Glossary: Homeostasis and Human Urinary System

This chapter's terms in context

Chapter 13 links five terms that together explain how the human body keeps its internal environment stable while producing urine: homeostasis, nephron, ultrafiltration, osmoregulation and deamination. Homeostasis is the maintenance of a stable internal environment within narrow limits despite changes in external conditions, achieved through negative feedback mechanisms that detect a deviation from a set point and trigger a corrective response to return the body towards that set point, blood glucose regulation and body temperature regulation, covered in nearby chapters, are both applications of this single underlying principle, which is why homeostasis functions as a unifying concept across several chapters rather than belonging only to this one.

The nephron is the basic structural and functional unit of the kidney, and each kidney contains roughly a million of them working in parallel; a nephron consists of a cup-shaped Bowman's capsule surrounding a knot of capillaries called the glomerulus, followed by a long, coiled tubule that leads to a collecting duct. Urine formation happens in two main stages along this structure, and examiners expect both stages to be named and explained correctly rather than treated as one blended process.

Ultrafiltration is the first stage: blood pressure forces water, glucose, amino acids, urea, salts and other small molecules out of the glomerulus capillaries and into the Bowman's capsule, while blood cells and large protein molecules are too big to pass through and remain in the blood. The resulting fluid, called the glomerular filtrate, is not yet urine, it still contains useful substances such as glucose and amino acids that the body cannot afford to lose, which is exactly why selective reabsorption must follow along the tubule, returning useful substances such as all of the glucose and most of the water back into the blood, leaving only urea, excess salts and excess water to continue on as urine. A frequent exam error is describing ultrafiltration as though it already produces urine, when in fact urine only exists after reabsorption has removed the substances the body still needs.

Osmoregulation is the specific homeostatic control of the water content and, correspondingly, the concentration of body fluids, achieved mainly by adjusting how much water is reabsorbed from the tubule back into the blood; this is controlled by antidiuretic hormone (ADH), released by the pituitary gland in greater amounts when blood water potential is low, which increases the permeability of the tubule wall to water and produces a smaller volume of more concentrated urine, and in smaller amounts when blood water potential is high, producing a larger volume of more dilute urine.

Deamination is a distinct, separate process that occurs in the liver, not the kidney: it is the removal of the amino group from excess amino acids that cannot be stored, converting them into ammonia (quickly converted to the less toxic urea) and a keto acid that can still be respired for energy. A common structural confusion is placing deamination inside the kidney or nephron, when it is in fact a liver function that supplies the urea the kidney then excretes, the kidney filters and concentrates urea, but it is the liver that produces it in the first place. Keeping the liver's role and the kidney's role clearly separate, while recognising that the kidney's job depends entirely on what the liver has already supplied, is the structural insight this chapter is really testing beneath its list of names.

Paper 2 questions on this chapter frequently ask a candidate to interpret a table comparing the composition of blood plasma, glomerular filtrate and urine, and the strongest answers explain each difference by naming the specific stage responsible, the near-total absence of protein in the filtrate is explained by ultrafiltration, while the near-total absence of glucose in normal urine is explained by selective reabsorption, not filtration. Treating every difference between these three fluids as evidence of the same single process is a common and avoidable source of lost marks.