Excretion

3. The urinary system

The kidneys are part of the urinary system, which filters the blood and removes water-soluble waste in the form of urine. The complete system has five named parts:

A tip for telling the two tubes apart: the ureter is one of two tubes that goes between upper organ (kidney) and bladder, while the urethra is the single tube that ends at the thigh.

4. Inside the kidney

A vertical cross-section through a kidney shows three regions:

• Cortex is the outer layer. Contains the Bowman's capsules, glomeruli, proximal convoluted tubules and distal convoluted tubules (covered in section 5).
• Medulla is the inner darker layer. Contains the loops of Henle and most of the collecting ducts.
• Renal pelvis is the funnel-shaped cavity at the very centre. Urine drains into the renal pelvis from all the collecting ducts and flows from there into the ureter.

Each kidney contains roughly one million tiny filtering units called nephrons. Each nephron starts in the cortex, dips down into the medulla, and ends at the renal pelvis. Together they filter the entire blood plasma volume around 60 times every day.

5. The nephron

The nephron is the functional unit of the kidney: every step of urine formation happens along its length. A single nephron has these parts, in order along the route the filtrate takes:

1. Bowman's capsule: a cup-shaped structure in the cortex that surrounds the glomerulus, a tight knot of capillaries. The Bowman's capsule is where blood is filtered (ultrafiltration, section 6).
2. Proximal convoluted tubule (PCT): a coiled tube also in the cortex. Selective reabsorption of glucose happens here.
3. Loop of Henle: a long hairpin loop that dips deep down into the medulla and back up. Reabsorbs some water and salts.
4. Distal convoluted tubule (DCT): a second coiled tube, back in the cortex. Fine-tunes the salt content of the filtrate.
5. Collecting duct: runs back down through the medulla to the renal pelvis. The site of most water reabsorption, controlled by the hormone ADH (section 7).

Around each nephron runs a dense network of capillaries that picks up the substances reabsorbed from the filtrate and returns them to the bloodstream.

6. How urine is formed

Three main processes turn blood into urine as the filtrate flows along the nephron:

1. Ultrafiltration in the glomerulus / Bowman's capsule (filtration under pressure)
2. Selective reabsorption in the PCT (taking useful molecules back)
3. Water reabsorption in the collecting duct (controlled by ADH)

Ultrafiltration

The Bowman's capsule surrounds a small knot of capillaries called the glomerulus. Two clever design features set up high-pressure filtration here:

• The afferent arteriole (the vessel bringing blood in) is wider than the efferent arteriole (the vessel taking blood out). This produces a bottleneck that raises the blood pressure inside the glomerulus to several times the normal capillary pressure.
• The wall of the glomerular capillary and the wall of the Bowman's capsule together form an extremely thin, partially-permeable filter.

The high blood pressure forces the small components of plasma out of the capillaries and across the filter into the Bowman's capsule:

• Water
• Glucose
• Amino acids
• Urea
• Mineral ions (Na⁺, K⁺, Cl⁻, etc.)

Larger molecules and cells are too big to cross the filter and stay in the blood:

• Red blood cells
• White blood cells
• Platelets
• Plasma proteins (such as albumin)

The fluid that collects inside the Bowman's capsule is called the glomerular filtrate. It is blood plasma without the proteins and cells.

High blood pressure in the glomerulus can damage the filter, allowing proteins to get through. Finding protein in urine is therefore an early sign of high blood pressure or kidney disease.

Selective reabsorption in the PCT

The glomerular filtrate is full of valuable molecules that the body cannot afford to wash away. As the filtrate flows along the proximal convoluted tubule (PCT), these useful substances are pulled back into the bloodstream through the surrounding capillaries:

• All of the glucose is reabsorbed (by active transport, against its concentration gradient)
• All of the amino acids are reabsorbed
• Most of the mineral ions are reabsorbed
• A large amount of water is reabsorbed too, by osmosis (following the reabsorbed solutes)

It is selective because only some of the molecules are pulled back: useful ones move into the blood while waste substances (such as urea) are left behind in the filtrate.

Active transport of glucose against its gradient needs ATP, so the cells lining the PCT are packed with mitochondria. The wall of the PCT also has microvilli on its inner surface, dramatically increasing the surface area for reabsorption.

In a healthy person, all of the glucose in the filtrate is reabsorbed in the PCT, so there should be no glucose in the urine at all. If glucose is found in the urine, it usually means blood glucose has risen so high that the PCT cannot keep up. This is a classic sign of diabetes.

Water reabsorption in the loop of Henle and collecting duct

After the PCT, the filtrate flows down through the loop of Henle into the medulla and back up. The loop of Henle uses a clever counter-current arrangement (beyond this level in detail) to make the surrounding tissue of the medulla very salty. This means that when the filtrate later flows through the collecting duct back down through the same salty tissue, water moves out of the filtrate by osmosis into the blood.

The amount of water that is reabsorbed here depends on how much the body needs to keep, and that is controlled by the hormone ADH (section 7).

What is left in the collecting duct at the end of all of this becomes urine: water + urea + excess ions + a few other waste products. The urine flows into the renal pelvis, down the ureter, into the bladder, and eventually out through the urethra.

7. ADH and osmoregulation

Osmoregulation is the control of the water content and salt concentration of the body fluids within narrow limits

Body cells only function properly within a small range of water content. Too little water in the blood and cells lose water by osmosis and shrink. Too much water and cells swell and may burst. The kidneys keep things in balance by adjusting how much water they leave in the urine.

