CONCEPT OF THE CIRCULATORY SYSTEM

1.2

BIOLOGY CHAPTER 1 : TRANSPORT

Circulatory system in humans and animals

• Circulatory system (CS) includes:– Medium fluid that flows in CS (eg: animal :

blood; insects: haemolymph)– Vesels arteries, veins and capillaries– Pump muscular heart

Erythrocytes

Erythrocytes• Small, biconcave disc• Have no nucleus• Great quantities of haemoglobin (which contains iron)

• Site of production: bone marrow• Life span: 120 days• Site of destruction: liver and spleen (by phagocytes)• Ratio of erythrocytes to leucocytes 1000 : 1 (in normal person)

(Cells become bright red)

Leucocytes (White blood cells)

Erythrocytes

Leucocytes

Leucocytes (White blood cells)

• Less numerous than eryhtrocytes.• Have nuclei• Do not have haemoglobin• Larger than erythrocytes and do not have

fixed shapes.• Site of production : bone marrow• Site of growth and development: thymus

gland or lymph nodes

Cont.

Granulocytes (have granular cytoplasm and lobed nuclei) • Neutrophils • Eosinophils • Basophils

Agranulocytes (clear cytoplasm and no lobed) • Monocytes (largest)• Lymphocytes (smallest)

Basic types of leucocytes:

Platelets

• Small, irregularly shaped

• F(x) : blood clotting• Life span: 5 -9 days

Plasma

• Pale, yellow liquid• Made up of 90% water & 10% dissolved solutes

(gases, minerals, hormones, plasma proteins and excretory wastes)

Plasma

Function of blood in transport

• Transport oxygen from the lungs to other parts of the body

• Transport absorbed food materials from the digestive tract to body tissues

• Transport waste products – Eg: carbon dioxide from body tissues to the lungs– Urea to the kidneys

• Transport heat, hormones and water

Transport of heat, hormones & water

• Body temperature can be regulated by blood by distributing heat from heat-producing sites (eg:muscles) to the skin.

• Hormones (eg:insulin & glucagon) produced by endocrine glands (pancreas) transported by blood to target organs (liver).

• Water is important to provide medium for biochemical reaction.

Function of haemolymph

• Circulating blood-like fluid found in invertebrates with open-circulatory systems

• Tubular heart pumps the haemolymph into haemocoel (body cavity).

• Haemolymph – bathes the tissues and internal organ directly.

• Nutrients and hormones diffuse from haemolypmh into the cells

• Waste products diffuse out from the cells into haemolymph.

Structure of human blood vessels

Arteries Capillaries Veins Transport blood away from the heart

Connect arterioles to venules

Transport blood to the heart

Transport oxygenated blood (except pulmonary artery)

Act as the sites for exchange of substances with the cells

Transport deoxygenated blood (except pulmonary vein)

Thick muscular wall Thinnest wall, one cell thickness

Thinner wall

No valves except semilunar valves at the base of the aorta and pulmonary artery

No valves Valves present to prevent back flow of blood

Blood flows in pulses under high pressure

No pulses. Pressure lower than arteries but higher than veins

No pulses. Blood flows under lower pressure than arteries.

Blood vessels : tubes that transport blood from one part to another.

Artery, vein and capillary

How blood is propelled through the human circulatory system

• Organ responsible to pump the blood : heart

• The human heart has four chambers :• Atria (atrium) the upper chambers which receive blood

returning to the heart– Ventricle the lower chambers which pump blood out of the heart

• The Septum seperates the right chambers from the left chambers

• The valves in the heart ensure blood flow only in one direction :– Semilunar Valve , at the base of the aorta and pulmonary artery– Bicuspid valve , between left atrium and left ventricle– Tricuspid valve , between right atrium and right ventricle

• The heart is made up of cardiac muscles. It is myogenic (auto rhtyhm) because it is not controlled by the nervous system

