Human Biology

  • Describe the structure of blood and that it contains cells and dissolved substances

  • Describe the structure of the human excretory (renal) system

  • Describe the function of the kidney to filter blood and excrete urea in urine.

  • Discuss the strengths and limitations of a sieve model of the kidneys.

  • Know that chromosomes contain genes, made of DNA, and that genes contribute to the determination of an organism's characteristics.

  • Describe the fusion of gametes to produce a fertilised egg with a new combination of DNA.

  • Describe the inheritance of sex in humans in terms of XX and XY chromosomes.

  • Describe how dissolved substances can travel between mother’s blood and the fetus through the placenta and umbilical cord

  • Discuss how fetal development is affected by the health of the mother, including the effect of diet, smoking and drugs.

1. Structure of blood

  • Blood is a tissue made up of cells suspended in a liquid called plasma.

  • Plasma: a pale-yellow liquid that transports dissolved substances such as glucose, amino acids, hormones, carbon dioxide, urea, and heat.

  • Red blood cells: biconcave discs without nuclei; packed with haemoglobin to transport oxygen.

  • White blood cells: part of the immune system; defend against pathogens (e.g., phagocytes engulf bacteria, lymphocytes make antibodies).

  • Platelets: cell fragments that help clot blood to prevent bleeding.

2. Structure of the human excretory (renal) system

  • Kidneys: filter waste products like urea from blood.

  • Ureters: narrow tubes that carry urine from the kidneys to the bladder.

  • Bladder: muscular sac that stores urine until excretion.

  • Urethra: tube that carries urine out of the body.

  • Renal artery: supplies blood rich in oxygen and waste to the kidney.

  • Renal vein: carries filtered blood away from the kidney.

3. Function of the kidney

  • Main role: filter blood, keeping useful substances and removing waste.

  • Filtration: urea, excess water, and salts are removed from the blood.

  • Reabsorption: glucose, most water, and essential ions are reabsorbed into the blood.

  • Excretion: remaining waste products form urine, which leaves the body via the ureters, bladder, and urethra.

4. Strengths and limitations of the sieve model of the kidneys

  • Strengths:

    • Helps visualise filtration (blood being “sieved” to remove waste).

    • Emphasises separation of large useful molecules (like proteins) from smaller waste molecules.

  • Limitations:

    • Oversimplifies kidney function – kidneys don’t just filter, they also reabsorb useful substances like glucose and water.

    • Doesn’t show active transport or hormonal regulation (e.g., ADH).

    • Makes it seem like kidneys are passive filters rather than active organs.

5. Chromosomes, genes, and DNA

  • Chromosomes are long DNA molecules coiled into structures, found in the nucleus.

  • Genes are sections of DNA coding for proteins.

  • Genes determine many characteristics of an organism (e.g., eye colour, blood type).

  • Humans have 46 chromosomes (23 pairs) – one set from each parent.

6. Fusion of gametes

  • Gametes: sex cells – sperm (male) and egg (female).

  • Each gamete has half the number of chromosomes.

  • At fertilisation, sperm and egg fuse to form a zygote (fertilised egg) with 46 chromosomes.

  • This results in a new combination of DNA, giving genetic variation.

7. Inheritance of sex in humans

  • Females: XX chromosomes.

  • Males: XY chromosomes.

  • The father’s sperm determines the sex of the child:

    • Sperm carrying an X → female (XX).

    • Sperm carrying a Y → male (XY).

8. Dissolved substances between mother and fetus

  • Placenta: allows exchange between mother’s blood and fetal blood without direct mixing.

  • Oxygen and nutrients (like glucose, amino acids) diffuse from mother to fetus.

  • Waste products (like carbon dioxide and urea) diffuse from fetus to mother.

  • Umbilical cord: contains blood vessels that carry substances between fetus and placenta.

9. Effect of mother’s health on fetal development

  • Diet: poor nutrition can cause low birth weight or developmental problems; lack of folic acid can cause spinal defects.

  • Smoking: reduces oxygen supply (carbon monoxide binds haemoglobin), increasing risk of premature birth, low birth weight, and breathing problems.

  • Alcohol: can cause fetal alcohol syndrome – learning difficulties, abnormal growth, facial deformities.

  • Drugs: harmful chemicals can cross the placenta, leading to abnormal development, miscarriage, or addiction in newborns.