Core Concepts in Biology: Reproduction, Homeostasis, and Gas Exchange

Posted on Oct 18, 2025 in Biology

Human Reproductive System Anatomy

Male Reproductive System

The male reproductive system components and their functions:

Testicles: Produce sperm and testosterone.
Epididymis: The reservoir for sperm, communicating with the testicles.
Scrotum: Holds the testicles outside the body to maintain a temperature lower than core body temperature, which is essential for sperm viability.
Seminal Duct (Vas Deferens): Transports sperm from the epididymis to the urethra.
Urethra: A duct where the seminal ducts converge with the urinary duct from the bladder.
Accessory Glands (Seminal Vesicle and Prostate): Release fluids that form semen. This fluid is slightly basic to neutralize the acidic environment of the female reproductive system.
Penis: The male erectile organ that ensures sperm is deposited near the cervix in the female body.

Female Reproductive System

The female reproductive system components and their functions:

Ovaries: The female gonads, where eggs (ova), estradiol, and progesterone are produced. These two hormones are essential for regulating the menstrual cycle. The hormones enter the bloodstream, while the egg travels to the oviduct.
Oviduct (Fallopian Tube): Captures the egg during ovulation, provides the site for fertilization, and then moves the embryo to the uterus.
Uterus: Provides the necessary environment for the embryo and the fetus during pregnancy.
Cervix: Protects the fetus during pregnancy and then dilates to provide a birth canal.
Vagina: Stimulates the penis to cause ejaculation and serves as the birth canal.
Vulva: Protects the internal parts of the female reproductive system.

Menstrual Cycle: Ovarian and Uterine Changes

The menstrual cycle occurs in women from puberty until menopause, stopping only during pregnancies or periods of ill health. It consists of the uterine cycle and the ovarian cycle combined. The cycles are regulated by hormones, which are molecules secreted into the blood that coordinate different bodily functions. The cycle lasts approximately 28 days.

Menstrual Cycle = Uterine Cycle + Ovarian Cycle

Ovarian Cycle Phases

The cycle starts counting from the first day of menstruation.

Follicular Phase (Day 1 to Day 14):
- Follicles begin to develop in the ovary.
- An egg within the most developed follicle is stimulated to grow.
- The most developed follicle ruptures, releasing its egg to the oviduct.
Ovulation (Around Day 14): The release of the egg from the follicle.
Luteal Phase (Post-Ovulation):
- The wall of the ruptured follicle develops into a structure called the corpus luteum.
- The corpus luteum is rich in lipid reserves and produces estradiol (a type of estrogen) and progesterone during this phase.
- If fertilization does not occur, the corpus luteum breaks down, and the ovary returns to the follicular phase.

Hormonal Regulation of the Cycle

FSH (Follicle-Stimulating Hormone): Rises to a peak towards the end of the menstrual cycle (around Day 7) and stimulates the development of follicles. FSH also stimulates the secretion of Oestradiol by the follicle wall. It has a peak on Day 14.
LH (Luteinizing Hormone): Rises to a peak towards the end of the follicular phase. It stimulates the completion of meiosis in the oocyte and promotes the post-ovulation development of the follicle wall into the corpus luteum, which secretes Oestradiol and progesterone.
Oestradiol (Estrogen): Has a peak towards the end of the follicular phase. It stimulates the repair and thickening of the endometrium. When it reaches high levels, Oestradiol inhibits the secretion of FSH and stimulates LH.
Progesterone: Levels rise at the start of the luteal phase, reach a peak, and then drop back to a low level by the end of the phase. This hormone promotes the thickening and maintenance of the endometrium and inhibits FSH and LH secretion by the pituitary gland (hypophysis).

Plant Reproduction and Structure

Flower Anatomy

The receptacle and the sepal provide protection for the flower. The stalk is similar to the stigma in function (support/connection). Sepals protect the flower when it is budding, and petals attract pollinators.

Female Flower Parts (Carpel)

Stigma: Responsible for catching the pollen.
Style: Connects the stigma to the ovary. It is variable in length and usually has a hollow center.
Ovary: Contains the ovules and holds them.
Carpel: Refers to all the female parts (stigma, style, and ovary).

