Organism level systems Revision Notes

Complexity and Delicacy of the Brain (Higher Tier)

The human brain is an extremely complex organ, composed of billions of neurons interconnected in intricate networks. Its delicate nature means that it is easily damaged, and even minor injuries or surgical interventions can lead to significant and irreversible consequences. This delicacy makes direct experimentation on living brains extremely risky and often unethical due to the high potential for harm and the vital functions it controls, such as personality, memory, and motor skills. The sheer number of connections and different types of cells makes it difficult to map out all pathways and understand how they interact in real-time.

Ethical and Practical Limitations (Higher Tier)

Ethical guidelines severely restrict the types of experimental procedures that can be performed on living human brains. This means researchers cannot directly manipulate brain structures or sever connections for study, as they might in other organs, without clear medical necessity. Practical limitations include the difficulty of accessing the brain, which is protected by the skull. The use of post-mortem brains provides structural information but lacks the dynamic, functional aspects of a living brain, making it unsuitable for studying active processes like thought or sensation. Animal models offer some insight, but their brains are not identical to human brains, limiting the direct applicability of findings.

Interconnectedness and Irreversibility of Damage (Higher Tier)

Brain regions are highly interconnected, meaning that damage to one specific area can have widespread effects across multiple functions, rather than just affecting an isolated capacity. This makes it challenging to pinpoint the exact function of a single region through studies of damage. Furthermore, once brain tissue is damaged, it often does not regenerate, meaning that any experimental error or accidental harm can be permanent. This irreversibility means that the risks associated with invasive brain research are exceptionally high, compelling researchers to pursue non-invasive methods whenever possible, despite their limitations.

Initial Phase: Follicle Development and Oestrogen Production

The menstrual cycle begins with the pituitary gland releasing Follicle Stimulating Hormone (FSH). FSH stimulates several follicles in the ovary to grow and mature. As these follicles develop, they produce increasing amounts of oestrogen. Oestrogen, at low to moderate concentrations, exerts a negative feedback effect on the pituitary gland, inhibiting further FSH secretion. This ensures that usually only one follicle continues to develop fully.

Mid-Cycle: Ovulation Triggered by LH Surge

As oestrogen levels continue to rise from the developing follicle, they eventually reach a high threshold. At this point, oestrogen's effect on the pituitary gland switches from negative to positive feedback, causing a sudden and significant increase in the release of Luteinising Hormone (LH). This LH surge is crucial as it triggers ovulation – the rupture of the most mature follicle and the release of the egg into the fallopian tube. The LH surge typically occurs around day 14 of a 28-day cycle.

Post-Ovulation: Progesterone Dominance and Uterine Preparation

After ovulation, the ruptured follicle transforms into a temporary endocrine gland called the corpus luteum, primarily due to the action of LH. The corpus luteum secretes large amounts of progesterone and some oestrogen. Progesterone is vital for preparing and maintaining the uterine lining (endometrium) for potential embryo implantation. Both progesterone and oestrogen, during this phase, exert strong negative feedback on the pituitary gland, inhibiting the release of FSH and LH. This prevents the development of new follicles and further ovulation.

Cycle End: Menstruation or Continuation of Pregnancy

If fertilisation and implantation do not occur, the corpus luteum begins to degenerate towards the end of the cycle. This degeneration causes a sharp decline in both progesterone and oestrogen levels. The withdrawal of these hormones removes the negative feedback on the pituitary gland, allowing FSH and LH levels to gradually rise again, initiating the next cycle. The drop in progesterone also leads to the breakdown and shedding of the uterine lining, resulting in menstruation. If pregnancy occurs, human chorionic gonadotropin (hCG) is produced, which maintains the corpus luteum and thus progesterone levels, preventing menstruation.

