsorption.

Normal urine analysis looks for physical and chemical components:
- Appearance, odor, pH, concentration.
Key abnormal findings:
- Presence of proteins or blood cells indicates potential kidney issues.

Preview of fluid balance topic to be discussed next week.
Emphasis on practical labs to reinforce learning with models and physical assessments.