Higher Human Biology Unit 2: Physiology and Health Overview

Gamete Production and Fertilisation

Ever wondered exactly where sperm and eggs come from? Understanding reproductive anatomy is crucial for grasping how human reproduction actually works.

Male reproductive organs have specific jobs: the testes produce sperm in tiny tubes called seminiferous tubules, whilst interstitial cells pump out testosterone. Think of accessory glands like the seminal vesicle and prostate gland as the support crew - they create fluids packed with fructose and enzymes to keep sperm moving and healthy.

Female reproductive organs work differently: ovaries contain thousands of immature eggs surrounded by protective follicles. When an egg matures, it's released into the oviduct where fertilisation might happen. The endometrium (uterus lining) becomes the perfect home for a developing embryo.

Key Point: Remember that follicles don't just protect eggs - they also secrete oestrogen, making them hormone factories too!

Male Hormone Control

Your body uses a clever negative feedback system to control male reproduction - it's like a thermostat that keeps everything balanced.

The pituitary gland releases two key hormones: FSH stimulates sperm production in those seminiferous tubules, whilst ICSH tells the interstitial cells to make testosterone. Here's the brilliant bit - when testosterone levels get too high, they actually switch off FSH and ICSH production.

This creates a perfect cycle: high testosterone stops hormone production, testosterone drops, the pituitary starts releasing hormones again, and testosterone rises. It's your body's way of maintaining steady sperm production without overdoing it.

Remember: The hypothalamus kicks off puberty by releasing hormones that wake up the pituitary gland - that's when this whole system starts running!

Female Hormonal Control

Unlike males, female reproduction follows a monthly cycle with two distinct phases that you need to understand clearly.

The follicular phase starts when FSH from the pituitary stimulates follicle development and oestrogen production. Oestrogen does two crucial jobs: it thickens the endometrium preparing for possible pregnancy and makes cervical mucus easier for sperm to swim through. When oestrogen peaks, it triggers a massive LH surge that causes ovulation.

The luteal phase begins after ovulation when the empty follicle becomes the corpus luteum and starts pumping out progesterone. This hormone further develops the endometrium's blood supply. If no pregnancy occurs, the corpus luteum breaks down, progesterone crashes, and menstruation begins.

Top Tip: If fertilisation happens, the corpus luteum keeps producing progesterone - that's why periods stop during pregnancy!

Fertility and Assisted Reproduction

Modern medicine offers amazing solutions when natural conception doesn't work - these techniques have helped millions of families.

Women show cyclical fertility (only fertile for a few days each cycle) whilst men have continuous fertility. You can spot ovulation because body temperature rises 0.5°C and cervical mucus becomes thin and watery - useful for natural family planning.

Fertility treatments include several options: ovulatory drugs that override negative feedback to stimulate egg production (sometimes causing multiple births), artificial insemination for low sperm counts, and ICSI where individual sperm are injected directly into eggs.

IVF involves surgically removing eggs, fertilising them in laboratory dishes, then transferring healthy embryos back to the uterus. Modern techniques include pre-implantation genetic diagnosis to screen for genetic disorders before pregnancy begins.

Did You Know: IVF embryos are grown to at least 8 cells before transfer - this gives them the best chance of successful implantation.

Contraceptive Methods

Contraception works by blocking different stages of reproduction - understanding the mechanisms helps you remember how each method functions.

Physical barriers like condoms prevent sperm reaching eggs, whilst IUDs (copper-containing T-shaped devices) stop embryos implanting in the endometrium. These methods don't affect your natural hormone cycles.

Chemical barriers work by manipulating hormones: the combined oral contraceptive pill contains synthetic oestrogen and progesterone that mimic negative feedback, preventing FSH and LH release. No LH surge means no ovulation. The progesterone-only pill thickens cervical mucus, making it harder for sperm to swim through.

Emergency contraception (morning-after pill) prevents or delays ovulation when taken within 72-120 hours after unprotected sex, depending on which type you use.

Key Point: The pill doesn't just stop ovulation - progesterone also changes the uterine lining, making implantation less likely.

Prenatal Screening

Prenatal screening helps detect potential problems early in pregnancy - knowing when and why different tests are used is essential knowledge.

