Form 4 · Paper 2 essay guide

Homeostasis and Human Urinary System, Paper 2 essay guide

How Homeostasis and Human Urinary System appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Urine formation in the nephron: Define homeostasis and name the urinary system as an excretory organ system. → Describe ultrafiltration: afferent wider than efferent arteriole, high hydrostatic pressure, filtrate entering the Bowman capsule, proteins and blood cells retained. → Describe selective reabsorption along the tubule: all glucose, most water and controlled salts returned to the blood. → Conclude by tracing what remains as urine down the ureter to the bladder.
Osmoregulation and the role of ADH: State that osmoregulation keeps blood water potential steady. → Explain detection by osmoreceptors in the hypothalamus. → Explain ADH release from the pituitary and its effect on collecting duct permeability. → Contrast the response when the body is short of water with the response when it has excess water, framed as negative feedback.
Negative feedback across temperature and blood glucose: State the general loop: receptor, control centre, effector, response that opposes the change. → Apply it to thermoregulation with vasodilation, vasoconstriction, sweating and shivering. → Apply it to blood glucose with insulin and glucagon acting as an opposite pair. → Explain why holding conditions within narrow limits protects enzyme and cell function.
Health issues of the urinary system: Link a kidney stone and a urinary tract infection to concentrated urine and bacterial growth. → Explain glomerulonephritis as damage to the filtration membrane. → Describe kidney failure and its treatment by haemodialysis or transplant. → Suggest keeping urine dilute by adequate water intake as a protective measure.

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Intro	Define homeostasis and name the condition being controlled
Point 1	Detect: name the receptor and the change it senses
Point 2	Process: name the control centre and the signal sent
Point 3	Respond: name the effector and the response produced
Point 4	Correct: show the response opposes the change and restores the set point
Conclusion	State why stable conditions matter for enzymes and cells

Key terms to include

homeostasis
negative feedback
ultrafiltration
selective reabsorption
glomerulus
Bowman capsule
nephron
ADH
osmoregulation
collecting duct
insulin
glucagon
vasodilation
haemodialysis

A model essay outline

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.

Frequently asked questions

What is homeostasis?

Homeostasis is the maintenance of a constant internal environment in the body, such as steady body temperature, blood glucose level, water content and pH, despite changes outside. It keeps conditions ideal for enzymes and cells to work, and is controlled by organs including the kidneys, liver, skin and the nervous and endocrine systems. Almost every homeostatic mechanism in the body follows the same negative feedback pattern of detecting, responding to and correcting a change.

How is urine formed in the nephron?

First, ultrafiltration: high pressure in the glomerulus forces water, glucose, salts and urea out of the blood into the Bowman's capsule, while large proteins and blood cells stay behind. Then selective reabsorption: as the filtrate flows along the tubule, useful substances such as all the glucose, some salts and much of the water are reabsorbed into the blood. What remains, mainly urea, excess salts and water, becomes urine. This urine then flows through the ureter to the bladder, where it is stored before being released.

Why is there normally no glucose in urine?

Glucose is small enough to be filtered out during ultrafiltration, but it is a useful nutrient, so it is completely reabsorbed back into the blood during selective reabsorption. Healthy urine therefore contains no glucose. If glucose appears in urine, it often means blood glucose is too high, as in diabetes.