Circulatory System & Body Fluids

• The circulatory system is the body's transport network for trillions of cells.
• It has three parts: the heart (pump), blood vessels (pipes) and blood (fluid).
• It delivers oxygen, nutrients, water and hormones to every cell.
• It picks up carbon dioxide (carried to lungs) and wastes (carried to kidneys).
• It defends the body using disease-fighting white blood cells.
• Deep-lying cells cannot exchange directly with surroundings, so a delivery system is essential.

The circulatory system is the body's transport network for trillions of cells.

It has three parts: the heart (pump), blood vessels (pipes) and blood (fluid).

It picks up carbon dioxide (carried to lungs) and wastes (carried to kidneys).

It defends the body using disease-fighting white blood cells.

• In an open system, blood leaves vessels and bathes organs directly in spaces called haemocoel.
• The fluid in an open system is haemolymph; flow is slow and pressure is low.
• Open systems are seen in cockroaches and most insects; the cockroach heart has side-holes called ostia.
• In a closed system, blood always stays inside vessels, so flow is fast and pressure stays high.
• All vertebrates and a few invertebrates (like the earthworm) have closed circulation.
• In the earthworm, haemoglobin is dissolved in plasma and its blood cells are phagocytic.
• Blood stays in vessels — No — Yes
• Fluid name — Haemolymph — Blood
• Pressure / speed — Low / slow — High / fast
• Example — Cockroach — Earthworm, vertebrates

In an open system, blood leaves vessels and bathes organs directly in spaces called haemocoel.

The fluid in an open system is haemolymph; flow is slow and pressure is low.

Open systems are seen in cockroaches and most insects; the cockroach heart has side-holes called ostia.

In a closed system, blood always stays inside vessels, so flow is fast and pressure stays high.

All vertebrates and a few invertebrates (like the earthworm) have closed circulation.

In the earthworm, haemoglobin is dissolved in plasma and its blood cells are phagocytic.

• An atrium receives blood returning to the heart; a ventricle pumps blood out.
• Fishes have a 2-chambered heart with single circulation (blood passes the heart once).
• Amphibians and most reptiles have a 3-chambered heart, causing partial mixing of blood.
• The frog heart is wrapped in a pericardium and frogs have a well-developed lymphatic system.
• Crocodiles, birds and mammals have a 4-chambered heart, so blood never mixes.
• The crocodile is the famous reptile exception with 4 chambers.
• Fish — 2 — Single
• Amphibian / most reptiles — 3 — Incomplete double
• Crocodile — 4 — Double
• Bird / mammal — 4 — Double

An atrium receives blood returning to the heart; a ventricle pumps blood out.

Fishes have a 2-chambered heart with single circulation (blood passes the heart once).

Amphibians and most reptiles have a 3-chambered heart, causing partial mixing of blood.

The frog heart is wrapped in a pericardium and frogs have a well-developed lymphatic system.

Crocodiles, birds and mammals have a 4-chambered heart, so blood never mixes.

The crocodile is the famous reptile exception with 4 chambers.

• The human heart is a fist-sized muscular pump lying in the chest, tilted to the left.
• It has four chambers: right atrium, right ventricle, left atrium, left ventricle.
• A muscular wall called the septum keeps oxygen-rich and oxygen-poor blood from mixing.
• The right side handles deoxygenated blood; the left side handles oxygenated blood.
• The left ventricle has the thickest wall to pump blood around the whole body.
• The heart muscle is tireless, contracting and relaxing for life without true rest.

The human heart is a fist-sized muscular pump lying in the chest, tilted to the left.

It has four chambers: right atrium, right ventricle, left atrium, left ventricle.

A muscular wall called the septum keeps oxygen-rich and oxygen-poor blood from mixing.

The right side handles deoxygenated blood; the left side handles oxygenated blood.

The heart muscle is tireless, contracting and relaxing for life without true rest.

• Valves are one-way doors that snap shut to stop blood flowing backward.
• The tricuspid valve lies between the right atrium and right ventricle.
• The bicuspid (mitral) valve lies between the left atrium and left ventricle.
• Semilunar valves guard the exits into the pulmonary artery and the aorta.
• The familiar 'lub-dub' heart sounds are the valves snapping shut.
• Tricuspid — Right atrium → right ventricle
• Bicuspid (mitral) — Left atrium → left ventricle
• Semilunar — Exits to pulmonary artery & aorta

Valves are one-way doors that snap shut to stop blood flowing backward.

The tricuspid valve lies between the right atrium and right ventricle.

The bicuspid (mitral) valve lies between the left atrium and left ventricle.

Semilunar valves guard the exits into the pulmonary artery and the aorta.

The familiar 'lub-dub' heart sounds are the valves snapping shut.

• Heart muscle is auto-excitable – it generates its own electrical signals.
• The sino-atrial node (SAN) in the right atrium is the pacemaker, firing 70–75 times/min.
• Contraction is called systole and relaxation is called diastole.
• One complete heartbeat is a cardiac cycle, lasting about 0.8 seconds.
• An ECG records the heart's electrical activity as P, Q, R, S, T waves.
• A doctor listens to heart sounds with a stethoscope.

Heart muscle is auto-excitable – it generates its own electrical signals.

The sino-atrial node (SAN) in the right atrium is the pacemaker, firing 70–75 times/min.