The ovary contains one or more ovules. The ovules are ovoid in shape and multicellular. A cell inside the ovule divides by meiosis to produce four haploid nuclei. One of these divides three times by mitosis to produce eight haploid nuclei, one of which is the female gamete (egg), and the others serve as helpers.

Male Flower Parts (Stamen)

Anther: The pollen-producing organ of the flower (pollen is the male gamete of a flowering plant).
Filament: The stalk that supports the anther.
Stamen: Refers to all the male parts (anther and filament).

Diploid cells inside the anther divide by meiosis to produce four haploid cells, each of which develops into a pollen grain. The nucleus inside a pollen grain divides by mitosis to produce three haploid nuclei. Two of these are male gametes, and the third is a helper cell.

Pollination and Fertilization

Pollination is the transfer of pollen from an anther to a stigma by wind or animals.

Fertilization:

Each pollen grain produces a pollen tube that extends into the ovary and towards an ovule.
Male gametes are released and fuse with female gametes to create a zygote.
The zygote develops into an embryo, which is contained within a seed.

Methods of Promoting Cross-Pollination

Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of a different flower, resulting in more genetic variation than self-pollination. Mechanisms to prevent self-pollination include:

Using wind or animal pollinators to carry pollen to different plants.
Having separate male and female flowers on the same plant (monoecious) or on separate plants (dioecious).
Male (anther) and female (stigma) structures developing at different times.
Self-Incompatibility: Mechanisms by which a pollen grain fails to develop when it reaches the ovary of the same plant, promoting genetic variation.

Dispersal and Germination of Seeds

Seed dispersal reduces competition between the parent plant and the offspring. Different fruits have different dispersal mechanisms:

Fleshy and Tasty Fruits: Animals eat them and disperse the seeds in their stool (feces).
Covered in Hooks: Carried elsewhere by animals on their fur.
Winged or Feathery: Carried by the wind.

Fertilization, pollination, and seed dispersal are separate processes.

Vascular Transport in Plants

Xylem and phloem are the two types of transport tissue found in vascular plants. The flow of water is from the roots to the leaves.

Phloem: Transports food (sugars).
Xylem: Transports water and minerals from the root to the leaf in a unidirectional flow upward.

For the plant, water transport is a passive process; the energy needed comes from the thermal energy (heat) that drives transpiration.

Leaf Structure and Function

The leaf structure facilitates photosynthesis and gas exchange:

Cuticle: A waxy layer that minimizes water loss and allows light to pass through.
Palisade Mesophyll: Full of chloroplasts, optimized for photosynthesis.
Spongy Mesophyll: Contains air spaces for the circulation of water vapor, CO₂, and O₂.
Stoma: An opening regulated by two guard cells, which open and close the pore.
Vein: Contains the vascular tissues (xylem and phloem).

Homeostasis and Regulation

Homeostasis is the maintenance of a constant internal environment within an organism despite changes in the external environment. This includes regulating factors like body temperature, blood glucose levels, pH, and water balance to keep conditions optimal for enzyme activity and cellular function.

Positive Feedback: Increases the gap between the original level and the new level (e.g., contractions during childbirth).
Negative Feedback: Loops that counteract changes to return the system to a set point (e.g., regulation of glucose).

Diabetes Mellitus

Diabetes is a condition characterized by constantly elevated blood glucose levels, even during prolonged fasting, leading to the presence of glucose in the urine. Elevated glucose can lead to damage in tissues, particularly to proteins.

Diabetes Type 1

Characterized by an inability to produce sufficient insulin. It is an autoimmune disease arising from the destruction of beta cells in the pancreas by the body’s own immune system. Treatment involves:

Testing blood glucose concentration regularly.
Injecting insulin when levels are too high or likely to become high.
Insulin pumps can also be used to release insulin into the blood as necessary.

Diabetes Type 2

Characterized by an inability to process or respond to insulin due to a deficiency of insulin receptors or glucose transporters on target cells. It is rare in people under 50 and common in those over 65. Causes include high intake of sugar or fat diets, obesity due to overeating and lack of exercise, along with genetic factors. Treatment includes:

Adjusting the diet to reduce the peaks and troughs of blood glucose.
Eating small amounts of food frequently rather than infrequent large meals.

Thermoregulation Mechanisms in Humans

Thermoregulation is the control of internal body temperature.