The hormone that controls this adjustment is ADH (antidiuretic hormone). The word "antidiuretic" means "against urine production", so high ADH makes the kidneys produce less urine.

How ADH works (negative feedback)

The body uses a negative feedback loop to keep blood water content steady. The loop has these stages:

1. Detection. Special cells called osmoreceptors in the hypothalamus of the brain monitor the water concentration of the blood as it flows past.
2. Signalling. The hypothalamus tells the pituitary gland (just beneath it) to release more or less ADH into the blood.
3. Effect on the kidney. ADH circulates to the kidneys, where it changes how permeable to water the walls of the collecting ducts are.
4. Reabsorption. With more permeable walls, more water moves out of the filtrate back into the blood, leaving less in the urine. With less permeable walls, less water is reabsorbed and more ends up in the urine.

When blood water is too LOW (dehydrated)

For example, after exercise on a hot day, or after eating a salty meal:

1. Osmoreceptors detect that the blood is too concentrated (low water).
2. Hypothalamus signals the pituitary to release more ADH.
3. ADH makes the collecting ducts more permeable to water.
4. More water is reabsorbed from the filtrate back into the blood.
5. The body produces a small volume of concentrated, dark yellow urine.
6. Blood water rises back to normal. (You also feel thirsty and drink water, which adds more.)

When blood water is too HIGH (over-hydrated)

For example, after drinking a litre of water:

1. Osmoreceptors detect that the blood is too dilute (high water).
2. Hypothalamus signals the pituitary to release less ADH.
3. ADH levels drop. Collecting ducts become less permeable to water.
4. Less water is reabsorbed; more passes into the urine.
5. The body produces a large volume of dilute, pale yellow urine.
6. Blood water falls back to normal.

Key mark-scheme phrase: "ADH increases the permeability of the collecting duct to water." Exam mark schemes expect this exact wording.

8. The composition of urine

In a healthy person, urine is roughly:

Urine should not normally contain:

• Glucose (all reabsorbed in the PCT)
• Amino acids (all reabsorbed)
• Proteins (too large to be filtered)
• Blood cells (too large to be filtered)

Finding any of these in urine is a sign of disease. Glucose suggests diabetes; protein suggests kidney damage or very high blood pressure.

What changes the colour and volume of urine

The volume and concentration of urine can change rapidly:

• More fluid drunk = larger volume of paler urine
• Hot weather or vigorous exercise = more water lost as sweat, so a smaller volume of darker urine
• Cold weather = less sweating, so more urine
• Salty diet = body needs to lose excess salt, often as concentrated urine
• Caffeine and alcohol = both block or reduce ADH release, producing larger volumes of paler urine and contributing to dehydration

9. Kidney failure

Each kidney has so much spare filtration capacity that a person can live a normal life with only one working kidney. But if both kidneys fail (a condition called renal failure), urea, water and ions accumulate in the blood within days, and the patient will die without treatment. There are two main treatments: dialysis and kidney transplant.

Dialysis (an artificial kidney)

Dialysis is a machine-based treatment that artificially performs the kidney's filtration job. The most common type is haemodialysis:

1. Blood from one of the patient's arteries flows out of the body and into the dialysis machine.
2. Inside the machine, the blood passes through a long network of partially permeable tubes (the artificial filter).
3. Outside these tubes flows a dialysis fluid designed to match the normal levels of glucose, mineral ions and water that should be in healthy blood. The fluid contains no urea.
4. Urea, excess ions and excess water diffuse out of the blood through the partially permeable membrane into the dialysis fluid (down a concentration gradient).
5. Useful substances such as glucose and the right amount of ions stay in the blood, because the dialysis fluid already has the same concentration of these (no gradient → no net diffusion).
6. The cleaned blood returns to a vein, back into the body.

A typical haemodialysis session lasts about 4 hours and is done three times a week at a hospital or clinic. Patients have to:

• Watch their diet closely (especially salt and protein intake, since the machine is not as efficient as a real kidney at controlling these)
• Limit how much fluid they drink between sessions
• Accept the time commitment, the restrictions on travel, and the long-term risks of infection at the access point

Dialysis keeps people alive, sometimes for many years, but it is not a cure. Without it, kidney failure is fatal within days.

Kidney transplant

A kidney transplant replaces the failed kidney with a healthy one from a donor. The donor can be:

• A close living relative (who can donate one kidney and survive normally with the other)
• A recently deceased person whose family has consented to organ donation

The surgery places the donor kidney into the lower abdomen, connects its blood vessels to the patient's arteries and veins, and connects its ureter to the patient's bladder. If everything works, the patient's body fluid balance returns to normal within days.

Advantages of a transplant over dialysis:

• A working transplanted kidney is far more efficient than a dialysis machine
• The patient does not need to attend the clinic three times a week
• Diet and fluid restrictions can usually be relaxed
• The cost per year is much lower in the long run

Disadvantages and challenges of a transplant:

• The donor kidney is recognised by the recipient's immune system as foreign because its antigens do not match. The recipient's lymphocytes can attack the transplant, a process called rejection.
• To reduce the chance of rejection, the donor must be a tissue match (the donor's antigens must be as close to the recipient's as possible). Close relatives are usually the best matches.
• The recipient must take immunosuppressant drugs for life to dampen the immune response. This makes them more vulnerable to infections and some cancers.
• There is a shortage of donor kidneys, and the waiting list is often long. Many patients spend years on dialysis before a suitable kidney becomes available.