Transport Systems in Mammals (Humans)
Transport systems in mammals (humans)
Mammals are highly active organisms that require a specialised, efficient transport system to deliver oxygen, nutrients and signalling molecules and to remove wastes rapidly from all tissues. Humans have two interlinked transport systems: a closed cardiovascular system that circulates blood and an open lymphatic system that returns tissue fluid to the blood and supports immunity.
Circulatory systems in animals
Open circulatory system
An open circulatory system is one in which blood (or haemolymph) leaves the vessels and flows freely through body cavities and sinuses, bathing organs directly.
- Blood vessels open into body cavities and form networks of channels and spaces.
- Blood (haemolymph) moves freely and more slowly within the body cavity.
- Open systems are common in many invertebrates such as most arthropods (for example, many insects, crabs and spiders).
Closed circulatory system
In a closed circulatory system the blood always remains within a continuous series of vessels (arteries, capillaries and veins). It circulates in one direction and is pumped by a heart.
- Blood flows in vessels and does not fill the general body cavity.
- Flow is continuous and generally faster and more efficient than in open systems.
- All vertebrates have closed circulatory systems.
| Feature | Open circulatory system | Closed circulatory system |
|---|---|---|
| Typical organisms | Invertebrates: many arthropods (insects, crabs, spiders) | Vertebrates |
| Location of circulating fluid | Body fluid/haemolymph fills body cavities and spaces | Blood remains inside arteries, capillaries and veins |
| Efficiency | Slow and limited exchange | Fast and efficient exchange of gases and nutrients |
Transport systems in humans
Humans have two principal transport systems:
- Cardiovascular (blood) system - a closed circulatory system that transports blood throughout the body.
- Lymphatic system - an open system of lymph vessels and nodes that returns excess tissue fluid to the blood and plays a major role in immune defence.
Cardiovascular circulation: single and double circulation
Vertebrate cardiovascular systems are organised as either single or double circulation.
Single circulation
Present in fishes. The heart receives deoxygenated blood and pumps it to the gills where gas exchange occurs; oxygenated blood then flows to the tissues. Blood passes through the heart only once in each complete circuit.
Double circulation
Present in amphibians, most reptiles, birds and mammals. The heart pumps blood through two separate circuits:
- Pulmonary circulation - between the heart and the lungs, for gas exchange.
- Systemic circulation - between the heart and the rest of the body, supplying tissues with oxygen and nutrients.
Because blood passes through the heart twice in a complete circuit (once to the lungs and once to the body), this arrangement is called double circulation and allows higher metabolic rates and better oxygen delivery to tissues.
Pulmonary circulation
Pulmonary circulation carries deoxygenated blood from the heart to the lungs and returns oxygenated blood to the heart. The pathway is:
- Right ventricle → pulmonary arteries → lungs
- Gas exchange in lung capillaries (blood picks up O2 and releases CO2)
- Pulmonary veins → left atrium
Systemic circulation
Systemic circulation carries oxygenated blood from the heart to all body tissues and returns deoxygenated blood to the heart. A simplified pathway is:
- Left ventricle → aorta and systemic arteries → arterioles → capillaries in tissues
- Exchange of gases, nutrients and wastes at tissue capillaries
- Venules → veins → superior and inferior vena cava → right atrium
Blood vessels are often named for the organs or regions they serve. Examples:
- Pulmonary - lungs
- Cardiac - heart
- Hepatic - liver
- Gastric - stomach
- Renal - kidney
- Mesenteric - intestines
The heart
The heart is a muscular pump located in the thoracic cavity behind the sternum and between the lungs. It pumps blood through the pulmonary and systemic circulations.
External structure
- The heart has an apex that points downwards and to the left.
- It is enclosed by the pericardium, a double-layered sac. The space between the layers contains pericardial fluid that reduces friction as the heart beats.
- Large blood vessels (aorta, pulmonary trunk, superior and inferior venae cavae) enter and leave the broader base of the heart and help hold it in position.
Internal structure
- The heart is hollow and composed mainly of cardiac muscle (myocardium) with an inner smooth lining called the endocardium.
- It has four chambers: two upper atria (right and left) and two lower ventricles (right and left). Atria are thinner-walled and receive blood; ventricles are thicker-walled and pump blood out.
- Valves enforce unidirectional flow:
- The tricuspid valve separates the right atrium and right ventricle.
- The bicuspid (mitral) valve separates the left atrium and left ventricle.
- Semilunar valves at the bases of the pulmonary artery and aorta prevent backflow into ventricles after contraction.
- Chordae tendineae (tendinous cords) attach valve leaflets to papillary muscles in the ventricles and prevent valve prolapse during ventricular contraction.
- The septum separates the left and right sides of the heart, preventing mixing of oxygenated and deoxygenated blood in normal anatomy.
The cardiac cycle
The cardiac cycle is the sequence of events in one complete heartbeat, during which the heart chambers contract and relax to move blood through the heart and to the lungs and body.
Key terms:
- Systole - contraction of heart muscle (atria or ventricles).
