Transport in mammals

Different types of circulatory system Open circulatory system The open circulatory system is common to molluscs and arthropods. Open circulatory systems (evolved in crustaceans, insects, molluscs and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. Closed circulatory system Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities.

The Structure of the Heart

The Cardiac Cycle Your heart beats around 70 times a minute. The cardiac cycle is the sequence of events which makes up one heart beat. The cycle is made up of 3 main stages: Heart relaxes Chambers fill with blood Atria contract Blood forced into ventricles Ventricles contract Blood forced into arteries Atria start to re-fill

Pressure Changes during the Cardiac Cycle Questions 1.What is the longest stage in the cardiac cycle? 2.Why do you think this is? 3.How long does it last? 4.Why do you think Ventricular systole lasts longer than Atrial systole? 5.When is the pressure in the Aorta at its highest?

Blood Vessels Blood is pumped by the heart into thick-walled vessels called ARTERIES, these split up into smaller vessels called ARTERIOLES, from which the blood passes into the CAPILLARIES. The capillaries form a networks which penetrate all the tissue and organs of the body. Blood from the capillaries collects into VENULES, which in turn empty blood into VEINS, which return it to the heart. Arteries and Veins have the same basic three layers but their proportions vary: The innermost layer (ENDOTHELIUM or TUNICA INTIMA) is one cell thick and provides a smooth lining with minimum resistance to blood flow The middle layer (TUNICA MEDIA) is made of elastic fibres and smooth muscle. It is thicker in arteries than in veins. The outer layer (TUNICA EXTERNA) is made of collagen fibres and is resistant to over stretching.

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The Blood Blood is a tissue made up of cells in a fluid plasma. It is composed of the following: 55% plasma: mostly water, carries CO 2, glucose, urea, amino-acids, hormones, heat and cells! 45% cells: all made in bone marrow; short life; destroyed in liver; only WBC’s have nucleus, reproduce RBCs (Erythrocytes) carry O 2 ; no nucleus; biconcave (↑ SA); contain haemoglobin; small (8μm so ↑mobility) WBCs (Luecocytes): fight disease; have nucleus, larger than RBCs; 2 main groups: Lymphocytes (secrete antibodies); Phagocytes (‘eat’ bacteria); Platelets (Thrombocytes): fragments of cells vital for blood clotting (and tissue repair)

Photomicrographs of blood cells:

The Lymphatic System When blood reaches the arteriole end of a capillary it is under pressure due to the pumping action of the heart. The hydrostatic pressure forces the fluid part of the blood through the capillary walls into the spaces between the cells. This tissue fluid at the venous end picks up CO 2 and other excretory products. Blood pressure, diffusion and osmosis are the forces involved in the movement of water and solutes IN and OUT of capillaries. Most fluid passes back into the capillaries but some drains into the lymphatic system and is returned to the blood later.

Lymph Lymph is the tissue fluid that drains into blind-ending lymphatic capillaries among the tissues that join up to form larger vessels. It is moved by contractions of muscles through which the vessels pass. Tissue fluid flows into them through tiny valves (which allow it to flow IN but not out). There are lymph glands and nodes associated with lymph vessels which play an important role in the making of lymphocytes. Lymph vessels transport the lymph back to the large veins which run just beneath the collarbone (subclavian veins).

The Role of Haemoglobin Oxygen Carriage Respiratory pigment such as haemoglobin have a high affinity for O 2 when the concentration is high, but a low affinity when the concentration is low. When exposed to a gradual increase in oxygen partial pressure (oxygen tension) haemoglobin absorbs O 2 rapidly at first but more slowly as the partial pressure increases further, [see Fig. 3, p.129 in text & graph on handout]. The release (offloading) of O 2 is facilitated by the presence of CO 2 – this called the Bohr Effect. When the partial pressure of O 2 is high (i.e. in lungs), it joins with haemoglobin to form oxyhaemoglobin When the partial pressure of oxygen is low (i.e. in muscles), the O 2 dissociates from the haemoglobin When the partial pressure of CO 2 is high, haemoglobin is less efficient at taking up (uploading) O 2 and more efficient at releasing (offloading) it.

Partial Pressure (Oxygen Tension) Partial Pressure is the pressure exerted by one gas in a mixture of gases. Atmospheric pressure (at sea level) is about 100 kPa. One fifth of the air is oxygen. So one fifth of atmospheric pressure is due to oxygen. Therefore the PARTIAL PRESSURE OF OXYGEN is about 20 kPa. (Partial pressure of oxygen can be written as ppO 2 ) Some key values for ppO 2 are: Atmospheric20 kPa Alveolar13 kPa And a typical value for blood flowing through tissues of the body: 5kPa

Review of Haemoglobin Structure So to function efficiently, haemoglobin must attract oxygen under certain conditions, but lose it under other conditions. Uploading/offloading is a reversible reaction, in which one haemoglobin (Hb) can attract up to 4 oxygen molecules: Hb + 4O 2  HbO 8 Haemoglobin is a conjugated quaternary protein comprising: two alpha-globulins two beta-globulins four haem groups, each with an iron atom at its core It is the iron atoms which bind oxygen.

Oxygen dissociation curve What happens in the lungs and in the tissues? partial pressure of oxygen (kPa) Saturation of blood with O2 (%) partial pressure of oxygen (kPa) In the upper part of the graph Hb is loaded with oxygen. In the lower part Hb is unloading its oxygen. lungs tissues Saturation of blood with O2 (%) OXYGEN DISSOCIATION CURVES are graphs which show the relationship between the ppO 2 and the degree of haemoglobin saturation.

