The kidney and urea removalEdexcel International A Level Biology: Revision notes

Mammals have two bean-shaped kidneys. Blood arrives from the aorta by the renal artery and leaves in the renal vein. Urine leaves each kidney in the ureter, which carries it to the bladder.

A section through the kidney shows three regions:

• The cortex, the outer region, is dark red and granular because it contains many capillaries, Bowman's capsules and convoluted tubules.
• The medulla, the inner region, is paler and striped. It contains the loops of Henle and collecting ducts, arranged in pyramids.
• The renal pelvis is a funnel-shaped cavity where the collecting ducts empty urine before it enters the ureter.

The functional unit of the kidney is the nephron. Each kidney has around a million. Its parts, in order, are:

• Bowman's capsule, a cup-shaped structure that surrounds the glomerulus.
• The glomerulus, a knot of capillaries inside the capsule.
• The proximal convoluted tubule (PCT), lined with cuboidal cells with microvilli and many mitochondria.
• The loop of Henle, with a descending limb and an ascending limb dipping into the medulla.
• The distal convoluted tubule (DCT).
• The collecting duct, which several nephrons share.

Blood reaches the glomerulus in the afferent arteriole and leaves in the narrower efferent arteriole, which then forms a capillary network around the tubules.

Humans cannot store excess amino acids. In the liver, they undergo deamination: the amino group (–NH₂) is removed, forming ammonia, and the remaining carbon skeleton is respired or converted to carbohydrate or fat.

Ammonia is very soluble and toxic, so the liver combines it with carbon dioxide in the ornithine cycle (you do not need the details) to form urea, CO(NH₂)₂. Urea is much less toxic. It passes from the liver cells into the blood plasma and is carried to the kidneys for removal.

Blood in the glomerulus is at a high hydrostatic pressure because the afferent arteriole is wider than the efferent arteriole. This forces fluid and small solutes out of the capillary in ultrafiltration. The filtration barrier has three layers:

• The capillary endothelium has gaps (fenestrations) between cells.
• The basement membrane, a mesh of collagen and glycoproteins, acts as the main filter and holds back molecules larger than about 69 000 daltons, such as plasma proteins.
• The podocytes of the capsule wall have slit-like gaps between their foot-like processes.

The glomerular filtrate contains water, glucose, amino acids, urea and ions at the same concentration as plasma. It contains no blood cells and no large plasma proteins.

Filtration happens because the pressure forcing fluid out exceeds the pressures opposing it. The opposing forces are the hydrostatic pressure of fluid in the capsule and the osmotic pressure of the plasma proteins, which stay in the blood.

Worked example: glomerular hydrostatic pressure 8.0 kPa, capsule pressure 2.0 kPa, plasma protein osmotic pressure 3.5 kPa.

Net filtration pressure = 8.0 − (2.0 + 3.5) = 2.5 kPa, so fluid is forced out.

Narrowing the efferent arteriole raises the glomerular pressure further, so more filtrate is formed. A healthy adult forms about 125 cm³ of filtrate per minute.