Structure of the human heart

Flow of blood

Pulmonary Circulation

Systemic Circulation

Pulmonary Artery

Pulmonary Vein

Lungs

Whole body

Left atrium

Left ventricleRight atrium

Right ventricle

Pulmonary veins

Aorta

Bicuspid valve

Tricuspid valve

Vena Cava

Pulmonary Artery

O2 blooddeO2 blood

• What are SAN & AVN?– The sino - atrium node (SAN) is a specialized bundle of tissue, located

in the right atrium wall, near the entrance of the anterior vena cava• It acts as a pacemaker which generate a wave of electric impulse. The

blood is then spread to the atria, causing them to contract simultaneously. Hence blood is forced into the ventricle

– The impulse also stimulates the second node ; atrio – ventricular node (AVN) lying at the base of the right atrium• Impulses from the AVN are conducted by specialized muscle fibres called

Bundle of His and purkinje fibres and bundle fibres to the ventricular walls. This causes the contraction of both ventricles to pump blood out of the heart.

• The right ventricle pumps the blood into the pulmonary artery, which forces the blood to the lung

• The left ventricle which is thicker and more muscular than the right ventricle, pumps blood into the aorta and then to the whole body

How does the heart pump blood?

The pumping of the heart

Sinoatrial node

Atrio-ventricular node

Bundle of His containing Purkinje tissue

Interventricular septum

How does the blood in the veins flow back to the heart?

• Cardiac cycle is the series of events that occur during one complete heartbeat :– Contraction (systole)– Relaxation (diastole)

• 1 systole & 1 diastole equal to 1 heartbeat (0.8 sec)• The pumping of the heart generates sufficient force to move blood

through the artery, arterioles and capillaries• However, when the blood reaches the vein, the blood pressure

produced by the heart is insufficient to force it back into the heart• When the body moves, the skeletal muscles around the veins contract

and press on the veins. • The veins contract and blood pressure increases, to open the valves

and the push the blood towards the heart.

Contraction of skeletal muscles around veins

Regulatory Mechanism of Blood Pressure

• SAN can initiate the heartbeat on its own. But the heart rate may be modified by certain external factors.

• The sympathetic nerve carrying impulse to the heart can increase the heart rate and the parasympathetic nerve can slow it down

• Heart rate increase when : – an increase in the secretion of hormone (adrenaline) when

someone is excited– Body temperature is elevated– Increase in the partial pressure of carbon dioxide in the

blood

• Blood pressure is the force of the blood exerted on the walls of the arterial blood vessels

• Arterial blood pressure is highest during ventricular systole and lowest during diastole– Normal blood pressure is 120 (systolic) / 80

(diastolic) mm Hg• Stretch – sensitive receptors known as

baroreceptors are located in the walls of the aorta and carotid arteries branch out from the aorta– They monitor the pressure of blood flowing to the

body and to the brain

An increase in blood

pressure stretches the baroreceptors

Impulses are sent to the

cardiovascular control centre in the medulla oblongata of

the brain

Impulses are then sent via

the parasympathetic nerve to

the heart

This slows down the heartbeat,

resulting in a decrease of

blood pressure

Thus, regular

heartbeat

Decrease in blood pressure increases stimulation of the

SAN by the sypathetic nerve

This increases the contraction of the cardiac muscles of

the heart and smooth muscles of the arteries

Blood pressure returns to the Normal level

Circulatory system in insects

1. When the heart relax, haemolymph re-enters ostia

2. Valves ensure the haemolymph flow in one direction

Material exchange occurs here. Haemolymph in haemocoel carry nutrients and waste products

Circulatory system in fish

• Single circulatory system.• Deoxygenated blood

leaves the heart at high pressure and passes through the gills where the gaseous exchange occurs.

• Oxygenated blood flows through the organs and blood pressure drops.

Sinuses

Circulatory system in amphibians (eg:frogs)

• Double circulatory system:– Pulmonary circulation– Systemic circulation

• Have three-chambered heart (2 atria & 1 ventricle)

• Mixing of oxygenated and deoxygenated blood in ventricle. The mixed blood enters the systemic circulation.

Circulatory system in humans

Circulatory system in humans

• Double circulatory system (blood passes through the heart twice for each circuit)– Pulmonary circulation– Systemic circulation

• Two divisions of heart: – Right side pump deoxygenated blood to the lungs– Left side pump oxygenated blood to the body (except lungs)

• Advantage: blood returns to the heart to be pumped again will increase the blood pressure and flow rate, thereby speeding up delivery O2 to the tissues and organs.