Contraction is called systole and relaxation is called diastole.

One complete heartbeat is a cardiac cycle, lasting about 0.8 seconds.

An ECG records the heart's electrical activity as P, Q, R, S, T waves.

A doctor listens to heart sounds with a stethoscope.

• Stroke volume is the blood pumped per beat by a ventricle, about 70 mL.
• A healthy heart performs about 72 cardiac cycles per minute.
• Cardiac output = stroke volume × heart rate, roughly 5 litres per minute.
• An athlete's cardiac output is higher because stroke volume and heart rate can rise.
• The body increases output during exercise to meet greater oxygen demand.
• Heart rate — ~72 beats/min
• One cardiac cycle — ~0.8 sec
• Stroke volume — ~70 mL
• Cardiac output — ~5 L/min

Stroke volume is the blood pumped per beat by a ventricle, about 70 mL.

Cardiac output = stroke volume × heart rate, roughly 5 litres per minute.

An athlete's cardiac output is higher because stroke volume and heart rate can rise.

• Double circulation means blood passes through the heart twice per full body trip.
• The pulmonary circuit carries blood from the heart to the lungs and back.
• The systemic circuit carries oxygen-rich blood from the heart to the body and back.
• Mammals and birds have complete double circulation due to a 4-chambered heart.
• This keeps oxygenated and deoxygenated blood fully separated for efficient delivery.

Double circulation means blood passes through the heart twice per full body trip.

The pulmonary circuit carries blood from the heart to the lungs and back.

The systemic circuit carries oxygen-rich blood from the heart to the body and back.

• Arteries carry blood away from the heart and have thick, elastic walls.
• Veins carry blood back to the heart and have valves to prevent backflow.
• Capillaries are the thinnest vessels where exchange of gases and nutrients occurs.
• Most arteries carry oxygenated blood; the pulmonary artery is the exception (carries deoxygenated).
• Most veins carry deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood.
• In bypass surgery a vein graft (often the saphenous vein) reroutes blood past blocked arteries.
• Artery — Away from heart — Pulmonary artery (deoxy.)
• Vein — Toward heart — Pulmonary vein (oxy.)
• Capillary — Exchange site — –

Arteries carry blood away from the heart and have thick, elastic walls.

Veins carry blood back to the heart and have valves to prevent backflow.

Capillaries are the thinnest vessels where exchange of gases and nutrients occurs.

Most arteries carry oxygenated blood; the pulmonary artery is the exception (carries deoxygenated).

Most veins carry deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood.

• Blood is a fluid connective tissue made of plasma plus different blood cells.
• Plasma is the liquid part (~55%) carrying water, proteins, salts, nutrients and wastes.
• Red blood cells (RBCs) carry oxygen using the pigment haemoglobin.
• White blood cells (WBCs) fight infection and provide immunity.
• Platelets help blood clot to stop bleeding.
• Total blood volume in a normal adult is about 5 litres.
• Plasma — Liquid transport medium
• RBC — Carry oxygen (haemoglobin)
• WBC — Immunity / fight germs
• Platelet — Blood clotting

Blood is a fluid connective tissue made of plasma plus different blood cells.

Plasma is the liquid part (~55%) carrying water, proteins, salts, nutrients and wastes.

Red blood cells (RBCs) carry oxygen using the pigment haemoglobin.

White blood cells (WBCs) fight infection and provide immunity.

Platelets help blood clot to stop bleeding.

Total blood volume in a normal adult is about 5 litres.

• Haemoglobin is an iron-containing red pigment that carries oxygen in blood.
• It gives blood its red colour and does not provide immunity (that is the WBCs' role).
• Some animals (e.g. cockroach, many insects) lack red blood and have colourless 'white' blood.
• Animals like the cockroach have no red blood cells; their haemolymph carries no haemoglobin.
• In the earthworm, haemoglobin is dissolved in plasma rather than packed in cells.
• Low haemoglobin causes anaemia, reducing oxygen delivery.

Haemoglobin is an iron-containing red pigment that carries oxygen in blood.

It gives blood its red colour and does not provide immunity (that is the WBCs' role).

Some animals (e.g. cockroach, many insects) lack red blood and have colourless 'white' blood.

Animals like the cockroach have no red blood cells; their haemolymph carries no haemoglobin.

In the earthworm, haemoglobin is dissolved in plasma rather than packed in cells.

Low haemoglobin causes anaemia, reducing oxygen delivery.

• The ABO system gives four blood groups: A, B, AB and O.
• Group O is the universal donor; group AB is the universal acceptor.
• Blood must match in both ABO type and Rh factor for safe transfusion.
• Rh-positive blood has the Rh antigen; Rh-negative blood lacks it.
• Mismatched transfusion causes clumping (agglutination) of red cells, which can be fatal.
• The Cath lab (catheterization lab) in a hospital is part of the cardiology department.
• O — All groups (universal donor) — O
• A — A, AB — A, O
• B — B, AB — B, O
• AB — AB — All (universal acceptor)

The ABO system gives four blood groups: A, B, AB and O.

Blood must match in both ABO type and Rh factor for safe transfusion.

Rh-positive blood has the Rh antigen; Rh-negative blood lacks it.

The Cath lab (catheterization lab) in a hospital is part of the cardiology department.