Introduction to the Cardiovascular System

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<h1>Introduction to the Cardiovascular System</h1>
<h2>Lecture 1: Fluids & Vessels | Dr. Robert Hunter</h2>

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???

**Timing overview:**
- Introduction: why does the body need a CVS? (~5 min)
- Section 1: Body Fluids (~12 min)
- Section 2: Capillary Exchange (~12 min)
- Section 3: Blood Vessels (~11 min)

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![:scale_l 70%](images/causes_of_death.png)

???

But we are not actually going to talk about CVS disease today.  We are talking about the CVS because it is so important for pretty much all of medicine!

---

.left-panel.left-panel.amber[

#### Today:

1) body fluids  
2) capillary exchange  
3) blood vessels

#### Tomorrow:

4) the heart  
5) cardiac output  
6) integrated control   
]

# Learning outcomes

1. **Explain** why multicellular organisms require a circulatory system, and **list** the major functions of the cardiovascular system
2. **Identify** the main body fluid compartments, their approximate volumes, and key differences in ionic composition
3. **Explain** the forces governing capillary fluid exchange (Starling's forces) and **describe** what happens when these forces are disrupted
4. **Describe** the structural and functional properties of each vessel type and **relate** structure to function

<br>

.callout[
**Context:** these lectures form a skeleton of knowledge you will build in in semester 2 ("Sustaining Life") and year 2 ("Renal & Urology")
]

---

# Why do you have a cardiovascular system?

![:scale_c 40%](images/amoeba.png)

<br>

Diffusion is extraordinarily fast over micrometres... ...but hopelessly slow over
millimetres:

- diffusion time increases with the **square** of distance
- effective only over distances of **less than ~1 mm**
- tissue in the centre of any organism larger than a pin-head would suffocate before
  oxygen reached it by diffusion

---

![:scale_c 40%](images/stroke.png)

???

DWI sequence in acute ischaemic stroke.

---

# Functions of the cardiovascular system

The cardiovascular system exists to solve the diffusion-distance problem — and, once
built, evolution put it to work doing a great deal more besides.

1. **Delivery** of O₂ and nutrients to tissues
2. **Removal** of CO₂ and metabolic waste
3. **Gas exchange** with the environment (pulmonary circulation)
4. **Maintenance** of body fluid volume and composition
5. **Dynamic redistribution** of blood flow to where it's needed
6. **Delivery** of hormones
7. **Immune defence** and tissue repair
8. **Thermoregulation**

.callout.callout-bottom[
The cardiovascular system **delivers**, **removes**, and
**redistributes** — almost everything else it does is a variation on these three
themes.
]

???

You don't need to memorise this list verbatim. If you can group it into deliver / remove / redistribute, you've got the important structure.

---

# Our circulation

![:scale_c 80%](images/plumbing_analogy.jpeg)

???

Hence structure of these two lectures (but working from the bottom up): fluids, tubes, pumps.

---

# Our circulation

The systemic and pulmonary circulations are **in series:**

- Left ventricle `$\rightarrow$` **systemic circulation** (body) `$\rightarrow$` right atrium `$\rightarrow$` right ventricle `$\rightarrow$` **pulmonary ciculation** (lungs) `$\rightarrow$` left atrium `$\rightarrow$` left ventricle `$\rightarrow$` repeat...

<br>

Within the systemic circulation, blood is distributed to different tissues in **parallel**.

]

???

- this is big pictcure wiring diagram (will go into details later)  
- trace out circuit

---
class: section, middle

# Part 1
## Body Fluids

???

**Aim:** Signpost the start of Section 1 (~12 minutes).

**Script cue:** "Let's start with the fluid itself — before we talk about pipes, we
need to know what's flowing through them, and where it all sits."

---

# You are mainly water

- **Total body water** ≈ **60% of body weight in men**, ≈ **55% in women**
- The difference is due to body composition, not sex per se: muscle is water-rich,
  fat is relatively water-poor, and men have proportionally more muscle mass
- Total body water **declines with age** as lean mass falls

.callout.callout-bottom[
**Worked example.** In a 70 kg man: total body water ≈ 70 × 0.6 = **42 litres**.
That's roughly enough to fill a large fish tank — moving around inside you, right
now.
]

---

# Fluid compartments

.pull-left-40[
.compact-table[
| Compartment | Volume | Key solutes |
|:---|:---|:---|
| ICF | 24 L | High K⁺, proteins |
| ECF | 14 L | High Na⁺ |
| — Plasma | 3 L | Na⁺, albumin |
| — Interstitial | 9 L | Na⁺, no protein |
| — Transcellular | 2 L | CSF, synovial, ocular |
| **Blood volume** | **5 L** | Plasma + red cells |
]

![](images/body-fluid-compartments.png)

]

???

Point out that blood volume (5 L) is *not* a separate compartment in
the additive sense — it's plasma (part of ECF) plus the cellular contents of blood.

