Circulatory Systems, Heart & Blood Vessels

Circulatory System Overview

• The circulatory system is an organ system that transports blood around the body.
• Components include **blood vessels**, a **pump** (heart), and **valves** for one-way flow.
• Fish have a **two-chambered heart** and **single circulation** (blood passes through heart once per circuit).
• Mammals have a **four-chambered heart** and **double circulation** (blood passes through heart twice per circuit).
• Double circulation: **right side** pumps deoxygenated blood to lungs (pulmonary circulation); **left side** pumps oxygenated blood to body (systemic circulation).
• Advantage: blood pressure lost in lung capillaries is restored by the heart before blood travels to the body, ensuring faster delivery of oxygen and glucose.

The Mammalian Heart

• The heart is a muscular pump; its own blood supply comes from the **coronary arteries**.
• The **septum** separates the right and left sides, preventing mixing of oxygenated and deoxygenated blood.
• Right side: receives deoxygenated blood from body via **vena cava**; pumps it to lungs via **pulmonary artery**.
• Left side: receives oxygenated blood from lungs via **pulmonary vein**; pumps it to body via **aorta**.
• **Atria** have thinner walls; **ventricles** have thicker walls (left ventricle thickest as it pumps to whole body).
• Valves: **atrioventricular valves** (tricuspid on right, bicuspid on left) prevent backflow into atria; **semilunar valves** in pulmonary artery and aorta prevent backflow into ventricles.

Heart Function & Cardiac Cycle

• Deoxygenated blood enters right atrium → passes through tricuspid valve into right ventricle → ventricle contracts → blood pushed through semilunar valve into pulmonary artery → to lungs.
• Oxygenated blood returns from lungs via pulmonary vein → enters left atrium → passes through bicuspid valve into left ventricle → ventricle contracts → blood pushed through semilunar valve into aorta → to body.
• Atrioventricular valves open when atria contract; close when ventricles contract to prevent backflow.
• Semilunar valves open when ventricles contract; close to prevent blood flowing back into heart.

Monitoring Heart Activity & Exercise

• Heart activity monitored by **ECG**, **pulse rate** (bpm), or listening to valve sounds with a **stethoscope**.
• During exercise, heart rate increases to deliver more oxygen and glucose to muscles and remove waste faster.
• After exercise, heart rate remains elevated to remove excess waste and repay **oxygen debt** (break down lactic acid).
• Pulse rate gradually returns to resting level over several minutes.

Coronary Heart Disease (CHD)

• **Coronary arteries** supply oxygen and nutrients to heart muscle.
• CHD occurs when these arteries become narrowed or blocked by fatty **plaques** (mainly cholesterol).
• Partial blockage causes **angina** (chest pain); complete blockage leads to **heart attack** (muscle cells die).
• Risk factors: smoking, high-fat diet, lack of exercise, obesity, high blood pressure.
• Prevention: quit smoking, eat less animal fat, exercise regularly.

Blood Vessels: Arteries, Veins, Capillaries

• **Arteries** carry blood **away** from heart at **high pressure**; thick muscular walls with elastic fibres; narrow lumen; mostly oxygenated (except pulmonary artery).
• **Veins** carry blood **towards** heart at **low pressure**; thin walls; wide lumen; contain **valves** to prevent backflow; mostly deoxygenated (except pulmonary vein).
• **Capillaries** are one cell thick with 'leaky' walls for diffusion; very narrow lumen; connect arterioles to venules.
• Arteries divide into **arterioles** then capillaries; capillaries merge into **venules** then veins.

Circulation Around the Body

• Key blood vessels: **aorta** (to body), **vena cava** (from body), **pulmonary artery** (to lungs), **pulmonary vein** (from lungs).
• **Renal artery** carries blood to kidney; **renal vein** carries blood from kidney.
• **Hepatic artery** brings oxygenated blood to liver; **hepatic portal vein** brings deoxygenated blood from gut to liver; **hepatic vein** carries blood away from liver to heart.

Components of Blood

• Blood consists of **plasma**, **red blood cells**, **white blood cells**, and **platelets**.
• **Plasma** transports carbon dioxide, digested nutrients, urea, hormones, and heat.
• **Red blood cells** contain **haemoglobin** and transport oxygen as **oxyhaemoglobin**; no nucleus; biconcave disc shape.
• **White blood cells** defend against infection: **phagocytes** engulf pathogens (phagocytosis); **lymphocytes** produce antibodies and antitoxins.
• **Platelets** are cell fragments involved in **blood clotting**.

Blood Clotting

• When skin is broken, platelets release chemicals that convert soluble **fibrinogen** into insoluble **fibrin**.
• Fibrin forms a mesh that traps red blood cells, creating a **clot**.
• The clot dries to form a **scab**, preventing blood loss and entry of pathogens.

Cross-sections of an artery, vein and capillary showing differences in wall thickness and lumen size.

Structure of the mammalian heart showing chambers, septum, and major blood vessels.

Blood components: red blood cells, white blood cells, platelets, and plasma.