Thermoregulation: Responding to High Temperatures (Higher)

When body temperature rises above the normal range (around 37°C), the thermoregulatory centre in the hypothalamus detects this change. The body initiates several mechanisms to cool down. Blood vessels near the skin surface undergo vasodilation, widening to increase blood flow to the skin. This allows more heat to radiate away from the body. Sweat glands are stimulated to produce sweat, which evaporates from the skin surface, taking heat with it through latent heat of vaporisation. Hair erector muscles relax, causing hairs to lie flat, reducing the insulating layer of trapped air above the skin. These responses work together to increase heat loss and bring body temperature back down.

Thermoregulation: Responding to Low Temperatures (Higher)

If body temperature falls below the normal set point, the hypothalamus triggers mechanisms to conserve heat and generate more heat. Blood vessels near the skin surface undergo vasoconstriction, narrowing to reduce blood flow to the skin. This minimizes heat loss through radiation. Muscles start to shiver involuntarily; these rapid contractions generate heat. Hair erector muscles contract, causing hairs to stand on end, trapping a thicker layer of insulating air close to the skin surface, which reduces heat loss. Metabolic rate can also be increased by hormones to generate more heat internally. These coordinated actions help to raise body temperature back to the set point.

Osmoregulation: Responding to Low Water Levels (Dehydration) (Higher)

When the body is dehydrated, the concentration of solutes in the blood (blood osmolarity) increases. Osmoreceptors in the hypothalamus detect this change. This triggers the posterior pituitary gland to release more Antidiuretic Hormone (ADH) into the bloodstream. ADH travels to the kidneys and increases the permeability of the collecting ducts (and to a lesser extent, the distal convoluted tubules) to water. This leads to increased reabsorption of water from the filtrate back into the blood. Consequently, less water is lost in urine, and a small volume of concentrated urine is produced, helping to conserve water and restore blood osmolarity to normal levels.

Osmoregulation: Responding to High Water Levels (Over-hydration) (Higher)

If the body has an excess of water, the concentration of solutes in the blood decreases (blood osmolarity decreases). Osmoreceptors in the hypothalamus detect this dilution. In response, the release of Antidiuretic Hormone (ADH) from the posterior pituitary gland is reduced. With less ADH, the collecting ducts in the kidneys become less permeable to water, leading to less water reabsorption back into the blood. As a result, more water remains in the filtrate and is excreted in a large volume of dilute urine. This process helps to remove excess water from the body and restore blood osmolarity to the normal range.

General Structure of the Brain

The brain is the main control organ of the central nervous system, responsible for processing information and coordinating responses. It is protected by the skull and layers of membranes called meninges, and floats in cerebrospinal fluid. The brain has several distinct regions, each with specialised functions. The three main parts to focus on are the cerebrum, cerebellum, and medulla oblongata. These regions work together to ensure the body functions correctly and responds appropriately to stimuli.

Functions of the Cerebrum

The cerebrum is the largest part of the brain and is responsible for conscious thoughts, intelligence, memory, and language. It also controls voluntary actions, such as walking, speaking, and artistic expression. The cerebrum is divided into two hemispheres, and its surface is highly folded, forming gyri (ridges) and sulci (grooves), which increases its surface area to accommodate more nerve cells and enhance its processing capabilities. Different areas within the cerebrum are specialised for sensory input, motor control, and association functions.

Functions of the Cerebellum

Located at the back of the brain, underneath the cerebrum, the cerebellum is primarily involved in coordinating muscular activity, balance, and posture. It receives information from the sensory systems, the spinal cord, and other parts of the brain and uses this input to fine-tune motor activity. This coordination allows for smooth, precise movements and helps maintain equilibrium. Activities like riding a bicycle, playing a musical instrument, or catching a ball heavily rely on the cerebellum's function.

Functions of the Medulla Oblongata

The medulla oblongata, located at the base of the brainstem, is crucial for regulating essential involuntary bodily functions. These include controlling heart rate, breathing rate, and blood pressure. It also regulates reflexes such as swallowing, coughing, sneezing, and vomiting. Because it controls these vital life-sustaining processes, damage to the medulla can have severe, often fatal, consequences.