Lecture 12: circulation 3

Where does all exchange occur

In the capillaries

What molecules use passive transport through the thin capillary endothelial wall

Very fast diffusion occurs by:

• Lipid soluble (lipophilic) substances

• Dissolved gases (O2 & CO2)

What is active transport in capillaries

Bulk flow of molecules through intercellular spaces between endothelial cells

What are fenestrated capillaries

Capillaries with small gaps which small lipophobic molecules go through

What are sinusoid or discontinuous capillaries

Capillaries with gaps which red blood cells & large lipophobic molecules go through

What is special about brain capillaries

Have no gaps or fenestration's between cells

Active transport in the brain

Brain capillaries have tight junctions which close intercellular spaces forming the blood-brain barrier

What molecules diffuse slowly through tight junctions

Small lipophobic molecules

How do tight junctions protect the brain

By preventing unwanted molecules from entering

Why do tight junctions require a lot of mitochondria

It takes time and energy for small lipids to diffuse through

What does the exchange of substances occur between

Blood and interstitial fluid

What pushes fluid out of the capillaries

Blood pressure in the capillaries

What pulls fluid back into the capillaries

Blood proteins create osmotic pressure which pulls the fluid back in

Is there an overall net loss or gain of fluid from capillaries

Net loss: more fluid is driven out of the capillary

Does osmotic pressure increase or decrease through exchange

It remains the same

Fluid movement & pressure at the arterial end of the capillary

• Blood pressure higher than osmotic pressure

• Therefore fluid moves out of capillary

Fluid movement and pressure at the venous end of capillary

• Blood pressure lower than osmotic pressure

• Therefore fluid moves in to capillary

Blood pressure levels through capillary

Blood pressure levels drop from arterial end to venous end

Where does the lost fluid from the capillary go

To the lymphatic system to be returned to veins as lymph

Where does the lymphatic system drain into

Veins in the neck

What are present in lymph vessels to prevent backflow

Valves

What is the space between the blood capillary & lymphatic capillary named

The interstitial space

Oedema

Swelling caused by disruptions in the flow of lymph

What are lymph nodes

Organs which filter lymph & play a role in the body's defence

What happens to lymph nodes when the body is fighting an infection

May become swollen & tender

What happens to air when it is inhaled through the nostrils

It is filtered, warmed, humidified & can be sampled for odours

What do cilia and mucus that line the air ducts do

Move dirt particles up toward the pharynx which cleans the respiratory system

Route of air flow

Nasal cavity or mouth → pharynx → larynx (oesophagus) → trachea → right & left bronchus → bronchioles → alveoli

What structure covers alveoli

Dense Capillary bed

SEM

Scanning electron microscopy

What does the lungs fill up most of

The rib cage

What does the pleural membrane of the lungs consist of

Visceral pleural (inner)

Parietal pleural layer (outer)

Pleural cavity (Space between)

What is contained in the pleural cavity /space

Pleural fluid

Role of pleural fluid

• Prevents friction

• Allows lungs to expand & contract

• Causes layers of pleural membrane to adhere to one another (surface tension)

What is the name of the area between the lungs

Mediastinum

Why is the left lung smaller than the right

To make space for the heart

What type of epithelium lines the alveoli

Simple squamous

How many alveoli are in an adult lung

Around 500 million

Structure of alveoli

Form honeycombs and have a large surface area

What does diffusion rate in the alveoli depend on

• Surface area

• Distance

• Concentration difference (high to low difference is the driving force of diffusion)

Diffusion distance in alveoli

Short (0.2 - 0.6 um)

Where does the respiratory gas exchange occur

Across alveolar & capillary walls

What allows alveolar expansion when breathing in

Fluid with surfactant in the outside of the lung and on the alveolus lining reduces surface tension and allows expansion to reduce friction

Direction of oxygen diffusion

Alveolar epithelium → interstitial space → capillary endothelium

What happens during inhalation

• Lungs expand to fill enlarged thoracic cavity and air is drawn into lungs

• The diaphragm contracts (moves down)

• Rib cage expands as rib muscles contract

Pressure during inhalation

Negative pressure so air is drawn in

What happens during exhalation

• The intercostal (rib) muscles relax & rib cage gets smaller

• The diaphragm relaxes & moves up

What drives air out of the lungs when exhaling

The elasticity of the lungs

Normal rest breathing rate

15 breaths per minute

What regulates breathing rate

Sensors which detect oxygen & carbon dioxide blood levels

Normal blood pH

Around 7.4

What does carbon dioxide do to blood pH

Makes it more acidic

Stages of breathing regulation

1. Blood pH levels fall as CO2 levels rise in tissues (such as when exercising)

2. Medulla detects pH decrease in cerebrospinal fluid

3. Major blood vessels (carotid arteries & aorta) have sensors which detect pH drop

4. Medulla receives signals from major blood vessels

5. Medulla sends signals to rib muscles and diaphragm to increase depth of ventilation

6. Blood pH rises and CO2 levels fall

What is haemoglobin

A protein consisting of 4 subunits that each have a polypeptide chain & a harm group. it is found in all vertebrates & contained within red blood cells

What does each haem group contain

An iron atom to which an oxygen molecule can bind

When is haemoglobin 100% saturated

When all sites have been bound with oxygen

What depends haemoglobin's readiness to bind to oxygen

It combines reversible with oxygen & this depends upon surrounding O2 conditions

mmHg

mm of mercury

pO2

Oxygen partial pressure

Oxygen partial pressure in air

160 mmHg

Oxygen partial pressure in lungs

100 mmHg

Oxygen partial pressure in tissues

• 40 mmHg at rest

• <20mmHg during exercise

Barometric pressure (atmospheric pressure)

760 mmHg

Oxygen partial pressure in venous blood

40 mmHg

Oxygen partial pressure in arteries

95 mmHg

02 saturation of haemoglobin in lungs

98%

02 saturation of haemoglobin in tissues at rest

75% as some 02 unloaded to tissues

02 saturation in tissues during exercise

<20% as haemoglobin has given most 02 to tissues

What are the 3 ways in which CO2 is transported

1. 5% is dissolved in the blood plasma

2. 5% enter RBC’s and attach to haemoglobin&other blood proteins

3. 90% enter RBC’s and are transported as bicarbonate ions

What happens to the CO2 which bind to haemoglobin

Forms carbamino compounds by attaching to the terminal NH2 group:

HbNH2 + CO2 = Hb.NH.CO2-+H+

How are carbonic acids formed

Most CO2 binds with water in the RBC's:

CO2+H2O=H2CO3

What happens to the carbonic acid that forms

Catalysed by carbonic anhydrase within RBC's:

H2CO3=H+ + HCO3-(bicarbonate ion which diffuses)