Circulatory Systems in Animals (Grade 10 NSC Matric Life Sciences): Revision Notes

Circulatory Systems in Animals

Introduction to transport systems

All living organisms need oxygen and nutrients to survive, and they must remove carbon dioxide and other waste products. Transport systems are essential for moving these materials around the body. The circulatory system does much more than just transport nutrients and gases - it also carries hormones to target organs and helps the immune system by transporting white blood cells and antibodies throughout the body.

Simple, single-celled organisms can rely on diffusion to move materials in and out of their cells. However, larger, more complex organisms need specialised circulatory systems to transport materials efficiently to all parts of their bodies.

Types of circulatory systems

There are two main types of circulatory systems found in animals: open and closed systems.

Open circulatory systems

In an open circulatory system, the blood vessels transport fluid into a body cavity where it flows freely around the organs. The blood directly bathes the organs, supplying them with oxygen and nutrients while removing waste products. This type of system moves blood quite slowly because it lacks the muscular walls found in closed systems.

Most invertebrates, including crabs, insects, and snails, have open circulatory systems. The blood flows at a much slower pace due to the absence of contained vessels throughout the body.

Closed circulatory systems

In a closed circulatory system, blood never leaves the blood vessels. Instead, materials are transferred from one blood vessel to another without the blood entering body cavities. Blood flows in one direction only, efficiently delivering oxygen and nutrients to cells while removing waste products.

Closed systems can be further divided into single and double circulation systems, depending on how many times blood passes through the heart in one complete circuit.

Single and double circulation systems

Single circulation systems

Fish have single circulation systems with a two-chambered heart consisting of one atrium and one ventricle. Here's how it works:

• The heart pumps deoxygenated blood to the gills
• At the gills, the blood picks up oxygen and releases carbon dioxide
• The now-oxygenated blood flows directly to the rest of the fish's body
• Deoxygenated blood returns to the heart to complete the circuit

Double circulation systems

Birds and mammals, including humans, have double circulation systems with four-chambered hearts. This system is called "double" because blood passes through the heart twice in one complete circuit.

The four chambers consist of:

• Left atrium and left ventricle (left side of heart)
• Right atrium and right ventricle (right side of heart)

Human circulatory system

The human circulatory system consists of two separate circuits: the pulmonary circuit and the systemic circuit.

Pulmonary circulation

The pulmonary circulation carries deoxygenated blood from the heart to the lungs and returns oxygenated blood back to the heart.

Here's the pathway:

• Deoxygenated blood leaves the right ventricle through the pulmonary artery
• The pulmonary artery is the only artery that carries deoxygenated blood
• Blood travels to the lung capillaries where gas exchange occurs
• Carbon dioxide moves out of the blood into the air spaces (alveoli)
• Oxygen moves from the alveoli into the blood
• Oxygenated blood returns to the left atrium via the pulmonary veins

Systemic circulation

Systemic circulation transports oxygenated blood from the heart to all body organs and returns deoxygenated blood back to the heart.

The pathway works like this:

• Oxygenated blood leaves the left ventricle through the aorta (the body's largest artery)
• The aorta branches into smaller arteries that supply different organs
• These arteries branch further into tiny capillaries
• In the capillaries, oxygen moves into cells while carbon dioxide and waste products move into the blood
• Deoxygenated blood flows into venules, then larger veins
• Blood returns to the right atrium via two major veins: the superior vena cava (from head and arms) and inferior vena cava (from lower body)

Heart structure and function

External structure of the heart

The heart is a powerful muscle about the size of your clenched fist. It's located in your chest cavity, just behind your breastbone, in a space called the pericardial cavity. The heart is surrounded by a protective double membrane called the pericardium, with pericardial fluid between the layers to reduce friction during heartbeats.

The heart muscle itself is called the myocardium and consists of four chambers:

• Atria (singular: atrium) - the two upper chambers that receive blood
• Ventricles - the two lower chambers that pump blood out of the heart

Internal structure of the heart

The heart's internal structure includes several important features that ensure blood flows in the correct direction:

Heart chambers:

• Right atrium: receives deoxygenated blood from the body
• Right ventricle: pumps deoxygenated blood to the lungs
• Left atrium: receives oxygenated blood from the lungs
• Left ventricle: pumps oxygenated blood to the body

Heart valves prevent blood from flowing backwards:

• Tricuspid valve: between right atrium and right ventricle
• Bicuspid valve (also called mitral valve): between left atrium and left ventricle
• Semi-lunar valves: at the base of the aorta and pulmonary artery

The valves are attached to strong tendinous cords (chordae tendineae) that connect to papillary muscles. These structures prevent the valves from opening the wrong way when the ventricles contract.

Circulation of blood through the heart

Blood follows a specific pathway through the heart in one complete circuit:

1. Deoxygenated blood enters the right atrium from the superior and inferior vena cava
2. Blood flows through the tricuspid valve into the right ventricle
3. The right ventricle pumps blood through the pulmonary valve into the pulmonary artery
4. Blood travels to the lungs for gas exchange
5. Oxygenated blood returns to the left atrium via pulmonary veins
6. Blood flows through the bicuspid valve into the left ventricle
7. The left ventricle pumps blood through the aortic valve into the aorta
8. Blood travels to the rest of the body through the systemic circulation

Major organs and blood supply

Every organ in the body receives its own blood supply through specific arteries and returns blood through corresponding veins. This ensures all cells receive the oxygen and nutrients they need for proper function.