- Diastole - relaxation of heart muscle (atria and ventricles).
Approximate durations in a normal resting heartbeat:
- Atrial systole: about 0.1 seconds.
- Ventricular systole: about 0.3 seconds.
- General diastole (both atria and ventricles relaxed): about 0.4 seconds.
- Deoxygenated blood returns from the body to the right atrium via the superior and inferior venae cavae.
- Right atrium contracts (atrial systole) and blood flows through the tricuspid valve into the right ventricle.
- Right ventricle contracts (ventricular systole), forcing blood through the pulmonary valve into the pulmonary arteries and on to the lungs.
- After gas exchange in the lungs, oxygenated blood returns to the left atrium via the pulmonary veins.
- Left atrium contracts and blood flows through the bicuspid (mitral) valve into the left ventricle.
- Left ventricle contracts, sending oxygenated blood through the aortic valve into the aorta and systemic arteries to the tissues.
Control of the heartbeat
The heartbeat is coordinated by specialised cardiac muscle regions that generate and conduct electrical impulses.
- The sino-atrial (SA) node, located in the wall of the right atrium, acts as the primary pacemaker. It initiates an electrical impulse that spreads across both atria, causing atrial systole.
- The impulse reaches the atrioventricular (AV) node, then passes through the atrioventricular bundle to the ventricular muscle, causing ventricular systole. This conduction delay at the AV node ensures atrial contraction is completed before ventricular contraction begins.
- The coordinated opening and closing of valves prevents backflow during these contractions.
Exercise and heart rate
Exercise increases heart rate and cardiac output by two main mechanisms:
- Chemical stimulation: Increased cellular respiration raises the blood concentration of carbon dioxide and other metabolites. Chemoreceptors in the carotid arteries and aorta detect the change and send impulses to the medulla oblongata, which increases sympathetic output (via the accelerans nerve) to raise heart rate.
- Mechanical stimulation: Increased muscular activity increases venous return to the heart. Stretch receptors in the right atrium sense the increased volume and trigger reflexes that raise heart rate to cope with the extra volume.
The autonomic nervous system balances sympathetic stimulation (increasing rate and force) with parasympathetic input (primarily via the vagus nerve) which slows the heart when required.
Blood vessels
Structure and functions
There are three main types of blood vessels: arteries, veins and capillaries. Their structure reflects their functions.
Arteries
- Carry blood away from the heart (usually oxygenated except pulmonary arteries).
- Walls have three layers: an inner endothelium, a thick middle layer of smooth muscle and elastic connective tissue, and an outer connective tissue layer. The thick muscular and elastic wall withstands high pressure from ventricular contraction.
- Large arteries branch into smaller arterioles and then capillaries.
- Arteries close to the skin surface are where a palpable pulse can be felt.
Veins
- Carry blood toward the heart (usually deoxygenated except pulmonary veins).
- Walls also have three layers but the middle muscle layer is thinner than in arteries. Veins have a larger lumen and thinner walls because blood pressure is lower.
- Many veins contain one-way semilunar valves that prevent backflow and assist return of blood to the heart, especially from the lower limbs against gravity.
- Smaller veins are called venules which drain capillary networks.
Capillaries
- Microscopic vessels that connect arterioles and venules.
- Walls are a single layer of endothelial cells (one cell thick), providing a short diffusion distance for exchange of gases, nutrients and wastes between blood and tissue fluid.
- Capillaries form extensive branching networks to increase surface area for exchange.
Comparative structure of vessel walls
| Vessel | Main wall features | Primary function |
|---|---|---|
| Artery | Thick muscular and elastic middle layer; narrow lumen; endothelium | Transport blood under high pressure from the heart |
| Vein | Thin muscular layer; larger lumen; valves present; endothelium | Return blood to the heart under low pressure |
| Capillary | Single-cell endothelium; very thin wall | Exchange of gases, nutrients and waste with tissues |
Blood
Blood is a fluid connective tissue that consists of a liquid plasma in which formed elements are suspended.
Main components:
- Plasma - the liquid portion (about 55% of blood volume), containing water, dissolved gases, nutrients, hormones, ions and plasma proteins.
- Erythrocytes (red blood cells) - carry oxygen bound to haemoglobin; have no nucleus in mature human cells and are specialised for gas transport.
- Leucocytes (white blood cells) - part of the immune system; several types defend against infection and foreign material.
- Platelets (thrombocytes) - cell fragments involved in blood clotting and wound repair.
The lymphatic system
The lymphatic system complements the cardiovascular system. It is a network of lymph capillaries, vessels, ducts, lymph nodes and lymphoid organs that collect excess tissue fluid (lymph), filter it and return it to the bloodstream. It also produces and transports immune cells.
Lymph vessels and ducts
- Lymph capillaries lie between tissue cells in most organs (except the central nervous system) and have blind-ended beginnings that collect tissue fluid.
- Capillaries join to form larger lymphatic vessels, which in turn drain into two main ducts:
- Thoracic duct - the largest lymphatic vessel; receives lymph from the lower body, left arm and left side of head and neck; empties into the left subclavian vein so lymph re-enters the blood plasma.
