Kidney Structure, Ultrafiltration, Selective Reabsorption & ADH
The kidneys are essential homeostatic organs that maintain water and ion balance (osmoregulation) in the blood while excreting toxic urea produced from excess amino acids in the liver. Inside each kidney, millions of microscopic tubules called nephrons filter the blood under high pressure (ultrafiltration) and selectively reabsorb 100% of the glucose alongside required amounts of water and mineral ions. Examiners frequently test the negative feedback loop of Anti-Diuretic Hormone (ADH) secreted by the pituitary gland, as well as evaluating kidney dialysis against kidney transplantation.
- Deamination & Urea Formation
- Occurs in the liver. Excess amino acids from digested proteins are deaminated to form ammonia (which is toxic). The liver immediately converts ammonia into urea, which is released into the bloodstream to be filtered out by the kidneys.
- Ultrafiltration in the Kidney
- High blood pressure in the knot of capillaries (glomerulus) forces small molecules (water, glucose, urea, and mineral ions) out of the blood and into the start of the kidney tubule (Bowman's capsule). Large proteins and blood cells are too big to pass through the filter and remain in the blood.
- Selective Reabsorption
- As the glomerular filtrate flows along the kidney tubule (nephron), useful substances are reabsorbed back into the bloodstream: 100% of glucose is reabsorbed by active transport and diffusion, along with the exact amount of water (by osmosis) and mineral ions needed by the body. Leftover water, excess ions, and all urea form urine.
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Heart (Double Circulatory Pump)
Structure & Function:
Pumps blood in a double circulatory system: the right ventricle pumps deoxygenated blood to the lungs, and the thick muscular left ventricle pumps oxygenated blood at high pressure around the whole body.
Key Exam Mark-Scheme Point:
Exam tip: The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood at high pressure all the way around the entire body, not just to the lungs.
What happens if this organ fails or is removed?
Without pressure from the heart, blood flow stops immediately. Oxygen and glucose cannot reach tissues, and waste carbon dioxide cannot be removed.
Tip: Use the “Simulate Organ Failure” button above to see how each organ system depends on the others to maintain homeostasis and cellular respiration.
Key Concepts & Exam Definitions (6)
- Deamination & Urea Formation
- Occurs in the liver. Excess amino acids from digested proteins are deaminated to form ammonia (which is toxic). The liver immediately converts ammonia into urea, which is released into the bloodstream to be filtered out by the kidneys.
- Ultrafiltration in the Kidney
- High blood pressure in the knot of capillaries (glomerulus) forces small molecules (water, glucose, urea, and mineral ions) out of the blood and into the start of the kidney tubule (Bowman's capsule). Large proteins and blood cells are too big to pass through the filter and remain in the blood.
- Selective Reabsorption
- As the glomerular filtrate flows along the kidney tubule (nephron), useful substances are reabsorbed back into the bloodstream: 100% of glucose is reabsorbed by active transport and diffusion, along with the exact amount of water (by osmosis) and mineral ions needed by the body. Leftover water, excess ions, and all urea form urine.
- ADH (Anti-Diuretic Hormone) & Negative Feedback
- Receptors in the hypothalamus detect the water content of the blood. If blood water content is too low (e.g., due to sweating or dehydration), the pituitary gland secretes more ADH into the blood. ADH makes the walls of the kidney tubules more permeable to water, so more water is reabsorbed into the blood, producing a small volume of concentrated urine.
- Kidney Dialysis Machine
- An artificial machine that filters a kidney failure patient's blood across a partially permeable membrane. Dialysis fluid contains normal blood plasma levels of glucose and mineral ions (preventing net loss of glucose) and zero urea (creating a steep concentration gradient so urea diffuses out of the blood). Counter-current flow maintains the gradient.
- Kidney Transplant vs Dialysis
- Kidney transplant replaces the failed kidney with a healthy donor kidney: it offers a normal lifestyle free from 3-times-weekly 4-hour hospital sessions and strict diets, and is cheaper long-term, but carries surgical risks, donor shortages, and the risk of tissue rejection (requiring lifelong immunosuppressant drugs).
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Why is osmoregulation (maintaining water and ion balance in the blood) essential for cells?
Name three substances filtered into the kidney tubule during ultrafiltration, and two that remain in the blood.
Which substance is 100% reabsorbed during selective reabsorption in the kidney tubule?
Describe the ADH response when a person drinks two litres of water on a cold day.
How is the risk of tissue rejection reduced after a kidney transplant?
