Chapter 3: Apply transport in plants and animals

The transport systems in plants and animals are fundamental to sustaining life by enabling the movement of essential substances such as nutrients, gases, and waste products. For applied biology professionals in Kenya, understanding these systems is crucial for fields such as agriculture, healthcare, and environmental management, where optimizing physiological functions can improve crop yields, animal health, and ecosystem sustainability. This chapter explores the mammalian circulatory system, highlighting its types, the cardiovascular system, and the lymphatic system, providing a comprehensive view of how transport mechanisms operate in complex organisms.

3.1 Mammalian Circulatory Systems

Cells need a constant supply of oxygen and nutrients, and they produce wastes that must be taken away. In large animals, diffusion alone is far too slow to cover the distances involved, so a transport system carries substances around the body. In mammals this is the circulatory system, which consists of a fluid (blood), vessels, a pump (the heart) and a companion network, the lymphatic system. Besides the transport of respiratory gases, nutrients, wastes and hormones, it helps to defend the body against disease, to regulate body temperature and to clot blood after injury. A technician who takes blood samples from laboratory animals, or examines the circulatory organs of a specimen, needs a clear picture of how the parts fit together. This topic starts with the main types of circulatory system, then describes the mammalian cardiovascular system, and ends with an overview of the lymphatic system.

3.1.1 Types of Circulatory Systems

Animals have two basic types of circulatory system, open and closed, and these also differ in the number of times the blood passes through the heart in one complete circuit.

3.1.1.1 Open Circulatory System

In an open circulatory system the transport fluid, called haemolymph, is pumped by a heart into open spaces (the haemocoel) and bathes the organs directly. It then returns to the heart through small openings.

• Where it is found: arthropods such as insects, spiders and crustaceans, and most molluscs such as snails.
• Advantages: it needs little energy to run and is enough for small animals with modest demands.
• Limitations: pressure is low, and the flow cannot easily be directed to organs that need more.
• Role in gas transport: in insects the haemolymph carries nutrients, wastes and immune cells but carries little oxygen, because air reaches the tissues through a network of tubes, the tracheae.

Example: at a field station apiary, a worker bee has a tubular heart along the back, which pumps haemolymph forward into the head region. The fluid then flows back through the body cavity around the organs.

3.1.1.2 Closed Circulatory System

In a closed circulatory system the blood stays within vessels and the heart pumps it through arteries, capillaries and veins. Materials are exchanged with the tissues across the thin walls of the capillaries.

• Where it is found: vertebrates, earthworms and other annelids, and cephalopod molluscs such as squid.
• Advantages: higher pressure and faster flow, and the flow to organs can be adjusted by narrowing or widening vessels.
• Components: blood, a heart, and arteries, veins and capillaries.
• Suitability: the speed and control suit large and active animals, which have high demands for oxygen.

Example: an earthworm used in a teaching practical shows a closed system in which blood moves through a dorsal vessel and a ventral vessel, with several pairs of muscular vessels in the front of the body that act as hearts.

Single and double circulation

Closed systems differ in the route of the blood.

• Single circulation: the blood passes through the heart once in each circuit. In a fish, the heart pumps blood to the gills and the blood then goes on to the body, so the pressure is low after the gills.
• Double circulation: in mammals the blood passes through the heart twice. The right side pumps deoxygenated blood to the lungs (pulmonary circuit) and the left side pumps oxygenated blood to the body (systemic circuit).
• Advantage of double circulation: the blood is pumped again after the lungs, so it reaches the tissues at high pressure, and oxygenated and deoxygenated blood do not mix in a four-chambered heart, which supports the high metabolic rate of warm-blooded animals.

3.1.2 The Cardiovascular System

The cardiovascular system is made up of the blood, the blood vessels and the heart. It moves blood around the body so that oxygen, nutrients and hormones reach every cell and carbon dioxide and wastes are removed.

3.1.2.1 Blood and Its Components

Blood is a fluid connective tissue made of plasma and cells. In an adult human, plasma makes up a little over half of the volume of blood and the cells slightly less than half, and the proportions differ between species.

• Plasma: a straw-coloured fluid that is mainly water. It carries dissolved nutrients such as glucose and amino acids, hormones, carbon dioxide (mostly as hydrogencarbonate ions), urea, plasma proteins and heat.
• Red blood cells (erythrocytes): biconcave discs without a nucleus in mammals, packed with the pigment haemoglobin, which combines with oxygen to form oxyhaemoglobin and carries it to the tissues. The shape gives a large surface for gas exchange. They are made in the bone marrow and live for about 120 days in humans.
• White blood cells (leucocytes): defend the body. Neutrophils and monocytes engulf microorganisms by phagocytosis, and lymphocytes make antibodies or attack infected cells. Eosinophils and basophils take part in responses to parasites and allergy.
• Platelets: small cell fragments that help to clot blood. At a wound they stick together and release substances that set off a chain of reactions in the plasma, which converts the soluble protein fibrinogen into insoluble fibrin threads that trap cells and form a clot.

Example: in a teaching laboratory, a technician spreads a drop of rabbit blood on a slide, stains it and counts the different white blood cells under the microscope, which is called a differential count. A change in the proportions may indicate infection or parasite burden.

3.1.2.2 Blood Vessels

Blood is carried in three main types of vessel, with smaller forms between them.