Response to Heat (Cooling Mechanisms)

Vasodilation: When the body is overheated, smooth muscle cells in the walls of arterioles in the skin relax, causing the vessels to widen. More blood flows to the skin surface. This increase in temperature difference between the skin and the external environment causes more heat to be lost from the body.
Sweating: Sweat is secreted by glands in the skin and passes through ducts to the surface. The evaporation of this water (which contains salts) cools the body. This process is controlled by the hypothalamus.

Response to Cold (Heat Retention/Generation)

Vasoconstriction: The muscular rings forming the walls of the arterioles contract, reducing the circumference of the arteriole and narrowing the lumen. Less blood flows to the capillaries in the skin, allowing the skin to cool below core body temperature, thus reducing heat loss.
Shivering: Muscles contract to generate movement, which generates heat. Sometimes involuntary contractions and relaxations are carried out rapidly solely to generate heat.
Uncoupled Respiration: Metabolic processes produce heat instead of ATP energy.
Hair Erection (Piloerection): Traps a layer of air near the skin for insulation (less effective in humans than in furred animals).

Gas Exchange and the Respiratory System

Gas exchange occurs when organisms absorb one gas from the environment and release another. Humans absorb oxygen for cellular respiration and ATP production, releasing the carbon dioxide produced. Gas exchange takes place between the capillaries and the alveoli in the lungs.

Properties of Gas Exchange Surfaces

Permeable: Oxygen and carbon dioxide can diffuse across freely.
Large Surface Area: The total surface area is large in relation to the volume of the organism.
Moist: The surface is covered by a film of moisture in terrestrial organisms so gases can dissolve before diffusion.
Thin: Gases must diffuse only a short distance, typically through a single layer of cells.

Maintenance of Concentration Gradient

Diffusion of gases only happens if concentration gradients exist.

Oxygen moves from the air in the alveoli into the capillaries because there is less oxygen in the blood than in the air.
Carbon dioxide diffuses from the blood to the air in the alveoli because there is a lower concentration of carbon dioxide in the air.

Mammals breathe in and out to keep oxygen levels high and carbon dioxide levels low in the lungs, adjusting their breathing rate based on carbon dioxide levels in the blood. Fish move water over their gills in one direction while blood flows the opposite way (countercurrent exchange), which helps maintain high oxygen and low carbon dioxide concentrations near the gills.

The Human Lung Structure

Mammals (e.g., whales, dolphins) use lungs to exchange oxygen and carbon dioxide. Air enters through the trachea, then into the bronchi, then bronchioles, and finally into tiny air sacs called alveoli. Each alveolus is very small and has a thin wall adjacent to thin blood capillaries. This structure makes gas movement between air and blood easy and fast.

Adult lungs contain about 300 million alveoli, providing a huge surface area for gas exchange (about 40 times bigger than the skin’s surface).
Elastic fibers help the lungs stretch and return to normal during breathing.
Surfactant (a special liquid) coats the alveoli, reducing surface tension so the alveoli do not collapse when we breathe out.
Type I Pneumocytes (AT1): Thin cells that allow rapid gas exchange.
Type II Pneumocytes (AT2): Cells that secrete the surfactant.

Chronic Obstructive Pulmonary Disease (COPD)

COPD is a long-term lung disease that makes breathing difficult. It happens mostly because of smoking, but also from long exposure to air pollution, dust, or chemicals, causing the airways and alveoli to become damaged and inflamed. Symptoms include coughing, wheezing, shortness of breath, and chest tightness. It worsens over time, but treatments (like inhalers, oxygen, or lifestyle changes) can help control it.

Lung Volumes and Capacities

These measurements describe the amount of air the lungs can hold:

Tidal Volume: The volume of air inhaled or exhaled during regular, quiet breathing.
Inspiratory Reserve Volume: The maximum volume of air that can be inhaled after a normal tidal inspiration.
Expiratory Reserve Volume: The maximum volume of air that can be exhaled after a normal tidal expiration.
Residual Volume: The volume of air that stays inside the lung after a maximal expiratory effort.
Vital Capacity: The total volume of air that can be exchanged (Tidal Volume + Inspiratory Reserve Volume + Expiratory Reserve Volume).