Why do dialysis machines flow blood and dialysis fluid in opposite directions (counter-current flow)?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Describe how the kidneys filter the blood and produce urine in a healthy person, explaining what happens to proteins, glucose, water, and urea. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Blood enters the kidney capillaries under high pressure, forcing small molecules (water, glucose, mineral ions, and urea) into the kidney tubule (ultrafiltration) [1 mark]. 2. Blood cells and large plasma proteins are too large to pass through the capillary membranes, so they remain in the blood [1 mark]. 3. As the filtrate flows along the kidney tubule, selective reabsorption takes place: 100% of the glucose is reabsorbed back into the blood by active transport [1 mark]. 4. The required volume of water is reabsorbed by osmosis (regulated by ADH) along with needed mineral ions [1 mark]. 5. None of the toxic urea is actively reabsorbed, so urea, excess water, and excess ions pass down the ureter to the bladder as urine [1 mark].
- Q2
Explain how the pituitary gland and kidneys work together via negative feedback to restore blood water levels when a marathon runner becomes dehydrated through heavy sweating. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Heavy sweating causes water loss, making the blood plasma more concentrated (lower water potential) [1 mark]. 2. Osmoreceptors in the hypothalamus of the brain detect that the water content of the blood is too low [1 mark]. 3. This stimulates the pituitary gland to secrete more ADH (anti-diuretic hormone) into the bloodstream [1 mark]. 4. ADH travels to the kidneys and causes the walls of the kidney tubules (collecting ducts) to become more permeable to water [1 mark]. 5. More water is reabsorbed from the tubules back into the blood by osmosis, restoring blood water concentration towards normal and producing a small volume of concentrated urine [1 mark].
- Q3
Explain how a kidney dialysis machine removes urea from a patient's blood without removing glucose. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The patient's blood flows alongside dialysis fluid separated by a partially permeable membrane that allows small molecules to diffuse across but blocks blood cells and proteins [1 mark]. 2. The dialysis fluid contains zero urea, creating a steep concentration gradient so urea diffuses out of the patient's blood into the dialysis fluid [1 mark]. 3. The dialysis fluid contains the exact same concentration of glucose (and normal mineral ions) as healthy blood plasma [1 mark]. 4. Because there is no concentration gradient for glucose between the blood and the dialysis fluid, there is no net diffusion of glucose out of the patient's blood [1 mark].
- Q4
Evaluate the use of a kidney transplant compared to regular haemodialysis for treating end-stage kidney failure. [6 marks]
Click “Check Mark Scheme” to compare your answer, 1. Advantages of transplant: The patient does not need to spend 3 to 4 hours attached to a dialysis machine 3 times every week, giving them a much better quality of life and freedom to travel [1 mark]. 2. Transplant patients can eat and drink normally, whereas dialysis patients must follow a strict low-protein and low-salt diet and restrict fluid intake [1 mark]. 3. Although transplant surgery is expensive initially, it is much cheaper for the healthcare system over the long term than years of continuous dialysis [1 mark]. 4. Disadvantages of transplant: There is a severe shortage of compatible donor kidneys (long waiting lists, during which dialysis is essential to stay alive) [1 mark]. 5. The patient's immune system may recognise the donor kidney antigens as foreign and reject the organ, requiring lifelong immunosuppressant drugs that increase vulnerability to infections [1 mark]. 6. Transplant requires major surgery under general anaesthetic (risk of bleeding/infection), and a transplanted kidney only lasts around 9 to 12 years on average before needing replacement [1 mark].
Common Exam Questions (4)
How is urea made in the human body?
Excess amino acids from digested proteins cannot be stored in the body. In the liver, amino acids undergo deamination: their amino groups are removed to form toxic ammonia, which the liver immediately converts into a safer waste molecule called urea. Urea is then transported in the blood to the kidneys for excretion in urine.
Why is there zero glucose in the urine of a healthy person, and none in blood cells?
Blood cells and large plasma proteins are too big to pass through the capillary and Bowman's capsule membranes during ultrafiltration, so they never enter the kidney tubule. Glucose is small enough to be filtered into the tubule, but 100% of the glucose is reabsorbed back into the blood by selective reabsorption (using active transport).
How does ADH control the concentration and volume of urine?
When blood water levels drop (blood becomes too concentrated), the pituitary gland releases more ADH into the blood. ADH increases the permeability of the kidney tubules (collecting ducts), so more water is reabsorbed back into the blood by osmosis, producing a small volume of concentrated (dark yellow) urine.
Why does dialysis fluid contain the same concentration of glucose and mineral ions as healthy blood plasma, but zero urea?
Having the same concentration of glucose and mineral ions as healthy blood ensures there is no net diffusion gradient for glucose, so the patient does not lose essential glucose, while any excess ions diffuse out. Having zero urea maintains a steep concentration gradient so urea continuously diffuses out of the patient's blood into the dialysis fluid.
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