Transport in Mammals

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Cambridge International AS & A Level Biology (9700)

Transport in Mammals

The big idea: A closed double circulation, driven by a four-chambered heart, carries blood through vessels whose structure matches their pressure. Haemoglobin’s S-shaped dissociation curve and the Bohr shift let red blood cells load oxygen in the lungs and release it where respiration is fastest.

Summary — What This Chapter Covers

Closed double circulation; pulmonary artery, pulmonary vein, aorta, vena cava.
Structure and function of elastic and muscular arteries, veins and capillaries; recognising them.
Red blood cells, monocytes, neutrophils and lymphocytes; water in blood.
Formation and functions of tissue fluid.
Transport of O₂ and CO₂: carbonic anhydrase, haemoglobinic acid, carbaminohaemoglobin, chloride shift, plasma.
The oxygen dissociation curve and the Bohr shift.
Heart structure, wall thickness, the cardiac cycle and its control by the SAN, AVN and Purkyne tissue.

1. The circulatory system

Overview

A closed double circulation

The mammalian circulatory system is a closed double circulation made of a heart, blood and blood vessels: arteries, arterioles, capillaries, venules and veins.

Closed: blood stays inside vessels.
Double: blood passes through the heart twice for each complete circuit of the body: the pulmonary circulation (heart → lungs → heart) and the systemic circulation (heart → body → heart). Blood returning from the lungs is re-pressurised before going to the body.

Vessel	Carries	From → to
Pulmonary artery	Deoxygenated blood	Right ventricle → lungs
Pulmonary vein	Oxygenated blood	Lungs → left atrium
Aorta	Oxygenated blood	Left ventricle → body (systemic circulation)
Vena cava	Deoxygenated blood	Body → right atrium

Blood vessels

Structure related to function

Walls have up to three layers: the tunica intima (endothelium, a single layer of flat cells), the tunica media (smooth muscle and elastic fibres) and the tunica externa (mainly collagen fibres).

Vessel	Structure	Related to function
Elastic artery (e.g. aorta)	Thick wall; tunica media with many elastic fibres; small lumen relative to wall	Elastic fibres stretch as blood surges out in systole and recoil in diastole, smoothing the pressure and keeping blood flowing; thick wall withstands high pressure
Muscular artery (distributing artery)	Thick tunica media with much smooth muscle, fewer elastic fibres	Smooth muscle contracts or relaxes to narrow or widen the lumen, controlling how much blood reaches each organ
Capillary	Wall is endothelium only, one cell thick; lumen ≈7 µm; gaps between cells; very many, forming networks	Short diffusion distance; red cells squeezed against the wall and flow slowed for exchange; gaps let fluid leak out to form tissue fluid; huge surface area
Vein	Thin wall with little muscle or elastic tissue; wide lumen; valves	Blood is at low pressure, so thick walls are unnecessary; the wide lumen reduces resistance; valves prevent backflow. Contraction of surrounding skeletal muscles squeezes blood towards the heart

Recognising vessels in sections

Artery Vein Capillary

Blood

Cells and the role of water

Cell	Appearance	Function
Red blood cell	Biconcave disc, about 7 µm, no nucleus	Transport of O₂ (and some CO₂)
Neutrophil	Lobed nucleus (2–5 lobes), granular cytoplasm, about 12 µm	Phagocyte
Monocyte	Largest
Lymphocyte

Water

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Also in the full note

How it forms and what it does
Carrying O₂ and CO₂
S-shaped loading and unloading
External and internal structure
Pressure changes and valves

What's inside

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Transport in Mammals