In certain "fight or flight" situations, the hormone adrenaline can also be released to boost blood glucose levels, providing a quick burst of energy.
Exam Tip: Be prepared to distinguish between Type 1 and Type 2 diabetes. Type 2 diabetes mellitus occurs when the body exhibits insulin resistance or insufficient production, leading to persistently high blood glucose levels.
The Nervous System and Reflex Actions
While hormones provide long-term, widespread regulation, the nervous system provides rapid, localized responses. Comprising the brain, spinal cord, and nerves, the nervous system coordinates bodily functions by transmitting electrical signals.
The simplest and fastest form of response is the reflex action. This is an involuntary, automatic response designed to protect the organism from harm. A reflex follows a specific pathway:
- Receptors detect a stimulus (e.g., heat).
- Sensory neurones carry the impulse to the central nervous system.
- Relay neurones (located in the spinal cord or brain) process the information.
- Motor neurones carry the impulse to an effector (a muscle or gland).
- The effector responds (e.g., the muscle contracts to pull your hand away).
Excretion and Osmoregulation
Living organisms produce metabolic waste products, such as nitrogenous compounds like urea, which must be removed to prevent toxicity. This process is called excretion. The human excretory system consists of the kidneys, ureter, bladder, and urethra.
The Nephron and Kidney Function
The functional unit of the kidney is the nephron. It facilitates excretion through two main processes:
- Ultra-filtration: High-pressure filtration of blood to remove small molecules.
- Selective reabsorption: The process of taking back useful substances (like glucose and some salts) into the blood.
Osmoregulation
Beyond removing waste, the kidneys are essential for osmoregulation—the regulation of salt and water levels to maintain a constant water potential in the body. This is controlled by anti-diuretic hormone (ADH). If you are dehydrated, ADH levels increase, causing the kidneys to reabsorb more water, resulting in concentrated urine.
Clinical Note: If the kidneys fail to function properly, a patient may require dialysis, a medical procedure that mechanically filters the blood to remove waste and excess salts.
Specialized Sensory and Regulatory Systems
The Human Eye
The eye is a complex sensory organ that coordinates vision by producing focused images on the retina. It can respond to light intensity through the pupil reflex, where the pupil constricts in bright light and dilates in dim light to protect the retina or allow more light in. Furthermore, humans possess binocular vision, using two eyes to provide depth perception.
Thermoregulation
The skin plays a vital role in thermoregulation (maintaining body temperature), a process coordinated by the hypothalamus in the brain.
- If too hot: The body triggers sweat glands to release moisture and alters blood flow near the skin surface to dissipate heat.
- If too cold: The body may trigger shivering to generate heat and constrict blood vessels to conserve it.
Digestion and Nutrient Processing
To fuel these coordination systems, the body must efficiently process food through the alimentary canal. Digestion follows a sequence: ingestion, digestion, absorption, assimilation, and egestion.
The Digestive Process
Digestion relies on enzymes—biological catalysts that speed up chemical reactions without being consumed. For example, amylase breaks down starch, while proteases break down proteins. Movement through the alimentary canal is facilitated by peristalsis, which consists of rhythmic, wave-like muscle contractions.
Absorption and the Role of the Liver
Once food is broken down into small, soluble molecules, it must be absorbed. In the small intestine, the inner wall is covered in tiny, finger-like projections called villi. These villi, along with capillaries and lacteals, significantly increase the surface area to maximize efficient absorption.
Once absorbed, nutrients are transported via the hepatic portal vein directly to the liver. The liver performs several critical functions:
- Converting glucose to glycogen (for storage) and vice versa.
- Metabolizing amino acids and forming urea.
- Breaking down alcohol and hormones.
- Assisting in fat digestion.