---

# Osmotic & oncotic pressure

#### Osmosis:
The movement of water across a semi-permeable membrane
from a region of **low** solute concentration to a region of **high** solute
concentration. The driving force can be expressed as an **osmotic pressure**.

]

![:scale_l 60%](images/oncotic_pressure.png)

#### Oncotic pressure:
`$\pi$`: a *special case* of osmotic pressure,
generated specifically by large, impermeable, charged solutes — principally plasma
proteins such as **albumin**. (The "Donnan" effect.)
]

---

# The Na-K-ATPase

- The earliest life evolved in a **potassium-rich** primordial environment —
  intracellular chemistry still reflects this ancient inheritance
- Later, multicellular life evolved bathed in **seawater**, which is
  **sodium-rich** — extracellular fluid still resembles diluted seawater today
- The **Na⁺/K⁺-ATPase pump** actively maintains this ancient divide against the
  natural tendency of ions to diffuse down their gradients (pumps 3Na<sup>+</sup> out and 2K<sup>+</sup> in per cycle, powered by ATP hydrolysis)
]

.callout-question.callout-bottom[
**Apply your knowledge:**<br>
What would happen to your cells if this pump failed?
]

???

"Every cell in your body is a living fossil record of two different
oceans — an ancient potassium-rich one on the inside, a more modern sodium-rich one
on the outside. The wall between them is one small protein pump, working
constantly, your whole life, to keep those oceans apart." Point to the diagram's
3-out/2-in stoichiometry explicitly — this detail is often examined.

Cells are packed with large, negatively charged proteins that **cannot cross the
membrane**. Left unchecked, these exert an osmotic pull on water.

---

# Part 2
## Capillary Exchange

???

**Aim:** Signpost transition into Section 2 (~12 minutes).

**Script cue:** "We've established the compartments and the physical forces
available. Now let's watch those forces do their work at the single most important
interface in the entire circulation: the capillary wall."

---

# Starling forces: a tug-of-war

![:scale_c 40%](images/starling_forces.png)

- **Capillary hydrostatic pressure** (`$P_c$`)
- **Interstitial oncotic pressure** (`$\pi_{if}$`)
]

- **Plasma oncotic pressure** (`$\pi_c$`)
- **Interstitial hydrostatic pressure** (`$P_{if}$`)
]

---

# Starling forces

<br>

`$$J_v = K[(P_c - P_{if}) - \sigma(\pi_c - \pi_{if})]$$`

<br>

.compact-table[
| Term | Meaning |
|:---|:---|
| `$J_v$` | Net fluid movement |
| `$K$` | Hydraulic permeability |
| `$P_c - P_{if}$` | Hydrostatic gradient (out) |
| `$\sigma$` | Reflection coefficient for protein |
| `$\pi_c - \pi_{if}$` | Oncotic gradient (in) |
]

<br>

.callout[
**Under normal conditions**, these forces are nearly balanced, with a small net
filtration drained by the **lymphatic system** (~2–4 L/day).
]

???

What does `$\sigma$` mean?  Lets take a closer look at capillary walls...

---

# The glycocalyx

- A thin layer of **glycosaminoglycans** lining the luminal surface of capillary
  endothelium
- Functions to **retain plasma proteins within the vessel**, helping to preserve
  the oncotic pressure  
- Damage to the glycocalyx (e.g. in sepsis, major trauma) can cause **protein leak**
  and contribute to oedema even when plasma albumin is normal

<br>

![:scale_l 55%](images/glycocalyx.png)

---

# Oedema & ascites

![:scale_l 80%](images/oedema_four.jpeg)

---

# Oedema & ascites

.table-compact[
| Mechanism | Example | 
|:---|:---|:---|
| ↑ Capillary hydrostatic pressure | Venous obstruction (e.g. DVT, heart failure) | 
| ↓ Plasma oncotic pressure | Low albumin (e.g. liver disease, malnutrition) | 
| ↓ Capillary integrity | Stripping of glycocalyx (e.g. sepsis) |
| Lymphatic obstruction | Lymph node blockage (e.g. malignancy, filariasis) | 
]

.callout.callout-bottom[
**Key take-home message.** Oedema occurs when fluid accumulates in the interstitium
**faster than it can be removed** — by reabsorption or by lymphatic drainage.
]

---

# 9 yo boy with puffy eyes

**History**
- 3-day history of periorbital swelling, now with bilateral leg and ankle swelling
- Parents report frothy urine
- No significant past medical history

**Examination findings**
- Bilateral pitting oedema of the lower limbs
- Periorbital oedema

**Investigations**
- Urinalysis: heavy proteinuria (3+ protein)
- Serum albumin: **18 g/L** (normal 35–50 g/L)

.callout-bottom.callout-question[
**Apply your knowledge.**<br>
Why has he developed oedema (fluid outside the vascular space)?  Remember Starling forces.  
]

---

---

# Part 3
## Blood Vessels

---

# The vascular tree

| Vessel | Diameter | Property | Function |
|:---|:---|:---|:---|
| Arteries | ~25 mm | High elastance | Distribute |
| Arterioles | ~100 µm | High resistance | Regulate flow |
| Capillaries | ~8 µm | Thin walls, huge SA | Exchange |
| Veins | ~25 mm | High compliance | Reservoir |

<br>

![](images/vessel_analogies.png)

---

# Compliance *vs* elastance

Tendency to **recoil** when stretched (i.e. accommodate changes in pressure without changes in volume).