Ultrasound scans happen twice: the dating scan (8-14 weeks) confirms pregnancy stage and due date, whilst the anomaly scan (18-20 weeks) checks for physical abnormalities. Blood and urine tests monitor chemical markers, but timing matters - wrong timing can give false positives.

Amniocentesis (offered 15-20 weeks) involves extracting amniotic fluid with a needle guided by ultrasound. It's 98-99% accurate but carries small risks including miscarriage, infection, and fluid leakage.

Chorionic Villus Sampling (CVS) happens earlier (10-13 weeks) by sampling placental tissue. It tests for conditions like Down syndrome, Edwards syndrome, and cystic fibrosis. CVS has higher miscarriage risk (1 in 100) but gives earlier results than amniocentesis.

Important: Both tests can produce karyotypes showing chromosome pairs - this helps diagnose many genetic conditions.

Genetics and Inheritance Patterns

Understanding genetic terminology and inheritance patterns helps you predict how traits pass between generations - essential for medical genetics.

Key terms you must know: alleles are different versions of genes, dominant alleles always show their effect, recessive alleles only appear when paired together. Homozygous means identical alleles, heterozygous means different alleles. Carriers don't show symptoms but can pass on recessive alleles.

Autosomal recessive conditions (like cystic fibrosis) often skip generations and affect males and females equally - sufferers need two recessive alleles. Autosomal dominant conditions (like Huntington's disease) appear in every generation because you only need one dominant allele.

Sex-linked recessive traits (like colour blindness) mainly affect males because they have only one X chromosome. Affected males can't pass the trait to sons but all daughters become carriers.

Memory Trick: If a condition skips generations and affects both sexes equally, think autosomal recessive!

Blood Vessel Structure and Function

Your circulatory system is perfectly designed for different jobs - each vessel type has specific features that match its function.

Arteries carry blood away from your heart under high pressure. They need thick, muscular walls with elastic fibres to handle pressure surges after each heartbeat. The elastic walls recoil, helping push blood forward smoothly.

Capillaries are where the real action happens - their walls are just one cell thick, allowing easy exchange of nutrients, oxygen, and waste between blood and tissues. They're so narrow that red blood cells pass through single file.

Veins return blood to your heart under low pressure, so they have thinner walls and larger lumens than arteries. Crucially, they contain valves to prevent blood flowing backwards - essential when blood travels uphill against gravity.

Key Process: Pressure filtration forces plasma through capillary walls to create tissue fluid, which bathes your cells with nutrients and removes waste.

Heart Function and Control

Your heart is an amazing pump with its own electrical system - understanding how it works helps explain many cardiovascular diseases.

The cardiac cycle has three stages: diastole (relaxation - blood fills chambers), atrial systole (atria contract, pushing blood into ventricles), and ventricular systole (ventricles contract, pumping blood out). The opening and closing of AV valves and semilunar valves creates the heartbeat sounds you hear.

The sinoatrial node (SAN) acts as your heart's natural pacemaker, generating electrical impulses that spread through the atria, then to the atrioventricular node (AVN), and finally through the ventricle walls. This creates coordinated contractions.

Your medulla controls heart rate through the autonomic nervous system: sympathetic nerves release noradrenaline to speed up your heart, whilst parasympathetic nerves release acetylcholine to slow it down.

Clinical Connection: ECGs detect these electrical impulses - abnormal patterns help doctors diagnose heart problems.

Cardiac Cycle and Blood Pressure

Understanding how your heart pumps blood efficiently is crucial for grasping cardiovascular health and disease.

Cardiac output equals heart rate × stroke volume - both ventricles must pump identical volumes to prevent blood backing up in your lungs or body. The heart has separate circuits: right side pumps deoxygenated blood to lungs, left side pumps oxygenated blood to body tissues.

During the cardiac cycle, diastole allows blood to flow into relaxed atria and ventricles. Atrial systole forces remaining blood into ventricles through AV valves. Ventricular systole closes AV valves and opens semilunar valves, pushing blood into the aorta and pulmonary artery.

Blood pressure changes throughout the cycle: it peaks during ventricular systole (systolic pressure ~120mmHg) and drops during diastole (diastolic pressure ~80mmHg). A sphygmomanometer measures these pressures using an inflatable cuff.

Health Alert: Hypertension (high blood pressure) is a major risk factor for coronary heart disease and stroke - that's why it's routinely monitored.