Ultrafiltration

Where it happens

Ultrafiltration takes place at the glomerulus, a knot of capillaries inside the Bowman's capsule at the start of each nephron in the kidney.

Inputs and outputs

• Input: blood arriving through the renal artery and entering the glomerulus by the afferent arteriole, carrying water, glucose, amino acids, mineral salts, urea, plasma proteins and blood cells.
• Input: high hydrostatic pressure generated by the difference in width between the afferent and efferent arterioles.
• Output to the Bowman's capsule: glomerular filtrate containing water, glucose, amino acids, urea and mineral salts.
• Output through the efferent arteriole: blood that has lost water and small solutes but keeps its blood cells and plasma proteins, now more concentrated.
• End result: the filtrate flows into the proximal convoluted tubule, where selective reabsorption begins.

The steps

1. Blood enters the glomerulus through the afferent arteriole, which is wider than the efferent arteriole leaving it.
2. This difference in arteriole width builds up a high hydrostatic (blood) pressure inside the glomerular capillaries.
3. The high pressure forces water and small dissolved substances out through the capillary walls and the wall of the Bowman's capsule.
4. Blood cells and plasma proteins are too large to be forced out, so they remain in the blood.
5. The fluid that is filtered out, called the glomerular filtrate, collects in the Bowman's capsule and flows into the renal tubule.
6. The filtrate passes through three layers in sequence: the pores in the capillary endothelium, the basement membrane, which is the main filter that stops proteins, and the slits between the podocyte cells of the Bowman's capsule wall.
7. Blood leaving through the efferent arteriole flows on into the capillary network around the tubule, where it later reabsorbs the useful substances that were filtered out.

Why it matters and how it is controlled

Ultrafiltration produces the filtrate that later passes along the renal tubule, where useful substances such as glucose and most water are reabsorbed into the blood while waste, including urea, is left to form urine.

Ultrafiltration is non-selective. It separates molecules by size only, so useful substances such as glucose and amino acids leave the blood along with urea.

The kidney relies on the tubule to reclaim them, which is why glucose appears in the urine only when the reabsorption capacity is exceeded, as in untreated diabetes mellitus.

The rate of filtration depends on the pressure in the glomerulus. The afferent and efferent arterioles can narrow or widen, adjusting this pressure so that filtration continues at a fairly steady rate even when the body's blood pressure changes during exercise or rest.

A large fall in blood pressure reduces filtration, and the body responds by constricting the efferent arteriole to raise glomerular pressure again.

Because the plasma proteins stay behind, the blood leaving the glomerulus has a lower water potential than the filtrate. This difference pulls some water back into the capillaries around the tubule and is one of the reasons reabsorption of water is so efficient.

How it is examined

You may be asked to explain why the afferent arteriole is wider than the efferent arteriole, to state which substances are and are not present in the glomerular filtrate, or to distinguish ultrafiltration from selective reabsorption.

Structured questions usually give a labelled diagram of a nephron, or a table comparing the composition of blood plasma, glomerular filtrate and urine. You are expected to explain why protein is present in plasma but absent from filtrate, why glucose is present in filtrate but absent from urine, and why the concentration of urea rises from filtrate to urine.

Essay questions may ask you to describe the formation of urine, combining ultrafiltration with reabsorption. A high-scoring answer keeps the two stages separate, names the site and driving force of each, and states which substances move at each stage.

Common misconceptions

Worked exam-style question

Question. The table below shows the concentration of three substances in blood plasma and in glomerular filtrate collected from a healthy person. Plasma protein: plasma 7.0 g per 100 cm³, filtrate 0.

Glucose: plasma 0.1 g per 100 cm³, filtrate 0.1 g per 100 cm³. Urea: plasma 0.03 g per 100 cm³, filtrate 0.03 g per 100 cm³.

(a) Explain why the concentration of plasma protein in the filtrate is zero. (b) Explain why glucose and urea have the same concentration in plasma and filtrate.

(c) Explain how the structure of the arterioles supplying the glomerulus makes ultrafiltration possible.

Model answer. (a) Plasma proteins have a large molecular size, so they cannot pass through the basement membrane and the pores of the capillary wall; they remain in the blood. (b) Glucose and urea are small molecules that pass freely through the filter with the water.

Ultrafiltration is non-selective and separates by size only, so the filtrate has the same concentration of small solutes as plasma. (c) The afferent arteriole is wider than the efferent arteriole.

Blood enters faster than it can leave, so a high hydrostatic pressure builds up in the glomerular capillaries. This pressure forces water and small solutes out into the Bowman's capsule.

Source:SRC-DSKP-EN

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