Homeostasis

• Homeostasis and its principles: stimuli, receptors, nervous and endocrine coordination, effectors, negative feedback.
• Urea is made in the liver by deamination of excess amino acids.
• Kidney and nephron structure; ultrafiltration in the Bowman’s capsule and selective reabsorption in the proximal convoluted tubule.
• Osmoregulation by the hypothalamus, posterior pituitary, ADH, aquaporins and collecting ducts.
• Cell signalling by glucagon via G-protein, adenylyl cyclase, cAMP and an enzyme cascade; blood glucose control by insulin and glucagon.
• Glucose test strips and biosensors (glucose oxidase and peroxidase).
• Stomata: daily rhythms, guard cell mechanism, abscisic acid and calcium ions as a second messenger.

What homeostasis is and why it matters

Homeostasis is the maintenance of a constant internal environment (the tissue fluid around cells) within narrow limits, despite changes in the external or internal environment.

• Core temperature: enzymes work fastest near their optimum; too low slows metabolism, too high denatures enzymes and other proteins.
• Blood glucose: cells need a steady supply for respiration (brain cells especially); too high lowers the water potential of blood and tissue fluid, so cells lose water by osmosis.
• Water potential of blood: stops cells gaining or losing water by osmosis and bursting or shrinking.
• pH and the concentration of ions are kept constant too, for enzyme activity and nerve function.

The principles

• A stimulus is a change in the internal or external environment (e.g. a rise in blood glucose; a fall in air temperature).
• Receptors detect the change (e.g. thermoreceptors in the hypothalamus and skin; α and β cells in the islets of Langerhans).
• Coordination systems pass information on: the nervous system (fast, short-lived, electrical impulses) and the endocrine system (hormones in the blood, slower, longer-lasting).
• Effectors (muscles and glands) bring about a response.
• Negative feedback: the response reverses the change, bringing the factor back towards the set point. Values fluctuate around the set point rather than staying perfectly constant. Having separate mechanisms to correct a rise and a fall gives finer control.

Urea and the kidney

Excess amino acids cannot be stored. In the liver they are deaminated: the amine group is removed as ammonia, and the rest of the molecule is respired or converted to carbohydrate or fat. Ammonia is very toxic and soluble, so it is combined with CO₂ in the ornithine cycle to form urea, which is less toxic and is carried in the blood to the kidneys.

Gross structure of the kidney

Formation of urine