Homeostasis and Human Urinary System, revision notes

Overview

Homeostasis is the maintenance of a stable internal environment inside the body despite changes in the surroundings. This chapter opens with the general principle of negative feedback that underlies homeostasis, then applies it in detail to the urinary system, where the kidneys filter blood and produce urine through ultrafiltration and selective reabsorption in the nephron.

You are also expected to know how the same feedback principle controls body temperature and blood glucose level, since these are commonly tested alongside the nephron in the same chapter. Being able to describe a control mechanism as a cycle, a change is detected, a response is triggered, and the condition is corrected back toward normal, helps with all three examples.

The urinary system content is examined in structural detail: the kidney, ureter, bladder and urethra as organs, and the glomerulus, Bowman's capsule and tubules as the working parts of a single nephron. You should be able to trace a substance's journey from blood entering the kidney to urine leaving the body, and explain why certain substances are filtered out and then reabsorbed.

The final content standard covers health issues related to the urinary system, including kidney stones, urinary tract infection, glomerulonephritis and kidney failure requiring dialysis or a transplant. A good exam answer connects each condition to what goes wrong in a specific part of the system rather than describing it as a vague illness.

Homeostasis and negative feedback (13.1)

Homeostasis is the maintenance of a stable internal environment, body temperature, blood glucose, water and salt content and blood pH, so that enzymes and cells keep working within their optimum range. Every control mechanism in this chapter runs on the same loop called negative feedback, so learning the pattern once lets you answer three different examples with one framework.

In a negative feedback loop a receptor detects a change away from the set point, a control centre such as the hypothalamus or an endocrine gland processes the information, and an effector produces a response that opposes the original change and returns the condition toward normal. Because the response always counteracts the deviation instead of amplifying it, the internal environment is held within narrow limits.

The most reliable way to score is to write the loop as detect, process, respond and correct, then name the receptor, control centre and effector for the specific example asked.

Thermoregulation and blood glucose control (13.1)

Body temperature is monitored by the hypothalamus and by temperature receptors in the skin. When the body is too hot, skin arterioles undergo vasodilation so more blood flows near the surface and heat is lost by radiation, and sweat glands release more sweat, which cools the skin as it evaporates.

When the body is too cold, these arterioles undergo vasoconstriction to reduce heat loss, sweating stops, and skeletal muscles contract rapidly in shivering to release heat from respiration.

Blood glucose is regulated by the pancreas through two hormones with opposite effects. After a meal, a rise in blood glucose triggers insulin, which makes liver and muscle cells take up glucose and store it as glycogen, lowering the level.

Between meals, a fall in blood glucose triggers glucagon, which breaks stored glycogen back into glucose and raises the level. Both examples are negative feedback loops that keep each condition close to its set point.

The urinary system and the nephron (13.2)

The urinary system removes urea, excess water and excess salts from the blood. Blood enters each kidney through the renal artery, is filtered and processed, and leaves cleaned through the renal vein; the urine produced drains down the ureter to the bladder, where it is stored until it leaves the body through the urethra.

Urea is the nitrogenous waste formed in the liver from the breakdown of excess amino acids, which is why the urinary system is classed as an excretory system.

The working unit of the kidney is the nephron, and each kidney holds a very large number of them working in parallel. A single nephron begins at the glomerulus, a knot of capillaries enclosed by the cup-shaped Bowman capsule, then continues through the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and finally the collecting duct.

Being able to trace a substance along this route, and to name where filtration and reabsorption each happen, is the core skill this content standard tests.

Urine formation: ultrafiltration and selective reabsorption (13.2)

Urine is formed in two stages. First comes ultrafiltration at the glomerulus: the afferent arteriole bringing blood in is wider than the efferent arteriole taking blood out, so a high hydrostatic pressure builds up and forces water, glucose, salts and urea out of the blood and into the Bowman capsule as glomerular filtrate.

Large plasma proteins and blood cells are too big to pass through the filtration membrane and stay in the blood.

The second stage is selective reabsorption as the filtrate flows along the tubule. All of the glucose, much of the water and a controlled amount of salts are reabsorbed back into the surrounding blood capillaries, while urea and excess salts and water are left behind to become urine.

The process is described as selective because different substances are reabsorbed to different extents according to the body needs, which is why healthy urine contains no glucose at all.

Osmoregulation and the role of ADH (13.2)

Osmoregulation keeps the water content of the blood steady, and it is controlled by antidiuretic hormone (ADH). Osmoreceptors in the hypothalamus detect the water potential of the blood.

When the body is short of water, the blood becomes too concentrated, so more ADH is released from the pituitary gland. ADH makes the walls of the distal tubule and collecting duct more permeable to water, so more water is reabsorbed into the blood and a small volume of concentrated urine is produced.