Key organ blood supplies:

• Brain: supplied by carotid and vertebral arteries; drained by jugular veins (receives 15% of total heart output)
• Liver: receives blood from hepatic artery and hepatic portal vein; drained by hepatic vein
• Kidneys: supplied by renal arteries; drained by renal veins (philtre waste from blood)
• Heart muscle: supplied by coronary arteries; drained by cardiac veins

The cardiac cycle

The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. At rest, each heartbeat takes approximately 0.8 seconds, and the normal heart rate is about 72 beats per minute.

Phases of the cardiac cycle

Phase 1: Atrial systole (0.1 seconds)

• Both atria contract simultaneously
• Blood flows from the superior and inferior vena cava into the right atrium
• Blood flows from the pulmonary veins into the left atrium
• Blood is forced through the tricuspid and bicuspid valves into the ventricles

Phase 2: Ventricular systole (0.3 seconds)

• Both ventricles contract while atria relax
• Blood is forced upward, closing the tricuspid and bicuspid valves (creates "lubb" sound)
• Blood flows through semi-lunar valves into the pulmonary artery and aorta
• Atria begin filling with blood during this time

Phase 3: General diastole (0.4 seconds)

• All four chambers relax
• Ventricular pressure drops, closing the semi-lunar valves (creates "dubb" sound)
• Blood continues flowing into the atria from veins
• The cycle begins again

Heart sounds and pulse

The heart produces two distinct sounds during each beat:

• "Lubb" sound: caused by tricuspid and bicuspid valves closing when ventricles contract
• "Dubb" sound: caused by semi-lunar valves closing when ventricles relax

Your pulse is the wave of pressure created by the left ventricle pumping blood into the aorta. It can be felt at various points where arteries are close to the skin surface, such as the wrist or neck.

Heart rate control and electrical activity

Control mechanisms

The heart has its own built-in pacemaker system that controls the cardiac cycle:

• Sinoatrial node (SA node): located in the right atrium wall, acts as the heart's natural pacemaker
• Atrioventricular node (AV node): located between atria and ventricles, delays electrical signals briefly

The electrical impulse starts at the SA node, spreads through both atria (causing them to contract), then reaches the AV node. After a brief pause, the signal spreads through specialised conducting fibres (bundles of His and Purkinje fibres) to both ventricles, causing them to contract.

Electrocardiogram (ECG)

The heart's electrical activity can be measured using an electrocardiogram (ECG). A normal heart shows a very regular rhythm, while abnormal patterns indicate heart problems:

• Bradycardia: heart rate too slow (less than 60 beats per minute)
• Tachycardia: heart rate too fast (more than 100 beats per minute)
• Arrhythmia: irregular heart rhythm

Stroke volume and blood pressure

Stroke volume is the amount of blood pumped by the heart during each beat. During exercise, the heart increases both its stroke volume and heart rate to meet the body's increased demand for oxygen and nutrients.

Blood pressure is the force that blood exerts on blood vessel walls. Normal blood pressure is 120/80 mmHg, where:

• 120 is the systolic pressure (when ventricles contract)
• 80 is the diastolic pressure (when ventricles relax)

Blood vessels

Blood vessels form a network throughout the body, with each type specialised for its specific function.

Arteries

Arteries carry blood away from the heart and generally transport oxygenated blood (except for the pulmonary artery). They have thick walls with three layers:

• Outer layer: connective tissue for protection
• Middle layer: smooth muscle that can contract to control blood flow and pressure
• Inner layer: smooth endothelium that reduces friction

Arteries branch into smaller arterioles, which then branch into microscopic capillaries.

Capillaries

Capillaries are the smallest blood vessels, consisting of just a single layer of endothelial cells. This thin wall allows efficient exchange of materials between blood and tissues:

• Oxygen and nutrients move from blood into tissue fluid
• Carbon dioxide and waste products move from tissue fluid into blood
• Blood cells never directly contact body cells - exchange occurs through tissue fluid

Capillaries form intricate networks throughout tissues, ensuring every cell is close to a blood supply.

Veins

Veins carry blood back to the heart and generally transport deoxygenated blood (except for pulmonary veins). They have thinner walls than arteries because blood pressure is much lower by the time it reaches veins.

Key features of veins:

• Two-layer walls: outer connective tissue and inner endothelium
• Large internal diameter to accommodate slower-moving blood
• Semi-lunar valves at intervals to prevent blood flowing backwards
• Located closer to the skin surface to release excess heat

Small capillaries join to form venules, which merge into larger veins that eventually return blood to the heart.

Comparison of blood vessel types

Feature	Arteries	Capillaries	Veins
Blood flow direction	Away from heart	Through tissue beds	To the heart
Wall structure	Thick, three layers with muscle	Single layer of endothelium	Thin, two layers
Blood pressure	High pressure	Pressure drops significantly	Low pressure
Valves	Only in aorta and pulmonary artery	None	Semi-lunar valves present
Pulse detection	Strong pulse felt	No pulse	No pulse detected
Location	Deeper in tissues	At tissue level	Near skin surface

Lymphatic system

The lymphatic system is part of the circulatory system, consisting of a network of vessels that carry a clear fluid called lymph back to the heart. This system serves important functions:

• Fluid balance: returns excess tissue fluid to the bloodstream
• Immune function: transports white blood cells and philtres out pathogens
• Fat absorption: absorbs fats from the digestive system

Practical investigations

Understanding how the circulatory system works involves hands-on investigations that demonstrate key concepts.