Effects of changes in ppO partial pressure of oxygen (kPa) What happens to %saturation when there is a small change in ppO 2 a) in the lungs? b) in the tissues? See how actively respiring tissues promote more O 2 unloading. lungs tissues Saturation of blood with O2 (%)

Carbon Dioxide Carriage The amount of oxygen haemoglobin carries is affected not only by the partial pressure of oxygen (ppO 2 ) but also by the partial pressure of carbon dioxide (ppCO 2 ). CO 2 is transported in blood cells and plasma in 3 ways: 85% as hydrogencarbonate, 5% in solution in plasma and 10% in RBCs combined to haemoglobin as carbaminohaemoglobin. CO 2 diffuses into red cells where it becomes carbonic acid (H 2 CO 3 ) – catalysed by carbonic anhydrase This acid dissociates into protons (H + ) and hydrogencarbonate ions (HCO 3 - ) HCO 3 - ions diffuse out of the cell & are transported in the plasma Chloride ions (Cl - ) diffuse inwards from plasma to maintain pH – the chloride shift Protons left inside cell are mopped up by haemoglobin forming haemoglobinic acid (HHb). This forces the haemoglobin to release its O 2 – hence the Bohr Shift. See Fig. 5, p.129 in text

CO 2 reacting with water inside cells: CO 2 CO 2 from respiring tissues enters the red blood cell and combines with water, forming carbonic acid. The reaction is accelerated by CARBONIC ANHYDRASE. carbonic anhydrase H 2 CO 3 + H 2 O

Dissociation of Carbonic acid: CO 2 CO 2 + H 2 O H 2 CO 3 Carbonic acid dissociates and HCO 3 - is transported out of the RBC, in exchange for Cl -. (This is the CHLORIDE SHIFT) carbonic anhydrase HCO 3 - H+H+ Cl -

Offloading of O 2 CO 2 CO 2 + H 2 O H 2 CO 3 H + displaces O 2 from haemoglobin, forming HHb - reduced haemoglobin. The O 2 is liberated to the tissues. carbonic anhydrase HCO 3 - H+H+ Cl - HbO 2 HHb O2O2

Explanation of Bohr Shift: CO 2 CO 2 + H 2 O H 2 CO 3 The more CO 2, the greater the displacement of O 2 from Hb. CO 2 reduces the affinity of Hb for O 2. This explains the Bohr effect. carbonic anhydrase HCO 3 - H+H+ Cl - HbO 2 HHb O2O2

The release of CO 2 : it’s reversible! CO 2 CO 2 + H 2 O carbonic anhydrase HCO 3 - H+H+ Cl - HbO 2 HHb O2O2 H 2 CO 3 In the lung capillaries ppO 2 is higher and ppCO 2 is lower, so now O 2 binds to Hb and this results in the release of CO 2 Don’t worry - you don’t have to learn these reactions!

Questions partial pressure of oxygen (kPa) If the black curve is ‘normal’, which curve represents the effect of elevated levels of carbon dioxide? Saturation of blood with O2 (%) If the black curve is human maternal haemoglobin, which curve represents foetal Hb?

Shift to the right - the Bohr effect of higher ppCO partial pressure of oxygen (kPa) What happens to %saturation when there is a small change in ppO 2 a) in the lungs? b) in the tissues? See how actively respiring tissues promote more O 2 unloading. lungs tissues Saturation of blood with O2 (%)

Summary Most oxygen is transported bound to haemoglobin, as oxyhaemoglobin The oxygen saturation of haemoglobin is affected by: ppO 2 ppCO 2 pH temperature CO 2 is transported in three ways: dissolved in plasma bound to haemoglobin as carbamino-haemoglobin converted to hydrogencarbonate ions in the red cells High levels of CO 2 facilitate O 2 offloading from haemoglobin through the formation of hydrogen ions. The effect of increased CO 2 / decreased pH on O 2 offloading is called the Bohr effect. High levels of O 2 facilitate CO 2 offloading from the blood.

Problems with Oxygen Transport The efficient transport of oxygen around the body can be impaired by many different factors. We will focus on one particular problem, this is surviving at high altitude. At sea level, the partial pressure of oxygen in the atmosphere is just over 20kPa, and the partial pressure of oxygen in the alveoli is around 13kPa. At altitudes of 6500 metres or above, the air pressure is much less, and the ppO2 is about 10kPa, dropping to 5.3kPa in the lungs. This results in haemoglobin becoming only about 75% saturated in the lungs – resulting in less oxygen being carried around the body. This can cause altitude sickness – where the sufferer begins to feel ill, dizzy, weak and short of breath. It can even be fatal! Luckily the body can adapt to gradual changes in altitude by producing more red blood cells and haemoglobin, (increasing from 40% or 50% up to 70%). However, this can take several weeks.

Questions: 1.Explain how an increase in the number of red blood cells can help to compensate for the lack of oxygen in the air at high altitudes 2.Athletes often prepare themselves for important competitions by spending several months training at high altitude. Explain how this could improve their performance Answers: 1.More RBCs = more haemoglobin  greater oxygen carriage. Compensates for the lower saturation on haemoglobin in each cell. 2.Over the months their blood will produce more RBCs and haemoglobin, which will increase the amount of oxygen they can carry in their blood to their respiring muscle cells whilst exercising