When the body has too much water, the blood is too dilute, so less ADH is released. The collecting duct becomes less permeable, less water is reabsorbed, and a large volume of dilute urine is produced instead.

This is another negative feedback loop: the correction always opposes the original change in blood water potential and restores it toward the set point.

Health issues of the urinary system (13.3)

When urine is concentrated for long periods, dissolved substances can crystallise into a hard kidney stone that causes sharp pain as it passes through the urinary tract. A urinary tract infection happens when bacteria multiply in the bladder or urethra, giving pain and a frequent urge to urinate.

Glomerulonephritis is inflammation of the glomeruli that damages the filtration membrane, so proteins and blood cells leak into the urine.

Kidney failure is the serious, widespread loss of filtering function; waste and excess water then build up in the blood and must be removed by treatment. Haemodialysis passes the patient blood through a machine where a partially permeable membrane lets urea and excess salts diffuse out into dialysis fluid while useful substances are kept, and a kidney transplant replaces the failed organ with a healthy donor kidney.

Keeping urine dilute by drinking enough water lowers the risk of both stones and infection.

Key concepts to master

• Homeostasis, Homeostasis is the maintenance of a constant internal environment, temperature, blood glucose, water content and pH, despite changes outside the body. It keeps conditions stable enough for enzymes and cells to function normally, and is controlled through negative feedback involving the nervous and endocrine systems. Without homeostasis, small outside changes could disrupt enzyme activity and cell metabolism throughout the body.
• Urinary system, The urinary system removes urea and excess water and salts from the blood. The kidneys filter the blood continuously; ureters carry the urine they produce to the bladder, which stores it; the urethra then carries urine out of the body during urination. This removal of urea, a waste product of protein breakdown, is why the urinary system is described as an excretory organ system.
• The nephron, The nephron is the kidney's individual filtering and processing unit, made of the glomerulus, Bowman's capsule and a system of tubules. Each kidney contains a very large number of nephrons working in parallel, which is why kidney function can still be adequate even if some nephrons are damaged. This same arrangement explains why a person can stay healthy with only one functioning kidney.
• Ultrafiltration, High blood pressure in the glomerulus forces water, glucose, salts and urea out of the blood and into Bowman's capsule, forming the glomerular filtrate. Large plasma proteins and blood cells are too big to pass through the filter and stay in the blood. This high pressure exists because the vessel carrying blood into the glomerulus is wider than the one carrying it out.
• Selective reabsorption, As the filtrate flows along the tubule, useful substances are reabsorbed back into the blood: all of the glucose, most of the water, and some of the salts, depending on the body's needs. What remains becomes urine. This process is described as selective because different substances are reabsorbed to different extents rather than all being treated the same way.
• Osmoregulation and kidney health, Antidiuretic hormone (ADH) adjusts how much water the tubules reabsorb, concentrating or diluting urine to keep blood water content steady. When the kidneys fail, waste and excess water build up in the blood, and treatment is by dialysis or a kidney transplant. More ADH is released when the body is short of water, producing a smaller volume of more concentrated urine.
• Negative feedback, Negative feedback is the general control pattern behind homeostasis: a receptor detects a change away from the normal level, a control centre triggers a response, and an effector corrects the change so the condition returns toward the set point. Thermoregulation, osmoregulation and blood glucose control all follow this same pattern, which is why learning one example well makes the others easier to explain.
• Thermoregulation, The skin and the hypothalamus in the brain work together to keep body temperature constant. When the body is too warm, skin blood vessels dilate and sweat glands become more active to lose heat; when it is too cold, skin blood vessels constrict and muscles may shiver to generate heat. Hair standing on end (goosebumps) is a related response that traps a thin layer of warm air near the skin.
• Blood glucose regulation, The pancreas keeps blood glucose within a narrow range using two hormones with opposite effects: insulin lowers blood glucose by increasing its uptake into cells and its storage as glycogen, while glucagon raises blood glucose by breaking glycogen back down. Glycogen is stored mainly in the liver and muscles, acting as a short-term reserve of glucose.
• Kidney stones and urinary tract infection, A kidney stone is a hard, crystallised deposit that forms when substances in concentrated urine build up, causing pain as it passes through the urinary tract. A urinary tract infection occurs when bacteria multiply in the bladder or urethra, which can cause pain and a frequent urge to urinate. Drinking enough water so urine stays more dilute is a simple factor linked to lowering the risk of both conditions.

Quick recall checklist

1. Can you define and explain Homeostasis?
2. Can you define and explain Urinary system?
3. Can you define and explain The nephron?
4. Can you define and explain Ultrafiltration?
5. Can you define and explain Selective reabsorption?
6. Can you define and explain Osmoregulation and kidney health?
7. Can you define and explain Negative feedback?
8. Can you define and explain Thermoregulation?
9. Can you define and explain Blood glucose regulation?
10. Can you define and explain Kidney stones and urinary tract infection?

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