Blood composed of and what kidney filter
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Here's a concise overview of both topics, sourced from medical textbooks:
π©Έ What Blood is Composed Of
Blood consists of a protein-rich fluid called plasma, in which three cellular elements are suspended:
| Component | Details |
|---|---|
| Plasma | ~55% of total blood volume; contains proteins, electrolytes, nutrients, hormones, waste products |
| Red Blood Cells (Erythrocytes) | Carry oxygen via hemoglobin |
| White Blood Cells (Leukocytes) | Immune defense; normally 4,000β11,000 per Β΅L |
| Platelets (Thrombocytes) | Involved in clotting |
- Total circulating blood volume β 8% of body weight (~5,600 mL in a 70 kg person)
- Blood cells are produced in the bone marrow (in adults), from hematopoietic stem cells (HSCs)
β Ganong's Review of Medical Physiology, 26th Edition
π« How the Kidneys Filter Blood
The kidney's filtration works in two main steps:
1. Glomerular Filtration
- Blood is filtered at the glomerulus (the filtering unit inside each nephron)
- The Glomerular Filtration Rate (GFR) = filtration rate per nephron Γ number of nephrons in both kidneys
- Normal GFR: ~130 mL/min/1.73 mΒ² (men) / ~120 mL/min/1.73 mΒ² (women)
- GFR is the best overall indicator of kidney function
2. Tubular Reabsorption & Secretion
- After filtration, the renal tubules selectively reabsorb useful substances (water, glucose, electrolytes) back into the blood
- Some substances are secreted into the filtrate for excretion
- This process maintains homeostasis (fluid balance, pH, electrolytes)
β National Kidney Foundation Primer on Kidney Diseases, 8th Edition
In Summary
Blood = plasma + red cells + white cells + platelets. The kidneys filter blood through the glomerulus, then tubules fine-tune what stays in the body and what gets excreted as urine. GFR is the key measure of how well the kidneys are doing this job.
Why only plasma thing get filter
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Great question. The reason only plasma (fluid) gets filtered β not blood cells or large proteins β comes down to the physical structure of the glomerular filtration barrier. It acts like a series of progressively finer sieves.
Why Only Plasma Gets Filtered
The glomerular capillary wall has 3 layers, each blocking different things:
Layer 1 β Fenestrated Endothelium (pores 70β100 nm)
- Has large pores β allows water, dissolved solutes, and plasma proteins to pass
- Pores are too small for blood cells (red cells, white cells, platelets) β they stay in the capillary
- Also covered by a glycocalyx (negatively charged sugar-protein coat) that repels large proteins
Layer 2 β Glomerular Basement Membrane (GBM)
- A thick, dense mesh of collagen and proteins (type IV collagen, laminin, heparan sulfate)
- Acts as the most significant barrier β blocks large plasma proteins (e.g. albumin)
- Also carries a net negative charge, which repels negatively charged proteins like albumin (like charges repel)
Layer 3 β Podocytes with Slit Diaphragms
- Specialized cells with "foot processes" that interdigitate to form filtration slit diaphragms
- These slits are the final barrier against large proteins
- Key proteins (nephrin, podocin) maintain the slit structure β mutations cause massive protein leakage into urine
So Why Not Blood Cells?
Blood cells are simply too large to squeeze through the endothelial pores. The pores (70β100 nm) are much smaller than a red blood cell (~6,000β8,000 nm in diameter).
Summary Table
| What's in Blood | Gets Filtered? | Why |
|---|---|---|
| Water & small solutes (NaβΊ, urea, glucose) | β Yes | Small enough to pass all 3 layers |
| Small proteins (low MW) | β Small amount | Partly filtered, mostly reabsorbed |
| Albumin (large protein) | β Mostly No | Blocked by GBM + negative charge repulsion |
| Red/White blood cells, Platelets | β No | Too large to fit through endothelial pores |
If blood cells or albumin appear in urine, it signals glomerular damage (e.g. glomerulonephritis, nephrotic syndrome).
β Costanzo Physiology, 7th Edition | National Kidney Foundation Primer on Kidney Diseases, 8th Edition | Histology: A Text and Atlas
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