![:scale_c 40%](images/hose.jpg)

]

Ability to accommodate a change in volume **without** a large rise in pressure.

![:scale_c 40%](images/balloon.png)
]

.callout[
Arteries are build to **smooth** pressure; veins are built to **store** volume.<br>
Both are highly **dynamic**.    
]

???

~ 60% of the circulating volume sits in systemic veins at rest (= 3L).  Venoconstriction can mobilise ~ 1L of this back into the arterial system.

**Where the analogy breaks down.** A firefighter's hose is inert rubber; the arterial
wall is *living* smooth muscle and elastin that can actively change its own
properties (vasoconstriction/dilation).

Define **vasoconstriction** and **vasodilatation**.

---

# Poiseuille's law

.compact-table[
| Term | Meaning |
|:---|:---|
| `$Q$` | Flow rate |
| `$\Delta P$` | Pressure gradient |
| `$r$` | Vessel radius |
| `$l$` | Vessel length |
| `$\eta$` | Blood viscosity |
]

.callout-warn[
**The single most important detail.** Flow ∝ radius **to the fourth power**. Halve
the radius, flow falls to **1/16**, not one half.
]
]

???

Use concrete numbers: "Halve the radius: flow doesn't halve, it
drops to one-sixteenth. Look at how steep that curve is — this is the entire reason
your body regulates blood flow by adjusting arteriolar diameter rather than heart
rate or vessel length — it's simply the most powerful lever available.

---

# Vessel wall structure

.pull-left[
- **Tunica intima** — single layer of endothelial cells; present in all vessels
- **Tunica media** — smooth muscle and elastic fibres; **thick in arteries, thin
  in veins**
- **Tunica adventitia** — connective tissue; contains:
  - **Nervi vasorum** — nerve supply controlling vessel tone
  - **Vasa vasorum** — blood supply to the vessel wall itself
- **Veins only:** contain **valves** to prevent backflow — essential for venous
  return against gravity
]

---

# Capillary types

![:scale_c 70%](images/capillaries.png)

<br>

- **Continuous** — the default type; tightly joined endothelium
- **Fenestrated** — contain pores; found in kidney and gut
- **Sinusoidal** — large gaps; found in liver and bone marrow

---

# Why is flow so slow in capillaries?

**Parallel plumbing:**

- total cross-sectional area of all capillaries combined vastly exceeds that of
  the aorta
- total flow (cardiac output) must be conserved, so velocity falls sharply where
  cross-sectional area rises
- this dramatic slowing is not a flaw — it is **essential**: it allows enough time
  for diffusion of O₂, CO₂ and nutrients across the capillary wall

<br>

![:scale_c 45%](images/braided_river_delta.jpg)

]

???

Use a metaphor (river delta vs gorge) to explain an easily-confused
concept, reinforced by what is arguably the single highest-yield diagram in this
lecture — the cross-sectional-area/velocity graph is a classic and highly
examinable physiology figure.

Ask rhetorically: "If capillaries are the narrowest vessels in the
body, why is flow through them the *slowest*, not the fastest?" Pause briefly before
revealing the parallel-circuit explanation, then walk through both curves on the
graph explicitly, pointing out that they move in opposite directions at exactly the
same point — the capillary bed.

---

# Pumps

- **Heart**  
- **Skeletal muscle pump** — contraction compresses veins; valves ensure one-way
  flow toward the heart
- **Respiratory pump** — negative intrathoracic pressure during inspiration draws
  blood toward the heart

---

# Summary — lecture 1

#### Body fluids & capillary exchange

- **Body fluids** are `$\frac{2}{3}$` intracellular (`$\text{K}^+$`-rich); `$\frac{1}{3}$` extracellular (`$\text{Na}^+$`-rich)  
- fluid is **distributed** between compartments by osmosis and the **Na-K-ATPase** pump  
- fluid exchange across capillary walls is governed by **Starling forces**: hydrostatic and oncotic pressures and capillary wall integrity

<br>

#### Vessels

- **arteries** distribute and smooth flow, **arterioles** control flow, **capillaries** exchange, **veins** store

.callout-ref.callout-bottom[
**Remember**:
- tomorrow we will add in the heart and learn about how the whole system is controlled  
- all of this will be fleshed out in more detail (Sustaining Life, Semester 2)  
- see primers  
]

---

# Reflection

.callout-question[
Take 🕐 60 seconds for each of these three questions. <br>
Write down, in your own words...
]

<br>

- The most important thing I learned today

<br>

- One thing I am still unclear on

<br>

- One specific thing you will do to enhance your learning by the end of the week

---
class: section, middle

# Thank You

## Any questions?