- Right lymphatic duct - receives lymph from the right arm, right side of head and neck and right thorax; empties into the right subclavian vein.
- Lymph vessels have three-layered walls similar to blood vessels but much thinner and more transparent and contain more valves than veins.
Functions of the lymphatic system
- Return excess tissue fluid and leaked plasma proteins to the bloodstream.
- Transport absorbed fats and fat-soluble vitamins from the digestive system (chyle) into the circulation.
- Produce and transport lymphocytes and other immune cells; lymph nodes filter lymph, removing bacteria, foreign particles and damaged cells.
- Drainage and detoxification - remove dead cells and pathogens from tissues.
Blood plasma, tissue fluid and lymph
Blood plasma is forced out of capillaries into the surrounding tissues by blood pressure; this plasma becomes tissue fluid, which bathes cells and permits exchange of substances. Most tissue fluid is reabsorbed into venous ends of capillaries; the remainder enters lymphatic capillaries and becomes lymph.
- Lymph is similar to plasma but contains less protein and, in lymph draining from the intestine after a meal, higher concentrations of fats (chyle).
- Lymph contains lymphocytes (a type of white blood cell) that are important in immune responses.
| Feature | Cardiovascular system (blood) | Lymphatic system (lymph) |
|---|---|---|
| Function | Deliver oxygen, nutrients and hormones to tissues; remove metabolic wastes | Collect and remove excess tissue fluid, filter it, transport immune cells and absorbed fats |
| Pathway | Closed circuit: heart → arteries → capillaries → veins → heart | Open circuit: tissue spaces → lymph capillaries → lymph vessels → ducts → subclavian veins |
| Movement | Pumped by the heart; flow aided by arterial pressure | Not pumped by a central organ; flow aided by skeletal muscle action, breathing and valves in lymph vessels |
| Fluid | Plasma containing red and white blood cells and platelets | Clear or milky fluid (lymph) containing lymphocytes and some proteins and fats |
| Filtration | Blood is filtered through kidneys and liver | Lymph is filtered by lymph nodes distributed throughout the body |
Cardiovascular diseases and disorders
Many diseases affect the cardiovascular and lymphatic systems; lifestyle factors such as smoking, poor diet, obesity and lack of exercise increase risk. Common disorders include:
Anaemia
Insufficient red blood cells or haemoglobin (often due to iron deficiency) leading to pallor, weakness and reduced oxygen-carrying capacity of the blood.
Leukaemia
A group of cancers causing uncontrolled production of abnormal white blood cells in bone marrow. These abnormal cells crowd out normal blood cell production causing anaemia, infection risk and bleeding problems.
Hypertension and hypotension
Hypertension (high blood pressure) often results from salt and water retention, obesity and other factors; it increases the workload on the heart and raises the risk of stroke, heart attack and kidney disease. Hypotension (low blood pressure) can cause dizziness or fainting; in some contexts it is associated with good cardiovascular health but may be pathological if severe.
Angina, atherosclerosis, stroke and myocardial infarction
Atherosclerosis is the build-up of fatty deposits (plaques) in arterial walls, which narrows arteries and reduces blood flow. When coronary arteries are affected, reduced blood flow to the heart muscle can cause angina (chest pain). If a coronary artery becomes blocked, a region of heart muscle may be deprived of oxygen and suffer myocardial infarction (heart attack). If an artery to the brain is blocked or ruptures, it can cause a stroke with potential brain damage.
Treatment of heart disease
Treatments range from lifestyle modification and medication to surgical interventions:
- Coronary stenting - a catheter-mounted balloon and mesh stent are inserted into a narrowed coronary artery; inflation of the balloon expands the stent and opens the vessel, restoring blood flow.
- Heart valve replacement - damaged valves may be replaced with artificial or donor valves during open-heart surgery. When the heart's own pacemaker fails, an artificial pacemaker can be implanted to provide electrical pacing and maintain a normal heartbeat.
- Coronary artery bypass graft (CABG) - bypass surgery uses vessels from elsewhere in the body to bypass blocked coronary arteries. The heart is often stopped during the operation and a heart-lung machine temporarily performs circulation and gas exchange.
- Heart transplant - in severe, otherwise untreatable heart failure a diseased heart may be replaced with a healthy donor heart. Transplantation is complex, requires lifelong immunosuppression and is reserved for patients who meet strict criteria.
Prevention of many cardiovascular diseases emphasises a healthy lifestyle: balanced diet low in saturated fats and excessive salt, regular physical activity, avoiding tobacco, maintaining a healthy weight and monitoring blood pressure and cholesterol.
Summary
The human transport system comprises a closed cardiovascular system that supplies oxygen and nutrients and removes wastes, and an interconnected lymphatic system that returns tissue fluid and supports immunity. The heart, blood vessels and blood work together to maintain tissue perfusion. Understanding structure and function-plus common diseases and their treatments-helps in preventing and managing cardiovascular disorders.
