Major Organs & Body Systems

The two lungs fill most of the thoracic cavity. The right lung has three lobes; the left lung has two lobes (the left is slightly smaller to make room for the heart). Air enters through the nose or mouth → travels down the trachea (windpipe) → splits into two bronchi (one for each lung) → branches into smaller bronchioles → reaches tiny air sacs called alveoli. Alveoli are where gas exchange happens. There are about 480 million of them, giving the lungs a total surface area roughly the size of a tennis court. Their walls are just one cell thick, and they are surrounded by capillaries (tiny blood vessels). Oxygen diffuses from the air in the alveoli into the blood; carbon dioxide diffuses the other way to be breathed out. Clinically: - Pneumonia — infection fills alveoli with fluid and pus, blocking gas exchange. Patients become short of breath because oxygen cannot reach the blood effectively. - Pneumothorax — air enters the pleural space (around the lung), causing the lung to collapse. Breath sounds disappear on that side. - Asthma — bronchospasm (the bronchioles squeeze shut), making it hard to breathe out. Treated with bronchodilators (inhalers that relax the muscle around the airways).

The Liver The liver is the largest internal organ (about 1.5 kg), sitting in the right upper abdomen under the ribcage. It is one of the most metabolically active organs in the body — it performs over 500 functions. Key functions: - Detoxification — breaks down drugs, alcohol, and waste products (e.g. converts ammonia to urea) - Bile production — bile is stored in the gallbladder and released into the small intestine to help digest fats - Protein synthesis — makes clotting factors (without which blood won't clot) and albumin (the main protein in blood) - Glucose regulation — stores excess glucose as glycogen; releases it back when blood sugar drops Clinically: Liver disease (cirrhosis, hepatitis) impairs all these functions. A patient with liver failure may bleed easily (low clotting factors), have yellow skin and eyes (jaundice — bile pigment bilirubin builds up), develop fluid in the abdomen (ascites — low albumin), and become confused (hepatic encephalopathy — ammonia not cleared). The digestive tract Food travels: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus. - Stomach — a muscular bag that churns food and mixes it with acid (pH ~2) and enzymes. Kills most bacteria. Begins protein digestion. - Small intestine (about 6 metres long) — where most digestion and almost all nutrient absorption happens. Lined with finger-like projections called villi that massively increase surface area. - Large intestine (about 1.5 metres) — absorbs water and electrolytes from what remains. Compact into faeces. Contains billions of gut bacteria. - Pancreas — sits behind the stomach. Releases digestive enzymes into the small intestine and makes the hormones insulin and glucagon.

The two kidneys sit at the back of the abdomen (retroperitoneal — behind the peritoneum), one on each side of the spine, roughly at waist level. Each is about the size of a fist. What kidneys do: Every minute, about 1.2 litres of blood passes through both kidneys — that is the entire blood volume filtered roughly 60 times per day. The kidney's job is to: 1. Filter out waste products (urea, creatinine, uric acid) 2. Regulate water and salt balance (controlling blood volume and blood pressure) 3. Regulate blood pH (acidity) 4. Produce hormones — erythropoietin (EPO, signals bone marrow to make red blood cells) and activate vitamin D The functional unit of the kidney is the nephron — each kidney has about 1 million of them. Each nephron filters blood, reabsorbs what the body needs, and lets the rest become urine. Urine flows: kidney → ureter → bladder → urethra → out. Clinically: - Acute kidney injury (AKI) — sudden loss of kidney function. Common in hospital patients (dehydration, sepsis, drug toxicity). Detected by rising creatinine on blood tests. - Chronic kidney disease (CKD) — gradual loss of kidney function over years, often from diabetes or high blood pressure. Can progress to needing dialysis (a machine that filters the blood externally). - Kidney stones — crystals of calcium oxalate or uric acid form in the kidney and can block the ureter, causing severe flank pain (renal colic).

The brain is the control centre of the body, weighing about 1.4 kg and containing approximately 86 billion neurons. Four main regions: - Cerebrum — the large, folded outer portion. Divided into two hemispheres (left and right) and four lobes: frontal (decision-making, movement, personality), parietal (sensation, spatial awareness), temporal (hearing, memory, language), occipital (vision). - Cerebellum — sits at the back, below the cerebrum. Coordinates movement, balance, and fine motor control. A drunk person staggers because alcohol preferentially affects the cerebellum. - Brainstem — connects the brain to the spinal cord. Controls vital automatic functions: breathing, heart rate, and blood pressure. Damage here is catastrophic. - Diencephalon — includes the thalamus (relay station for sensory signals) and hypothalamus (controls body temperature, hunger, thirst, sleep, and the hormonal system via the pituitary gland). The spinal cord runs from the brainstem down through the vertebral column. It carries signals between the brain and the rest of the body, and controls simple reflexes locally (without needing the brain). Clinically: - Stroke — blood supply to a part of the brain is cut off (ischaemic) or a blood vessel bursts (haemorrhagic). Brain cells die within minutes. Because the left hemisphere controls the right side of the body (and vice versa), a left-sided stroke causes right-sided weakness. - Meningitis — infection of the meninges (the membranes covering the brain and spinal cord). Classic signs: severe headache, stiff neck, sensitivity to light, fever. A medical emergency.