# The mammalian circulatory system

The mammalian circulatory system is a **specialized mass transport system** essential for multicellular organisms, such as mammals, because **diffusion alone is too slow** to efficiently transport necessary substances to and from all body cells. Larger organisms have a **low surface area to volume ratio**, making a specialized system vital to carry raw materials like glucose and oxygen from exchange organs (e.g., lungs and digestive system) to body cells, and to remove metabolic waste products such as carbon dioxide. This system also helps in heat distribution and maintaining a constant internal environment (homeostasis).

* **System Type: Closed and Double Circulation**
  * Mammals have a **closed circulatory system**, meaning blood is always contained within a network of blood vessels.
  * It is a **double circulatory system** because blood passes through the heart twice for each complete circuit of the body.
  * The two circuits are:
    * **Pulmonary circulation**: Carries **deoxygenated blood** from the heart to the lungs and returns **oxygenated blood** to the heart.
    * **Systemic circulation**: Carries **oxygenated blood** from the heart to the rest of the body and returns **deoxygenated blood** to the heart.
  * This double circulation allows oxygenated blood to be pumped around the body at a **higher pressure** and **faster**, providing a more efficient oxygen supply to mammalian cells compared to single circulatory systems (e.g., in fish) where pressure is lost in gill capillaries.
* **The Heart: The Pump**
  * The heart is a **muscular, four-chambered organ** that acts as the pump for the circulatory system.
  * The **right side** pumps deoxygenated blood to the lungs, while the **left side** pumps oxygenated blood to the rest of the body.
  * The **ventricles** have thicker, more muscular walls than the atria because they need to pump blood at higher pressure over greater distances. The **left ventricle** has the thickest wall, as it pumps blood to the entire body, while the right ventricle only pumps to the nearby lungs.
  * Heart muscle is **myogenic**, meaning it contracts and relaxes automatically without external nerve impulses.
  * The **sinoatrial node (SAN)** in the right atrium acts as the natural **pacemaker**, initiating a wave of electrical activity that spreads across the atria, causing them to contract (atrial systole).
  * The impulse then reaches the **atrioventricular node (AVN)**, which, after a short delay (0.1-0.2 seconds), transmits it via the **bundle of His** and **Purkyne tissue** to the ventricles, causing them to contract from the bottom upwards (ventricular systole).
  * The **cardiac cycle** is the ongoing sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles, maintaining continuous blood circulation. Valves (atrioventricular and semilunar) open and close in response to pressure changes, ensuring **unidirectional blood flow** and preventing backflow.
* **Blood Vessels**
  * The circulatory system consists of **arteries, arterioles, capillaries, venules, and veins**.
  * **Arteries**: Carry blood *away* from the heart at **high pressure**. They have **thick, muscular, and elastic walls** to withstand high pressure and maintain blood flow by stretching and recoiling. The inner lining (endothelium) is folded to allow stretching. Most arteries carry oxygenated blood, except for the pulmonary arteries.
  * **Arterioles**: Smaller arteries that regulate blood flow to different body areas by contracting or relaxing their muscular walls.
  * **Capillaries**: The **smallest** vessels, forming extensive networks (capillary beds) near cells. They are adapted for **efficient exchange**: their walls are only **one cell thick** (squamous epithelium), providing a **short diffusion pathway** and a **large surface area**. Blood flow is slowest in capillaries, allowing more time for exchange.
  * **Venules**: Small veins that collect blood from capillaries.
  * **Veins**: Carry blood *back* to the heart under **low pressure**. They have wider lumens and thinner walls with less elastic and muscle tissue than arteries. Veins contain **valves** to prevent backflow of blood, especially against gravity, and blood flow is aided by surrounding body muscle contractions. Most veins carry deoxygenated blood, except for pulmonary veins.
* **Blood: The Transport Medium**
  * Blood is a complex fluid tissue composed of **plasma**, **red blood cells (erythrocytes)**, **white blood cells (leucocytes)**, and **platelets**.
  * **Plasma**: The liquid component (mostly water, 95%) that acts as a solvent for transporting nutrients (glucose, amino acids, lipids), waste products (urea, carbon dioxide), hormones, and heat. It also contains plasma proteins like albumin (regulating water potential and transporting lipids/iron) and antibodies.
  * **Red Blood Cells**: Biconcave discs with no nucleus to maximize space for **haemoglobin**, which carries oxygen. Each haemoglobin molecule can bind with four oxygen molecules.
  * **Oxygen Transport**: Oxygen loads onto haemoglobin in the lungs (high partial pressure of oxygen, pO2) to form oxyhaemoglobin and unloads in respiring tissues (low pO2). The **Bohr shift (or Bohr effect)** describes how increased carbon dioxide concentration and lower pH (more acidic conditions from respiration) reduce haemoglobin's affinity for oxygen, facilitating unloading in tissues.
  * **Carbon Dioxide Transport**: Primarily transported in plasma as **hydrogencarbonate ions** (HCO3-), with some bound to haemoglobin (carbaminohaemoglobin). The enzyme **carbonic anhydrase** in red blood cells catalyzes the reaction between CO2 and water to form carbonic acid, which then dissociates into H+ and HCO3-. The H+ ions are buffered by haemoglobin.
  * **White Blood Cells (Leucocytes)**: Involved in the **immune system**, including lymphocytes (form antibodies) and phagocytes (ingest bacteria/cell fragments).
  * **Platelets**: Cell fragments involved in **blood clotting**.
* **Tissue Fluid and Lymph**
  * **Tissue fluid** is formed from blood plasma that is forced out of capillaries (by high **hydrostatic pressure** at the arteriole end) and surrounds cells in tissues.
  * It contains oxygen, water, and nutrients, but generally lacks red blood cells and large plasma proteins (like albumin) because they are too large to pass through capillary walls.
  * Cells take in substances from tissue fluid and release metabolic waste into it.
  * Most water re-enters capillaries at the venule end by **osmosis** due to lower water potential in the capillaries (caused by the higher concentration of plasma proteins).
  * Any **excess tissue fluid** drains into the **lymphatic system**, a network of tubes that transports it back to the circulatory system via lymph vessels and lymph nodes. Lymph nodes filter the lymph and are sites for immune cells.
* **Cardiovascular Disease (CVD)**
  * CVD is a general term for diseases of the heart and blood vessels, often beginning with **atheroma formation** (fatty deposits in artery linings).
  * Atheromas can restrict blood flow, lead to blood clots (thrombosis), aneurysms, and potentially a **myocardial infarction (heart attack)** if a coronary artery is blocked.
  * Risk factors include high blood pressure, high cholesterol, poor diet, and smoking.
* **Comparison to Other Organisms**
  * **Single-celled organisms** do not need a specialized transport system as they can rely on **diffusion** due to their large surface area to volume ratio and short diffusion pathways.
  * **Fish** have a **single circulatory system** where blood passes through the heart only once per circuit, moving from the heart to the gills, then around the body, losing pressure in the gill capillaries.
  * **Plants** have different transport systems (**xylem** for water and mineral ions, **phloem** for organic substances like sucrose) and do not have a central pump like the heart. Plant transport relies on passive processes (e.g., transpiration in xylem) and active transport mechanisms in phloem loading, rather than a circulatory pump.