• Arteries: carry blood away from the heart, usually oxygenated, apart from the pulmonary artery. Their walls are thick, muscular and elastic, so they withstand the high pressure and the elastic recoil smooths the pulses. They branch into smaller arterioles, whose muscle controls how much blood reaches each organ.
• Capillaries: microscopic vessels with walls one cell thick, where substances are exchanged with the tissues. Fluid leaks out at the arterial end to form tissue fluid, bathing cells with oxygen and nutrients, and much of it is reabsorbed at the venous end.
• Veins: carry blood back to the heart, usually deoxygenated, apart from the pulmonary veins. Venules join to form veins. The walls are thinner and the lumen is wider, with low pressure. Valves stop backflow, and the squeezing of veins by nearby skeletal muscles helps to push blood towards the heart.

A portal vein is a vein that connects two capillary beds. The hepatic portal vein carries blood rich in absorbed nutrients from the intestine to the liver before the blood returns to the heart.

3.1.2.3 The Heart

The heart is a muscular pump made mostly of cardiac muscle, which contracts rhythmically without tiring. It lies in the chest and is enclosed in a protective membrane, the pericardium.

• Chambers: two thin-walled atria receive blood, and two thick-walled ventricles pump it out. The wall of the left ventricle is thicker than that of the right, as it must pump blood to the whole body, while the right ventricle sends blood only to the lungs.
• Valves: the atrioventricular valves (tricuspid on the right and bicuspid on the left) between atria and ventricles and the semilunar valves at the exits to the aorta and pulmonary artery stop blood flowing backwards. Their closing makes the heart sounds.
• Blood supply: the heart muscle is supplied with blood by the coronary arteries.
• Route of the blood: deoxygenated blood enters the right atrium through the vena cava, passes to the right ventricle and goes to the lungs through the pulmonary artery. Oxygenated blood returns by the pulmonary veins to the left atrium, passes to the left ventricle and leaves through the aorta.

The cardiac cycle has three phases. In atrial systole the atria contract and fill the ventricles. In ventricular systole the ventricles contract, the atrioventricular valves close and the semilunar valves open to eject blood. In diastole the whole heart relaxes and blood flows in again. The beat starts in the sinoatrial node, the pacemaker in the right atrium, spreads over the atria, reaches the atrioventricular node and then passes through special fibres to the ventricles. The nervous system and hormones such as adrenaline change the heart rate. In general, small mammals have much faster heart rates than large ones, so a mouse's heart beats many times faster than that of a cow.

3.1.3 Overview of the Lymphatic System

The lymphatic system is a network of vessels, nodes and organs that returns tissue fluid to the blood and takes part in defence against infection. It supports the cardiovascular system but, unlike it, has no pump.

Structure and components

• Lymph: the fluid in the lymphatic vessels. It is like plasma but with less protein, and it contains lymphocytes.
• Lymph capillaries and vessels: blind-ended capillaries collect excess tissue fluid and join to form larger vessels with valves. They drain into large veins near the heart, so that lymph rejoins the blood. Lymph moves by the squeezing of nearby skeletal muscles, the movement of breathing and the contraction of vessel walls, and the valves keep it flowing one way.
• Lymph nodes: small, bean-shaped structures along the vessels that filter lymph and hold lymphocytes and macrophages, which destroy microorganisms.
• Lymphoid organs: the bone marrow and thymus are the primary organs, in which lymphocytes form and mature. The spleen, tonsils and the patches of lymphoid tissue in the wall of the intestine are secondary organs, in which immune responses start. The spleen also filters the blood.
• Lacteals: lymph capillaries in the villi of the small intestine, which take up absorbed fats.

Functions of the lymphatic system

• Return of fluid: the system returns to the blood the tissue fluid that is not reabsorbed at the capillaries. In an adult human this amounts to a few litres a day.
• Defence: lymph nodes filter out microorganisms and foreign particles, and lymphocytes there respond to them.
• Fat absorption: lacteals carry fats and fat-soluble vitamins from the intestine to the blood.
• Maintaining blood volume: returning fluid keeps blood volume and pressure steady.

Relationship with the circulatory system

The lymphatic system is closely linked to the blood circulation. Tissue fluid is formed from plasma at the capillaries, and lymph is formed from tissue fluid, so the three are continuous. If lymph vessels are blocked or damaged, fluid builds up in the tissues and causes swelling called oedema. A technician examining livestock at a field station may feel for enlarged lymph nodes in front of the shoulder, since in cattle these swell in certain infections such as East Coast fever, and a swollen node shows that the lymphatic system is responding to an infection.

Chapter Summary

This chapter explored the mammalian circulatory systems, distinguishing between the open circulatory system where blood flows freely through body cavities and the closed circulatory system where blood circulates within vessels. The cardiovascular system was examined in detail, highlighting the composition of blood and its components, the structure and function of various blood vessels, and the anatomy and operation of the heart. An overview of the lymphatic system was provided, explaining its role in fluid balance and immune defense. The focus then shifted to plant transport systems, starting with the internal structures of roots and shoots that facilitate transport. The mechanisms of water and mineral uptake in plants were described, emphasizing the forces that drive these processes. Finally, the chapter covered transpiration as a key process for water movement and translocation for the distribution of nutrients within plants. Together, these topics provide a comprehensive understanding of how transport functions sustain life in both animals and plants.