The Kidney and Osmoregulation
The kidneys are vital organs responsible for excretion of metabolic waste, osmoregulation (control of blood water potential), and regulation of blood pH. Understanding the structure of the nephron and the mechanisms of ultrafiltration, selective reabsorption, and the role of ADH is essential for A-Level Biology.
Key Definition: Osmoregulation is the homeostatic control of the water potential of the blood, ensuring that cells neither gain nor lose water by osmosis to a degree that would disrupt their function.
Gross Structure of the Kidney
Each kidney contains approximately one million nephrons — the functional units of the kidney. The kidney has three main regions:
- Cortex — the outer region. Contains the Bowman's capsules, proximal and distal convoluted tubules, and the glomeruli.
- Medulla — the inner region. Contains the loops of Henle and collecting ducts arranged in pyramids.
- Pelvis — the central cavity where urine collects before passing down the ureter to the bladder.
The renal artery brings blood to the kidney; the renal vein carries filtered blood away; the ureter carries urine to the bladder.
Structure of the Nephron
The nephron is the functional unit of the kidney. Each nephron consists of:
1. Bowman's Capsule (Renal Capsule)
- A cup-shaped structure in the cortex that surrounds the glomerulus (a knot of capillaries).
- The inner wall is lined with specialised cells called podocytes, which have foot-like processes (pedicels) with gaps between them called filtration slits.
- The Bowman's capsule collects the filtrate.
2. Proximal Convoluted Tubule (PCT)
- A coiled tube in the cortex, immediately following the Bowman's capsule.
- The walls are composed of cuboidal epithelial cells with microvilli (brush border), many mitochondria, and co-transporter proteins.
- This is where most selective reabsorption occurs.
3. Loop of Henle
- A hairpin-shaped loop that dips into the medulla and returns to the cortex.
- Descending limb — permeable to water, impermeable to ions.
- Ascending limb — impermeable to water, actively pumps Na⁺ and Cl⁻ out into the medulla.
4. Distal Convoluted Tubule (DCT)
- A coiled tube in the cortex.
- Involved in fine-tuning the composition of the filtrate.
- Responds to hormones such as aldosterone (controls Na⁺ reabsorption).
5. Collecting Duct
- Passes through the medulla, collecting filtrate from several nephrons.
- Permeability to water is controlled by ADH (antidiuretic hormone).
- Plays a crucial role in producing concentrated or dilute urine.
Ultrafiltration
Ultrafiltration occurs in the Bowman's capsule, where blood is filtered under high pressure to produce the glomerular filtrate.
How High Pressure is Created
- The afferent arteriole (bringing blood to the glomerulus) has a wider diameter than the efferent arteriole (carrying blood away).
- This creates a high hydrostatic pressure in the glomerular capillaries, forcing fluid and small solutes out of the blood.
The Filtration Barrier
The filtrate must pass through three layers:
- Endothelium of the glomerular capillary — has small pores (fenestrations) that allow plasma and small molecules through but retain blood cells.
- Basement membrane — a fine mesh of glycoproteins and collagen that acts as a molecular filter, preventing molecules with a relative molecular mass greater than approximately 69,000 (e.g., plasma proteins) from passing through.
- Podocytes of the Bowman's capsule — their pedicels create filtration slits that allow the filtrate to pass into the capsule.
What is Filtered?
| Filtered (in glomerular filtrate) | Not filtered (remain in blood) |
|---|
| Water | Blood cells (red and white) |
| Glucose | Platelets |
| Amino acids | Large plasma proteins (e.g., albumin) |
| Urea | |
| Inorganic ions (Na⁺, K⁺, Cl⁻) | |
| Small hormones | |
Exam Tip: The presence of protein or blood cells in the urine indicates damage to the filtration barrier (e.g., kidney disease). Glucose in the urine suggests diabetes mellitus (blood glucose exceeds the reabsorptive capacity of the PCT).
Selective Reabsorption
Most of the glomerular filtrate is reabsorbed in the proximal convoluted tubule (PCT). This is selective because useful substances are reabsorbed, while waste products (e.g., urea) remain in the filtrate.
In the PCT (approximately 85% of filtrate reabsorbed here):